Mechanistic studies of ethylene hydrophenylation catalyzed by bipyridyl Pt(II) complexes

ARTICLE

pubs.acs.org/JACS

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by

Bipyridyl Pt(II) Complexes

Bradley A. McKeown,^§Hector Emanuel Gonzalez,^‡Max R. Friedfeld,^§T. Brent Gunnoe,*^,§

Thomas R. Cundari,*^,‡and Michal Sabat^§

^§Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

^‡Center for Advanced Scientiﬁc Computing and Modeling (CASCaM), Department of Chemistry, University of North Texas,

Denton, Texas 76203, United States

S Supporting Information

b

ABSTRACT: Cationic platinum(II) complexes [(^tbpy)Pt(Ph)-

(L)]⁺[^tbpy =4,4⁰-di-tert-butyl-2,2⁰-bipyridyl; L = THF, NC₅F₅,

or NCMe] catalyze the hydrophenylation of ethylene to gene-

rate ethylbenzene and isomers of diethylbenzene. Using ethy-

lene as the limiting reagent, an 89% yield of alkyl arene products

is achieved after 4 h at 120 °C. Catalyst eﬃciency for ethylene

hydrophenylation is diminished only slightly under aerobic

conditions. Mechanistic studies support a reaction pathway that

involves ethylene coordination to Pt(II), insertion of ethylene

into the Ptꢀphenyl bond, and subsequent metal-mediated

benzene CꢀH activation. Studies of stoichiometric benzene

(C₆H₆or C₆D₆) CꢀH/CꢀD activation by [(^tbpy)Pt(Ph-d_n)-

(THF)]⁺(n = 0 or 5) indicate a k_H/k_D= 1.4(1), while

comparative rates of ethylene hydrophenylation using C₆H₆and C₆D₆reveal k_H/k_D= 1.8(4) for the overall catalytic reaction.

DFT calculations suggest that the transition state for benzene CꢀH activation is the highest energy species along the catalytic

cycle. In CD₂Cl₂, [(^tbpy)Pt(Ph)(THF)][BAr⁰₄] [Ar⁰= 3,5-bis(triﬂuoromethyl)phenyl] reacts with ethylene to generate

[(^tbpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)][BAr⁰₄] with k_obs= 1.05(4) ꢁ 10^ꢀ3s^ꢀ1(23 °C, [C₂H₄] = 0.10(1) M). In the catalytic

hydrophenylation of ethylene, substantial amounts of diethylbenzenes are produced, and experimental studies suggest that the

selectivity for the monoalkylated arene is diminished due to a second aromatic CꢀH activation competing with ethylbenzene

dissociation.

’ INTRODUCTION

acylation followed by Clemmenson or WolﬀꢀKishner reduction.

Finally, achieving 1,2-, 1,3-, or 1,4-selectivity for the preparation

of dialkyl benzenes is a challenge with FriedelꢀCrafts-type

catalysts. Solid-state catalysts, largely based on zeolite materials,

have provided improvements, but these catalysts function by the

same general mechanism as traditional FriedelꢀCrafts catalysts

and have some of the same limitations.^3ꢀ11Despite the advance-

ments with zeolite-based catalysts, it has been estimated that

approximately 40% of global ethylbenzene is still produced using

processes with aluminum-based catalysts.⁵

The formation of alkyl arenes by transition metal-mediated

addition of aromatic CꢀH bonds across unsaturated CdC

bonds, oleﬁn hydroarylation, is of considerable importance

(eq 1).¹Several billion pounds of alkyl aromatics are produced

annually and used for plastics, fuels, pharmaceuticals, and

surfactants. Historically, FriedelꢀCrafts catalysts have been

used to prepare alkyl arenes including the large-scale synthesis

of ethylbenzene, cumene, and long-chain alkyl arenes.²However,

such systems suﬀer from drawbacks that are inherent to the

mechanism by which the catalysts function.³For example, the

use of a Brønsted acid cocatalyst, often HF, can be problematic

due to its corrosive nature, and recycling of the catalyst is not

possible due to catalyst decomposition upon neutralization of the

acidic solution during product isolation. In addition, polyalkyla-

tion is common with FriedelꢀCrafts catalysts, and the produc-

tion of monoalkylated arenes in high yield on an industrial scale

often requires a second step with a transalkylation catalyst.^3ꢀ6

Furthermore, with FriedelꢀCrafts catalysts, the selective pro-

duction of linear alkyl arenes from α-oleﬁns is not possible.

The synthesis of linear alkyl arenes requires a FriedelꢀCrafts

The development of transition metal-based catalysts for the

hydroarylation of oleﬁns that operate by oleﬁn insertion into

Received: June 30, 2011

Published: November 08, 2011

r

2011 American Chemical Society

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metalꢀaryl bonds followed by aromatic CꢀH activation oﬀers a

complementary route to alkyl arenes. It is possible that such

catalysts can be tuned to achieve selectivity including mono-

versus polyalkylation, linear versus branched alkyl chains, as well

as regioselective formation of dialkylated aromatic substrates.

Several transition metal systems have been demonstrated to catalyze

oleﬁn hydroarylation, but substrate scope is often limited to

heteroaromatic compounds or substrates that possess at least one

activating group.^1,12ꢀ17Homogeneous catalysts that initiate oleﬁn

hydroarylation of unactivated substrates (i.e., hydrocarbons) have

been reported, but these are limited to a few catalyst systems.^18ꢀ26

Despite the importance of aromatic alkylations, detailed

mechanistic studies of transition metal catalysts that are active

for conversion of alkenes and arenes to alkyl arenes remain

relatively rare.^{18,19,21,26ꢀ28}Ir(III) systems of the type (acac)₂-

Ir(Ph)(L) (acac = k²-O,O-acetylacetonate, L = H₂O or pyridine)

have been studied by Matsumoto et al. and Oxgaard et al.^20,28

Goldberg and co-workers have reported that (dmpp)PtMe₃

[dmpp = 3,5-dimethyl-2-(2-pyridyl)pyrrolide] successfully cata-

lyzes oleﬁn hydrophenylation to yield alkyl benzenes.²¹Tilley,

Bell, and Karshtedt found that a PtꢀAg catalyst system and

Pt(II) catalysts supported by pyridylꢀpyrrolyl ligands initiate

norbornene hydroarylation, but the observed product selectivity

suggests the possibility of a FriedelꢀCrafts-type pathway.^22,29

Particularly germane is a recent study that reveals catalytic oleﬁn

hydroarylation using Pt(II) triﬂate salts likely proceeds by

formation of triﬂic acid, which serves as the active catalyst.³⁰

Our groups reported a series of studies on TpRu(L)(NCMe)-

(Ph) [Tp = hydridotris(pyrazolyl)borate, L = CO, PMe₃, P(N-

pyrrolyl)₃, or P(OCH₂)₃CEt] catalysts for oleﬁn hydroary-

lation.^{18,19,23ꢀ26,31}The primary focus has been to elucidate the

eﬀect of the donor and steric properties of the ligand L on the key

steps of the catalytic cycle (i.e., oleﬁn insertion and aromatic/

oleﬁn CꢀH activation).¹⁹In an eﬀort to understand more

broadly transition metal catalyzed oleﬁn hydroarylation, we have

begun to probe structure/activity relationships for catalysts

outside the octahedral/d⁶motif. Platinum(II) seemed well suited

for this transformation as it is known to readily activate CꢀH

bonds as well as undergo oleﬁn insertion.^32ꢀ42In a preliminary

communication,⁴³we reported that in situ chloride removal from

(^tbpy)Pt(Ph)Cl (^tbpy =4,4⁰-di-tert-butyl-2,2⁰-bipyridyl) using

NaBAr⁰₄[Ar⁰= 3,5-bis(triﬂuoromethyl)phenyl] provides a cat-

alyst for oleﬁn hydroarylation. Herein, we report a more detailed

experimental/computational investigation of catalytic hydrophe-

nylation of ethylene by [(^tbpy)Pt(Ph)(L)]⁺(L = NCMe, NC₅F₅

or THF) complexes in an eﬀort to delineate the reaction

mechanism and develop a better understanding of the individual

steps in the catalytic cycle.

Figure 1. ORTEP of [(^tbpy)Pt(NCMe)(Ph)][BAr⁰₄] (3) (50% prob-

ability; H atoms and BAr⁰₄anion omitted for clarity). Selected bond

lengths (Å): PtꢀN1 2.092(4), PtꢀN2 2.000(4), PtꢀN3 1.969(4),

PtꢀC1 2.004(5). Selected bond angles (°): N1ꢀPtꢀN2 79.3(2),

C1ꢀPtꢀN3 88.3(2), PtꢀN3ꢀC7 169.7(4).

A crystal of complex 3 suitable for an X-ray diffraction study was

grown (Figure 1). The N1ꢀPtꢀN2 bond angle is compressed to

79.3(2)° relative to the ideal 90° bond angles, which is character-

istic of Pt(II) bipyridyl and diimine complexes.^35,44,45The PtꢀN2

bond is 0.09 Å shorter than the PtꢀN1 bond, indicative of a greater

trans influence of the phenyl ligand relative to acetonitrile.

We probed complexes 1ꢀ3 as catalyst precursors for the

hydrophenylation of ethylene. Heating a benzene solution of 1, 2,

or 3 in the presence of ethylene results in the production of

ethylbenzene and diethylbenzenes (Table 1). At 100 °C under

0.1 MPa of ethylene, a solution of 0.025 mol % 1 (relative to

benzene) results in 15.7 turnovers (TO) of ethylbenzene and 3.6

TO of diethylbenzenes after 4 h, corresponding to a turnover

frequency (TOF) of 1.3 ꢁ 10^ꢀ3s^ꢀ1(the formation of diethyl-

benzene is counted as a single catalytic TO; unless otherwise

noted, all TOFs are calculated based on product after 4 h of

reaction). After 16 h, 63.5 TO (ethylbenzene and diethyl-

benzenes) were observed. A solution of 1 (0.04 M, C₆D₆) was

pressurized with ethylene (0.1 MPa) and monitored by ¹H NMR

at 80 °C. Ethylbenzene was produced, and [(^tbpy)Pt(CH₂-

’ RESULTS AND DISCUSSION

1

CH₂Ph)(η²-C₂H₄]⁺was the only Pt complex observed by H

Catalytic Hydrophenylation of Ethylene. The complex

[(^tbpy)Pt(Ph)(THF)][BAr⁰₄] (1) has been isolated in 93% yield

from the reaction (ꢀ60 °C) of (^tbpy)Pt(Ph)₂with HBAr⁰₄in

tetrahydrofuran. In CD₂Cl₂, the ¹H and ¹³C NMR spectra of 1

reveal inequivalent pyridyl groups, and the resonances for

coordinated THF (¹H NMR) are observed as broad multiplets

at 4.14 and 1.86 ppm. From complex 1, [(^tbpy)Pt(Ph)(L)]⁺

variants can be prepared by substitution of THF with other

ligands. For example, stirring complex 1 in neat perfluoropyridine

or acetonitrile yields the complexes [(^tbpy)Pt(Ph)(NC₅F₅)]-

[BAr⁰₄] (2) and [(^tbpy)Pt(Ph)(NCMe)][BAr⁰₄] (3) (eq 2).

NMR spectroscopy during the catalytic reaction. The catalysis

appears to tolerate aerobic conditions. Setting up the reaction of

benzene and ethylene (100 °C, 0.025 mol % 1) on the benchtop

(in air) resulted in 17.4 and 55.7 total TO after 4 and 16 h,

respectively (compared to 19.3 and 63.5 total TO for the reaction

prepared under a dinitrogen atmosphere). Using a calibrated gas

buret, a catalytic reaction with a known quantity of ethylene (0.03

MPa, 4.2 mmol) as the limiting reagent was arranged. Heating

the reaction mixture at 120 °C for 4 h resulted in the production

of 119.3 TO of ethylbenzene, diethylbenzenes, and styrene,

which corresponds to an 89% yield (eq 3). In this reaction, since

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Table 1. Catalytic Hydrophenylation of Ethylene Using

Table 2. Impact of Temperature and Ethylene Concentration

on Catalysis Using [(^tbpy)Pt(Ph)(THF)][BAr⁰₄] (1).^a

Complexes 1ꢀ3 As Catalysts^a

^aCatalyst (0.025 mol %) dissolved in C₆H₆with hexamethylbenzene as

internal standard at 100 °C. ^bRatio of 1,2-, 1,3-, and 1,4-diethylbenzene

after 4 h. ^cTurnover frequency calculated after 4 h with the formation of

diethylbenzenes counted as one turnover. ^dTurnovers after 4 h as

determined by GC/MS (see Experimental Section for details). ^eNum-

bers in parentheses are turnovers after 16 h.

ethylene was the limiting reagent, diethylbenzene production

was counted as two TO.

^aCatalyst (0.025 mol %) dissolved in C₆H₆with hexamethylbenzene as

an internal standard at 100 °C. ^bRatio of 1,2-, 1,3-, and 1,4-diethylbenzene

after 4 h. ^cTurnover frequency calculated after 4 h; formation of

diethylbenzenes counted as one turnover. ^dTurnovers after 4 h as

determined by GC/MS. ^eNumbers in parentheses are turnovers after 16 h.

Substitution of THF with NC₅F₅reduces the extent of

ethylbenzene production after 4 h by approximately 50%

(Table 1). For example, using 2 at 100 °C results in 8.1 TO of

ethylbenzene and 2.4 TO of diethylbenzenes, which gives a TOF

ꢀ1

of 0.7 ꢁ 10^ꢀ3

s

. Complex 3 performed similarly to 2 with a

TOF of 0.6 ꢁ 10^ꢀ3s^ꢀ1and ∼9.2 TO after 4 h.

Using complex 1, we probed the inﬂuence of temperature and

pressure (Table 2). With increasing temperature, the TOF of

ethylbenzene production increases but catalyst decomposition is

accelerated. At 90 °C under 0.1 MPa of ethylene pressure, a

solution of 1 in benzene results in a TOF of 0.4 ꢁ 10^ꢀ3s^ꢀ1

.

Increasing the temperature to 100 °C eﬀectively triples the rate of

ethylene hydrophenylation compared to the reaction at 90 °C,

and at 120 °C the rate increases by a factor of ∼18 (compared to

Figure 2. Catalysis using complex 1, benzene, and dynamic ethylene

pressure maintained at 0.9 MPa (100 °C; R²= 0.99).

the same reaction at 90 °C) with a TOF of 7.1 ꢁ 10^ꢀ3s^ꢀ1

.

At 90 °C, 100 °C and 120 °C, increasing the ethylene pressure

from 0.1 to 0.3 MPa of ethylene results in a ∼63% to 75%

reduction in the amount of ethylbenzene.

species was varied. We used conditions where the rate of catalysis

was monitored through e5 TOs since data from early in the

reaction is not influenced by catalyst decomposition. To show

the dependence of rate on the concentration of each substrate,

we plotted the concentration of ethylbenzene, at a specified

time, versus the concentration of 1, ethylene or benzene

(Figures 3ꢀ5). The production of ethylbenzene displays a

first-order dependence on the concentration of 1 (Figure 3).

Catalytic reactions performed in NC₅F₅with varying concentra-

tions of benzene (0.1 to 2.7 M) display saturation kinetics

(Figure 4). Catalysis was studied over an ethylene concentration

range of 0.04 to 0.13 M, and catalyst activity decreases with

increasing ethylene concentration, and saturation kinetics are

observed at higher ethylene concentrations (Figure 5). Similar

effects of increased olefin concentration on rate of catalysis

have been observed with Ru(II) systems.¹⁸Comparative rates

Although increased ethylene pressure suppresses the rate of

catalysis, we postulated that catalyst longevity might be enhanced

by continuous presence of ethylene. A catalytic reaction under

dynamic ethylene pressure was analyzed over ∼70 h. After

purging the reaction vessel with ethylene and pressurizing with

N₂(0.9 MPa), the reaction was then exposed to a constant source

of ethylene (0.9 MPa) and heated at 100 °C. A plot of TO versus

time demonstrates a linear relationship with no evidence of

catalyst decomposition (Figure 2).

Kinetic Studies and Isotope Effects. To determine the

dependence of catalytic ethylbenzene production on the con-

centrations of 1, ethylene and benzene, three sets of kinetic

experiments were performed where the concentrations of two

species were held constant and the concentration of the third

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Figure 3. Plot of [ethylbenzene] (M) after 30 min as a function of [1]

(M) at 100 °C with 0.1 MPa of ethylene in neat benzene (R²= 0.99).

Figure 5. Plot of [ethylbenzene] (M) after 2 h as a function of

[ethylene] (M) at 100 °C in C₆D₆and [1] = 0.028 M.

4.6(4) ꢁ 10^ꢀ5s^ꢀ1. At low concentrations of C₆D₆, a ﬁrst-order

dependence on concentration of C₆D₆is observed, and as the

concentration of C₆D₆is increased, saturation conditions are

reached (Figure 7).

Two pathways for stoichiometric benzene CꢀH activation by

1 are shown in Scheme 2, which are distinguished by associative

versus dissociative pathways for benzene/THF exchange. Rate

laws were derived assuming that benzene CꢀH/D coordination

is the rate-limiting step and by applying the steady-state approxi-

mation (reaction intermediates are not observed). The small

primary KIE of 1.4(1) is consistent with similarly small KIEs

(k_H/k_D∼1.1ꢀ1.4) for CꢀH/D activation by d⁸metal centers,

which have been interpreted as rate-determining CꢀH coordina-

tion.^46,48,49Rate-limiting CꢀH/D activation by d⁸metals have

been reported to exhibit KIEs g2.5.^50,51Both pathways predict

an inverse dependence on the concentration of THF; however,

only the rate law for the dissociative pathway is consistent with

the observation of saturation kinetics for variation of benzene

concentration. Thus, we suggest that the dissociative pathway

with rate-determining CꢀH coordination is most likely. Studies

of CꢀH activation by other cationic Pt(II) systems with hydro-

carbyl ligands suggest similar pathways that likely involve three-

coordinate Pt complexes.⁴¹It should be noted that rapid pre-

equilibrium conditions would be consistent with either

pathway.⁵²The KIE (C₆H₆vs C₆D₆) for the catalytic cycle is

larger [k_H/k_D= 1.8(4)], but the deviation is also larger. Thus, the

KIE for the catalytic cycle is consistent with either rate-limiting

CꢀH coordination or CꢀH activation. A change of the rate-

determining step for stoichiometric benzene CꢀH activation

versus the catalytic cycle might be explained by a similar

magnitude for the two steps (i.e., CꢀH coordination and

CꢀH bond breaking) and a change in relative barrier height

for CꢀH activation by a Ptꢀphenyl versus PtꢀCH₂CH₂Ph.

Computational Study of Benzene CꢀH Activation by

[(^tbpy)Pt(CH₂CH₂Ph)]⁺. The mechanisms by which Pt(II) sys-

tems activate the CꢀH bonds of hydrocarbons have been studied

extensively,^41,53ꢀ55and the two mechanisms most commonly

invoked are electrophilic substitution and oxidative addition

(Scheme 3).^{32,41,46,56ꢀ59}In addition, recent studies suggest

that exchange of phenyl groups in [(NꢀN)Pt(Ph)(C₆H₆)]⁺

Figure 4. Plot of [ethylbenzene] (M) after 2 h as a function of

[benzene] (M) at 100 °C in perﬂuoropyridine with [ethylene] =

0.16(5) M and [1] = 0.03 M.

of ethylene hydrophenylation with C₆H₆and C₆D₆were used to

determine the kinetic isotope effect (KIE) for catalytic reactions.

In separate experiments that incorporated either C₆H₆or C₆D₆,

using the ratio of total TO after 4 h of reaction (taken from

multiple experiments under identical conditions) a KIE for the

catalytic reaction of 1.8(4) was determined.

To study stoichiometric benzene CꢀH activation by

[(^tbpy)Pt(R)]⁺systems, we used the reaction of [(^tbpy)Pt-

(Phꢀd_n)(THF)]⁺[n = 5 (1-d₅) or 0 (1)] with excess C₆H₆or

C₆D₆. The reaction of 1-d₅and C₆H₆at 21 °C in CD₂Cl₂leads to

the formation of 1 and C₆D₅H with an observed pseudo-ﬁrst-

order rate constant of 7.1(4) ꢁ 10^ꢀ5

s

^ꢀ1. A pseudo-ﬁrst-order

rate constant of 5.0(3) ꢁ 10^ꢀ5s^ꢀ1is observed for the same

reaction between 1 and C₆D₆to form 1-d₅and C₆H₅D. The

activation of a benzene CꢀH/D bond results in a KIE of 1.4(1)

(Scheme 1), which is statistically indistinguishable from the

catalytic KIE, and the magnitude of the KIE is similar to that

previously observed for arene and aliphatic CꢀH activation by

Pt(II) systems.^32,34,46,47The impact of the weakly coordinating

Lewis base THF on the rate of benzene CꢀD activation by 1 was

investigated. The reaction rate decreases with increasing THF

concentration (Figure 6). At 30 °C in CD₂Cl₂, the conversion of

1 to 1-d₅with 0.18 M C₆D₆proceeds with an observed rate of

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Scheme 1. Rates of Degenerate Benzene CꢀH/D Activation by 1 and 1-d₅in CD₂Cl₂([Pt] = 0.03 M, [benzene] = 0.5 M, 21 °C)

have been coined “σ-complex assisted metathesis” (σ-CAM) or

oxidative hydrogen migration.^27,61ꢀ67

In our computational modeling, we considered aromatic CꢀH

activation by [(^tbpy)Pt(CH₂CH₂Ph)]⁺, which is likely the

catalytically relevant species, via σ-bond metathesis and oxidative

addition/reductive elimination pathways. For all simulations, the

parent 2,2⁰-bipyridine (bpy) was used in place of ^tbpy. A two-step

oxidative addition/reductive elimination pathway was calculated

to be favored over a one-step σ-bond metathesis mechanism by

2.3 kcal/mol (Scheme 4). The core geometries of the three

transition states (i.e., transition states for σ-bond metathesis,

oxidative addition and reductive elimination), plus the ﬁve-

coordinate hydride intermediate, are shown in Figure 8. All of

these species are calculated to have short PtꢀH distances, with the

longest (1.68 Å) being the kite-shaped σ-bond metathesis transi-

Figure 6. Observed rate of benzene CꢀD activation by 1 as a function of

tion state, for which the H atom is transferred in the equatorial

plane from the aryl ring to the alkyl fragment. This distance is

THF concentration in CD₃NO₂([1] = 0.03 M, [C₆D₆] = 0.5 M, 45 °C).

similar to bona ﬁde bond distances of isolated Ptꢀhydride

complexes, which are in the range of 1.63ꢀ1.68 Å.^68,69

Ethylene Insertion. In contrast to Ni and Pd systems, Pt

complexes have not been reported to be efficient catalysts for

olefin polymerization,⁷⁰and five-coordinate Pt(II) complexes

with coordinated olefin have been characterized.^71ꢀ76Studies of

olefin insertion into Pt(II) hydrocarbyl bonds suggest that

insertion of unactivated olefins into PtꢀR bonds is feasible for

cationic systems when R = aryl;^74,75,77however, insertions appear

to have more substantial barriers when R = alkyl^74,75,77and for

overall charge neutral systems.^72,76Ruffo et al. have reported the

insertion of electron-deficient olefins into the Pt(II)ꢀMe bond of

a cationic system.⁷⁸In addition, Templeton and co-workers have

reported ethylene insertion into a Ptꢀphenyl bond at 80 °C, after

which intramolecular CꢀH activation of ethylbenzene occurs to

form a stable Pt(IV) ortho-metalated phenethyl complex.⁴²

The reaction of 1 or 2 with ethylene at room temperature in

dichloromethane produces [(^tbpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)]-

Figure 7. Observed rate of benzene CꢀD activation by 1 as a function

of benzene concentration in CD₂Cl₂. ([1] = 0.03 M, 30 °C).

[BAr⁰₄] (4) in 97% isolated yield for complex 1 (eq 4). In the ¹H

NMR spectrum (CD₂Cl₂, room temperature) of 4, a single

resonance is observed at 4.14 ppm for the coordinated ethylene,

which indicates a ﬂuxional process that is rapid on the NMR time

scale. A suitable crystal of 4 was grown for an X-ray diﬀraction

study (Figure 9). The PtꢀN1 bond trans to the η²-ethylene is

shortened by 0.06 Å relative to the PtꢀN2 bond trans to the alkyl

ligand, presumably due to the stronger trans inﬂuence of the alkyl

(NꢀN = ArNd CMeꢀCMedNAr, Ar = 2,6-Me₂C₆H₃) occurs

by a σ-bond metathesis pathway with a calculated activation

energy ∼5.0 kcal/mol lower than that calculated for oxidative

addition.⁶⁰The calculated transition state is similar to that

proposed for σ-bond metathesis-type transition states in which

the metal appears to interact with the activated hydrogen, which

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Scheme 2. Two Possible Mechanisms for Stoichiometric Benzene CꢀH Activation by 1 ([Pt] = [(^tbpy)Pt])

Scheme 3. Pt(II)-Mediated CꢀH Activation via Electrophi-

lic Substitution, Oxidative Addition or σ-Bond Metathesis

Scheme 4. Calculated Reaction Coordinate (ΔG in THF,

kcal/mol) Comparing a Two-Step (Oxidative Addition/

Reductive Elimination) Pathway via a Pt(IV)ꢀHydride with a

One-Step (σ-Bond metathesis, in red) Pathway for Benzene

CꢀH Activation from [(bpy)Pt(CH₂CH₂Ph)(η²-C₆H₆)]⁺

ligand.^45,79The phenyl ring is oriented over the cis-pyridyl ring,

and the PtꢀC44 bond is distorted by ∼11.7° out of the square

plane. The CꢀC bond of ethylene is oriented perpendicular to

the coordination square plane and only slightly elongated

[1.393(6) Å] relative to uncoordinated ethylene (1.339 Å),⁸⁰

which is more representative of a CdC moiety than a metalla-

cyclopropane, similar to that observed for other Pt(II)ꢀoleﬁn

complexes.^45,79

room temperature, and the reaction rate at this low temperature

implicates that the process is a viable step in the catalytic ethylene

hydrophenylation transformation at 100 °C. The dependence of

the rate of conversion of 1 and ethylene to 4 on ethylene

concentration (0.1 to 1.5 M) was probed. At low concentrations

Monitoring the rate of conversion of 1 (0.03 M) and ethylene

(0.1 M) to 4 in CD₂Cl₂under pseudo-ﬁrstꢀorder conditions at

23 °C reveals a k_obs= 1.07(2) ꢁ 10^ꢀ3s^ꢀ1(Figure 10). Thus,

complex 1 is capable of ethylene coordination and insertion at

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Figure 8. Calculated structures of pertinent stationary points (see Scheme 4) from [(bpy)Pt(CH₂CH₂Ph)(η²-C,C-C₆H₆)]⁺. Shown are the transition

states for benzene CꢀH oxidative addition (A), ethylbenzene reductive elimination (B), and σ-bond metathesis (C) in addition to a ﬁve-coordinate,

Ptꢀhydride intermediate (D) that is the successor to the oxidative addition transition state and the precursor to the reductive elimination transition

state. Only the two nitrogens of the bipyridyl ligand are shown to increase the clarity of the Pt core.

of ethylene, the rate increases approximately linearly with

increasing ethylene concentration, and at higher ethylene con-

centrations saturation is observed (Figure 11). The addition of 2,

5, and 10 equiv (0.06 to 0.3 M) of THF-d₈(relative to 1) had

negligible impact on the rate of insertion (Figure 12).

associative pathway, the lowest energy isomer of the ethylene

adduct, [(bpy)Pt(Ph)(THF)(η²-C₂H₄)]⁺, has the ethylene

bound to Pt with THF weakly associated, viz. [(bpy)Pt(Ph)-

(η²-C₂H₄)]⁺(THF). From [(bpy)Pt(Ph)(η²-C₂H₄)]⁺(THF),

3

the most stable calculated transition state for ethylene insertion has

Two pathways were considered for ethylene coordination and

insertion, an associative and a dissociative process. Scheme 5

shows the two reaction pathways and the corresponding rate

laws, which were derived using the steady-state approximation.

Both rate laws predict saturation kinetics as a function of ethylene

concentration; however, the independence of the rate on the

concentration of THF is most consistent with the associa-

tive mechanism (Scheme 5). At high concentrations of ethylene,

the rate law for the associative process becomes Rate = k₂[1].

Using the ﬁrst-order rate constant for the formation of 4 at 1.5 M

ethylene of 2.1(1) ꢁ 10^ꢀ3s^ꢀ1(23 °C) corresponds to a ΔG^q=

21.0(1) kcal/mol, which is similar to the ΔG^qof 19.2 kcal/mol

(64 °C) determined by Brookhart, Templeton, et al. for

ethylene insertion into the PtꢀH bond of [(NꢀN)Pt(H)-

(ethylene)]⁺[NꢀN = (2,6-Me₂C₆H₃)NdC(An)ꢀC(An)d

N(2,6-Me₂C₆H₃); An = 1,8-naphthalenediyl].⁴⁵

THF in the outer coordination sphere and 19.0 kcal/mol relative

to [(bpy)Pt(Ph)(η²-C₂H₄)]⁺(THF) (Scheme 6). Searches

3

for transition states for CdC insertion into the PtꢀPh bond

of [(bpy)Pt(Ph)(η²-C₂H₄)(THF)]⁺with THF in the inner

coordination sphere resulted in a high-energy transition state

with dissociation of one of the bpy arms. The dissociative

displacement of THF with ethylene is calculated to be exergonic

by 12.5 kcal/mol and features a free energy barrier of 19.7 kcal/mol

for ethylene insertion into the Ptꢀphenyl bond from 7⁰to

(7ꢀ8)⁰. Thus, the overall calculated ΔG^qfor the dissociative

pathway is 0.7 kcal/mol greater than that for the associative

pathway, which is insuﬃcient to diﬀerentiate the two pathways

solely on the basis of the calculations; however, the combination

of kinetic experiments and computational analysis suggests a

process that has associative character in what might be best

considered as an interchange associative pathway.⁸¹

Simulations were used to compare associative and dissociative

mechanisms for ethylene insertion starting from [(bpy)Pt(Ph)-

(THF)]⁺(1⁰) (Scheme 6). The associative pathway has a calcu-

lated ΔG^qof 19.0 kcal/mol, which is close to the experimentally

determined activation barrier of 21.0(1) kcal/mol (23.0 °C).

After evaluation of multiple coordination isomers for the

Proposed Mechanism for the Catalytic Cycle. Metal-

mediated olefin hydroarylation can proceed by a FriedelꢀCrafts

pathway or by metal-mediated olefin insertion into a metal-

phenyl bond followed by aromatic CꢀH activation. As antici-

pated for relatively electron-rich octahedral d⁶systems, results

from studies of Ru(II) and Ir(III) catalysts are consistent with a

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Figure 11. Plot of k_obsfor the conversion of 1 and ethylene to 4 as a

function of ethylene concentration (M) ([1] = 0.03 M, 23.0 °C).

Figure 9. ORTEP of [(^tbpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)][BAr⁰₄] (4)

(50% probability; H atoms and BAr⁰₄anion omitted for clarity) Selected

bond lengths (Å): PtꢀN1 2.069(4), PtꢀN2 2.127(4), PtꢀC3 2.052(5),

PtꢀC1 2.144(5), PtꢀC2 2.118(4), C1ꢀC2 1.393(6). Selected bond

angles (deg): N1ꢀPtꢀN2 78.4(1), PtꢀC3ꢀC4 111.3(3), C3ꢀC4ꢀC5

115.5(3). View on the bottom shows stacking of phenyl group of

phenethyl ligand and pyridyl of ^tbpy ligand.

Figure 12. Plot of k_obsfor the conversion of 1 and ethylene to 4 as

a function of THF concentration (M) ([1] = 0.03 M, [C₂H₄] =

0.11(2) M, 23.0 °C).

methoxy substituents.⁸²Evidence that the hydroarylation of

cyclohexene using L₂Pt(OTf)₂(L₂= 1,5-cyclooctadiene or ^tbpy,

OTf = trifluoromethanesulfonate) likely proceeds via the gen-

eration of HOTf, which is the active catalyst, has been

disclosed.³⁰

The hydrophenylation of ethylene (0.1 MPa) using0.025 mol %

HBAr⁰₄results in ∼1 TO of ethylbenzene after 16 h at 120 °C.

In addition, catalysis with 1 in the presence of 2,6-di-tert-butyl-4-

methylpyridine (1.5 equiv relative to 1), which gave 16.8 TO of

ethylbenzenes after 4 h, was not suppressed. These results provide

strong evidence against the in situ generation of a Brønsted acid

serving as the active catalyst using [(^tbpy)Pt(L)(Ph)]⁺systems.

We propose two possible mechanisms for ethylene hydro-

phenylation by [(^tbpy)Pt(Ph)]⁺(Scheme 7) with benzene CꢀH

activation as the rate-determining step in each (rate-limiting

CꢀH coordination is equally viable, but this change does not

impact the analysis of the proposed pathway). Both cycles are

initiated by the substitution of THF, NC₅F₅or NCMe from the

catalyst precursor with ethylene to give [(^tbpy)Pt(η²-C₂H₄)(Ph)]⁺.

Figure 10. Representative kinetic plot for the conversion of 1 to 4

([1] = 0.031 M, [C₂H₄] = 0.1 M, 23 °C).

non-FriedelꢀCrafts pathway.^{18ꢀ20,23,25,27,28}In contrast, reaction

pathways are likely to be less predictable with later transition

metals, such as Pt(II), that, in general, have enhanced electro-

philicity. For example, Vitagliano et al. have reported that

Ptꢀethylene complexes are susceptible to nucleophilic addition

by electron-rich aromatic compounds that are activated by

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Insertion of ethylene into the Ptꢀphenyl bond generates the

coordinatively unsaturated complex [(^tbpy)Pt(CH₂CH₂Ph)]⁺.

The coordination of ethylene to [(^tbpy)Pt(CH₂CH₂Ph)]⁺

forms [(^tbpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)]⁺(4), which has been

identiﬁed by ¹H NMR spectroscopy as the catalyst resting state.

In mechanism A, the catalyst resting state is incorporated into

the catalytic cycle, and the subsequent exchange of ethylene for

benzene occurs through an associative process to yield

[(^tbpy)Pt(CH₂CH₂Ph)(η²-C₆H₆)]⁺. In mechanism B, the for-

mation of 4 removes Pt(II) from the catalytic cycle. Dissocia-

tion of ethylene from 4 allows re-entry of Pt(II) into the cycle

and is followed by the coordination of benzene to a three-

coordinate intermediate, which precedes rate-determining

CꢀH activation. Subsequent aromatic CꢀH activation with

transfer of the H atom from benzene to the phenethyl ligand

produces coordinated ethylbenzene, which is displaced by

ethylene, regardless of the pathway (e.g., associative versus

dissociative ligand exchange), to regenerate the catalyst.

Rate laws for the two mechanisms in Scheme 7 were derived

using the KingꢀAltman method.⁸³The derived rate laws are

shown in Scheme 7 (for derivations, see Supporting Informa-

tion). Since kinetic experiments demonstrated kinetic saturation

at ∼2 M benzene (Figure 4), and the catalytic reactions are

performed in neat benzene (∼11 M), we assume that terms

containing concentration in benzene in the denominator would

dominate. This provides the simpliﬁed forms of the rate laws

shown in Scheme 7. Both of the rate laws predict saturation

kinetics for ethylene and a ﬁrst-order dependence on catalyst

concentration, which are consistent with experimental observa-

tions; however, the kinetic experiments do not allow us to

diﬀerentiate between mechanisms A and B.

Scheme 5. Two Possible Mechanisms for the Formation of 4

from 1 and Ethylene {[Pt] = [(^tbpy)Pt]}; Kinetic Data Are

Consistent with the Associative Pathway

Goldberg et al. have reported a non-FriedelꢀCrafts mechan-

ism for charge neutral Pt(II) catalysts supported by pyridylꢀ

pyrrolyl ligands.²¹Evidence suggests that ethylbenzene is pro-

duced from the reaction of (dmpp)Pt(CH₂CH₂Ph)(η²-C₂H₄)

with benzene via orthometalation of the phenethyl ligand to form

(dmpp)Pt(o-C₆H₄Et), followed by CꢀH activation of coordi-

nated benzene to release ethylbenzene. These results and

proposed mechanism are similar to those reported by De Renzi

et al. for stoichiometric reactions that involve ethylene insertion

into cationic Pt-Ph bonds.^77,84The ﬁndings of our study are

consistent with CꢀH activation of coordinated benzene being

the predominant mode of producing ethylbenzene, but the

isotopic labeling studies indicate that cyclometalation of the

phenethyl ligand is competitive (see Supporting Information for

details).

Scheme 6. Calculated Energetics (ΔG in THF, kcal/mol) for Two Possible Mechanisms for Ethylene Insertion into PtꢀPh Bond

Starting from [(bpy)Pt(Ph)(THF)][BAr⁰₄] (1⁰)

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Scheme 7. Two Proposed Mechanisms for Ethylene Hydrophenylation Catalyzed by [(^tbpy)Pt(Ph)]⁺

Calculated Energetics of Ethylene Hydrophenylation. The

energetics of the proposed catalytic cycle were probed using DFT

calculations (Scheme 8). Complex 4 has been identified experi-

mentally as the catalyst resting state, and our DFT calculations

suggest it is the lowest energy species in Scheme 8. Dissociation

of ethylene from 4⁰produces 8⁰, which has two calculated

conformers, a π complex (8⁰_π) and a β-agostic (8⁰_β) structure

(Figure 13). Complex 8⁰_πand 8⁰_βare 7.9 and 12.9 kcal/mol,

respectively, less favorable than 4⁰. In the latter conformer, one of

the benzylic CꢀH bonds of the phenethyl ligand occupies the

vacant site in the Pt coordination sphere in an agostic interaction

with the metal center, as opposed to η²-coordination through a

CdC bond of the phenyl group for 8⁰_π. Conformer 8⁰_πis

calculated to be more stable than 8⁰_βby 5 kcal/mol. The calculated

β-agostic structure for 8⁰_βshows a lengthening of one of the CꢀH

bonds (1.33 Å) of the β-CH₂group (Figure 13) by 0.2 Å versus a

typical CꢀH bond length for 8⁰_β. The C_sp3ꢀC_sp3bond length for

8⁰_βis calculated to be relatively short at 1.49 Å, suggesting some

[(bpy)Pt(H)(styrene)]⁺character for this stationary point.

The complex [(bpy)Pt(CH₂CH₂Ph)]⁺(either 8⁰_βor 8⁰_π)

may coordinate and subsequently activate benzene for CꢀH

bond cleavage. The ΔG for binding of benzene to form [(bpy)-

Pt(CH₂CH₂Ph)(η²-C₆H₆)]⁺is calculated to be endergonic at

+12.8 kcal/mol relative to 8⁰_πand +20.7 kcal/mol from 4⁰

(Scheme 8). The calculated ΔG^qfor benzene CꢀH activation by

an oxidative addition/reductive elimination pathway is 18.0 kcal/mol

from the benzene complex [(bpy)Pt(CH₂CH₂Ph)(η²-C₆H₆)]⁺.

Two low energy conformations of [(bpy)Pt(Ph)(ethylbenzene)]⁺

(9⁰) were isolated via calculations, one in which the ethylbenzene

coordinates by an agostic interaction with the CꢀH bond of its

methyl group, 9⁰_ag, and an η²-C,C π-bonded conformation, 9⁰_π. The

latter is calculated to be more stable by 7.9 kcal/mol (Scheme 9).

Thus, it appears reasonable to propose that 9⁰_agwill be the initial

product of benzene CꢀH activation by [(^tbpy)Pt(CH₂CH₂Ph)]⁺,

but that 9⁰_agwill likely convert to 9⁰_πor a related π-arene

adduct of ethylbenzene. The ethylbenzene product is released and

[(bpy)Pt(Ph)]⁺is reformed. Loss of ethylbenzene from 9⁰_πwas

calculated to be endergonic by only 4.4 kcal/mol (Scheme 8).

Consistent with the experimental observation of a KIE for catalysis

with perprotio versus perdeuterobenzene (see above), the transition

state for benzene CꢀH activation is calculated to be the highest

energy species on the reaction coordinate. As discussed above, the

primary KIE (k_H/k_D) of 1.8(4) is suﬃciently large to suggest rate-

determining CꢀH/D activation rather than CꢀH/D coordination;

however, the deviation of (0.4 does not allow deﬁnitive diﬀerentia-

tion between these two elementary reaction steps.

Study of Multiple Insertions of Ethylene. From complex 4,

insertion of ethylene into the PtꢀCH₂CH₂Ph bond and sub-

sequent benzene CꢀH activation would form butylbenzene. In

fact, using TpRu(CO)(NCMe)(Ph) as a catalyst precursor for

the hydrophenylation of ethylene produces small amounts of

butylbenzene at ethylene pressures g1.7 MPa.¹⁸Under the

conditions probed for the Pt systems, including ethylene pres-

sure up to 2.1 MPa at 100 °C, no evidence of butylbenzene is

detected. The lack of butylbenzene production by the Pt catalyst

is consistent with previous observations that suggest olefin

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Scheme 8. Comparison of Calculated Gibbs Free Energies (kcal/mol, in THF) for Proposed Intermediates and Transition States

in Catalytic Ethylene Hydrophenylation by [(bpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)]⁺

dissociation. The observation of Pt satellites for the resonance at

4.14 ppm indicates that ethylene dissociation is not occurring

and, hence, the exchange of ethylene hydrogen atoms is likely

due to fast rotation about the Ptꢀethylene bond, which has been

reported for related Pt(II) systems.⁸⁵Cooling a CD₂Cl₂solution

of 4 results in decoalescence of the single resonance for co-

ordinated ethylene into two broad resonances. At temperatures

below ꢀ60 °C, the Δδ for the two resonances of coordinated

ethylene does not change. The coalescence temperature is

observed at ꢀ33.4 °C (Figure 14), which gives k = 110 s^ꢀ1

and ΔG^qof 11.7 kcal/mol. The calculated energy of the rotamer

with ethylene parallel to the square plane is 12.4 kcal/mol (see

below), which is close to the experimental rotational barrier.

Upon addition of one equivalent of ethylene (as measured by

NMR integration) to a solution (CD₂Cl₂) of 4 at room temperature,

the coordinated ethylene peak shifts downﬁeld and a time-averaged

resonance for free and coordinated ethylene is observed at ∼4.8 ppm.

Decoalescence of the resonances is not observed after cooling

(ꢀ90 °C) the solution. These data are consistent with rapid exchange

between free and coordinated ethylene (eq 5). Using the known

chemical shifts for coordinated and free ethylene, an upper limit of ∼8

kcal/mol can be placed on the rate of ethylene exchange. ΔG^qvalues

for related systems ranged from ∼14.5 kcal/mol to <9.5 kcal/mol.⁸⁵

Figure 13. DFT calculated structures for [(bpy)Pt(Ph)(η²-C₂H₄)]⁺

(7⁰) and the two conformers of the resulting product [(bpy)Pt-

(CH₂CH₂Ph)]⁺(8⁰_βand 8⁰_π).

insertion into Pt(II)ꢀaryl bonds is more facile than insertion

into Pt(II)ꢀalkyl bonds.^74,75,77

From the solid-state structure of complex 4, ethylene insertion

would require initial rotation of the ethylene such that the CdC

bond axis, which is perpendicular to the square plane in 4 (see

Figure 9), becomes coplanar with the PtꢀC_sp3bond of the

1

phenethyl ligand. The H NMR spectrum of 4 at room tem-

perature reveals a single resonance due to the coordinated

ethylene at 4.14 ppm. This reveals either rapid ethylene rota-

tion on the NMR time scale or facile and reversible ethylene

We attempted to calculate the energetics of a second insertion of

ethylene from [(bpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)]⁺. A conformation

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Scheme 9. DFT Calculated Structures for [(bpy)Pt(η²-C,C-

C₆H₅Et)(Ph)]⁺(9⁰_π) and [(bpy)Pt(η²-C,H- C₆H₅Et)(Ph)]⁺

(9⁰_ag) with the Relative Energy Diﬀerence between Them

(kcal/mol)

Figure 14. Variable-temperature ¹H NMR spectra in the region of

resonances due to coordinated ethylene for [(^tbpy)Pt(CH₂CH₂Ph)(η²-

C₂H₄)][BAr⁰₄] (4).

β-Hydride Elimination. Although direct oxidative hydroary-

lation of olefins to form vinyl arenes is an attractive synthetic

target, the formation of vinyl arenes via β-hydride elimination is

potentially problematic when alkyl arenes are desired. Ethylene

hydrophenylation performed by 1ꢀ3 typically results in e1 TO

of styrene. The formation of styrene likely occurs via β-H

elimination from the phenethyl complex (4). The thermolysis

(80 °C) of 4 with ethylene (0.3 MPa) in nitromethane-d₃

1

produces styrene after approximately 10 h (∼78% yield by H

NMR). Although the initial reaction mixture is complicated, the

acetonitrile complex [(^tbpy)Pt(Et)(NCMe)][BAr⁰₄] (10) was

isolated (70% yield) after addition of NCMe to the reaction

solution (Scheme 11). The formation of 10 was verified by

comparison to an independently prepared sample. We presume

that β-hydride elimination from 4 results in the formation of

[(^tbpy)Pt(H)(η²-styrene)][BAr⁰₄]. The coordinated styrene is

displaced by ethylene, which subsequently inserts into the PtꢀH

bond. The coordination of an additional equivalent of ethylene

would form [(^tbpy)Pt(Et)(η²-C₂H₄)][BAr⁰₄], but we have not

been able to isolate this complex. These data are consistent with

the following possibility: during catalysis, β-hydride elimination

occurs but is reversible; however, eventually styrene dissociation

occurs (regardless of the exact pathway), and under catalytic con-

ditions, the resulting Ptꢀhydride decomposes. β-Hydride elim-

ination from 8⁰_πwas calculated to have a ΔG^qof 5.0 kcal/mol

and a ΔG of ꢀ3.9 kcal/mol (Scheme 12). The formation of

of the catalyst resting state in which the ethylene is rotated by

approximately 90° was found, and this rotamer is calculated to be

12.4 kcal/mol higher than 4⁰, consistent with the measured VT-

NMR rotational barrier (ΔG^q= 11.7 kcal/mol). The subsequent

transition state wherein the ethylene inserts to form a phenbutyl

ligand was not found computationally. Multiple conformational

searches for the transition state all yield either transition states

already observed or dissociation of the ethylene moiety. The most

plausible explanation for these results is steric hindrance brought

about by the hydrogen atoms of the phenethyl ligand, which are

absent on the phenyl sp²carbon for CdC insertion into the

PtꢀPh bond. The ΔG for a consecutive insertion of ethylene is

calculated to be 15.6 kcal/mol relative to the resting state, 4⁰

(Scheme 10).

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Scheme 10. Calculated Gibbs Free Energies (kcal/mol) for

Consecutive Ethylene Insertion from 4⁰

Scheme 12. ΔG’s (kcal/mol) for Styrene Production via

β-Elimination from 8⁰_β

CꢀH activation (compared with the ΔG^qfor ethylene insertion)

by TpRu(PMe₃)(η²-C₂H₄)(Ph), this system is not a catalyst for

ethylene hydrophenylation.¹⁹For 7⁰, the large kinetic preference

for CdC insertion over ethylene CꢀH activation suggests sub-

stantial latitude with respect to potential strategies for catalyst

reﬁnement before vinylic CꢀH activation becomes problematic

for this family of Pt(II) catalysts, thus providing one major

advantage over reported Ru(II) catalysts (Scheme 14).

Scheme 11. Heating 4 in CD₃NO₂Produces Styrene and a

PtꢀEthyl Complex

Polyalkylation of Benzene. Ethylene hydrophenylation

using complexes 1ꢀ3 produces significant quantities of diethyl-

benzenes that account for approximately 20% of the total alkyl

arene products (Table 1). For the Pt catalysts [(^tbpy)Pt(Ph)-

(L)]⁺, the ratio of diethylbenzenes to ethylbenzene is approxi-

mately invariant as a function of the identity of the ligand L. In

contrast, only 3% of the total alkyl arene product is diethylben-

zene for ethylene hydrophenylation using a Ir(III) catalyst,^20,28

and catalytic ethylene hydrophenylation using TpRu(CO)-

(NCMe)(Ph) produces only trace amounts of diethylben-

zene.^18,20Polyalkylated benzenes produced during catalytic

ethylene hydrophenylation using the Pt catalyst precursor

(dmpp)PtMe₃[dmpp = 3,5-dimethyl-2-(2-pyridyl)pyrrolide]

comprise ∼4% of the total alkyl arene product.²¹One of the

potential advantages of transition metal-catalyzed olefin hydro-

arylation (compared to FriedelꢀCrafts catalysis) is the inhibition

of polyalkylation. Thus, we sought to determine the reason that

[(^tbpy)Pt(Ph)(L)]⁺catalysts produce substantial quantities of

diethylbenzenes. The simplest explanation is that the reaction of

the Pt catalyst with ethylbenzene is substantially faster than the

reaction with benzene. However, as delineated below, experi-

mental data do not support this scenario.

three-coordinate [(bpy)Pt(H)]⁺via styrene dissociation is un-

favorable with a calculated ΔG of 7.6 kcal/mol from [(bpy)Pt-

(H)(η²-styrene)]⁺.

CꢀH Bond Activation of Ethylene. A potentially problematic

transformation for non-FriedelꢀCrafts olefin hydroarylation is

CꢀH activation of the coordinated olefin. Evidence for ethylene

CꢀH activation has not been observed experimentally using

complexes 1ꢀ3, but we have investigated this process computa-

tionally. Simulations suggest that vinyl CꢀH activation via σ-

bond metathesis is energetically more favorable by 1.4 kcal/mol

over an oxidative addition/reductive elimination pathway. After

the binding of ethylene to [(bpy)Pt(Ph)]⁺to form complex 7⁰, a

transition state for the activation of the vinylic CꢀH bond is

calculated to occur with a ΔG^qof 36.0 kcal/mol (Scheme 13).

The resulting vinyl complex, [(bpy)Pt(η²-C,C-C₆H₆)(CHd

CH₂)]⁺, has a ΔG of +17.5 kcal/mol relative to 7⁰. The ensuing

release of benzene and formation of [(bpy)Pt(CHdCH₂)]⁺has

a calculated ΔG of 22.4 kcal/mol relative to 7⁰.

This barrier to vinyl CꢀH activation is approximately 16.3

kcal/mol higher than the calculated activation barrier for the

insertion of CdC into the PtꢀPh bond (vide supra), which is

nearly double the calculated ΔΔG^qof 8.6 kcal/mol for the same

two reactions with TpRu(CO)(η²-C₂H₄)(Ph) (Scheme 14).¹⁹

For TpRu(PMe₃)(η²-C₂H₄)(Ph) the calculated ΔΔG^qfor ethy-

lene insertion and ethylene CꢀH activation is only 3.1 kcal/mol.

As a result of the relatively low activation barrier for ethylene

The catalytic conversion of ethylbenzene and ethylene (0.1

MPa) to diethylbenzenes by 1 at 100 °C results in 5.8 turnovers

after 4 h, corresponding to a TOF of 0.4 ꢁ 10^ꢀ3s^ꢀ1; whereas the

hydrophenylation of ethylene by 1 under identical conditions

occurs with a TOF of 1.3 ꢁ 10^ꢀ3

s

^ꢀ1. Thus, the rate of

ethylbenzene production from benzene is ∼3 times faster than

the rate of diethylbenzene formation from ethylbenzene, and the

production of signiﬁcant quantities of diethylbenzenes during the

hydrophenylation of ethylene cannot be accounted for by reac-

tion of the Pt catalyst with free ethylbenzene formed during the

reaction.

A plausible mechanism to explain the production of diethyl-

benzenes is depicted in Scheme 15. The key steps follow benzene

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Scheme 13. Comparison of calculated Gibbs free energies (kcal/mol, THF) for ethylene insertion and ethylene CꢀH activation

by [(bpy)Pt(Ph)(η²-C₂H₄)]⁺(7⁰)

Scheme 14. Comparison of the Calculated ΔG^q(kcal/mol)

for Ethylene Insertion vs Ethylene CꢀH Activation between

Several Reported Ethylene Hydroarylation Catalysts, with the

ΔΔG^qbetween the Two Transformations Highlighted

B requires the conversion of alkyl-coordinated ethylbenzene to

an arene-coordinated system. Such a rearrangement of CꢀH

coordinated alkyl arene to a π-arene complex has been observed

for late transition metals.⁸⁶From complex B, coordination and

insertion of a second equivalent of ethylene with subsequent

benzene CꢀH activation leads to diethylbenzene. At 100 °C and

0.1 MPa of ethylene, the ratio of ethylbenzene to diethylbenzenes

ranges from 4.4 to 4.8, which indicates that the rate of ethylben-

zene dissociation is only ∼4.5 times faster than diethylbenzene

formation. Hence, we suggest that relatively slow dissociation of

ethylbenzene, regardless of whether this occurs by a net associa-

tive or dissociative process, allows opportunity for a second

CꢀH activation and, hence, formation of diethylbenzene. It is

also feasible that diethylbenzene formation begins with an

initial cyclometalation of the phenethyl ligand, as reported by

Goldberg et al. for a related Pt system.²¹However, in this

pathway (Scheme 16) the formation of 1,3-diethylbenzene and

1,4-diethylbenzene would likely require that the Pt(II) complex

coordinate ethylbenzene with suﬃcient lifetime to allow a second

CꢀH activation (and ethylene insertion) to compete with net

dissociation of ethylbenzene; hence, complex A is still implicated

as an intermediate.

Catalysis using a 1:1 ratio (v:v) of C₆H₆and C₆D₆with C₂H₄

was anticipated to produce ethylbenzene-d_nwith n = 0, 1, 5 and 6,

which would arise from ethylene insertion into either PtꢀPh or

PtꢀPh-d₅followed by activation of either C₆H₆or C₆D₆

(Scheme 17), as observed for the same experiment using TpRu-

(CO)(NCMe)Ph as the catalyst.¹⁸However, all isotopologues of

ethylbenzene-d_nwith n = 0ꢀ6 (m/z = 106ꢀ112) were detected

by GC/MS using complex 2. The observation of substantial

H/D exchange is consistent with our proposed mechanism for

diethylbenzene formation in which relatively slow net dissocia-

tion of ethylbenzene from Pt(II) (regardless of the mechanism of

CꢀH activation to produce [(^tbpy)Pt(ethylbenzene)(Ph)]⁺(A

in Scheme 15). Dissociation of ethylbenzene from A completes

the catalytic cycle for ethylene hydrophenylation. However,

aromatic CꢀH activation of ethylbenzene generates [(^tbpy)Pt-

(C₆H₄Et)(C₆H₆)]⁺(B, 3 isomers are possible due to regioselec-

tivity of ethylbenzene CꢀH activation). The conversion of A to

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Scheme 15. Proposed Pathway by Which Diethylbenzenes

Are Formed during Catalytic Ethylene Hydrophenylation

Catalyzed by [(^tbpy)Pt(Ph)]^+a

Scheme 16. Alternative Pathway for Formation of Diethyl-

benzenes during Catalytic Ethylene Hydrophenylation Cata-

lyzed by [(^tbpy)Pt(Ph)]⁺

a

(Ar = C₆H₄Et; note: three isomers of the C₆H₄Et ligand of B are

possible).

Scheme 17. Expected Isotopologues of Ethylbenzene-d_n

(n = 0, 1, 5, 6) That Are Produced from Ethylene Insertion

into a PtꢀPh or PtꢀPh-d₅and Subsequent CꢀH/D Activa-

dissociation) provides a pathway for H/D scrambling to yield

ethylbenzene-d_nwith n = 2, 3, and 4 in addition to the n = 0, 1, 5,

and 6 isotopes that are expected (Scheme 17).

a

tion of Either C₆H₆or C₆D₆

Heating complex 4 in benzene (100 °C) results in quantitative

production of ethylbenzene (by GC/MS) (eq 6). Importantly,

GC/MS analysis of the reaction of 4 in C₆D₆reveals the

production of ethylbenzene-d_n(n = 1ꢀ6). NMR spectroscopy

and GC/MS analysis indicate that extensive deuteration does not

occur in the ethyl fragment, other than the expected formation of

ꢀCH₂CH₂D. If the conversion of A to B (Scheme 15) were

inaccessible, then heating 4 in C₆D₆should produce only

ethylbenzene-d₁. The mass spectrum of ethylbenzene features

peaks at mass range m/z = 106ꢀ113 and fragmentation at mass

range m/z = 91ꢀ97, which indicates that H/D scrambling

occurs predominantly into the phenyl ring of ethylbenzene

(Scheme 18). Ions in the range of m/z = 27ꢀ30 were observed,

which is indicative of the presence of ꢀCH₂CH₂D (m/z = 30)

fragments. There is no evidence of a peak at m/z = 31 (or greater)

due to ethyl-d_n(n g 2) fragments. The ²H NMR spectrum of the

reaction mixture supports the presence of a ꢀCH₂CH₂D frag-

^aIn addition to isotopologues with n = 0, 1, 5, and 6, those with n = 2, 3,

and 4 are also observed.

2

ment (δ = 1.03 ppm in the H NMR spectrum) and a lack of

extensive H/D scrambling into the ethyl fragment, since a

resonance consistent with deuterium incorporation into the

benzylic position is not observed (Figure 15).

m/z = 112 to m/z = 107. The addition of the Lewis base increases

the prevalence of the lower mass ions (m/z = 105ꢀ109)

compared to the reaction without its addition. The inhibition

of isotopic scrambling by the addition of a Lewis base is most

readily explained by an associative mechanism for exchange of

ethylbenzene with THF and mirrors the observations of

isotope exchange from protonation of (diimine)Pt(Me)₂

using DOTf, as well as positional o/m/p isomerization of Pt

tolyl and xylyl complexes where NCMe was used as the Lewis

base in both cases.^39,40

To conﬁrm the interpretation of the results of isotopic

labeling, [(^tbpy)Pt(CH₂CH₂Ph-d₅)(η²-C₂H₄)]⁺(4-d₅) was

synthesized with an all deutero Ph moiety. The reaction of

4-d₅in C₆H₆produces isotopologues with a mass range m/z =

106ꢀ111, with m/z = 106 having the greatest abundance,

which indicates substantial formation of per protio ethyl-

benzene by H/D exchange. The observed selectivity for

The addition of a Lewis base to the reaction of 4 with C₆D₆

might be expected to promote the loss of ethylbenzene from A

(Scheme 15) via associative displacement and suppress the

extent of H/D exchange. Indeed, the addition of THF (100 equiv

relative to 4) to the stoichiometric reaction of 4 in C₆D₆results

in a shift of the isotopic distribution from the major isotope of

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Scheme 18. Proposed Mechanism for H/D Scrambling that

Occurs during the Thermolysis of [(^tbpy)Pt(CH₂CH₂Ph)-

(η²-C₂H₄)][BAr⁰₄] in C₆D₆

Figure 15. ²H NMR spectrum for the product of thermolysis of 4 in

C₆D₆, highlighting the resonance (δ = 1.03 ppm) of the ethylbenzene

isotopologues possessing an ethyl fragment with a terminal ꢀCH₂D

group. Incorporation of deuterium into the benzylic position is not

observed.

Scheme 19. Comparison of the Calculated ΔG (kcal/mol;

gas phase) for Ethylbenzene Dissociation from Several

Reported Ethylene Hydroarylation Catalysts

isotopologues of ethylbenzene is similar to that observed for

the thermolysis of 4 in C₆D₆with H/D exchange occurring

predominantly into the phenyl ring of ethylbenzene. Similar to

4 and C₆D₆, the addition of the Lewis base increases the

prevalence of the higher mass ions (m/z > 108) compared to

the reaction without its addition.

Benzene coordination and CꢀH activation by [(bpy)Pt-

(CH₂CH₂Ph)]⁺forms [(bpy)Pt(Ph)(η²-C,C-C₆H₅Et)]⁺(A⁰).

Simulations were used to assess the potential for A⁰to (a)

undergo dissociation of ethylbenzene, (b) undergo a second

aromatic CꢀH activation event and (c) undergo sp³CꢀH

activation. The last process (i.e., c), which forms [(bpy)Pt-

(CH₂CH₂Ph)(η²-C,C-C₆H₆)]⁺, has a high calculated barrier

(ΔG^q= 27.5 kcal/mol), consistent with the absence of extensive

H/D scrambling into the ethyl fragment. The free energy

for dissociation of ethylbenzene from A⁰is calculated to be

12.5 kcal/mol (Scheme 19; these calculations are gas phase and

thus the free energy of ethylbenzene dissociation is diﬀerent from

that in Scheme 8), while arene CꢀH activation barriers are

calculated to be 16.2 kcal/mol (para CꢀH activation), 16.7 kcal/

mol (meta CꢀH activation) and 18.7 (ortho CꢀH activation).

Thus, the calculations suggest that the energetics of ethylbenzene

dissociation and a second CꢀH activation are of similar magni-

tude. As a benchmark, the energetics of ethylbenzene dissocia-

tion were compared with that for the charge-neutral complex

(dmpp)Pt(Ph)(η²-C,C-ethylbenzene) [dmpp =3,5-dimethyl-

2-(2-pyridyl)pyrrolide], which exhibits less extensive formation

of diethylbenzenes during catalytic hydrophenylation of ethy-

lene. The barrier to ethylbenzene dissociation from (dmpp)Pt-

(Ph)(η²-C,C-ethylbenzene) was calculated to be only 0.6 kcal/

mol, and ethylbenzene dissociation from TpRu(CO)(ethyl-

benzene)(Ph) is calculated to be exergonic by 8 kcal/mol

(Scheme 19).

A striking observation is the regioselectivity for diethylben-

zene formation as a function of aromatic substrate. For example,

the ortho:meta:para (o:m:p) selectivity for the production of

diethylbenzenes from 1 with ethylene and benzene is 1:2.6:1.6

after 4 h (Table 1). In contrast, for the reaction of ethylene with

ethylbenzene under identical conditions, the o:m:p selectivity is

1.0:34.8:24.0 (eq 7). Notably, the m/p selectivity ratio is

approximately the same as the reaction with benzene while the

ratio of o to m/p changes dramatically. The diﬀerent selectivity

suggests distinct pathways for diethylbenzene formation from

C₂H₄/C₆H₆and C₂H₄/ethylbenzene reactions (Scheme 20).

For the reaction of ethylene and benzene, the formation of

diethylbenzene is proposed to originate via benzene CꢀH

activation to give A, followed by sp²CꢀH activation of the

coordinated ethylbenzene (Scheme 15). In this scenario,

Pt(II) migrates from the methyl group of coordinated ethylben-

zene (C in Scheme 20) to an η²-C,C intermediate involving

the ortho carbon (D in Scheme 20). If the rate of aromatic

CꢀH activation is competitive with migration around the phenyl

ring, it is expected that ortho CꢀH activation, ultimately

resulting in 1,2-diethylbenzene, will be competitive. For the

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Scheme 20. Pathway for Diethylbenzene Formation from C₆H₆and C₂H₄Requires Pt to Interact with the 2-Position Carbon,

Which Provides a Route for 1,2-Diethylbenzene Formation

catalytic ethylation of toluene, the Pt(II) catalyst reported by

Goldberg et al. provided an o/m+p selectivity of 7:93 (the meta

and para isomers could not be separated).²¹

catalytic oleﬁn hydroarylation initiated by Pt(II)

systems,^30,82our results provide evidence that late transi-

tion metal systems can catalyze oleﬁn hydroarylation by a

pathway that involves oleﬁn coordination, oleﬁn inser-

tion, and metal-mediated aromatic CꢀH activation.

(2) In contrast to traditional FriedelꢀCrafts catalysts, start-

ing with ethylbenzene, the [(^tbpy)Pt(Ph)]⁺catalyst is

highly selective for 1,3- and 1,4-diethylbenzene over 1,2-

diethylbenzene.

(3) The platinum catalysts related to [(^tbpy)Pt(Ph)]⁺result

in high yields under conditions in which ethylene is the

limiting reagent. The catalyst also displays a tolerance

toward exposure to aerobic conditions. The formation of

vinyl arene via β-hydride elimination and oleﬁn dissocia-

tion occurs only to a minor extent even though com-

putational studies indicate that β-hydride elimination

from [(^tbpy)Pt(CH₂CH₂Ph)]⁺should have a low ΔG^q.

Thus, the lack of substantial styrene production may

result from facile reinsertion relative to the rate of styrene

dissociation.

(4) The competition between oleﬁn insertion and oleﬁn

CꢀH activation is a key parameter for oleﬁn hydroaryla-

tion catalysts. Compared with TpRu(L)(NCMe)Ph sys-

tems, calculations (supported by experimental studies)

suggest that the Pt catalysts have a larger ΔΔG^qthat

favors the desired catalysis.

For the formation of diethylbenzenes from free ethylbenzene

and ethylene, the regioselectivity is dictated by the coordination

and CꢀH activation of ethylbenzene. Here, free ethylbenzene

likely coordinates to Pt(II) via an initial η²-C,C complex, possibly

with a preferred orientation of the ethyl group away from the

Pt(II) center in order to reduce steric interactions. Hence, the

aromatic CꢀH activation can occur without Pt(II) directly

interacting with the ortho carbon, which contrasts with diethyl-

benzene formed from ethylene and benzene, since the proposed

pathway requires Pt to interact with the carbon ortho to the ethyl

group. This results in the decreased selectivity for meta and para

over ortho for diethylbenzene formation from benzene and

ethylene.

’ SUMMARY AND CONCLUSIONS

The cationic fragment [(^tbpy)Pt(Ph)]⁺serves as a catalyst for

the hydrophenylation of ethylene by a non-FriedelꢀCrafts path-

way. One of the overarching goals of this work was a comparison

of catalytic oleﬁn hydroarylation using a late transition metal

catalyst, traditionally considered electrophilic in character, with

catalysts based on d⁶systems that are considered more electron-

rich (e.g., Ru(II) and Ir(III) catalysts). On the basis of this study,

the following salient points are emphasized:

(5) The formation of diethylbenzene products using 1ꢀ3 as

catalyst precursors is hypothesized to be a result of a

second aromatic CꢀH activation that competes with the

dissociation of ethylbenzene. This is distinct from poly-

alkylation in FriedelꢀCrafts catalysis, which is thought to

originate from enhanced rates of reaction of free alkyl

arenes relative to the starting arene.³The barrier to

hydrocarbon dissociation using 1 is consistent with ob-

servations of other cationic, late transition metal

systems.^87,88Also relevant here are data consistent with

alkane intermediates involved in Pt(II) systems,^53,89and

implications of barriers for hydrocarbon dissociation

which facilitate the polyfunctionalization of methane, best

studied in cases of H/D exchange.⁵⁴While previous

studies of H/D exchange have implicated coordinated

hydrocarbons as intermediates,^46,90to our knowledge this

is a rare example of such an eﬀect impacting the selectivity

of a catalytic reaction, and it suggests an important

(1) Experimental studies for the catalytic conversion of

ethylene and benzene to ethylbenzene are consistent with

a cycle that involves ethylene coordination and insertion

followed by Pt-mediated benzene CꢀH activation with

the latter step, or the CꢀH coordination step, as the rate-

1

limiting step. In addition, monitoring catalysis by H

NMR spectroscopy reveals that [(^tbpy)Pt(CH₂CH₂Ph)-

(η²-C₂H₄)]⁺is likely the catalyst resting state. DFT

calculations are consistent with these conclusions. Thus,

despite studies that reveal alternative mechanisms for

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consideration when designing catalysts for oleﬁn hydro-

arylation using late transition metals. The present re-

search implies that the cationic charge on [(^tbpy)Pt-

(Ph)]⁺may play an important role in selectivity vis-ꢀa-vis

mono- versus polyalkylated products, a notion that is

consistent with the neutral catalyst (dmpp)Pt(CH₂-

CH₂Ph)(η²-C₂H₄) producing only small amounts of

diethylbenzenes.²¹This working hypothesis further hints

at improved design strategies for oleﬁn hydroarylation

catalysts, e.g. via attenuation of the electrophilicity of the

metal center with overall neutral catalysts.

8.09 (d, 1H, ⁴J_HH= 2 Hz, H³-^tbpy), 7.97 (d, 1H, ⁴J_HH= 2 Hz, H³-^tbpy),

3

4

7.78 (dd, 1H, J_HH= 6 Hz, J_HH= 2 Hz, H⁵-^tbpy), 7.71 (br s, 8H,

H_o(Ar⁰)), 7.55 [br s, 4H H_p(Ar⁰)], 7.46 [d, 1H, ³J_HH= 7 Hz, H_o(Ph)],

3

4

7.27 (dd, 1H, J_HH= 6 Hz, J_HH= 2 Hz, H⁵-^tbpy), 7.16 [m, 2H,

H_m(Ph)], 7.06 [m, 1H, H_p(Ph)], 4.14 (br m, 4H, H²-THF), 1.86 (br m,

4H, H³-THF), 1.46 (s, 9H, tert-butyl ^tbpy), 1.38 (s, 9H, tert-butyl ^tbpy).

3

¹H NMR (tetrahydrofuran-d₈) δ 8.54 (d, 1H, J_HH= 6 Hz, H⁶-^tbpy),

8.51 (s, 1H, H³-^tbpy), 8.39 (s, 1H, H³-^tbpy), 8.17 (d, 1H, ³J_HH= 6 Hz,

H⁶-^tbpy), 7.89 (d, 1H, ³J_HH= 6 Hz, H⁵-^tbpy), 7.80 [br s, 9H, H_o(Ar⁰)

3

and H⁵-^tbpy ], 7.58 [br s, 4H, H_p(Ar⁰)], 7.50 (d, 1H, J_HH= 7 Hz,

H_o(Ph)], 7.13 [m, 2H, H_m(Ph), 7.03 (m, 1H, H_p(Ph)], 1.47 (s, 9H, tert-

butyl ^tbpy), 1.39 (s, 9H, tert-butyl ^tbpy). Resonances due to coordinated

THF were not observed due to rapid exchange with THF-d₈. ¹³C NMR

(75 MHz, tetrahydrofuran-d₈) δ 166.5, 166.3, 158.6, 154.8, 153.9, 147.2,

140.8, 137.5, 136.8, 128.6, 125.4, 121.7, 121.2 (^tbpy and Ph aromatic),

’ EXPERIMENTAL SECTION

t

36.6 (tert-butyl, quaternary, bpy), 36.5 (tert-butyl, quaternary, bpy),

General Considerations. Unless otherwise noted, all synthetic

procedures were performed under anaerobic conditions in a nitrogen-

filled glovebox or by using standard Schlenk techniques. Glovebox purity

was maintained by periodic nitrogen purges and was monitored by an

oxygen analyzer (O₂< 15 ppm for all reactions). Tetrahydrofuran and

n-pentane were dried by distillation from sodium/benzophenone and

P₂O₅, respectively. Diethyl ether and perfluoropyridine were distilled

over CaH₂. Methylene chloride and benzene were purified by passage

through a column of activated alumina. Benzene-d₆, acetone-d₆, and CD₂Cl₂

were used as received and stored under a N₂atmosphere over 4 Å

molecular sieves. ¹H NMR spectra were recorded on a Varian Mercury

t

30.0 (tert-butyl-CH₃, bpy), 29.8 (tert-butyl-CH₃, bpy); 162.6 (q, Ar⁰,

¹J_BꢀCipso= 50 Hz), 135.4 (Ar⁰), 129.9 (q, m-Ar⁰, ²J_CꢀF= 32 Hz), 127.1

(q, CF₃-Ar⁰, J_CꢀF= 272 Hz), 118.0 (Ar⁰). ¹⁹F NMR (282 MHz,

1

CD₂Cl₂) δ ꢀ62.2 (s, CF₃-Ar⁰). Anal. Calcd for PtBON₂F₂₄C₆₀H₄₉(%):

C 48.82, H 3.35, N 1.90; found: C 48.27, H 3.13, N 2.12.

[(^tbpy)Pt(Ph-d₅)(THF)][BAr⁰₄] (1-d₅). This complex was synthe-

sized by the procedure used for 1. In the preparation of the initial starting

material, PhLi was substituted with PhLi-d₅to produce (^tbpy)Pt(Ph-

d₅)₂. ¹H and ¹³C NMR spectra are consistent with the all protio analogue

minus the resonances for the phenyl ring in the ¹H NMR spectrum.

[(^tbpy)Pt(NC₅F₅)(Ph)][BAr⁰₄] (2). To a cooled solution of (^tbpy)-

Pt(Ph)₂(0.14 g, 0.22 mmol) in THF (∼20 mL, ꢀ60 °C), [H(Et₂O)₂]-

[BAr⁰₄] (0.26 g, 0.22 mmol) in THF (∼15 mL, ꢀ60 °C) was added.

After the addition, the volatiles were removed in vacuo. The crude solid

was then reconstituted in perfluoropyridine (∼5 mL). The solution was

stirred at room temperature for 36 h. The volatiles were then removed.

The crude solid was treated with n-pentane (∼2 mL), which was

subsequently removed under vacuum to afford a fluffy, orange powder.

The resulting product was then dried in vacuo (0.33 g, 96%) . ¹H NMR

(NC₅F₅, acetone-d₆insert) δ 7.36 (s, 1H, H³-^tbpy), 7.30 (s, 1H,

1

300 or 500 MHz spectrometer. All H spectra are referenced against

residual proton signals of the deuterated solvents. ¹⁹F NMR (282 MHz

operating frequency) spectra were obtained on a Varian 300 MHz

spectrometer and referenced against an external standard of hexafluor-

obenzene (δ = ꢀ164.9 ppm). ²H NMR (77 MHz operating frequency)

spectra were obtained on a Varian Mercury 500 MHz spectrometer.

GC/MS was performed using a Shimadzu GCMS-QP2010 Plus system

with a 30 mm ꢁ 0.25 mm SHRXI-5MS column with 0.25 mm film

thickness using negative chemical ionization (NCI), which also allows

for simulated electron impact (SEI) ionization. Ethylene (99.5%) was

purchased in a gas cylinder from GTS-Welco and used as received. All

other reagents were used as purchased from commercial sources. The

preparation, isolation, and characterization of [H(Et₂O)₂][BAr⁰₄],⁹¹

(^tbpy)Pt(Ph)₂,⁴⁴and PhLi-d₅⁹²have been previously reported.

Computational Methods. All computations were performed

using 2,2⁰-bipyridyl as a model ligand for 4,4⁰-ditertbutyl-2,2⁰-bipyridyl.

The DFT calculations were carried out using the Gaussian 03 software

package. All optimizations were done with B3LYP^93ꢀ95using the

Stevens valence basis sets, CEP-31G (5d, 7f),^96ꢀ99and pseudopoten-

tials. Extra d polarization basis functions were added to the main group

elements using values taken from the 6-31G(d) all-electron basis sets.

Given that the majority of the species in this study are cationic, solvent

effects were assessed for a more accurate description of energetics.

CPCM, Conductor-like Polarized Continuum Method,^100ꢀ103was used

with both benzene and THF as the solvents selected to run the

calculations. Little qualitative difference in the various steps was seen

in the various solvent media. Reported free energies were determined at

298.15 K and 1 atm using unscaled B3LYP frequencies. Unless stated

otherwise, the calculated free energies are those modeled in THF

solvent.

3

H³-^tbpy), 7.06 (d, 1H, J_HH= 6 Hz, H⁶-^tbpy), 6.92 (d, 1H, J_HH

=

6 Hz, H⁶-^tbpy), 6.62 [br s, 10H, H_o(Ar⁰) and H_o(Ph)], 6.56 (d, 1H, ³J_HH

6 Hz, H⁵-^tbpy), 6.18 [br s, 5H, H_p(Ar⁰) and H⁵-^tbpy], 5.63 (t, 2H,

³J_HH=7 Hz, H_m(Ph)], 5.39 [t, 1H, ³J_HH= 7 Hz, H_p(Ph)], 0.36 (s, 9H,

tert-butyl ^tbpy), 0.29 (s, 9H, tert-butyl ^tbpy). ¹³C NMR (75 MHz, NC₅F₅

with acetone-d₆insert) δ 166.3, 156.0, 153.2, 150.6, 145.9, 126.3, 124.3,

123.9, 122.9, 119.1 (^tbpy and Ph aromatic), 34.6 (tert-butyl, quaternary,

^tbpy, coincidental overlap), 27.7 (tert-butyl-CH₃, ^tbpy), 27.6 (tert-butyl-

CH₃, ^tbpy); 160.9 (q, Ar⁰, ¹J_BꢀCipso= 50 Hz), 127.6 (q, m-Ar⁰, ²J_CꢀF= 31

Hz), 123.4 (q, CF₃-Ar⁰, ¹J_CꢀF= 272 Hz), 115.5 (Ar⁰). Resonances for

coordinated perfluoropyridine were not observed due to ligand ex-

change and resulting time averaged signals at room temperature.

t

Remaining five bpy, Ph, and Ar⁰aromatic resonances are obscured

due to broad NC₅F₅resonances or coincidental overlap. ¹⁹F NMR (282

MHz, CD₂Cl₂) δ ꢀ61.2 (s, 24F, CF₃-Ar⁰), ꢀ74.3 (m, 2F, ³J_PtꢀF= 316

Hz (Pt satellites), o-NC₅F₅), ꢀ118.9 (m, 1F, p-NC₅F₅), ꢀ154.2 (m, 2F,

m-NC₅F₅). Anal. Calcd for PtBN₃F₂₉C₆₁H₄₁(%): C 46.58, H 2.63, N

2.67; found: C 46.54, H 2.65, N 2.20.

[(^tbpy)Pt(NCMe)(Ph)][BAr⁰₄] (3). To a cooled solution of

(^tbpy)Pt(Ph)₂(0.090 g, 0.15 mmol) in THF (∼20 mL, ꢀ60 °C) was

added [H(Et₂O)₂][BAr⁰₄] (0.16 g, 0.15 mmol) in THF (∼15 mL,

ꢀ60 °C). After the addition, the volatiles were removed in vacuo. The

crude solid was then reconstituted in acetonitrile (∼5 mL). The solution

was stirred at room temperature for 15 min. The volatiles were then

removed. The crude solid was treated with n-pentane (∼2 mL), which

was subsequently removed under vacuum to afford a fluffy, yellow

powder. The resulting product was then dried in vacuo (0.20 g, 94%).

[(^tbpy)Pt(Ph)(THF)][BAr⁰₄] (1). To a cooled solution of (^tbpy)Pt-

(Ph)₂(0.095 g, 0.15 mmol) in THF (∼20 mL, ꢀ60 °C), [H(Et₂O)₂]-

[BAr⁰₄] (0.16 g, 0.15 mmol) in THF (∼15 mL, ꢀ60 °C) was added.

After the addition, the volatiles were removed in vacuo. The crude solid

was treated with n-pentane (∼2 mL), which was subsequently removed

under vacuum to afford a fluffy, yellow powder. The resulting product

was then dried in vacuo (0.21 g, 93%) . ¹H NMR (300 MHz, CD₂Cl₂) δ

8.36 (d, 1H, ³J_HH= 6 Hz, H⁶-^tbpy), 8.14 (d, 1H, ³J_HH= 6 Hz, H⁶-^tbpy),

3

¹H NMR (acetone-d₆) δ 9.02 (d, 1H, J_HH= 5.8 Hz, H⁶-^tbpy), 8.80

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(d, 1H, ⁴J_HH= 1.9 Hz, H³-^tbpy), 8.73 (d, 1H, ⁴J_HH= 2.1 Hz, H³-^tbpy),

8.25 (d, 1H, ³J_HH= 6.3 Hz, H⁶-^tbpy), 7.93 (dd, 1H, ³J_HH= 5.8, ⁴J_HH= 1.9

⁴J_HH= 2 Hz, H⁵-^tbpy), 7.57 [s, 4H, H_p(Ar⁰)], 2.61 (s, 3H, NCMe), 1.73

(q, 2H, ³J_HH= 8 Hz, CH₂-Et), 1.45 (s, 9H, tert-butyl ^tbpy), 1.44 (s, 9H,

tert-butyl ^tbpy), 1.00 (t, 3H, ³J_HH= 8 Hz, CH₃-Et). ¹³C NMR (125 MHz,

CD₂Cl₂) δ 166.3, 165.9, 158.6, 153.6, 149.1, 147.8, 125.9, 125.4, 120.9,

120.3 (^tbpy aromatic), 36.5 (tert-butyl, quaternary, ^tbpy), 30.3 (tert-butyl-

Hz, H⁵-^tbpy), 7.80 (br s, 8H, H_o(Ar⁰)), 7.72 (dd, 1H, ³J_HH= 6.3, ⁴J_HH

=

2.1, H⁵-^tbpy), 7.68 (br s, 4H, H_p(Ar⁰)), 7.39 (m, 2H, H_o(Ph)), 7.10 (m,

3H, H_m(Ph) and H_p(Ph)), 2.81 (s, 3H, NCMe), 1.48 (s, 9H, tert-butyl

^tbpy), 1.43 (s, 9H, tert-butyl ^tbpy). ¹³C NMR (75 MHz, acetone-d₆) δ

167.0, 166.9, 158.8, 155.2, 152.4, 149.7, 137.5, 128.9, 126.1, 125.9, 125.5,

122.8, 122.2, 120.9 (^tbpy and Ph aromatic), 69.5 (tert-butyl, quaternary,

t

CH₃, bpy, coincidental overlap), 16.7 (NCCH₃), 4.8 (CH₂-Et),

1.8 (CH₃-Et); 162.3 (q, Ar⁰, ¹J_BꢀCipso= 49 Hz), 135.4 (Ar⁰), 129.2

(q, m-Ar⁰, ²J_CꢀF= 30 Hz), 125.2 (q, Ar⁰, ²J_CꢀF= 272 Hz), 118.1 (Ar⁰).

¹⁹F NMR (282 MHz, CD₂Cl₂) δ ꢀ64.86 (s, CF₃-Ar⁰). Anal. Calcd

for PtBN₂F₂₄C₆₀H₄₁D₃(%): C 46.43, H 3.18, N 3.00; found: C 47.06,

H 3.29, N 2.86.

t

^tbpy), 68.2 (tert-butyl, quaternary, bpy), 37.0 (tert-butyl-CH₃, coin-

cidental overlap), 4.2 (NCMe, CH₃); 162.8 (q, Ar⁰, ¹J_BꢀCipso= 50 Hz),

135.7 (Ar⁰), 130.2 (q, m-Ar⁰, ²J_CꢀF= 31 Hz), 125.5 (q, CF₃-Ar⁰, ¹J_CꢀF

=

272 Hz), 118.6 (Ar⁰). ¹⁹F NMR (282 MHz, acetone-d₆) δ ꢀ60.6

(s, CF₃-Ar⁰). Anal. Calcd for PtBN₃F₂₄C₅₈H₄₄(%): C 48.21, H 3.08,

N 2.91; found: C 48.43, H 3.05, N 3.03.

Catalytic Ethylene Hydrophenylation. A representative cata-

lytic reaction is described. [(^tbpy)Pt(Ph)(THF)][BAr⁰₄] (1) (0.049 g,

0.033 mmol) was dissolved in 12.0 mL of benzene containing

0.025 mol % hexamethylbenzene (HMB) relative to benzene as an inter-

nal standard. The reaction mixture was placed in a stainless steel pressure

reactor, charged with ethylene, and heated to 100 °C. After 4 and 16 h,

the reaction mixture was cooled to room temperature and analyzed by

GC/MS. Peak areas of the products and the internal standard were used

to calculate product yields. Ethylbenzene production was quantified

using linear regression analysis of gas chromatograms of standard

samples. A set of five known standards were prepared consisting of

2:1, 3:1, 4:1, 5:1, and 6:1 molar ratios of ethylbenzene to hexamethyl-

benzene in benzene. A plot of the peak area ratios versus molar ratios

gave a regression line. For the GC/MS system, the slope and correlation

coefficient (R²) for ethylbenzene were 0.53 and 0.98, respectively.

Identical procedures were used to quantify the production of 1,3-

diethylbenzene, 1,4-diethylbenzene, and 1,2-diethylbenzene. The slope

and correlation coefficients (R²) for these species are 0.56, 0.99; 0.56,

0.99; 0.52, 0.99, respectively. At a minimum, reactions were performed

in triplicate for a given set of conditions.

[(^tbpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)][BAr⁰₄] (4). Complex 1 (0.033

g, 0.019 mmol) was dissolved in dichloromethane (∼5 mL). The

reaction mixture was transferred to a stainless steel pressure reactor

and pressurized with ethylene (0.3 MPa) for 16 h. The volatiles were

removed and pentane (∼2 mL) was added to the crude solid. The

pentane was removed under vacuum to afford an orange solid. The solid

was collected and dried in vacuo (0.027 g, 97%). ¹H NMR (CD₂Cl₂) δ

8.67 (d, 1H, ³J_HH= 6 Hz, H⁶-^tbpy), 8.21 (s, 1H, H³-^tbpy), 8.16 (s, 1H,

H³-^tbpy), 7.93 (d, 1H, ³J_HH= 6 Hz, H⁶-^tbpy), 7.79 (d, 1H, ³J_HH= 6 Hz,

H⁵-^tbpy), 7.72 [br s, 9H, H_o(Ar⁰) and H⁵-^tbpy], 7.55 [br s, 4H, H_p(Ar⁰)],

7.28ꢀ7.12 (m, 5H, Ph), 4.14 (br s with doublet (∼33%) due to ²J_PtꢀH

,

4H, ²J_PtꢀH= 69 Hz, C₂H₄), 2.70 (t, 2H, ³J_HH= 8 Hz, Pt-CH₂CH₂Ph),

1.48 and 1.43 (overlapping resonances, 20H, tert-butyl ^tbpy and

PtCH₂CH₂Ph). ¹³C NMR (75 MHz, CD₂Cl₂) δ 169.4, 167.0, 157.7,

154.5, 148.1, 145.5, 144.0, 129.2, 126.7, 126.5, 125.8, 121.0, 120.9 (^tbpy

and Ph aromatic), 70.2 (C₂H₄), 37.7 (PtCH₂CH₂Ph, ²J_PtꢀC= 28 Hz, Pt

satellites), 36.9 (tert-butyl, quaternary, ^tbpy), 36.6 (tert-butyl, quaternary,

t

^tbpy), 30.3 (tert-butyl-CH₃, bpy, coincidental overlap), 17.0 (PtCH₂-

Determination of Percent Yield for Catalytic Ethylene

Hydrophenylation. Complex 1 (0.040 g, 0.027 mmol) was dissolved

in 12.0 mL of benzene containing 0.02 mol % HMB relative to benzene

as an internal standard. The reaction mixture was placed inside a stainless

steel pressure reactor with a gas buret (300 mL) attached. The buret was

evacuated and backfilled with N₂(three times), evacuated, and pressur-

ized with C₂H₄(0.4 MPa). From the buret, the pressure reactor was

charged with ethylene (0.03 MPa). The buret was then evacuated and

backfilled with N₂(three times) and pressurized, along with the pressure

reactor, with N₂(0.8 MPa). The solution was then heated for 4 h at

120 °C, cooled to room temperature, and analyzed by GC/MS.

Kinetic Isotope Effect for Catalysis. A representative catalytic

reaction is described. [(^tbpy)Pt(Ph)(THF)][BAr⁰₄] (1) (0.021 g, 0.014

mmol) was dissolved in 5.0 mL of C₆D₆containing 0.025 mol %

hexamethylbenzene (HMB) relative to C₆D₆as an internal standard.

The reaction mixture was placed in a stainless steel pressure reactor,

charged with ethylene (0.1 MPa), and heated to 100 °C. After 4 and 16 h,

the reaction mixture was cooled to room temperature and analyzed by

GC/MS. Peak areas of the products and the internal standard were used

to calculate product yields. The reaction was performed five times,

and the average TOs of ethylbenzene, diethylbenzenes, and styrene

were determined. The catalytic KIE was determined from the ratio of

total alkylbenzene TO after 4 h produced from reaction in C₆H₆

versus C₆D₆. The reported error is the deviation from multiple

experiments.

CH₂Ph, ¹J_PtꢀC= 674 Hz, Pt satellites); 162.3 (q, Ar⁰, ¹J_BꢀCipso= 49 Hz),

135.4 (Ar⁰), 129.5 (q, m-Ar⁰, ²J_CꢀF= 32 Hz), 125.2 (q, Ar⁰, ²J_CꢀF= 272

Hz), 118.1 (Ar⁰). Remaining three ^tbpy and Ph aromatic resonances are

obscureddue tobroadeningorcoincidental overlap. ¹⁹F NMR (282 MHz,

CD₂Cl₂) δ ꢀ63.1 (s, CF₃-Ar⁰). Anal. Calcd for PtBN₂F₂₄C₆₀H₄₉(%): C

49.36, H 3.39, N 1.92; found: C 49.28, H 3.27, N 1.89.

[(^tbpy)Pt(CH₂CH₂Ph-d₅)(η²-C₂H₄)][BAr⁰₄] (4-d₅). This com-

plex was synthesized by the procedure used for 4. In the preparation of

the initial starting material, PhLi was substituted with PhLi-d₅to produce

1

(^tbpy)Pt(Ph-d₅)₂. H and ¹³C NMR spectra were consistent with the

spectra of the all-protio analogue minus the resonances for the phenyl

ring in the ¹H NMR spectrum.

[(^tbpy)Pt(Et)(NCMe)][BAr⁰₄] (10). Method A: Complex 4 (0.060 g,

0.040 mmol) was dissolved in nitromethane-d₃(0.4 mL) and transferred

to a high pressure NMR tube. The NMR tube was then degassed,

pressurized with ethylene (0.3 MPa), and heated at 80 °C for 16 h. To

the solution was added NCMe (200 μL). After 2 h, the solution was filtered

through Celite using dichloromethane. The volatiles were removed from

the filtrate in vacuo, and the resulting solid was collected (0.039 g, 70%).

The identity of the product was confirmed by comparison to a sample

prepared by Method B. Method B: To a cooled solution of (^tbpy)Pt(Et)₂

(0.017 g, 0.030 mmol) in THF (∼15 mL, ꢀ60 °C), [H(Et₂O)₂][BAr⁰₄]

(0.033 g, 0.030 mmol) in THF (∼5 mL, ꢀ60 °C) was added. After the

addition, the solution immediately changed from red-orange to pale yellow.

The solvent volume was reduced by ∼50% under vacuum. Acetonitrile

(1 mL) was added. All volatiles were removed in vacuo. The crude solid was

treated with n-pentane (∼1 mL), which was subsequently removed under

vacuum to afford a fluffy, yellow powder. The resulting product was dried in

vacuo (0.040 g, 92%). ¹H NMR (CD₂Cl₂) δ 8.79 (d, 1H, ³J_HH= 6 Hz,

H⁶-^tbpy), 8.57 (d, 1H, ³J_HH= 6 Hz, H⁶-^tbpy), 8.10 (d, 1H, ⁴J_HH= 2 Hz,

H³-^tbpy), 8.07 (d, 1H, ⁴J_HH= 2 Hz, H³-^tbpy), 7.74 [s, 8H, H_o(Ar⁰)], 7.70

(dd, 1H, ³J_HH= 6 Hz, ⁴J_HH= 2 Hz, H⁵-^tbpy), 7.67 (dd, 1H, ³J_HH= 6 Hz,

Kinetics of Benzene CꢀD Activation as a Function of C₆D₆

Concentration. A representative kinetic experiment is described.

Complex 1 (0.071 g, 0.048 mmol) and hexamethyldisilane (HMDS, 2 μL)

an internal standard, were dissolved in 1.3 mL of CD₂Cl₂. The solution

was then divided (0.38 mL for each sample) and added to three NMR

tubes. To each tube was added C₆D₆(0.040 mL, 0.42 mmol). The

sample was then placed into a temperature-equilibrated (30 °C) NMR

probe. The kinetic runs were performed in triplicate. ¹H NMR spectra

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Journal of the American Chemical Society

ARTICLE

were collected every 10 min with 16 scans and a 5.0 s pulse delay. The peaks

for complex 1 were integrated against that of HMDS, and from a plot of

ln([1]_t) vs time (seconds), the rate constants were extracted. The rate of

formation of complex 1-d₅, in the presence of 1.1 M C₆D₆, was 2.04(8) ꢁ

10^ꢀ4s^ꢀ1with a correlation coefficient (R²) of g0.98 for each plot.

Kinetic Isotope Effect for Stoichiometric Benzene CꢀH/D

Activation. A representative kinetic experiment is described. Complex

1-d₅(0.063 g, 0.043 mmol), C₆H₆(0.063 mL, 0.71 mmol), and HMB

(0.0014 g) were dissolved in 1.4 mL of CD₂Cl₂. The solution was then

divided (0.40 mL for each sample) and added to three NMR tubes. The

sample was then placed into a temperature-equilibrated (21 °C) NMR

probe. The kinetic runs were performed in triplicate. ¹H NMR spectra

were collected every 10 min with 16 scans and a 5.0 s pulse delay. The

peaks for complex 1 were integrated against that of HMB, and from a

plot of ln(1ꢀ [1]_t/[1-d₅]_o) vs time (seconds), the rate constants were

extracted. The rate of formation of complex 1, in the presence of

0.49 M C₆H₆, was 7.1(4) ꢁ 10^ꢀ5s^ꢀ1with a correlation coefficient (R²)

of g0.98 for each plot. The conversion of 1 to 1-d₅was monitored under

identical conditions, and from a plot of ln([1]_t) vs time (seconds), the

rate constants were extracted.. The rate of formation of complex 1-d₅

was 5.0(3) ꢁ 10^ꢀ5s^ꢀ1with a correlation coefficient (R²) of g0.98 for

each plot. From these two reactions, a KIE (k_H/k_D) value of 1.4(1) was

determined.

Catalytic Ethylene Hydrophenylation using 1:1 (v:v) C₆H₆

and C₆D₆. [(^tbpy)Pt(NC₅F₅)(Ph)][BAr⁰₄] (2) (0.009 g, 0.006 mmol)

was dissolved in 2.0 mL of benzene (1:1 C₆H₆and C₆D₆). The

reaction mixture was placed in a stainless steel pressure reactor,

charged with ethylene (0.1 MPa), and heated to 100 °C. After 16 h,

the reaction mixture was cooled to room temperature and analyzed

by GC/MS in order to identify the resulting isotopologues of

ethylbenzene.

Catalytic Ethylene Hydrophenylation as a Function of the

Concentration of [(^tbpy)Pt(Ph)(THF)][BAr⁰₄] (1). Solutions

(12 mL) of benzene and 1 (0.001, 0.005, 0.01, 0.015, and 0.02 mol %

relative to benzene) containing equimolar amounts of HMB, relative to

1, were placed in stainless steel pressure reactors, charged with ethylene

(0.1 MPa), and heated to 100 °C. After 30 min, the reaction mixtures

were cooled to room temperature and analyzed by GC/MS. Peak areas

of the products and the internal standard were used to calculate product

yields. Each reaction was performed in triplicate.

Catalytic Ethylene Hydrophenylation as a Function of

Benzene Concentration. A representative experiment is described.

Complex 1 (0.014 g, 0.010 mmol), benzene (8 μL), and hexamethyldi-

silane (HMDS, 2 μL) were dissolved in NC₅F₅(292 μL). The reaction

mixture was transferred to a J-Young NMR tube along with an insert

tube containing acetone-d₆. The tube was purged and pressurized with

ethylene (0.1 MPa). A ¹H NMR spectrum was obtained, and the

ethylene concentration was extracted after integration relative to

HMDS. The solution was then heated at 100 °C for 2 h and cooled to

room temperature. An aliquot (50 μL) of the reaction mixture and HMB

(145 μL, 0.0104 M in DCM) were dissolved in DCM (2 mL) and

analyzed by GC/MS. Peak areas of the products and the internal

standard were used to calculate product yields.

Catalytic Ethylene Hydrophenylation as a Function of

Ethylene Concentration. A representative experiment is described.

A solution of complex 1 (0.01 g, 0.007 mmol) and C₆D₆(2.45 mL)

containing 0.025 mol % HMB (relative to C₆D₆) was distributed

(0.4 mL) among six J-Young NMR tubes. The tubes were purged and

pressurized with ethylene (0.3 MPa). The ethylene concentrations were

determined after integration of the ethylene signal relative to HMB. The

solutions were heated at 100 °C for 2 h, cooled to room temperature,

and analyzed by GC/MS.

an internal standard, were dissolved in 1.3 mL of CD₂Cl₂. The solution

was then divided (0.25 mL for each sample) and added to five high-

pressure NMR tubes. Each tube was degassed with a freezeꢀpumpꢀ

thaw cycle, pressurized with 0.1 MPa of ethylene, and cooled to ꢀ78 °C

until it was placed into a temperature-equilibrated (23 °C) NMR probe.

The temperature of the probe was determined by using a sample of

methanol-d₄.¹⁰⁴The kinetic runs were performed in triplicate. The

concentration of ethylene in solution was determined by integration

1

against the internal standard. H NMR spectra were collected every

2 min with 16 scans and a 2.0 s pulse delay. The product peaks were

integrated against that of HMDS, and from a plot of ln(1 ꢀ [4]_t/[1]_o) vs

time (seconds) were extracted the rate constants. The rate of formation

of complex 4, in the presence of 0.1(1) M C₂H₄, was 1.07(2) ꢁ 10^ꢀ3s^ꢀ1

with a correlation coefficient (R²) of 0.99 for each plot.

Thermolysis of [(^tbpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)][BAr⁰₄] (4)

in C₆H₆. To a stainless steel pressure reactor were added complex 4

(0.016 g, 0.011 mmol) and 1 mL of benzene (containing 0.1 mol %

HMB relative to benzene). The reactor was sealed, pressurized with

0.9 MPa of N₂, and heated at 100 °C for 2 h. At that time, the reactor was

allowed to cool to room temperature. The reaction mixture was analyzed

by GC/MS. Ethylbenzene was detected in 97% yield along with ∼2% of

1,3-diethylbenzene (no evidence for 1,4- or 1,2-diethylbenzene forma-

tion was obtained).

Thermolysis of [(^tbpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)][BAr⁰₄] (4)

in C₆D₆. To a screw-cap NMR tube were added complex 4 (0.014 g,

0.0094 mmol) and 0.5 mL of C₆D₆. The reaction mixture was heated at

100 °C for 1 h, and then analyzed using GC/MS.

Thermolysis of [(^tbpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)][BAr⁰₄] (4)

in C₆D₆with added Lewis Base. To a screw-cap NMR

tube were added complex 4 (0.020 g, 0.014 mmol), 0.5 mL of

C₆D₆, and tetrahydrofuran-d₈(100 equiv relative to 4, 111 μL).

The reaction mixture was heated at 100 °C for 24 h and analyzed

via GC/MS.

Thermolysis of [(^tbpy)Pt(CH₂CH₂Ph-d₅)(η²-C₂H₄)][BAr⁰₄]

(4-d₅) in C₆D₆. To a screw-cap NMR tube were added complex 4-d₅

(0.022 g, 0.015 mmol) and 0.5 mL of C₆D₆. The reaction mixture was

heated at 100 °C for 2 h and cooled to room temperature. The reaction

mixture was analyzed by GC/MS.

Thermolysis of [(^tbpy)Pt(CH₂CH₂Ph-d₅)(η²-C₂H₄)][BAr⁰₄]

(4-d₅) in C₆H₆. To a screw-cap NMR tube were added complex 4-d₅

(0.018 g, 0.012 mmol) and 0.5 mL of C₆H₆. The reaction mixture was

heated at 100 °C for 2 h and cooled to room temperature. The reaction

mixture was analyzed by GC/MS.

Thermolysis of [(^tbpy)Pt(CH₂CH₂Ph-d₅)(η²-C₂H₄)][BAr⁰₄]

(4-d₅) in C₆D₆with added Lewis Base. To a screw-cap NMR

tube were added complex 4-d₅(0.022 g, 0.015 mmol), 0.5 mL of C₆H₆,

and tetrahydrofuran (100 equiv relative to 4-d₅, 120 μL). The reaction

mixture was heated at 100 °C for 2 h, and analyzed via GC/MS.

Variable-Temperature ¹H NMR of [(^tbpy)Pt(CH₂CH₂Ph)-

(η²-C₂H₄)][BAr⁰₄] (4). A sample of [(^tbpy)Pt(CH₂CH₂Ph)(η²-C₂H₄)]-

[(BAr⁰₄)] in CD₂Cl₂was placed in a Varian 500 MHz spectrometer.

The temperature was cooled incrementally down to ꢀ80 °C, and

¹H NMR spectra were acquired after several minutes at each tempera-

ture to allow for equilibration. Temperatures were calibrated using a

methanol-d₄standard.

Variable-Temperature ¹H NMR of [(^tbpy)Pt(CH₂CH₂Ph)-

(η²-C₂H₄)][BAr⁰₄] (4) in the Presence of Free Ethylene. A ¹H

NMR spectrum of a solution of 4 in CD₂Cl₂with an internal HMB

standard was acquired on a Varian 500 MHz spectrometer. The solution

was then charged with ethylene by bubbling into a screw-cap NMR

1

tube. Acquisition of a H NMR spectrum revealed a single coalesced

resonance (4.77 ppm) for coordinated and free ethylene. Cooling

the NMR probe to ꢀ90 °C did not result in decoalescence of the

resonances.

Kinetics of Ethylene Insertion. A representative kinetic experi-

ment is described. Complex 1 (0.060 g, 0.041 mmol) and HMDS (5 μL),

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Journal of the American Chemical Society

ARTICLE

’ ASSOCIATED CONTENT

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Supporting Information. Crystallographic and GC/MS

b

data, additional experimental details, computational details, and

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Mechanistic studies of ethylene hydrophenylation catalyzed by bipyridyl Pt(II) complexes

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