Use of Ligand Steric Properties to Control the Thermodynamics and Kinetics of Oxidative Addition and Reductive Elimination with Pincer-Ligated Rh Complexes

Pincer-Ligated Rh Complexes

Article

Use of Ligand Steric Properties to Control the Thermodynamics and

Kinetics of Oxidative Addition and Reductive Elimination with

Shunyan Gu, Robert J. Nielsen,* Kathleen H. Taylor, George C. Fortman, Junqi Chen, Diane A. Dickie,

William A. Goddard III,* and T. Brent Gunnoe*

Cite This: https://dx.doi.org/10.1021/acs.organomet.0c00122

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sı Supporting Information

ABSTRACT: Oxidative addition and reductive elimination reactions are

central steps in many catalytic processes, and controlling the energetics of

reaction intermediates is key to enabling eﬃcient catalysis. A series of

oxidative addition and reductive elimination reactions using ( PNP)RhX

complexes (R = tert-butyl, isopropyl, mesityl, phenyl; X = Cl, I) was

studied to deduce the eﬀect of the size of the phosphine substituents.

Using ( PNP)RhCl as the starting material, oxidative addition of MeI was

observed to produce ( PNP)Rh(Me)(I)Cl, which was followed by

reductive elimination of MeCl to form ( PNP)RhI. The thermodynamics

and kinetics vary with the identity of the substituent R on phosphorus of

the PNP ligand. The presence of large steric bulk (e.g., R = tert-butyl,

mesityl) on the phosphine favors Rh(I) in comparison to the presence of two smaller substituents (e.g., R = isopropyl, phenyl). An

tBu

Eyring plot for the oxidative addition of MeI to ( PNP)RhCl in THF-d is consistent with a polar two-step reaction pathway, and

tBu

the formation of [( PNP)Rh(Me)I]I is also consistent with this mechanism. DFT calculations show that the steric bulk aﬀects the

−

reaction energies of addition reactions which generate six-coordinate complexes by tens of kcal mol . The ligand steric bulk is

−

calculated to have a reduced eﬀect (a few kcal mol ) on S 2 addition barriers, which only require access to one side of the square

plane.

INTRODUCTION

such as those based on Rh or Ir, which are anticipated to be

■

29,30

less electrophilic.

Using a putative catalytic cycle that

Oxidative addition and reductive elimination transformations

provide reactions that break and form bonds. These

fundamental reactions are incorporated into many catalytic

processes. For example, they are key reactions in catalytic C−C

mirrors the proposed Pt(II)/Pt(IV) cycle of Pt-catalyzed

Shilov-type reactions, a Rh catalytic cycle would involve C−H

oxidative addition to Rh(I) to form a Rh(III) intermediate

−6

III

followed by reductive elimination of the Rh −R bond

coupling reactions.

As a result, understanding how the

features of the catalyst (e.g., metal identity, metal oxidation

state, ligand donor ability, ligand steric parameters) dictate the

energetics of oxidative addition and reductive elimination

reactions is critical to understanding catalyst activity for better

catalyst design. In particular, understanding energy proﬁles of

catalytic intermediates is important to designing molecular

(Scheme 1). Thus, regardless of the speciﬁc mechanistic

details, a key step in rhodium-catalyzed alkane functionaliza-

tion using Shilov-type chemistry is the reductive elimination of

III

RX (X = halide, pseudohalide) from an X−Rh −R

2,31

intermediate (Scheme 2).

While C−H activation by Rh

32−41

complexes has been reported,

elimination of alkyl halide from Rh complexes are not

examples of reductive

−16

III

catalysts for light alkane partial oxidations.

Given the inert nature of alkanes, their selective function-

42−46

common.

−11,17−20

alization is a substantial challenge.

discovery of Pt-mediated alkane functionalization,

trophilic late-metal complexes have been developed for

Since Shilov’s

In addition to alkane functionalization, reductive elimination

reactions are important in industrial processes such as

8,21,22

elec-

−16,21−26

catalyzing the functionalization of alkanes.

Received: February 19, 2020

signiﬁcant limitation of this approach is the ability of weakly

basic solvents to coordinate to the metal and inhibit catalyst

3−28

activity.

While several electrophilic catalysts are eﬃcient

for methane functionalization in superacidic oleum (H SO +

SO ), they are less active or inactive in more weakly acidic

solvents. A possible strategy is to use earlier transition metals,

XXXX American Chemical Society

Organometallics XXXX, XXX, XXX−XXX

Organometallics

phosphine substituents (Figure 1 ). Examples are known where

Article

Scheme 1. Possible Shilov-Type Catalytic Cycle for Rh-

Catalyzed Hydrocarbon C−H Functionalization

Some recent reports have focused on reductive elimination

reactions from Rh , Pd , Pt , and Au complexes.

The Milstein group has reported on mechanistic studies with a

22,47

III

42,44,58−61

focus on ligand inﬂuence for the reductive elimination of MeX

III

(

X = halide, pseudohalide) from Rh ₄complexes ligated by

2−44

both PCP ligands and PNP ligands.

With our recent

reports of Rh-mediated C−H activation and functionaliza-

37,62−69

tion,

we have become interested in understanding how

to modify ligands to control the energetics of redox bond-

breaking and bond-forming processes that convert among Rh ,

III

IV 63

Rh , Rh , and Rh . Although understanding ligand eﬀects

on reductive eliminations can be more subtle than σ-donor

0,71

III

eﬀects,

one strategy to shift Rh /Rh equilibria toward Rh

is to alter the metal electron density on the basis of ligand

donor ability. For example, our group studied reductive

elimination from a series of ( terpy)Rh(Me)(Cl)(I) complexes

Scheme 2. General Energy Diagram of Rh-Mediated

Hydrocarbon Functionalization That Operates via a Rh /

III

(R = tert-butyl, NO ) in acidic media where the electron-

Rh Pathway

withdrawing nitro group facilitates reductive elimination of the

45,46

methyl ligand.

Most relevant here, steric eﬀects could also

be used to facilitate two-electron reductive eliminations from

III

octahedral d Rh complexes to form square-planar d Rh

III

complexes, where Rh complexes can be destabilized with

bulky ligands. For example, “axial steric bulk”, which is deﬁned

as the ligand sterics above/below the coordination plane of a

square-planar complex, could be used to destabilize octahedral

Rh complexes relative to square-planar Rh complexes. PNP

pincer ligands, which provide a tridentate binding mode, oﬀer

an opportunity to control axial steric bulk on the basis of

III

The relative energetics are based on unpublished calculations of

III

Figure 1. Generic structure of a metal complex with a “pincer” ligand

where substituents R can be modiﬁed to control the steric proﬁle.

several Rh complexes. The Rh intermediate can be thermodynami-

cally favored, resulting in a high activation barrier for the reductive

elimination of RX (in the red box).

steric factors are central in reactions of pincer-ligated

3,44,72−75

_m_e_t_h_a_n_o_l_c_a_r_b_o_n_y_l_a_t_i_o_n_a_n_d_o_l_e_ﬁ_n_h_y_d_r_o_f_o_r_m_y_l_a_t_i_o_n_.8−52 In

transition-metal complexes.

With an interest in

controlling the thermodynamics of bond-breaking/-forming

reactions that cycle through formal Rh oxidation states as a

means to potentially optimize the kinetics of catalytic

reactions, we have investigated the axial steric eﬀect on

oxidative addition and reductive elimination reactions of a

s e r i e s o f ( P N P ) R h ( P N P

dihydrocarbylphosphinomethyl)pyridine) complexes with

various steric bulks (Scheme 4). Initial computational

nonradical reductive elimination, S 2 (polar two-step) and

concerted mechanisms are the two most common pathways

III/I

3−57

(Scheme 3).

In the S 2 pathway, a charged intermediate

is commonly formed upon dissociation of an anionic ligand

from the metal center. This two-step mechanism is often

accelerated in polar solvents, where the intermediate can be

stabilized. In the concerted pathway, a three-center transition

state is typically invoked.

2 , 6 - b i s -

(

III

Scheme 4. Destabilization of Octahedral Rh Complexes by

Tuning “Axial Steric Bulk” in ( PNP)Rh Complexes

Scheme 3. Common Pathways for Reductive Elimination of

RX from a Metal Center

55,57

R = hydrocarbyl and X = formally anionic species such as hydride,

hydrocarbyl, halide, etc.

R = tert-butyl, isopropyl, mesityl, phenyl.

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Article

−

III

Scheme 5. Computed Activation and Reaction Free Energies (kcal mol at 298 K) for S 2 Elimination Reactions from Rh

Methyl Complexes

Representative transition state structures with explicit solvent molecules are shown on the right with the Rh−CH units represented by spheres.

Table 1. Crystal Structures (Top) of (^t^B^uPNP)RhCl (1a), ( PNP)RhCl (2a), ( PNP)RhCl (3a), and ( PNP)RhCl (4a) and

iPr

Mes

the Visualization of Shielding of Diﬀerent Ligands on the Rh Center (Bottom)

The crystal structure has been previously reported.⁷⁹⁻⁸¹^bThe PNP ligand shieldings are indicated in blue, and the Cl contributions are indicated

Mes

in red. In ( PNP)RhCl, since there is no C symmetry in the crystal structure, the shielding is diﬀerent from above and below the square plane in

the crystal structure. Thus, two images are provided.

79−81

modeling studies suggested that large substituents on the

phosphine ligands would favor, thermodynamically and

kinetically, square-planar Rh complexes over octahedral Rh

steric eﬀects of larger phosphine substituents and to assess

what free energy surfaces might be aﬀorded by a more diverse

set of ligands.

III

complexes. Experimental studies are consistent with this

hypothesis. Herein, we report the details of these combined

computational/experimental studies.

The R = methyl cases show that elimination is not kinetically

or thermodynamically favorable if steric or electronic factors

are not speciﬁcally employed to drive the reaction (Scheme 5).

Consistent with this, reductive elimination of methyl halides

III

RESULTS AND DISCUSSION

from (PNP)Rh complexes has previously been observed

■

when trapping agents (e.g., CO) are included to sequester Rh

Density functional theory calculations were used to predict ΔG

⧧

or CH X products, a tactic incompatible with catalytic cycles.

and ΔG for the elimination of CH X products from

Oxygen-based nucleophiles such as triﬂuoroacetate and water

oﬀer advantages over halides in C−H activation reactions,

since their basicity in polar media drives the removal of

protons from hydrocarbons. However, these harder nucleo-

(

PNP)Rh(Me)(X)₂intermediates in order to gauge the

likelihood of extending this transformation from catalytic

cycles based on more electrophilic metals to rhodium.

Sterically unencumbered and electron-deﬁcient pincer ligands

6,77

have been observed to form multinuclear

or fac-

philes lead to higher S

2 elimination barriers in comparison to

coordinated complexes. Nonetheless, we modeled R = Me,

iodide. Increasing the size of the phosphine substituent to tert-

−

CF , tert-butyl variants as mer pincer complexes to isolate the

butyl imparts an additional 20 kcal mol driving force to the

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Article

Figure 2. H NMR (left) and ³¹P{ H} NMR (right) spectra of the reaction of (^t^B^uPNP)RhCl (1a) and MeI to form (^t^B^uPNP)RhI (1c) in C D

III

(

from bottom to top: without MeI, 15 min, 50 min). Within 15 min of MeI addition (second from the bottom), the Rh complex

PNP)Rh(Me)(Cl)(I) was observed at 50.3 ppm in P{ H} NMR with J = 101 Hz.

tBu

PRh

elimination of methyl triﬂuoroacetate, while the barrier is

spectroscopy indicates that 1a undergoes oxidative addition to

−

III

reduced by 1.4 kcal mol . Rendering the rhodium center more

electrophilic via triﬂuoromethylphosphine substituents beneﬁts

both the activation barrier and reaction energy of elimination,

making the elimination of methanol and methyl triﬂuoroace-

tate exergonic. Predicted activation barriers range from roughly

generate a Rh complex as an intermediate that quickly

tBu

eliminates MeCl to form ( PNP)RhI (1c ) (Figures 2 and 3)

−

0 to 40 kcal mol , and only reactions at the low end of this

range are suitable for hydrocarbon functionalization cycles,

even at temperatures high for homogeneous catalysis. The

steric and electronic properties of the ligand and nucleophile

must therefore be limited by this constraint.

On the basis of these computational modeling results, in this

work we sought to quantify, experimentally and computation-

ally, the impact of the axial steric eﬀect on oxidative addition

and reductive elimination reactions for this series of pincer-

ligated Rh complexes. Multiple small molecules were tested,

and MeI and CH Cl were chosen as suitable substrates due to

Figure 3. ORTEP drawing of (^t^B^uPNP)RhI (1c). Ellipsoids are drawn

at the 50% probability level, and hydrogen atoms are omitted for

clarity. Selected bond lengths for 1c (Å): Rh1−N1 2.049(6), Rh1−P1

clean reactivity.

tert-Butyl and mesityl were selected as the larger phosphine

substituents, while isopropyl and phenyl were selected as the

smaller substituents, allowing for the comparison of both alkyl

groups and aryl groups. Although there is not a model that can

perfectly quantify axial steric bulkiness, the solid-G software is

able to visualize the shielding of diﬀerent phosphine

substituents perpendicular to the square plane centered around

2.2815(16), Rh1−P1 2.2815(16), Rh1−I1 2.6077(8). Selected bond

angles for 1c (deg): N1−Rh1−P1 83.49(4), N1−Rh1−P2 83.49(4),

P1−Rh1−P2 166.99(8), N1−Rh1−I1 180.00(2), P1−Rh1−I1

96.51(4), P2−Rh1−I1 96.51(4).

and MeCl. With excess MeI added to 1c, an equilibrium

III

among 1c, MeI, and 1d′ was established (Scheme 6). The Rh

Rh (Table 1). This analysis, based on structures obtained

from single-crystal X-ray diﬀraction, provides a view of the

steric shielding above/below the Rh square plane. The

visualization of steric shielding is shown in rows 4 and 6 of

Table 1, where the blue indicates the area blocked by the PNP

intermediate was not successfully isolated, but NMR data are

consistent with the expected oxidative addition product

PNP)Rh(Me)(Cl)(I). A doublet with J = 101 Hz was

observed in the P{ H} NMR spectrum, indicating the

tBu

(

RhP

III

83,84

formation of Rh species.

Two diastereotopic PCH peaks

iPr

ligand. In comparison with ( PNP)RhCl and ( PNP)RhCl,

were observed at 4.39 and 4.02 ppm in the H NMR spectrum,

indicating the loss of a mirror plane. Attempts to generate 1b

in situ were made by charging MeCl to THF solutions of 1c,

tBu

Mes

as expected, ( PNP)RhCl and ( PNP)RhCl have larger

ligand shielding (blue area) of the Rh center.

Reactions of ( PNP)RhCl (1a). The complex ( PNP)-

RhCl (1a) was prepared by mixing

C H ) Cl] in THF according to a published procedure. In a

tBu

but no reaction was observed, indicating that the Rh species

tBu

PNP and [Rh(μ-

1c is likely favored thermodynamically over 1b.

4 2

Complex 1c has been characterized by a single-crystal X-ray

diﬀraction study (Figure 3). There is only a slight diﬀerence in

the bond lengths and bond angles between 1c and the

THF solution of 1a, MeI was added dropwise. With an

increasing amount of added MeI, an orange precipitate was

formed, which was found to be consistent with the reported

previously reported structure of 1a. In 1a, the N−Rh bond

length is 2.036(3) Å, while the same bond distance for 1c is

2.049(6) Å.

tBu

NMR spectra of [( PNP)Rh(Me)I]I (1d′). In situ

observation of the reaction of 1a with MeI by H NMR

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Article

Scheme 6. Oxidative Addition and Reductive Elimination Reactions Using (^t^B^uPNP)RhCl (1a) as the Starting Complex^a

All of the reactions were performed in THF. 1b is a proposed structure without specifying the exact isomer.

The kinetics of reaction between 1a and MeI were probed

from 27 to 54 °C. The concentrations of 1a and MeI were

Scheme 7. Proposed S 2-Type Mechanism for the Oxidative

Addition of MeI to ( PNP)RhCl (1a)

tBu

monitored with respect to time using integration of H NMR

spectra (Figure 4 ). The results demonstrate a good ﬁt to a

bonding with the Rh center, and it also applies to the case of

tBu

the intermediate, [( PNP)Rh(Me)(Cl)](I) (1b′).

Due to the low solubility of 1d′ in THF-d , determination of

the equilibrium constant as a function of temperature between

c/MeI and 1d′ was performed in CD Cl . The equilibrium

Figure 4. Concentration changes for (^t^B^uPNP)RhCl (1a), MeI, and

constant of this reaction, at 27 °C, is 78(7) (note that the K

tBu

(

PNP)RhI (1c) with respect to time. Conditions: 0.5 mL of THF-

and standard deviation are the result of three independent

experiments). Using the variable-temperature data and

experimentally determined K_e_qvalues, a van’t Hoﬀ plot was

created (Figure 6). The ΔH and ΔS values of the reaction

tBu

^d8^,¹⁶^m^M⁽ PNP)RhCl (1a), 20 mM MeI, HMDSO

hexamethyldisiloxane) as the internal standard, 319 K.

(

second-order reaction, ﬁrst order in both 1a and MeI,

−1

were calculated to be −15.2(6) kcal mol and −42(2) cal

mol K , respectively.

according to eq 1. The k value was determined to be

−1 −1

obs

−

−1 −1

⧧

.2(5) × 10

a₁t 300 K, which corresponds to a ΔG

−

value of 20.6 kcal mol .

[A]

[B]

[A]₀

[B]₀

= kt + ln

[

A] − [B]

(1)

An Eyring plot was obtained using variable-temperature

NMR experiments from 27 to 54 °C (Figure 5 ). The Eyring

Figure 6. van’t Hoﬀ plot of the equilibrium among (^t^B^uPNP)RhI (1c),

tBu

MeI, and [( PNP)Rh(Me)I]I (1d′). Conditions: 0.5 mL of CD Cl ,

2.6 mM 1d′, HMDSO (hexamethyldisiloxane) in C D in a capillary

as the internal standard, 300−324 K. All data points and standard

deviations are the results of at least three independent experiments.

Figure 5. Eyring plot for the oxidative addition of MeI to

iPr

tBu

(

PNP)RhCl (1a). All data points and standard deviations are the

Reactions of ( PNP)RhCl (2a). ( PNP)RhCl (2a) was

iPr

results of at least three independent experiments.

synthesized by mixing PNP and [Rh(μ-C H ) Cl] in

THF. The reaction of MeI with 2a in THF-d generates

two Rh complexes, both exhibiting J = 99 Hz in P{ H}

NMR spectra. The identities of the complexes have been

⧧

III

plot gives a good linear ﬁt, and the ΔH and ΔS values of the

PRh

−

reaction were determined as 10.4(8) kcal mol and −34(2)

−

−1

⧧

iPr

cal mol K , respectively. The large and negative ΔS is

conﬁrmed by X-ray crystallography to be isomers of ( PNP)-

consistent with a S 2-type mechanism with a ﬁve-coordinate

Rh(Me)(I)(Cl), and both isomers possess a methyl at the

position cis to the pyridyl (2b-1 and 2b-2, Figure 7). The two

isomers cocrystallize, with 2b-1 being the dominant isomer

(2b-1:2b-2 = 86:14). Complex 2b-1 has a chlorine atom trans

to the pyridyl N, giving a 173.0(2)° N−Rh−Cl bond angle,

square-pyramidal complex forming as an intermediate (Scheme

4,85

tBu

(

1b′), is consistent with the reported crystal structure of 1d′,

which also has a ﬁve-coordinate square-pyramidal structure

The proposed structure, [( PNP)Rh(Me)(Cl)](I)

with one iodide appearing as a counterion. This might be due

to the large axial steric bulk, which could prevent iodide from

while 2b-2 shows a 98.0(3)° N−Rh−Cl bond angle. In the H

NMR spectra of 2b-1 and 2b-2, both complexes display C_s

Organometallics XXXX, XXX, XXX−XXX

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Article

tBu

that for ( PNP)Rh(Me)(Cl)(I) (1b). Mixing MeI with 2c

iPr

results in a rapid oxidative addition to form trans-( PNP)-

Rh(Me)I (2d), turning the solution from dark red to yellow

tBu

(

Scheme 8). The reaction of ( PNP)RhI (1c) with MeI

results in a rapid oxidative addition as well; however, this is

followed by immediate reductive elimination, forming a rapid

equilibrium between the two species. In comparison with the

reaction of 1c, the ﬁnal product of 2c oxidative addition (2d) is

a six-coordinate Rh complex with both iodides binding to the

Rh center. Assuming a mechanism similar to that for the

iPr

Figure 7. ORTEP drawings of two isomers of ( PNP)Rh(Me)(I)-

Cl) (2b; 2b-1 on the left, 2b-2 on the right), which are found as a

(

iPr

cocrystallized mixture. Ellipsoids are drawn at the 50% probability

level, and hydrogen atoms are omitted for clarity. Selected bond

lengths for 2b (Å): Rh1−N1 2.050(4), Rh1−C1 2.124(5), Rh1−P1

reaction of 1c, [( PNP)Rh(Me)I]I, with the trans position to

the methyl vacant, might be an intermediate of the oxidative

addition reaction. Heating a THF-d solution of 2d and 10

.308(1), Rh1−P2 2.353(1). Selected bond lengths for 2b-1 (Å):

equiv of CD I to 90 °C in a J. Young tube did not give the

Rh1−Cl1 2.362(4), Rh1−I1 2.8424(6). Selected bond lengths for 2b-

iPr

product of CD I exchange, ( PNP)Rh(CD )I , after 7 days,

: Rh1−Cl1A 2.45(1), Rh1−I1A 2.460(8). Selected bond angles for

b (deg): N1−Rh1−C1 87.2(2), N1−Rh1−P1 82.4(1), C1−Rh1−

indicating that reductive elimination of MeI from 2d does not

likely occur under these conditions.

Scheme 9 contains the calculated free energy surfaces for

P1 86.8(1), N1−Rh1−P2 82.3(1), C1−Rh1−P2 91.0(1), P1−Rh1−

P2 164.68(5). Selected bond angles for 2b-1 (deg): N1−Rh1−Cl1

73.0(2), C1−Rh1−Cl1 86.2(2), P1−Rh1−Cl1 95.0(2), P2−Rh1−

reactions of 1a and 2a with CH I in THF. The conversion of

Cl1 100.0(2), N1−Rh1−I1 98.0(1), C1−Rh1−I1 174.8(1), P1−

Rh1−I1 94.80(4), P2−Rh1−I1 88.79(3), Cl1−Rh1−I1 88.7(2).

Selected bond angles for 2b-2 (deg): N1−Rh1−Cl1A 98.0(3), C1−

Rh1−Cl1A 171.3(4), P1−Rh1−Cl1A 100.8(4), P2−Rh1−Cl1A

the Rh −Cl species 1a to the Rh −I species 1c is accomplished

by the S 2 addition of CH I, the rearrangement of halogen

atoms, and S 2 elimination of CH Cl. The immediate product

of CH I addition is the ﬁve-coordinate ion pair 1b, which is

2.8(4), N1−Rh1−I1A 172.1(2), C1−Rh1−I1A 85.3(2), P1−Rh1−

I1A 94.7(2), P2−Rh1−I1A 100.3(2), Cl1A−Rh1−I1A 89.7(3).

III

also predicted to be the most stable geometry for this Rh

composition. The rearrangement that exchanges iodine and

chlorine is slightly uphill (1b-3) and allows the reaction to

symmetry, as determined from the four peaks due to the

proceed via CH Cl elimination. The octahedral intermediates

isopropyl and the diastereotopic PCH units.

−1

b-2 and 1b-1 lie 6.1 and 14.5 kcal mol higher in free energy

We were unable to determine the rate of the oxidative

addition of MeI to 2a because the reaction is complete in <10

min at −78 °C. An oxidative reaction experiment of 2a and

MeI using the same starting concentration as that for the

reaction for 1a (Figure 4) was performed in a screw-cap NMR

in comparison to 1b, showing that the axial metal−halide bond

is only metastable and that the equatorial position is less

hindered and is better suited to host the larger iodide anion.

We have not sought transition states for the halide rearrange-

ment but take the energy of the octahedral complexes to

indicate that rearrangement may occur without barriers as high

tube at −78 °C. After 10 min, 2a was not observable by H

NMR spectroscopy. Assuming that the reactions undergo the

same mechanism for 1a and 2a and that more than 95% of the

starting material was consumed in 10 min, a rate constant can

as those for the S 2 reactions. The transition state for

−

elimination of CH Cl (TS2, G = 21.4 kcal mol ) is higher

−

−1

than that for CH I addition (TS1). We interpret that TS2 is

be estimated to be ≥0.032 M

at 195 K. Thus, an upper

⧧

−1

⧧

responsible for the eﬀective barrier of ΔG = 20.6 kcal mol

from experimental results (see above) and that the Rh

intermediates are not observed spectroscopically because

limit on ΔG for this reaction can be estimated as ∼13 kcal

mol . Heating a THF-d solution of the mixture of 2b-1 and

III

−

b-2 to 90 °C does not give the reductive elimination product

iPr

they are endergonic from the reactants. Addition of another

(

PNP)RhI (2c), and the product of MeCl reductive

iPr

CH I molecule to 1c proceeds with a geometry (TS3) and

elimination followed by MeI oxidation addition, ( PNP)Rh-

barrier similar to the ﬁrst, yielding the ﬁve-coordinate 1d′. An

alternate pathway from 1b-3 to 1d′ is intermolecular outer-

sphere halide exchange; however, the observation of 1c and

MeCl is consistent with our proposed pathway. Since this

addition is predicted to be endergonic, the reaction appears to

be driven by Le Chatelier’s principle and the precipitation of

product. Computed thermochemistry (using the M06-L and

B3PW91-d3 functionals) is compared to experimental data

(

Me)(I) , was not observed after 100 h at 90 °C. Thus, in

contrast to the Bu variant 1b, 2b is stable against MeX (X =

Cl, I) reductive elimination. ( PNP)RhI (2c) was then

synthesized independently by mixing 2a and NaI in acetone.

Charging MeCl to a THF-d solution of complex 2c gives three

isomers of 2b according to NMR spectra.

The observations of reactivity of 1a and 2a are consistent

with the hypothesis that a reduced axial steric bulk stabilizes

Rh relative to Rh and that the ΔG value for reductive

elimination from ( PNP)Rh(Me)(Cl)(I) (2b) is larger than

iPr

III

⧧

from this and related work in order to estimate expected

errors in the ESI.

iPr

Scheme 8. Oxidative Addition of MeI and MeCl Using ( PNP)RhCl (2a) and ( PNP)RhI (2c) as the Starting Complexes

All of the reactions were performed in THF. 2b refers to a mixture of isomers of this complex.

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Article

−

Scheme 9. Computed Free Energy Surface (kcal mol at 298 K) for Reactions of 1a (in Black) and 2a (in Blue) with CH I in

THF

The inset shows the geometry of S 2 and reductive elimination pathways for CH Cl elimination from 1b.

Scheme 10. Oxidative Addition of MeI to (^M^e^sPNP)RhCl (3a) to Form ( PN_aP)Rh(Me)(I)(Cl) (3b) as a Proposed

Mes

Intermediate and Reductive Elimination of MeCl to Form ( PNP)RhI (3c)

An unidentiﬁed species and methane were also observed as products. All of the reactions were performed in THF.

−

−1

We sought transition states for the elimination of CH Cl

(CH )(Cl)] I lies 12.3 kcal mol above the octahedral 2b-2

from 1b with reductive elimination character. TSRE-1 and

TSRE-2, with free energies roughly 20 and 40 kcal mol

in free energy. The iodide ion prefers to be bound to Rh, in

−

iPr

III

qualitative contrast to 1b′. The ( PNP)Rh states are

thermodynamic sinks, consistent with the lack of experimen-

tally observed elimination reactions. In addition to being

smaller than tert-butyl groups, the isopropyl substituents can

rotate to best accommodate each other and axial ligands as

needed.

higher than that of the S 2-style TS2, may be described as

migration of a methyl group along the Rh−Cl bond with

coordination of the outer-sphere iodide. Such paths are

apparently not kinetically relevant.

When the tert-butylphosphine substituents are replaced with

isopropylphosphine groups, the activation barriers for addition

Reactions of (^M^e^sPNP)RhCl (3a). (^M^e^sPNP)RhCl (3a) was

of CH I to the square-planar 2a,b are lowered by about 4 kcal

synthesized by stirring a THF solution of [Rh(μ-C H ) Cl]

and PNP in a 1:2 ratio. H NMR spectrum of 3a indicates a

2 4 2 2

−

Mes

mol from the analogous tert-butyl values. The dramatic

diﬀerence between the two free energy surfaces is the stability

molecule with four symmetry-equivalent mesityl groups. The

III

of Rh states relative to Rh . The octahedral product of CH I

JPRh value of 143 Hz for 3a in the P{ H} NMR spectrum is

−

addition 2b-2 is predicted to be 17.4 kcal mol exergonic, in

contrast to the 7.5 kcal mol endergonic for 1b-2. The

octahedral ( PNP)Rh(CH )(I) complex 2d is similarly

stable. Furthermore, the ﬁve-coordinate ion pair [( PNP)Rh-

consistent with a Rh oxidation state. To a THF solution of 3a

−

was added MeI dropwise, resulting in a color change from red

iPr

III

to yellow. Two Rh complexes were observed by P{ H}

iPr

NMR spectroscopy, each with J

= 99 Hz, indicating the

PRh

Organometallics XXXX, XXX, XXX−XXX

Organometallics

formation of a similar product, ( PNP *)Rh(Cl)(I) (3b ″).

Article

likely formation of isomers of (^M^e^sPNP)Rh(Me)(I)(Cl) (3b).

(Me)(I) (3d). This resonance is absent when 3c is reacted

III

Mes

Stirring the yellow solution of the Rh products results in a

with CD I. Also, the H NMR spectrum of ( PNP)Rh(Me)-

color change back to red after ∼30 min, and H NMR and

(I) (3d) is consistent with the loss of a mirror plane after

P{ H} NMR spectra are consistent with the formation of

reacting with MeI, in comparison to 3c. A doublet with J

106 Hz was observed in the P{ H} NMR spectrum. Methane

(0.19 ppm, singlet) was observed as a product along with the

PRh

Mes

(

PNP)RhI (3c) and another unidentiﬁed species as a minor

product (Scheme 10). 3c was synthesized independently by

mixing 3a and NaI in acetone, and the structure of 3c was

conﬁrmed by X-ray crystallography (Figure 8). Monitoring the

reaction by NMR spectroscopy indicates the formation of

MeCl and CH ₄ .

formation of 3d″, and for the reaction of 3c with CD

I, the

observed septet at 0.14 ppm the H NMR spectrum is assigned

to CD H (Figure 9). It is notable that in the reaction of 3a and

MeI, methane release was also observed, which suggests the

Mes

Figure 8. ORTEP drawings of (^M^e^sPNP)RhCl (3a, left) and

Mes

(

PNP)RhI (3c, right). Ellipsoids are drawn at the 50% probability

level, and hydrogen atoms are omitted for clarity. Selected bond

lengths for 3a (Å): Rh1−N1 2.056(3), Rh1−P2 2.2703(9), Rh1−P1

.2963(9), Rh1−Cl1 2.3757(8). Selected bond angles for 3a (deg):

N1−Rh1−P2 84.66(9), N1−Rh1−P1 84.38(9), P2−Rh1−P1

64.43(3), N1−Rh1−Cl1 176.23(9), P2−Rh1−Cl1 92.43(3), P1−

Rh1−Cl1 98.00(3). Selected bond lengths for 3c (Å): Rh1−N1

.053(9), Rh1−P1 2.281(3), Rh1−P2 2.283(3), Rh1−I1 2.591(1).

Figure 9. (top) Peaks assigned to Rh−CH (left) and CH (right) in

Selected bond angles for 3c (deg): N1−Rh1−P1 84.9(3), N1−Rh1−

P2 85.3(3), P1−Rh1−P2 167.0(1), N1−Rh1−I1 173.7(2), P1−

Rh1−I1 94.27(7), P2−Rh1−I1 94.62(8).

the H NMR spectrum of the reaction of 3c with CH I. (bottom) H

NMR spectrum of the reaction of 3c with CD I. Reactions were

performed in THF-d at room temperature.

In an attempt to generate (^M^e^sPNP)Rh(Me)(I) (3d)

Mes

through oxidative addition of MeI to ( PNP)RhI (3c), the

product of intramolecular mesityl methyl C−H activation,

Mes

(

PNP*)Rh(I) (3d″), was obtained (Scheme 11). The

Scheme 11. C−H Activation of the Mesityl Substituent of

Mes

(

PNP)RhI (3c) in THF

structure of 3d″ was determined by X-ray crystallography

(

Figure 10). The P{ H} NMR spectrum of 3d″ shows a two-

Figure 10. ORTEP drawing of (^M^e^sPNP*)Rh(I)

(3d″). Ellipsoids are

bond P−P coupling constant of 462 Hz, which is within the

range of common J values between two trans phosphorus

Similar C−H activations of the methyl of mesityl or

mesitylene by late-transition-metal complexes has been

drawn at the 50% probability level, and hydrogen atoms are omitted

for clarity. Selected bond lengths for 3d″ (Å): Rh1−N1 2.093(5),

Rh1−C1 2.105(7), Rh1−P1 2.262(2), Rh1−P2 2.481(2), Rh1−I1

2.6622(7), Rh1−I2 2.8175(8). Selected bond angles for 3d″ (deg):

N1−Rh1−C1 87.0(2), N1−Rh1−P1 81.3(2), C1−Rh1−P1 82.6(2),

N1−Rh1−P2 81.8 (2), C1−Rh1−P2 94.1(2), P1−Rh1−P2

8,89

atoms.

1,90−94

Mes

reported.

The oxidative addition product, ( PNP)-

Rh(Me)(I) (3d), could be observed by NMR spectroscopy,

but attempts to isolate it were not successful due to the

62.92(6), N1−Rh1−I1 175.7(2), C1−Rh1−I1 92.4(2), P1−Rh1−

conversion to 3d″ and methane. The H NMR spectrum of the

I1 94.45(5), P2−Rh1−I1 102.45(4), N1−Rh1−I2 92.5(2), C1−

Rh1−I2 173.1(2), P1−Rh1−I2 90.53(5), P2−Rh1−I2 92.68(5), I1−

Rh1−I2 87.59(2), C3−P1−C10 107.1(3).

reaction between 3c and MeI reveals a triplet of doublets at

Mes

.59 ppm assigned to the Rh−CH group of ( PNP)Rh-

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Reactions of ( PNP)RhCl (4a). ( PNP)RhCl (4a) was

synthesized by stirring a THF solution of [Rh(μ-C H ) Cl]

product is assigned as another isomer of ( PNP)Rh(Me)-

(Cl)(I), but eﬀorts to conﬁrm the structure by X-ray

diﬀraction were not successful. Similar to the case for 2b,

4 2

81,95

with PNP (2 equiv).

groups and four PCH protons in the H NMR spectrum of

PNP)RhCl (4a) is consistent with the formation of 4a, and

value of 150 Hz for 4a is consistent with a Rh

oxidation state. Adding MeI to a benzene solution of

The equivalency of four phenyl

heating a THF-d suspension of 4b at 90 °C did not give the

(

corresponding reductive elimination product.

the J

PRh

Oxidative Addition of CH Cl to ( PNP)RhCl. CH Cl

was added to solutions of the four ( PNP)RhCl complexes in

order to compare oxidative addition reactions to form

III

(

PNP)RhCl (4a) gives two Rh products (Scheme 12).

(

PNP)Rh(CH Cl)(Cl) . The oxidative addition of CH Cl

2 2 2 2

Scheme 12. Oxidative Addition of MeI to ( PNP)RhCl

only occurred with smaller substituents on phosphorus (i.e., for

R = Pr, Ph). In the reaction of ( PNP)RhCl (2a), two

isomers, cis-( PNP)Rh(CH Cl)Cl (2e-cis) and trans-

(

4a) in Benzene

iPr

(

PNP)Rh(CH Cl)Cl (2e-trans), were formed in a 2:1

2 2

ratio, according to the integration of the 3,5-H of pyridyl in the

H NMR spectrum (Scheme 13 and Figure 12). The structures

of 2e-cis and 2e-trans have been conﬁrmed by a single-crystal

X-ray diﬀraction study (Figure 13). In the reaction of

The major product (4b-1) is one isomer of ( PNP)Rh(Me)-

Cl)(I), with methyl and iodide trans to each other, which is

(

PNP)RhCl (4a), only cis-( PNP)Rh(CH Cl)Cl (4e-cis)

2 2

(

was observed by H NMR spectroscopy, and the structure was

conﬁrmed by X-ray crystallography (Figure 11 ). The other

conﬁrmed by a single-crystal X-ray diﬀraction study (Figure

3). There was no reaction between CH Cl and ( PNP)RhCl

when R = Bu, Mes. We attempted to gain evidence for the

reversible reductive elimination of CH Cl from complexes 2e

(

a mixture of 2e-cis and 2e-trans) and 4e-cis by heating

III

solutions of the Rh complexes in CD Cl . Reversible

reductive elimination should produce free CH Cl and Rh−

CD Cl complexes; however, no reductive elimination was

observed, indicating that the oxidative addition products are

III

likely highly favored for these Rh complexes.

Activation and reaction free energies were calculated for the

S 2 addition of CH Cl to the four ( PNP)RhCl complexes

(

Scheme 14). As for the addition of CH I to ( PNP)RhCl, the

phosphine substituent exerts a far greater inﬂuence on the

calculated exothermicity of the reaction than on the activation

Figure 11. ORTEP drawing of ( PNP)Rh(Me)(Cl)(I) (isomer 4b-

⧧

−1

barrier. The ΔG value for R = Mes is 19.3 kcal mol , 5 kcal

). Ellipsoids are drawn at the 50% probability level, and hydrogen

atoms are omitted for clarity. Selected bond lengths for 4b-1 (Å):

Rh1−N1 2.059(6), Rh1−C1 2.10(2), Rh1−P2 2.298(2), Rh1−P1

.307(2), Rh1−Cl1 2.359(2), Rh1−I1 2.686(1). Selected bond angles

for 4b-1 (deg): N1−Rh1−C1 86.1(6), N1−Rh1−P2 82.9(2), C1−

Rh1−P2 89.6(5), N1−Rh1−P1 83.9(2), C1−Rh1−P1 89.3(5), P2−

Rh1−P1 166.83(8), N1−Rh1−Cl1 178.6(2), C1−Rh1−Cl1 93.6(5),

P2−Rh1−Cl1 95.74(9), P1−Rh1−Cl1 97.43(9), N1−Rh1−I1

0.3(2), C1−Rh1−I1 175.7(5), P2−Rh1−I1 87.77(7), P1−Rh1−I1

2.45(7), Cl1−Rh1−I1 90.00(7).

−

mol higher than that for the phenyl analogue. For R = Bu,

⧧

−1

ΔG = 18.8 kcal mol , only 2 kcal mol higher than that for

R = Pr. This small diﬀerence may not explain the distinct

reactivity observed for the two alkyl substituents. The standard

reaction free energies for the bulky substituents R = Bu, Mes

−1

are +2.0 and −8.5 kcal mol , respectively, while the reactions

with R = Pr and Ph are irreversibly exergonic at ΔG = −26.7

−

III

and −25.6 kcal mol . Again, the stability of the Rh

intermediates is consistent with the absence of elimination

reactions for the smaller substituents.

Scheme 13. Attempted Oxidative Addition Reactions of CH Cl with the Rh Complexes ( PNP)RhCl

R = Bu, Pr, Mes, Ph.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

iPr

Figure 12. H NMR (pyridyl region, left) and P{ H} NMR (right) spectra from the reaction of ( PNP)RhCl (2a) and CH Cl to form cis-

iPr

(

PNP)Rh(CH Cl)Cl (2e-cis, 36.7 ppm, J = 97 Hz in P{ H} NMR) and trans-( PNP)Rh(CH Cl)Cl (2e-trans, 39.5 ppm, J = 96 Hz

in P{ H} NMR). From bottom to top: starting complex 2a and reaction after 4 h and after 15 h. Reaction conditions: 60 °C, C D as the solvent.

2 2 PRh 2 2 PRh

iPr

Figure 13. ORTEP drawings of ( PNP)Rh(CH Cl)Cl (2e; 2e-cis on the left and 2e-trans in the middle) and cis-( PNP)Rh(CH Cl)Cl (4e-cis,

right). Ellipsoids are drawn at the 50% probability level, and hydrogen atoms are omitted for clarity. 2e refers to the mixture of 2e-cis and 2e-trans.

Selected bond lengths for 2e (Å): Rh1−N1 2.143(3), Rh1−P1 2.3434(9), Rh1−P2 2.3472(9), Rh1−Cl2 2.3669(8). Selected bond lengths for 2e-

cis (Å): Rh1−C20A 2.08(8), Rh1−Cl1A 2.41(2). Selected bond lengths for 2e-trans (Å): Rh1−C20 2.099(5), Rh1−Cl1 2.365(1). Selected bond

angles for 2e (deg): N1−Rh1−P1 81.96(7), N1−Rh1−P2 82.02(7), P1−Rh1−P2 163.95(3), P1−Rh1−Cl2 84.35(3). Selected bond angles for 2e-

cis (deg): C20A−Rh1−N1 87.0(3), C20A−Rh1−P1 94.(3), C20A−Rh1−P2 86.(3), C20A−Rh1−Cl2 176.(3), N1−Rh1−Cl2 89.29(7), P2−

Rh1−Cl2 94.52(3), C20A−Rh1−Cl1A 85.(3), N1−Rh1−Cl1A 171.3(4), P1−Rh1−Cl1A 100.9(4), P2−Rh1−Cl1A 95.1(4), Cl2−Rh1−Cl1A

9.1(4). Selected bond angles for 2e-trans (deg): C20−Rh1−N1 177.9(2), C20−Rh1−P1 97.4(2), N1−C20−Rh1−P2 98.6(2), C20−Rh1−Cl1

7.5(1), N1−Rh1−Cl1 90.53(7), P1−Rh1−Cl1 95.13(4), P2−Rh1−Cl1 85.95(4), C20−Rh1−Cl2 92.7(1), N1−Rh1−Cl2 89.29(7), P2−Rh1−

Cl2 94.52(3), Cl1−Rh1−Cl2 179.47(4). Selected bond lengths for 4e-cis (Å): Rh1−N1 2.081(13), Rh1−C1 2.13(2), Rh1−P1 2.311(3), Rh1−Cl1

.346(4), Rh1−Cl2 2.536(5). Selected bond angles for 4e-cis (deg): N1−Rh1−C1 92.6(8), N1−Rh1−P1 83.31(7), C1−Rh1−P1 90.5(9), C1−

Rh1−P1 90.1(9), P1−Rh1−P1 166.61(14), N1−Rh1−Cl1 180.0, C1−Rh1−Cl1 87.4(8), P1−Rh1−Cl1 96.69(7), N1−Rh1−Cl2 89.0(2), P1−

Rh1−Cl2 95.8(2), P1−Rh1−Cl2 84.0(2), Cl1−Rh1−Cl2 91.0(2), N1−Rh1−Cl2 89.0(2), P1−Rh1−Cl2 84.0(2), P1−Rh−Cl2 95.8(2), Cl1−

Rh1−Cl2 91.0(2), Cl2−Rh1−Cl2 178.0(3).

CONCLUSIONS

a function of the substituents. When CH Cl is used as the

2 2

■

oxidative addition reagent, the reactions only occur with PNP

ligands that possess the smaller substituents isopropyl and

phenyl. An Eyring analysis for the oxidative addition of MeI to

We speculated that the axial steric bulk of PNP ligands could

be used to control the energetics of Rh oxidative addition/

reductive elimination sequences. The hypothesis was validated

by combining computational modeling and experimental

studies. For reactions of MeI with ( PNP)RhCl complexes,

I/III

tBu

⧧

(

PNP)RhCl (1a) indicates that the ΔH and ΔS values are

−1 −1 −1

0.4(8) kcal mol and −34(2) cal mol K , respectively.

⧧

−1

The ΔG value at −78 °C is ∼17 kcal mol . The rate of

the results vary with the identity of the substituents R. For

large substituents such as tert-butyl and mesityl, MeI oxidative

addition is slow relative to that for PNP ligands with smaller

substituents, and subsequent MeCl reductive elimination is

observed. In contrast, the smaller substituents isopropyl and

phenyl facilitate MeI oxidative addition, and the reductive

elimination of MeCl or MeI was not observed due to the high

iPr

oxidative addition of MeI to ( PNP)RhCl (2a) at −78 °C is

too fast to be measured by NMR spectroscopy, but an upper

⧧

−1

limit for the ΔG of ≤13 kcal mol was estimated. Thus, the

⧧

ΔΔG value for MeI oxidative addition to 1a and 2a is

−

proposed to be ≥4 kcal mol . Calculated thermochemistry

corroborates that the activation barriers for S 2 addition

reactions at square-planar Rh , which require access to only

one site, are perturbed by 2−5 kcal mol when the steric bulk

is varied. The stability of six-coordinate products, however,

III

−1

stability of Rh products. While the results are consistent with

steric inﬂuence being the most important factor, it is not

possible to eliminate a potential role for ligand donor ability as

−

changes by more than 25 kcal mol . These results indicate

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

t B u

i P r

M e s

P h

prepare

PNP,

PNP, and [Rh-

Scheme 14. Computed Activation and Reaction Free

88,95,97−99

−

(C H ) Cl] .

Energies (kcal mol^aI ^t²⁹⁸^Kⁱⁿ^T ^H ^F ⁾^f^o^r^t^h^e^A^d^dⁱ^tⁱ^oⁿ^o^f

tBu

Synthesis and Characterization. ( PNP)RhCl (1a). A proce-

dure modiﬁed from the literature was applied. In a glass vial, a THF

5 mL) solution of PNP (210 mg, 531 μmol) was added to a THF

5 mL) solution of [Rh(μ-C H ) Cl] (110 mg, 283 μmol) and the

CH Cl to ( PNP)Rh Cl Complexes

tBu

(

mixture was stirred for 10 min. The mixture turned from orange to

red. The solvent was removed under vacuum, the solid was washed

with pentane (5 mL × 2) and then dried under vacuum, and the pure

tBu

product of ( PNP)RhCl was obtained as a red solid. The isolated

yield was 81%. H NMR (C D , 600 MHz): δ 6.89 (t, 1H, J = 8

Hz, py H-4), 6.27 (d, 2H, J = 8 Hz, py H-3,5), 2.57 (vt, 4H,

CH P), 1.46 (vt, 36H, J = 6 Hz, PC(CH ) ). C{ H} NMR

3 3

(

C D , 151 MHz): δ 164.4 (vt, py C-2,6), 128.7 (s, py C-4), 119.8

6 6

vt, J = 5 Hz, py C-3,5), 36.5 (vt, CH P), 34.9 (vt, PC(CH ) ),

3 3

9.6 (vt, PC(CH ) ). P{ H} NMR (C D , 243 MHz): δ 58.1 (d,

3 3 6 6

= 145 Hz).

PRh

(ⁱ^P^rPNP)RhCl (2a). A procedure modiﬁed from the literature was

iPr

used. A THF solution (3 mL) of PNP (95.6 mg, 282 μmol) was

added to a stirred solution of [Rh(μ-C H ) Cl] (54.8 mg, 141 μmol)

in THF (5 mL), leading to a color change from orange to dark red.

The reaction mixture was stirred at ambient temperature for 30 min

followed by solvent removal under vacuum to give a red solid. The

solid was washed with cold pentane (5 mL × 2). The isolated yield

was 72%. Crystals suitable for X-ray analysis were obtained by layering

iPr

a concentrated THF solution of ( PNP)RhCl with pentane.

NMR (C D , 600 MHz): δ 6.90 (t, J = 8 Hz, 1H, py 4-H), 6.30 (d,

J_H_H= 8 Hz, 2H, py 3,5-H), 2.44 (vt, J = 4 Hz, 4H, PCH ), 2.08

svt, J = 8 Hz, 4H, CH), 1.52 (dvt, 12H, Me of Pr), 1.07 (dvt,

(

2H, Me of Pr). C{ H} NMR (C D , 151 MHz): δ 163.9 (vt, py C-

6 6

,6), 128.9 (s, py C-4), 112.0 (vt, py C-3,5), 35.9 (vt, CH P), 24.3 (vt,

that the rates of Rh^I^/^I^I^Ioxidative addition/reductive elimination

processes can be controlled through variation of axial steric

bulk (Scheme 15). Adjusting the steric properties of the

intermediates in the proposed mechanism of hydrocarbon

oxidation (Scheme 1) oﬀers another strategy to tune the

energetics of both C−H activation and reductive elimination.

CHMe

NMR (C D

), 19.4 (vt, CH(CH

, 243 MHz): δ 47.4 (d, JPRh = 145 Hz).

PNP)RhCl (3a). In a 100 mL round-bottom ﬂask, a THF (20

) ), 18.0 (s, vt, CH(CH ) ). P{ H}

3 2 3 2

6 6

Mes

(

Mes

mL) solution of PNP (31.7 mg, 492 μmol) was added to a THF

20 mL) solution of [Rh(μ-C H ) Cl] (95.6 mg, 246 μmol) and the

(

mixture stirred for 3 h. The mixture turned from orange to red. The

solvent was removed under vacuum, and the solid was washed with

pentane (10 mL × 2) and then dried under vacuum. The pure

EXPERIMENTAL SECTION

Mes

■

product of ( PNP)RhCl was obtained as a red solid with 62% yield.

General Methods. Unless otherwise noted, all reactions and

manipulations were performed under a nitrogen atmosphere in a

glovebox or using standard Schlenk techniques with dried and

degassed solvents. All solvents were reagent grade or better.

Deuterated solvents were used as received. All the solvents were

kept in the glovebox over 4 Å molecular sieves. All NMR spectra were

recorded on a Varian Inova 600 or 500 MHz spectrometer or a Bruker

Avance III 800 MHz spectrometer. The operating frequency for

Crystals suitable for X-ray analysis were obtained by slowly

evaporating pentane into a concentrated THF solution of ( PNP)-

RhCl. H NMR (THF-d , 600 MHz): δ 7.52 (t, 1H, J = 8 Hz, py

H-4), 7.04 (d, 2H, J = 8 Hz, py H-3,5), 6.73 (s, 8H, mesityl H-

3,5), 4.00 (vt, 4H, PCH ), 2.19 (s, 12H, mesityl 4-Me), 2.15 (s, 24H,

mesityl 2,6-Me). C{ H} NMR (DCM-d , 151 MHz): 163.6 (vt, py

C-2,6), 141.1 (vt, Mes C-2,6), 138.8 (s, Mes C-4), 133.2 (vt, Mes C-

1), 130.8 (vt. Mes C-3,5), 128.9 (s, py C-4), 121.1 (vt, py C-3,5), 48.0

(vt, CH P), 25.2 (vt, Mes 2,6-CH ), 21.1 (s, Mes 4-CH ). P{ H}

Mes

C{ H} NMR was 150 MHz (on the 600 MHz instrument) or 201

MHz (on the 800 MHz instrument) and for P{ H} NMR was 243

MHz (on the 600 MHz instrument) or 202 MHz (on the 500 MHz

NMR (THF-d , 243 MHz): δ 11.8 (d, J = 143 Hz). Acceptable

8 RhP

elemental analysis results for 3a could not be obtained. The NMR

spectra of 3a appear in Figures S7 −S9 .

instrument). All H and C{ H} NMR spectra are referenced against

residual H resonances ( H NMR) or C{ H} resonances ( C{ H}

( PNP)RhCl (4a). A procedure modiﬁed from the literature was

NMR) of the deuterated solvents. P{ H} NMR spectra are

referenced with 85% H PO in H O (used as an external standard).

used. In a 100 mL round-bottom ﬂask, a THF (20 mL) suspension

of PNP (270 mg, 560 μmol) was added to a THF (5 mL) solution

All spectra were recorded at 25 °C unless otherwise indicated. For the

of [Rh(C H ) Cl] (120 mg, 309 μmol) and the mixture stirred for

90 min. The reaction mixture turned to a red suspension. The mixture

was ﬁltered, and the precipitate was washed with pentane (5 mL × 2)

variable-temperature NMR experiments, the temperature was

calibrated using methanol. Literature procedures were used to

Scheme 15. Axial Steric Eﬀect on the Oxidative Addition and Reductive Elimination of ( PNP)Rh Complexes

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

iPr

and dried. The product ( PNP)RhCl was isolated as a reddish

to an acetone (0.5 mL) solution of ( PNP)RhCl (72.7 mg, 152

μmol). The reaction mixture was stirred for 10 min, and the solvent

was removed under vacuum. A few drops of benzene were added to

fully dissolve the red product, and the mixture was ﬁltered through a

ﬁne-porosity frit loaded with Celite. Benzene was removed under

vacuum, and the product was washed with cold pentane (10 mL × 2).

orange solid in 43% yield. H NMR (C D , 600 MHz): δ 8.17 (8H,

m, phenyl H-2,6), 7.06 (t, 8H, J = 7 Hz, phenyl H-3,5), 7.00 (t,

H, J = 7 Hz, phenyl H-4), 6.76 (t, 1H, J_H_H= 8 Hz, py H-4), 6.28

(

t, 2H, J = 8 Hz, py H-3,5), 3.21 (vt, 4H, CH P). C{ H} NMR

C D , 151 MHz): δ 162.3 (vt, py C-2,6), 135.7 (vt, Ph C-1), 133.5

vt, Ph C-2,6), 130.6 (s, Ph C-4), 129.7 (s, py C-4), 128.7 (vt, Ph C-

,5), 120.5 (vt, py C-3,5), 44.0 (vt, CH P). P{ H} NMR (C D , 243

MHz): δ 22.2 (d, J = 150 Hz).

A red solid was obtained in 85% yield. H NMR (600 MHz, C D ): δ

6.98 (t, J = 8 Hz, 1H, py H-4), 6.38 (d, J = 8 Hz, 2H, py H-

HH HH

3,5), 2.48 (vt, 4H, PCH ), 2.19 (m, 4H, PCH(CH ) ), 1.53 (dvt, J

2 3 2 HH

= 7 Hz, 12H, PCH(CH ) ), 0.97 (dvt, J_H_H= 7 Hz, 12H,

PCH(CH ) ). C{ H} NMR (151 MHz, C D ): δ 163.7 (vt, py

3 2 6 6

C-2,6), 130.0 (s, py C-4), 120.1 (vt, py C-3,5), 36.9 (vt, PCH ), 24.6

PRh

iPr

(

PNP)Rh(Me)(Cl)(I) (2b). In a glass vial, 0.5 mL of MeI was added

3 2

iPr

dropwise into a stirred THF (10 mL) solution of ( PNP)RhCl (54.1

mg, 113 μmol). The solution turned from red to yellow in 5 s. The

solvent was removed under vacuum. The product was dissolved in a

minimal amount of DCM (3 mL), crashed out with addition of

pentane (50 mL), and collected by ﬁltration. The solid was washed

with pentane (10 mL × 2) and then dried under vacuum. The

product ( PNP)Rh(Me)(Cl)(I) was obtained as a yellow solid. The

isolated yield was 78%. Crystals suitable for X-ray analysis were

obtained by slowly evaporating MeI into a benzene solution of

(vt, PCH(CH ) ), 19.8 (br s, PCH(CH ) ), 18.0 (br s, PCH(CH ) ).

3 2

P{ H} NMR (243 MHz, C D ): δ 51.1 (d, J = 138 Hz). Anal.

Calcd for C H INP Rh: C, 40.09; H, 6.20; N, 2.46. Found: C,

39.93; H, 6.17; N, 2.54.

^M^e^sPNP)RhI (3c). In a glass vial, an acetone solution (3 mL) of NaI

(24.6 mg) was added to an acetone suspension (5 mL) of

PRh

iPr

(

Mes

(

PNP)RhCl (64.2 mg). The reaction mixture was stirred for 30

iPr

(

PNP)RhCl. The NMR results are the data for one isomer of 2b-1

and 2b-2, and the elemental analysis data are for the mixture of the

min, and it quickly turned from an orange suspension into a dark red

solution. After the solvent was removed under vacuum, the mixture

was dissolved in 5 mL of benzene and was ﬁltered through frits loaded

with Celite to remove NaCl. Benzene was removed under vacuum,

and the product was washed using pentane (5 mL × 2). A dark red

solid was obtained as the product after it was dried under vacuum.

The yield was 89%. Crystals suitable for X-ray analysis were obtained

by vapor diﬀusion of pentane into a concentrated THF solution of

two complexes. H NMR (C D , 600 MHz): δ 6.70 (t, J = 8 Hz,

H, py 4-H), 6.39 (d, J = 8 Hz, 2H, py 3,5-H), 4.05 (m, 2H, CH),

.86 (dvt, J = 17 Hz, 2H, PCH ), 3.01 (dvt, J = 17 Hz, 2H,

PCH ), 2.45 (m, 2H, CH), 1.68 (q, J = 8 Hz, 6H, Me of Pr), 1.52 (td,

J_R_h_H= 2 Hz, 3H, Rh−Me), 1.33 (q, J = 7 Hz, 6H, Me of Pr), 1.19 (q,

J = 7 Hz, 6H, Me of Pr), 1.07 (q, 6H, Me of Pr). C{ H} NMR

THF-d , 151 MHz): 163.9 (vt, py C-2,6), 136.0 (s, py C-4), 120.8

vt, py C-3,5), 40.2 (vt, PCH ), 30.6 (vt, PCHMe ), 23.6 (vt,

Mes

(

PNP)RhI. H NMR (C D , 600 MHz): δ 6.72 (m, 9H, py H-4

and mesityl H), 6.16 (d, 2H, J = 7 Hz, py H-3,5), 3.90 (vt, 4H,

6 6

PCHMe ), 20.1 (s, CH(CH ) ), 19.9 (s, CH(CH ) ), 19.8 (s,

PCH ), 2.80 (s, 24H, mesityl 2,6-Me), 2.05 (s, 12H, mesityl 4-Me).

CH(CH ) ), 19.4 (s, CH(CH ) ). P{ H} NMR (C D , 243 MHz):

C{ H} NMR (C D , 151 MHz): 162.3 (vt, py C-2,6), 141.1 (vt,

6 6

δ 36.0 (d, J

= 100 Hz). Anal. Calcd for C H ClINP Rh: C,

Mes C-2,6), 138.4 (s, Mes C-4), 133.4 (vt, Mes C-1), 130.9 (vt. Mes

PRh

20 38

8.76; H, 6.18; N, 2.26. Found: C, 38.98; H, 6.33; N, 2.28.

C-3,5), 128.4 (s, py C-4), 120.3 (vt, py C-3,5), 50.1 (vt, CH P), 26.4

(

PNP)Rh(Me)(Cl)(I) (4b-1). In a 20 mL glass vial, ( PNP)RhCl

(vt, Mes 2,6-CH ), 20.9 (s, Mes 4-CH ). P{ H} NMR (C D , 243

3 3 6 6

(

82.9 mg, 135 μmol) was dissolved in 10 mL of CH Cl . The reaction

MHz): δ 14.2 (d, J

= 140 Hz). Acceptable elemental analysis

RhP

mixture was stirred overnight, and the solvent was evacuated under

vacuum. The product was dissolved in a minimal amount of DCM (3

mL), precipitated with addition of pentane (50 mL), and collected by

ﬁltration. The product was washed with pentane (10 mL × 2). A

yellow solid was obtained as the ﬁnal product. The isolated yield was

results for 3c could not be obtained. The NMR spectra of 3c appear

in Figures S25 −S27 .

tBu

[( PNP)Rh(Me)I]I (1d′). In a 50 mL round-bottom ﬂask, 5 mL of

tBu

MeI was added to a THF (20 mL) solution of ( PNP)RhCl (92.3

mg, 172 μmol). The reaction mixture turned from a dark red solution

to a orange suspension in 20 s. The reaction mixture was stirred

overnight. The mixture was ﬁltered, and the solid was washed with a

mixture of MeI and pentane (1/3 v/v, 10 mL) and dried under

vacuum. The product was obtained as an orange solid, and the NMR

9%. Crystals suitable for X-ray analysis were obtained by slowly

evaporating MeI into a benzene solution of ( PNP)RhCl. H NMR

(600 MHz, DCM-d ): δ 7.95 (m, 4H, phenyl H), 7.74 (t, 1H, J

Hz, py H-4), 7.68 (m, 4H, phenyl H), 7.52 (d, 2H, J = 8 Hz, py

H-3,5), 7.41 (m, 12H, phenyl H), 4.80 (vdt, 2H, J = 5 Hz, PCH ),

spectra are consistent with published results. The yield was 65%. H

RhH

.39 (vdt, 2H, J = 5 Hz, PCH ), 0.72 (td, 3H, J = 2 Hz, J

NMR (600 MHz, DCM-d ): δ 8.01−7.93 (m, 3H, py H-3,4,5), 4.30

RhH

Hz, Rh−CH ). C{ H} NMR (202 MHz, DCM-d ): 163.1 (vt),

38.6, 134.8 (vt), 132.8 (vt), 131.4, 130.9, 129.1 (vt), 128.5 (vt),

(dvt, J = 18 Hz, 2H, PCH ), 4.04 (dvt, J = 18 Hz, 2H, PCH ),

HH 2 HH 2

2.30 (vtd, J = 3 Hz, 3H, RhCH ), 1.56 (vt, 18H, PC(CH ) ), 1.38

RhH 3 3 3

22.3 (vt), 42.9 (vt). P{ H} NMR (243 MHz, DCM-d ): δ 23.2 (d,

(vt, 18H, PC(CH ) ). C{ H} NMR (DCM-d , 151 MHz): 165.1

^JPRh = 106 Hz). Anal. Calcd for C H ClINP Rh: C, 50.85; H, 4.00;

(vt, py C-2,6), 140.1 (s, py C-4), 124.1 (vt, py C-3,5), 39.2 (vtd, J

RhC

N, 1.85. Found: C, 50.82; H, 3.91; N, 1.91.

= 2 Hz, PCH ), 38.1 (vt, PC(CH ) ), 37.5 (vt, PC(CH ) ), 31.3 (vt,

2 3 3 3 3

tBu

(

PNP)RhI (1c). In a 50 mL round-bottom ﬂask, 5 mL of MeI was

PC(CH ) ), 29.8 (vt, PC(CH ) ), 9.2 (dt, J = 25 Hz, J = 4 Hz,

3 3 3 3 RhC PC

tBu

added to a THF (20 mL) solution of ( PNP)RhCl (92.3 mg, 0.172

μmol). The reaction mixture turned from a dark red solution to an

orange suspension in 20 s. After the suspension was stirred for 20 min,

the solvent was removed under vacuum, and another 20 mL of THF

was added. The resulting red solution was again left under vacuum to

remove the solvent. The addition of THF and removal of solvent was

repeated three times, resulting in dark red solid as the product. The

isolated yield was 99%. Crystals suitable for X-ray analysis were

RhCH₃). P{ H} NMR (243 MHz, DCM-d ): δ 51.8 (d, J = 100

2 PRh

Hz). Leaving the sample under vacuum for 2 weeks did not remove all

of the THF, and 0.08 molecule of THF is included in the calculation

of theoretical value of elemental analysis, on the basis of integration in

the H NMR spectrum. Anal. Calcd for C₂₄_.₃₂H I NP RhO₀_.₀₈: C,

46.64 2

37.78; H, 6.08; N, 1.81. Found: C, 38.03; H, 6.25; N, 1.87.

iPr

cis-( PNP)Rh(Me)(I) (2d). The procedure was modiﬁed from the

iPr

literature. In a 100 mL round-bottom ﬂask, ( PNP)RhI (54.2 mg,

95 μmol) was dissolved in THF, and 0.5 mL of MeI was added. The

solution quickly turned from red to bright yellow. The reaction

mixture was stirred for 10 min, and then the solvent was removed

under vacuum. The resulting solid was dissolved in a small amount of

DCM, and 50 mL of pentane was added to precipitate the product.

tBu

obtained by layering a concentrated THF solution of ( PNP)RhI

with pentane. H NMR (CD Cl , 600 MHz): δ 7.55 (t, 1H, J = 8

Hz, py H-4), 7.04 (d, 2H, J = 8 Hz, py H-3,5), 3.07 (vt, 4H,

CH P), 1.43 (vt, 36H, PC(CH ) ). C{ H} NMR (C D , 151

MHz): δ 164.0 (vt, py C-2,6), 130.0 (s, py C-4), 119.7 (vt, J = 5

Hz, py C-3,5), 37.7 (vt, J = 5 Hz, CH P), 35.4 (vt, J = 6 Hz,

The mixture was ﬁltered through a ﬁne-porosity frit, and a yellow

iPr

PC(CH ) ), 30.0 (vt, J_P_C= 4 Hz, PC(CH ) ). P{ H} NMR

CD Cl , 243 MHz): δ 60.5 (d, J = 138 Hz).

solid, cis-( PNP)Rh(Me)I , was collected. The product was washed

(

with pentane (5 mL × 2). The isolated yield was 88%. H NMR

PRh

iPr

(

PNP)RhI (2c). A reported procedure was used. In a glass vial,

(C D , 600 MHz): δ 6.69 (t, J = 8 Hz, 1H, py 4-H), 6.38 (d, J

6 6 HH HH

an acetone (0.6 mL) solution of NaI (45.5 mg, 304 μmol) was added

= 8 Hz, 2H, py 3,5-H), 3.97−3.90 (m, 4H, CH and PCH ), 3.08 (dvt,

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

²J_H_H= 17 Hz, 2H, PCH ), 2.47 (m, 2H, CH), 1.71 (q, J = 8 Hz, 6H,

PCH ), 1.27−1.20 (m, 24H, PCH(CH ) ). C{ H} NMR (201

3 2

Me of Pr), 1.40 (td, J

= 2 Hz, 3H, Rh−Me), 1.37 (q, J = 7 Hz,

MHz, Benzene-d ): δ 160.9 (vt, pyridine C-2,6), 137.4 (s, pyridine C-

RhH

H, Me of Pr), 1.22 (q, J = 7 Hz, 6H, Me of Pr), 1.06 (q, 6H, Me of

4), 120.5 (vt, pyridine C-3,5), 39.1 (vt, PCH

CP = 6 Hz, RhCH

19.2 (s, PCH(CH

), 34.0 (d, JCRh = 21 Hz,

), 19.6 (s, PCH(CH ),

). P{ H} NMR (201 MHz, benzene-d ): 39.5

(d, JPRh = 96 Hz). Anal. Calcd for C20 NP Rh: C, 42.69; H,

.63; N, 2.49. Found: C, 42.85; H, 6.72; N, 2.57.

Pr). C{ H} NMR (DCM-d , 151 MHz): δ 163.6 (vt, py C-2,6),

Cl), 26.0 (vt, PCHMe

)

3 2

37.9 (s, py C-4), 121.7 (vt, py C-3,5), 41.6 (vt, PCH ), 31.0 (vt,

)

CH(CH ) ), 24.6 (vt, CH(CH ) ), 21.5 (s, CH(CH ) ), 21.1 (s,

37Cl

3 2

CH(CH ) ), 20.3 (s, CH(CH ) ), 20.1 (s, CH(CH ) ), −0.5 (m,

3 2

cis-( PNP)Rh(CH Cl)Cl (4e-cis). ( PNP)RhCl (50 mg, 81.5

Rh−CH ). P{ H} NMR (C D , 243 MHz): δ 38.0 (d, J = 99

PRh

μmol) was dissolved in 10 mL of DCM, and the mixture was stirred

for 48 h. The solution gradually turned from red to yellow. The

solvent was removed under vacuum, and the resulting solid was

dissolved in a minimal amount of DCM (3 mL). A 50 mL portion of

pentane was added to the reaction mixture to precipitate the product.

After ﬁltration through a ﬁne-porosity frit, the yellow solid cis-

Hz). Calcd for C H I NP Rh: C, 33.78; H, 5.39; N, 1.97. Found: C,

38 2

4.32; H, 5.55; N, 2.06.

Mes

(

PNP*)RhI₂(3d″). In a glass vial, MeI (1 mL) was added

Mes

dropwise to a THF solution of ( PNP)RhI (34.2 mg, 39 μmol). The

solution turned from red to yellow quickly. The reaction mixture was

stirred for 24 h, and then the solvent was removed under vacuum and

pentane was used to wash the solid. H NMR (CD Cl , 500 MHz): δ

( PNP)Rh(CH Cl)Cl was washed with 10 mL of pentane and dried

under vacuum. The isolated yield was 92%. Crystals suitable for X-ray

.41 (t, J = 7 Hz, 1H, py H-4), 7.16 (d, J = 8 Hz, 1H, py H-3/

HH HH

analysis were obtained by vapor diﬀusion of DCM into a concentrated

), 7.06 (s, 1H, Mes H-3/5), 7.03 (s, 1H, Mes H), 7.00 (d, J = 7

benzene solution of ( PNP)Rh(CH Cl)Cl . H NMR (600 MHz,

Hz, 1H, py H-5/3), 6.88 (s, 1H, Mes H), 6.84 (s, 1H, Mes H), 6.79

s, 1H, Mes H), 6.66 (s, 2H, Mes H), 6.61 (s, 1H, Mes H), 6.04 (d,

CD Cl ): 8.02 (m, 4H, phenyl H), 7.81−7.74 (m, 5H, pyridine H-4

(

and phenyl H), 7.52 (d, J = 8 Hz, 2H, pyridine H-3,5), 7.50−7.40

^JHH = 13 Hz, 1H, RhCH ), 4.85 (d, J = 15 Hz, 1H, PCH ), 4.55

d, ²J_H_H= 12 Hz, 1H, PCH ), 4.38 (vtd, J = 15 Hz, J = 7 Hz,

(m, 12H, phenyl H), 4.68 (dvt, JHH = 17 Hz, 2H, PCH

), 4.58 (dvt,

), 3.91 (td, JHP = 8 Hz, JHRh = 3 Hz, 2H,

RhCH Cl). C{ H} NMR (201 MHz, CD Cl ): δ 162.7 (vt), 138.9

(

2 HH HP

JHH = 17 Hz, 2H, PCH

H, RhCH ), 4.02 (d, J = 18 Hz, 1H, PCH ), 3.41 (t, J = 12

2 HH 2 HH

Hz, 1H, PCH ), 3.19 (s, 3H, Me), 3.16 (s, 3H, Me), 2.33 (s, 3H, Me),

(

s), 134.6 (vt), 132.6 (vt), 131.3 (s), 131.3 (s), 131.1 (vt), 130.9 (vt),

.31 (s, 3H, Me), 2.29 (s, 3H, Me), 2.24 (s, 3H, Me), 2.09 (s, 3H,

129.2 (vt), 128.7 (vt), 122.5 (vt), 42.7 (dt, J

= 27 Hz, J = 6

): 27.1 (d, JPRh

CRh

Me), 2.06 (s, 3H, Me), 1.94 (s, 3H, Me), 1.91 (s, 3H, Me). C{ H}

NMR (CD Cl , 201 MHz): δ 162.4 (s), 160.6 (s), 157.6 (d, J = 28

Hz), 146.1 (d, J = 12 Hz), 144.3 (d, J = 21 Hz), 142.3 (s),

Hz), 42.3 (vt). P{ H} NMR (243 MHz, CD

103 Hz). Anal. Calcd for C32 NP

.00. Found: C, 55.20; H, 4.24; N, 2.10.

Kinetic Studies. Oxidative Addition of MeI to (^t^B^uPNP)RhCl (1a)

in THF-d . A J. Young NMR tube was charged with a THF-d solution

H29Cl

Rh: C, 55.00; H, 4.18; N,

41.7 (s), 141.5 (s), 141.5 (s), 141.3 (s), 141.1 (s), 140.1 (s), 139.1

(

d, J = 27 Hz), 138.2 (s), 133.6 (d, J = 26 Hz), 132.4 (s), 132.3

s), 131.6 (d, J = 10 Hz), 131.9 (d, J = 8 Hz), 131.8 (d, J = 8

CP CP

tBu

of ( PNP)RhCl (1a; 16 mM) and hexamethyldisiloxane (HMDSO,

internal standard) in the glovebox. After a H NMR spectrum was

obtained for the starting solution, the J. Young tube was brought back

into the glovebox, and approximately 1 μL of MeI was injected into

Hz), 129.5 (d, J = 6 Hz), 129.3 (s), 125.9 (s), 125.8 (s), 121.5 (d,

^JCP = 12 Hz), 121.2 (d, J = 12 Hz), 53.6 (s, RhCH ), 46.8 (d, J

19 Hz, PCH ), 32.8 (d, J = 19 Hz, PCH ), 26.9 (d, J = 6 Hz),

2 CP 2 CP

6.7 (s), 25.4 (s), 24.8 (d, J = 11 Hz), 23.7 (s), 21.3 (s), 21.1 (s),

1.1 (s), 21.0 (s), 20.3 (s). P{ H} NMR (CD Cl , 243 MHz): δ

1.0 (dd, J = 462 Hz, J = 113 Hz), 5.8 (dd, J = 462 Hz, J

the tube. The tube was then quickly analyzed and H NMR spectra

were collected every 1 min using an arrayed experiment. Each data

point consists of four scans with a delay time of 10 s. Concentrations

were determined by using the NMR integrals of the 3,5-H of pyridyl

of the starting and ﬁnal complexes. The reactions were run at constant

temperature. The reaction was run at 300, 309, 319, and 327 K in the

NMR instrument with the spectra recorded.

PRh

90 Hz). Acceptable elemental analysis results for 3d″ could not be

obtained. The NMR spectra of 3d″, including COSY and HSQC,

appear in Figures S34 −S38 .

iPr

cis-( PNP)Rh(CH Cl)Cl (2e-cis) and trans-( PNP)Rh(CH Cl)Cl

iPr

(2e-trans). In a 100 mL round-bottom ﬂask, ( PNP)RhCl (52.6 mg,

Equilibrium between (tBuPNP)RhI (1c) and [(tBu_P_N_P₎_R_h₍_M_e₎_I_]_I

1d′) in DCM-d . A J. Young NMR tube was charged with a DCM-d

10 μmol) was dissolved in DCM (30 mL), and the mixture was

(

stirred for 24 h. The reaction mixture gradually turned from red to

light orange. The solvent was removed under vacuum, and the

resulting solid was dissolved in a minimal amount of DCM (3 mL). A

tBu

solution of [( PNP)Rh(Me)I]I (1d′, 12 mM) and hexamethylben-

zene (HMB, internal standard) in the glovebox. After a H NMR

spectrum was obtained for the starting solution (at this time point

there was already an equilibrium established), the J. Young tube was

brought back into the glovebox, and MeI was injected into the tube.

Another NMR spectrum was taken, and another injection was done.

Four points were taken for each sample to calculate the equilibrium

constant. The reaction was run at 300, 309, 319, and 327 K in the

NMR instrument with the spectra recorded.

0 mL portion of pentane was added to precipitate a yellow solid. The

product was collected using ﬁltration, washed with pentane (5 mL ×

), and dried under vacuum. The product was a mixture of cis-

iPr

(

PNP)Rh(CH Cl)Cl and trans-( PNP)Rh(CH Cl)Cl . The iso-

2 2 2 2

lated yield was 87%. Crystals suitable for X-ray analysis were obtained

by vapor diﬀusion of pentane into a concentrated THF solution of

iPr

(

PNP)Rh(CH Cl)Cl . Data for cis-( PNP)Rh(CH Cl)Cl (2e-cis)

Computational Methods. The free energies of the species (1 M

are as follows. H NMR (497 MHz, benzene-d ): δ 6.73 (t, J = 8

Hz, 1H, pyridine H-4), 6.42 (d, J = 8 Hz, 2H, pyridine H-3,5),

at 298 K) were calculated as

.15 (td, J = 7 Hz, J

7 Hz, 2H, PCH ), 3.33 (dvt, J = 17 Hz, 2H, PCH ), 2.82 (m, 2H,

= 3 Hz, 2H, RhCH Cl), 3.67 (dvt, J

G = G_s_o_l+ ZPE + H − T(ΔS_v_i_b+ ΔS_e_l_e_c+ ΔS_t_r_a_n_s_/_r_o_t− k_B

HRh

ln 24.5)

PCHMe ), 2.68 (m, 2H, PCHMe ), 1.66 (dvt, J = 7 Hz, 6H,

PCH(CH ) ), 1.46 (dvt, J = 7 Hz, 6H, PCH(CH ) ), 1.30 (dvt,

^JHH = 7 Hz, 6H, PCH(CH ) ), 1.06 (dvt, ^JHH = 7 Hz, 6H,

PCH(CH ) ). C{ H} NMR (201 MHz, benzene-d ): δ 163.8 (vt,

including G_s_o_l(the sum of the self-consistently determined electronic

energy and solvation free energy), zero point energy, enthalpy and

entropy (including vibrational, electronic, translational, and rotational

contributions as well as the contribution by compression from 1 atm

to 1 M, which complements the solvation free energy from the

continuum model). Geometries were ﬁrst optimized under vacuum

pyridine C-2,6), 136.5 (s, pyridine C-4), 120.9 (vt, pyridine C-3,5),

9.2 (vt, PCH ), 38.19 (dt, J

= 28 Hz, J = 6 Hz, RhCH Cl),

CRh CP 2

4.7 (vt, PCHMe ), 24.2 (vt, PCHMe ), 20.2 (s, PCH(CH ) ), 19.6

3 2

100

(

s, PCH(CH ) ), 19.5 (s, PCH(CH ) ), 19.0 (s, PCH(CH ) ).

3 2 3 2 3 2

using the B3PW91-D3 functional,

including the D3 dispersion

101

102

P{ H} NMR (201 MHz, Benzene-d ): 36.7 (d, J = 97 Hz). Data

corrections, the 6-311G** basis set on organic elements with an

PRh

iPr

103

for trans-( PNP)Rh(CH Cl)Cl (2e-trans) are as follows. H NMR

extra “tight” d shell (α = 1.35) on phosphorus, 6-311G**++ on

104

(497 MHz, benzene-d ): δ 6.87 (t, J = 8 Hz, 1H, pyridine H-4),

chlorine, cc-PVTZ-PP

sans f functions for iodine, and the Los

.52 (d, J = 8 Hz, 2H, pyridine H-3,5), 5.75 (td, J = 5 Hz, J

Alamos small core eﬀective core potential (including 4s and 4p

HRh

3 Hz, 2H, RhCH Cl), 3.54 (m, 4H, PCHMe ), 3.49 (vt, 4H,

electrons explicitely to better describe shielding of the 4d valence

Organometallics XXXX, XXX, XXX−XXX

Organometallics

The Supporting Information is available free of charge at

Article

electrons) and triple-ξ valence functions for rhodium. Frequencies

major and minor conformations, respectively. Constraints were used

on the anisotropic displacement parameters of the disordered atoms.

In 4e-cis, constraints were used on the anisotropic displacement

computed at these geometries were used for the zero-point, enthalpy,

and entropy terms above. To obtain G , geometries were reoptimized

functional and we applied PBF implicit

solvation (using ε = 7.6 and solvent radius 2.52 Å for THF, ε =

.55 and solvent radius 2.45 Å for triﬂuoroacetic acid, ε = 80.37 and

solvent radius 1.40 Å for water, ε = 8.93 and solvent radius 2.33 Å for

DCM, ε = 20.7 and solvent radius 2.44 Å for acetone).

sol

with the M06-L

parameters of the symmetry-disordered CH /Cl substituents.

107

ASSOCIATED CONTENT

sı Supporting Information

■

https://pubs.acs.org/doi/10.1021/acs.organomet.0c00122.

We used the larger basis set 6-311G**++ for organic elements and

103

6-311++G-3df-3pd

for chlorine and phosphorus. For iodine we

used the cc-PVTZ-PP basis set with the Los Alamos eﬀective core

potential and we supplemented the valence basis functions with two f

NMR spectra of the complexes, NMR spectra and plots

of kinetics studies and equilibrium studies, details of the

computational methods, and crystal structure data

108

shells

Scheme 14), the frequency-dependent terms for all species were

110,111

for rhodium. For the reactions involving CH Cl₂(i.e.,

100,109

PDF )

Cartesian coordinates for the calculated structures

XYZ )

calculated using the B3LYP

functional and 6-31G**++

basis set on nonmetal elements and the Los Alamos eﬀective core

potential with double-ζ valence functions. For the survey of anions

and ligands in Scheme 5, frequency-dependent terms were calculated

Pasadena, California 91125, United States; orcid.org/0000-

−

using B3LYP and 6-31G**. Computed frequencies below 50 cm

Accession Codes

were replaced with a value of 50 cm⁻in the calculation of vibrational

entropy to avoid spurious contributions to entropy from numerical

error in the force constants. All calculations were performed with

CCDC 1948889−1948897 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

112

Jaguar 8.4.

Crystallographic Analysis. Single crystals of 1c, 2a, 2b, 2e, 3a,

c, 3d″, 4b-1, and 4e-cis were coated with Paratone oil and mounted

on a MiTeGen MicroLoops. The X-ray intensity data were measured

on a Bruker Kappa APEXII Duo system. The Incoatec Microfocus IμS

(

Cu Kα, λ = 1.54178 Å) and a multilayer mirror monochromator

AUTHOR INFORMATION

Corresponding Authors

■

were used for 4e-cis. A graphite monochromator and a Mo Kα ﬁne-

focus sealed tube (λ = 0.71073 Å) were used for all other crystals. All

frames were integrated with the Bruker SAINT software package

using a narrow-frame algorithm. Data were corrected for absorption

eﬀects using the Multi-Scan method (SADABS).

was solved and reﬁned using the Bruker SHELXTL software

113

Robert J. Nielsen − Materials and Process Simulation Center,

Department of Chemistry, California Institute of Technology,

0002-7962-0186 ; Email: robert.smith.nielsen@gmail.com

William A. Goddard III − Materials and Process Simulation

Center, Department of Chemistry, California Institute of

Technology, Pasadena, California 91125, United States;

orcid.org/0000-0003-0097-5716; Email: wag@

caltech.edu

T. Brent Gunnoe − Department of Chemistry, University of

Virginia, Charlottesville, Virginia 22904, United States;

orcid.org/0000-0001-5714-3887; Email: tbg7h@

virginia.edu

113

Each structure

114

113

115

package

within APEX3

and OLEX2.

Non-hydrogen atoms

were reﬁned anisotropically. Hydrogen atoms were placed in

geometrically calculated positions with U_i_s_o= 1.2U_e_q_u_i_vof the parent

atom (U_i_s_o= 1.5U_e_q_u_i_vfor methyl). During the structure solution of 1c

and 2a, electron density diﬀerence maps revealed that there was

disordered solvent that could not be successfully modeled with or

without restraints. Thus, the structure factors were modiﬁed using the

PLATON SQUEEZE technique, in order to produce a “solvate-

free” structure factor set. For 1c, PLATON reported a total electron

density of 201 e and a total solvent-accessible volume of 771 Å ,

likely representing one pentane molecule per asymmetric unit. For 2a,

PLATON reported a total electron density of 334 e and a total

solvent-accessible volume of 1477 Å . This corresponds to

approximately half of a pentane molecule per asymmetric unit. In

116

−

Authors

Shunyan Gu − Department of Chemistry, University of Virginia,

Charlottesville, Virginia 22904, United States

b, the halide atoms were reﬁned as a mixture of Cl and I. The

Kathleen H. Taylor − Department of Chemistry, University of

Virginia, Charlottesville, Virginia 22904, United States

George C. Fortman − Department of Chemistry, University of

Virginia, Charlottesville, Virginia 22904, United States

Junqi Chen − Department of Chemistry, University of Virginia,

Charlottesville, Virginia 22904, United States

Diane A. Dickie − Department of Chemistry, University of

Virginia, Charlottesville, Virginia 22904, United States;

orcid.org/0000-0003-0939-3309

relative occupancy was freely reﬁned, converging at 86/14 for the

major and minor conformations, respectively. Constraints were used

on the thermal displacement parameters of the disordered atoms, and

restraints were used on the Cl−Rh and I−Rh bonds in the minor

component of the disorder. Three independent sites of disorder were

identiﬁed in 2e. The relative occupancies of the sets of disordered

atoms were freely reﬁned, converging at major/minor ratios of 59/41

for the solvent disorder, 53/47 for the isopropyl disorder, and 92/8

for the chloride disorder. Constraints were used on the anisotropic

displacement parameters of the disordered atoms. In 3c, the THF

solvent molecule and one of the mesityl groups were each found to be

disordered over two positions. The relative occupancies of the two

positions of each fragment were freely reﬁned, converging at 68/32

for the major and minor conformations of the THF and 57/43 for the

mesityl. Constraints were used on the thermal displacement

parameters of the disordered atoms, and restraints were used on

bond lengths in the solvent. In 3d″, one void space within the crystal

was found to be partially occupied by a cocrystallized methyl iodide

molecule. Its occupancy was freely reﬁned, converging at 66%. In 4b-

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.0c00122

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

The Gunnoe group acknowledges support from the U.S.

National Science Foundation (1800173). The Goddard group

acknowledges support from the NSF (CBET-1805022).

, the I and CH substituents were disordered over two positions. The

relative occupancy was freely reﬁned, converging at 52/48 for the

Organometallics XXXX, XXX, XXX−XXX

Organometallics

to Methanol Catalyzed, with C −H Activation, by Homogeneous,

Article

Cationic Gold. Angew. Chem. 2004, 116, 4626 −4629.

REFERENCES

■

(

1) Johansson Seechurn, C. C. C.; Kitching, M. O.; Colacot, T. J.;

(24) Gretz, E.; Oliver, T. F.; Sen, A. Carbon-hydrogen bond

Snieckus, V. Palladium-Catalyzed Cross-Coupling: A Historical

Contextual Perspective to the 2010 Nobel Prize. Angew. Chem., Int.

Ed. 2012, 51, 5062−5085.

activation by electrophilic transition-metal compounds. Palladium-

II)-mediated oxidation of arenes and alkanes including methane. J.

Am. Chem. Soc. 1987, 109, 8109−8111.

25) Zimmermann, T.; Soorholtz, M.; Bilke, M.; Schu

2) Jana, R.; Pathak, T. P.; Sigman, M. S. Advances in Transition

Metal (Pd,Ni,Fe)-Catalyzed Cross-Coupling Reactions Using Alkyl-

th, F. Selective

organometallics as Reaction Partners. Chem. Rev. 2011, 111, 1417−

492.

3) Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling

Reactions of Organoboron Compounds. Chem. Rev. 1995, 95, 2457−

483.

4) Tellis, J. C.; Primer, D. N.; Molander, G. A. Single-electron

Methane Oxidation Catalyzed by Platinum Salts in Oleum at

Turnover Frequencies of Large-Scale Industrial Processes. J. Am.

Chem. Soc. 2016, 138, 12395−12400.

26) Bhalla, G.; Periana, R. A. C −H Activation of Alkanes and

Arenes Catalyzed by an O-Donor Bis(tropolonato)iridium(III)

(27) Kua, J.; Xu, X.; Periana, R. A.; Goddard, W. A., III Stability and

Complex. Angew. Chem., Int. Ed. 2005, 44, 1540−1543.

transmetalation in organoboron cross-coupling by photoredox/nickel

Thermodynamics of the PtCl Type Catalyst for Activating Methane

dual catalysis. Science 2014, 345, 433−436.

to Methanol: A Computational Study. Organometallics 2002, 21,

(

5) Font, M.; Acuna-Pares, F.; Parella, T.; Serra, J.; Luis, J. M.;

Lloret-Fillol, J.; Costas, M.; Ribas, X. Direct observation of two-

11−525.

electron Ag(I)/Ag(III) redox cycles in coupling catalysis. Nat.

Commun. 2014, 5, 4373.

28) Xu, X.; Kua, J.; Periana, R. A.; Goddard, W. A., III Structure,

Bonding, and Stability of a Catalytica Platinum(II) Catalyst: A

Computational Study. Organometallics 2003, 22, 2057−2068.

(

6) Ackermann, L.; Gunnoe, T. B. Catalytic hydroarylation of carbon-

carbon multiple bonds; Wiley-VCH: Weinheim, 2018.

7) Ahlquist, M.; Nielsen, R. J.; Periana, R. A.; Goddard, W. A., III

Product Protection, the Key to Developing High Performance

Methane Selective Oxidation Catalysts. J. Am. Chem. Soc. 2009,

31, 17110−17115.

8) Gol’dshleger, N. F.; Shteinma, A. A.; Shilov, A. E.; Eskova, V. V.

Reactions of alkanes in solutions of chloride complexes of platinum.

Russ. J. Phys. Chem. 1972, 46, 785−786.

9) Muehlhofer, M.; Strassner, T.; Herrmann, W. A. New catalyst

systems for the catalytic conversion of methane into methanol. Angew.

Chem., Int. Ed. 2002, 41 , 1745 −1747.

10) Lin, M.; Sen, A. Direct catalytic conversion of methane to acetic

acid in an aqueous medium. Nature 1994, 368, 613−615.

11) Wong-Foy, A. G.; Bhalla, G.; Liu, X. Y.; Periana, R. A. Alkane

(29) Oxgaard, J.; Tenn, W. J.; Nielsen, R. J.; Periana, R. A.; Goddard,

W. A., III Mechanistic Analysis of Iridium Heteroatom C −H

Activation: Evidence for an Internal Electrophilic Substitution

Mechanism. Organometallics 2007, 26, 1565−1567.

(31) Luinstra, G. A.; Wang, L.; Stahl, S. S.; Labinger, J. A.; Bercaw, J.

(30) Tenn, W. J.; Young, K. J. H.; Bhalla, G.; Oxgaard, J.; Goddard,

(

W. A., III; Periana, R. A. CH Activation with an O-Donor Iridium −

Methoxo Complex. J. Am. Chem. Soc. 2005, 127, 14172−14173.

E. C −H activation by aqueous platinum complexes: A mechanistic

(32) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H.

study. J. Organomet. Chem. 1995, 504, 75−91.

Selective Intermolecular Carbon-Hydrogen Bond Activation by

Synthetic Metal Complexes in Homogeneous Solution. Acc. Chem.

Res. 1995, 28, 154−162.

33) Jones, W. D.; Feher, F. J. Comparative reactivities of

C −H Activation and Catalysis by an O-Donor Ligated Iridium

hydrocarbon carbon-hydrogen bonds with a transition-metal complex.

Complex. J. Am. Chem. Soc. 2003, 125, 14292−14293.

Acc. Chem. Res. 1989, 22, 91−100.

12) Luinstra, G. A.; Labinger, J. A.; Bercaw, J. E. Mechanism and

34) Kloek, S. M.; Heinekey, D. M.; Goldberg, K. I. C −H Bond

stereochemistry for nucleophilic attack at carbon of platinum (IV)

alkyls: Model reactions for hydrocarbon oxidation with aqueous

platinum chlorides. J. Am. Chem. Soc. 1993, 115, 3004−3005.

Activation by Rhodium(I) Hydroxide and Phenoxide Complexes.

Angew. Chem. 2007, 119, 4820−4822.

(35) Janowicz, A. H.; Bergman, R. G. Carbon-hydrogen activation in

(

13) Periana, R. A.; Taube, D. J.; Evitt, E. R.; Loffler, D. G.;

completely saturated hydrocarbons: direct observation of M + R-

Wentrcek, P. R.; Voss, G.; Masuda, T. A mercury-catalyzed, high-yield

H.fwdarw. M(R)(H). J. Am. Chem. Soc. 1982 , 104 , 352 −354.

system for the oxidation of methane to methanol. Science 1993, 259,

(36) Jones, W. D.; Feher, F. J. Mechanism of arene carbon-hydrogen

(

40−343.

bond activation by (C Me )Rh(PMe )(H)Ph. Evidence for arene

14) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.;

precoordination. J. Am. Chem. Soc. 1982, 104, 4240−4242.

Fujii, H. Platinum catalysts for the high-yield oxidation of methane to

a methanol derivative. Science 1998, 280, 560−564.

15) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Homogeneous

oxidation of alkanes by electrophilic late transition metals. Angew.

Chem., Int. Ed. 1998, 37, 2180−2192.

16) Periana, R. A.; Mironov, O.; Taube, D.; Bhalla, G.; Jones, C. J.

(37) Vaughan, B. A.; Webster-Gardiner, M. S.; Cundari, T. R.;

Gunnoe, T. B. A rhodium catalyst for single-step styrene production

Y.-S.; Sun, W.-Y.; Yu, J.-Q. Rh(III)-Catalyzed meta-C −H Olefination

from benzene and ethylene. Science 2015, 348, 421−424.

(38) Gunnoe, T. B. Metal-Mediated Carbon-Hydrogen Bond

Activation. In Physical Inorganic Chemistry: Reactions, Processes, and

Applications; Bakac, A., Ed.; Wiley-VCH: Weinheim, 2010; pp

495549.

(39) Xu, H.-J.; Lu, Y.; Farmer, M. E.; Wang, H.-W.; Zhao, D.; Kang,

Directed by a Nitrile Template. J. Am. Chem. Soc. 2017, 139, 2200−

2203.

(40) Colby, D. A.; Tsai, A. S.; Bergman, R. G.; Ellman, J. A.

Rhodium Catalyzed Chelation-Assisted C −H Bond Functionalization

Reactions. Acc. Chem. Res. 2012, 45 , 814 −825.

(41) Cheng, C.; Hartwig, J. F. Rhodium-Catalyzed Intermolecular

C −H Silylation of Arenes with High Steric Regiocontrol. Science

2014, 343, 853−857.

(42) Frech, C. M.; Milstein, D. Direct Observation of Reductive

Elimination of Methyl Iodide from a Rhodium(III) Pincer Complex:

The Importance of Sterics. J. Am. Chem. Soc. 2006, 128, 12434−

12435.

(43) Feller, M.; Iron, M. A.; Shimon, L. J. W.; Diskin-Posner, Y.;

Leitus, G.; Milstein, D. Competitive C −I versus C −CN Reductive

Catalytic, oxidative condensation of CH to CH COOH in one step

via CH activation. Science 2003, 301, 814−818.

17) Olah, G. A.; Molnar, A. Hydrocarbon chemistry; Wiley:

(

Hoboken, NJ, 2003.

18) Caballero, A.; Per

chemistry: The final frontier. Chem. Soc. Rev. 2013 , 42 , 8809 −8820.

19) Munz, D.; Strassner, T. Alkane C −H functionalization and

oxidation with molecular oxygen. Inorg. Chem. 2015 , 54 , 5043 −5052.

20) Goldberg, K. I.; Goldman, A. S. Large-scale selective

functionalization of alkanes. Acc. Chem. Res. 2017, 50, 620−626.

21) Gol’dshleger, N. F.; Tyabin, M. B.; Shilov, A. E.; Shteinman, A.

ez, P. J. Methane as raw material in synthetic

A. Activation of Saturated Hydrocarbons-Deuterium-Hydrogen

Exchange in Solutions of Transition Metal Complexes. Russ. J. Phys.

Chem. 1969, 43, 1222−1223.

22) Shilov, A. E.; Shul’pin, G. B. Activation of C −H bonds by metal

complexes. Chem. Rev. 1997, 97, 2879−2932.

23) Jones, C. J.; Taube, D.; Ziatdinov, V. R.; Periana, R. A.; Nielsen,

R. J.; Oxgaard, J.; Goddard, W. A., III Selective Oxidation of Methane

(

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Elimination from a Rh ^I ^I ^I Complex. Selectivity is Controlled by the

Article

(64) Webster-Gardiner, M. S.; Piszel, P. E.; Fu, R.; McKeown, B. A.;

Nielsen, R. J.; Goddard, W. A., III; Gunnoe, T. B. Electrophilic Rh

Solvent. J. Am. Chem. Soc. 2008, 130, 14374−14375.

(44) Feller, M.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.;

catalysts for arene H/D exchange in acidic media: Evidence for an

electrophilic aromatic substitution mechanism. J. Mol. Catal. A: Chem.

2017, 426, 381−388.

(65) Vaughan, B. A.; Khani, S. K.; Gary, J. B.; Kammert, J. D.;

Webster-Gardiner, M. S.; McKeown, B. A.; Davis, R. J.; Cundari, T.

R.; Gunnoe, T. B. Mechanistic Studies of Single-Step Styrene

Production Using a Rhodium(I) Catalyst. J. Am. Chem. Soc. 2017,

139, 1485−1498.

(66) Webster-Gardiner, M. S.; Chen, J.; Vaughan, B. A.; McKeown,

B. A.; Schinski, W.; Gunnoe, T. B. Catalytic Synthesis of “Super ”

Linear Alkenyl Arenes Using an Easily Prepared Rh(I) Catalyst. J. Am.

Chem. Soc. 2017, 139, 5474−5480.

(67) Chen, J.; Nielsen, R. J.; Goddard, W. A., III; McKeown, B. A.;

Dickie, D. A.; Gunnoe, T. B. Catalytic Synthesis of Superlinear

Alkenyl Arenes Using a Rh(I) Catalyst Supported by a “Capping

Arene ” Ligand: Access to Aerobic Catalysis. J. Am. Chem. Soc. 2018,

140, 17007−17018.

Milstein, D. Direct Observation of Reductive Elimination of MeX (X

III

Cl, Br, I) from Rh Complexes: Mechanistic Insight and the

Importance of Sterics. J. Am. Chem. Soc. 2013, 135, 11040−11047.

45) O’Reilly, M. E.; Pahls, D. R.; Webb, J. R.; Boaz, N. C.;

(

Majumdar, S.; Hoff, C. D.; Groves, J. T.; Cundari, T. R.; Gunnoe, T.

B. Reductive functionalization of a rhodium(III)−methyl bond by

electronic modification of the supporting ligand. Dalton Trans. 2014,

46) O ’Reilly, M. E.; Pahls, D. R.; Cundari, T. R.; Gunnoe, T. B.

3, 8273−8281.

Reductive Functionalization of a Rhodium(III)−Methyl Bond in

Acidic Media: Key Step in the Electrophilic Functionalization of

Methane. Organometallics 2014 , 33 , 6504 −6510.

47) Crabtree, R. H. Alkane C −H activation and functionalization

with homogeneous transition metal catalysts: a century of progress a

new millennium in prospect. J. Chem. Soc., Dalton Trans. 2001, 2437−

2450.

48) Paulik, F. E.; Roth, J. F. Novel catalysts for the low-pressure

carbonylation of methanol to acetic acid. Chem. Commun. (London)

968, 1578a−1578a.

49) Ojima, I.; Tsai, C.; Tzamarioudaki, M.; Bonafoux, D. The

Hydroformylation Reaction. In Organic Reactions ; Wiley: 2004.

50) Forster, D. Kinetic and spectroscopic studies of the carbon-

(68) Zhu, W.; Luo, Z.; Chen, J.; Liu, C.; Yang, L.; Dickie, D. A.; Liu,

N.; Zhang, S.; Davis, R. J.; Gunnoe, T. B. Mechanistic Studies of

Single-Step Styrene Production Catalyzed by Rh Complexes with

Diimine Ligands: An Evaluation of the Role of Ligands and Induction

Period. ACS Catal. 2019, 9, 7457−7475.

(

(69) Liebov, N. S.; Zhu, W.; Chen, J.; Webster-Gardiner, M. S.;

Schinski, W. L.; Gunnoe, T. B. Rhodium-Catalyzed Alkenylation of

Toluene Using 1-Pentene: Regioselectivity To Generate Precursors

for Bicyclic Compounds. Organometallics 2019, 38, 3860−3870.

(70) Wang, D. Y.; Choliy, Y.; Haibach, M. C.; Hartwig, J. F.; Krogh-

Jespersen, K.; Goldman, A. S. Assessment of the Electronic Factors

Determining the Thermodynamics of “Oxidative Addition ” of C −H

and N −H Bonds to Ir(I) Complexes. J. Am. Chem. Soc. 2016, 138,

149−163.

ylation of methanol with an iodide-promoted iridium catalyst. J. Chem.

Soc., Dalton Trans. 1979 , 1639 −1645.

(51) Forster, D. On the mechanism of a rhodium-complex-catalyzed

carbonylation of methanol to acetic acid. J. Am. Chem. Soc. 1976, 98,

46−848.

52) Van Leeuwen, P. W.; Claver, C. Rhodium catalyzed hydro-

formylation; Springer Science & Business Media:Berlin, 2002.

53) Low, J. J.; Goddard, W. A., III Theoretical studies of oxidative

(

addition and reductive elimination. 2. Reductive coupling of

hydrogen-hydrogen, hydrogen-carbon, and carbon-carbon bonds

from palladium and platinum complexes. Organometallics 1986, 5,

(71) Krogh-Jespersen, K.; Czerw, M.; Zhu, K.; Singh, B.;

Kanzelberger, M.; Darji, N.; Achord, P. D.; Renkema, K. B.;

Goldman, A. S. Combined Computational and Experimental Study

(

09−622.

of Substituent Effects on the Thermodynamics of H , CO, Arene, and

Alkane Addition to Iridium. J. Am. Chem. Soc. 2002, 124, 10797−

10809.

(72) Albrecht, M.; van Koten, G. Platinum group organometallics

based on “pincer ” complexes: sensors, switches, and catalysts. Angew.

Chem., Int. Ed. 2001, 40, 3750−3781.

(73) Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S.

Dehydrogenation and Related Reactions Catalyzed by Iridium Pincer

Complexes. Chem. Rev. 2011, 111, 1761−1779.

54) Furuya, T.; Benitez, D.; Tkatchouk, E.; Strom, A. E.; Tang, P.;

Goddard, W. A., III; Ritter, T. Mechanism of C −F Reductive

Elimination from Palladium(IV) Fluorides. J. Am. Chem. Soc. 2010,

32, 3793−3807.

55) Hartwig, J. F.; Collman, J. P. Organotransition metal chemistry:

from bonding to catalysis. University Science Books: Sausalito, CA,

010.

56) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.

(

Principles and Applications of Organotransition Metal Chemistry;

University Science Books: Mill Valley, CA, 1980.

(

In The Organometallic Chemistry of the Transition Metals, 6th ed.;

Crabtree, R. H., Eds.; Wiley: Hoboken, NK, 2014; pp 163184.

Carbon Reductive Elimination and Carbon-Iodide Bond Formation

from a Pt(IV) Complex. J. Am. Chem. Soc. 1994, 116 , 1573 −1574.

Fluorine Bond-Forming Reductive Elimination from Palladium(IV)

Complexes. Angew. Chem., Int. Ed. 2012 , 51 , 3414 −3417.

Goldberg, K. I.; Heinekey, D. M. The importance of steric factors in

(74) Goldberg, J. M.; Wong, G. W.; Brastow, K. E.; Kaminsky, W.;

iridium pincer complexes. Organometallics 2015, 34, 753−762.

(75) Goldberg, J. M.; Cherry, S. D. T.; Guard, L. M.; Kaminsky, W.;

Goldberg, K. I.; Heinekey, D. M. Hydrogen Addition to (pincer)IrI-

(CO) Complexes: The Importance of Steric and Electronic Factors.

Organometallics 2016, 35, 3546−3556.

57) Crabtree, R. H., Oxidative Addition and Reductive Elimination.

58) Goldberg, K. I.; Yan, J. Y.; Winter, E. L. Competitive Carbon-

(76) Anderson, B. G.; Spencer, J. L. The Coordination Chemistry of

Pentafluorophenylphosphino Pincer Ligands to Platinum and

Palladium. Chem. Eur. J. 2014, 20, 6421−6432.

59) Racowski, J. M.; Gary, J. B.; Sanford, M. S. Carbon(sp3)−

(77) Gruver, B. C.; Adams, J. J.; Warner, S. J.; Arulsamy, N.;

Roddick, D. M. Acceptor Pincer Chemistry of Ruthenium: Catalytic

60) Mankad, N. P.; Toste, F. D. C(sp3)−F reductive elimination

CF3

from alkylgold(III) fluoride complexes. Chem. Sci. 2012 , 3 , 72 −76.

61) Scott, V. J.; Labinger, J. A.; Bercaw, J. E. Mechanism of

Alkane Dehydrogenation by ( PCP)Ru(cod)(H). Organometallics

2011, 30, 5133−5140.

Reductive Elimination of Methyl Iodide from a Novel Gold(III)−

(78) Adams, J. J.; Arulsamy, N.; Roddick, D. M. Acceptor PCP

Pincer Iridium(I) Chemistry: Stabilization of Nonmeridional PCP

Coordination Geometries. Organometallics 2011 , 30 , 697 −711.

(79) Guzei, I. A.; Wendt, M. An improved method for the

computation of ligand steric effects based on solid angles. Dalton

Trans. 2006, 3991−3999.

Monomethyl Complex. Organometallics 2010, 29, 4090−4096.

(

62) Webster-Gardiner, M. S.; Fu, R.; Fortman, G. C.; Nielsen, R. J.;

Gunnoe, T. B.; Goddard, W. A., III Arene C −H activation using

Rh(I) catalysts supported by bidentate nitrogen chelates. Catal. Sci.

Technol. 2015, 5, 96−100.

(

63) Fu, R.; O ’Reilly, M. E.; Nielsen, R. J.; Goddard, W. A., III;

(80) Feller, M.; Ben-Ari, E.; Gupta, T.; Shimon, L. J. W.; Leitus, G.;

Diskin-Posner, Y.; Weiner, L.; Milstein, D. Mononuclear Rh(II) PNP-

Type Complexes. Structure and Reactivity. Inorg. Chem. 2007, 46,

10479−10490.

Gunnoe, T. B. Rhodium Bis(quinolinyl)benzene Complexes for

Methane Activation and Functionalization. Chem. Eur. J. 2015, 21,

1286−1293.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

81) Hahn, C.; Sieler, J.; Taube, R. Synthesis of 2,6-bis-

(99) Cramer, R.; McCleverty, J. A.; Bray, J.; Parshall, G. W. Di-μ -

chlorotetrakis(ethylene)dirhodium(I), 2,4-Pentanedionatobis-

(ethylene)rhodium(I), and Di-μ -chlorotetracarbonyldirhodium(I).

Inorg. Synth. 2007, 14.

diphenylphosphinomethyl)pyridine-monoligand-rhodium(I) com-

plexes [Rh(PNP)L]X with L = pyridine, CH CN, DMSO and X =

CF SO , BF from the corresponding ethylene complex and

comparison of the structures to the piperidine complex (Lc=

(100) Becke, A. D. Density-Functional Thermochemistry: 3. The

Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648 −5652.

piperidine, X= BF4). Polyhedron 1998, 17, 1183−1193.

(101) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and

(82) Hermann, D.; Gandelman, M.; Rozenberg, H.; Shimon, L. J.

accurate ab initio parametrization of density functional dispersion

correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010,

W.; Milstein, D. Synthesis, Structure, and Reactivity of New Rhodium

and Iridium Complexes, Bearing a Highly Electron-Donating PNP

System. Iridium-Mediated Vinylic C −H Bond Activation. Organo-

metallics 2002, 21, 812−818.

102) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V.

32, 154104.

R. Efficient diffuse function-augmented basis sets for anion

calculations. III. The 3-21+ G basis set for first-row elements, Li −F.

J. Comput. Chem. 1983, 4, 294−301.

(83) Brown, T. H.; Green, P. J. Phosphorus-31 and rhodium-103

nuclear magnetic resonance spectra of some rhodium(I) and

rhodium(III) phosphine complexes. J. Am. Chem. Soc. 1970, 92,

(103) Frisch, M. J.; Pople, J. A.; Binkley, J. S. Self-consistent

molecular orbital methods 25. Supplementary functions for Gaussian

359−2362.

84) Nixon, J.F.; Pidcock, A. Phosphorus-31 Nuclear Magnetic

basis sets. J. Chem. Phys. 1984, 80, 3265−3269.

Resonance Spectra of Co-ordination Compounds. Annu. Rep. NMR

Spectrosc. 1969, 2, 345−422.

(104) Peterson, K. A.; Figgen, D.; Goll, E.; Stoll, H.; Dolg, M.

Systematically convergent basis sets with relativistic pseudopotentials.

II. Small-core pseudopotentials and correlation consistent basis sets

for the post-d group 16 −18 elements. J. Chem. Phys. 2003, 119,

(85) Goldberg, K. I.; Yan, J.; Breitung, E. M. Energetics and

Mechanisms of Carbon-Carbon and Carbon-Iodide Reductive

Elimination from a Pt(IV) Center. J. Am. Chem. Soc. 1995, 117,

1113.

105) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for

889−6896.

86) Gair, J. J.; Qiu, Y.; Chan, N. H.; Filatov, A. S.; Lewis, J. C.

Molecular Calculations - Potentials for K to Au Including the

Rhodium Complexes of 2,6-Bis(dialkylphosphinomethyl)pyridines:

Improved C −H Activation, Expanded Reaction Scope, and Catalytic

Direct Arylation. Organometallics 2017, 36, 4699−4706.

Outermost Core Orbitals. J. Chem. Phys. 1985 , 82 , 299 −310.

(106) Zhao, Y.; Truhlar, D. G. A new local density functional for

main-group thermochemistry, transition metal bonding, thermochem-

ical kinetics, and noncovalent interactions. J. Chem. Phys. 2006, 125,

(87) Separate entropies and enthalpies of activation are not

computed because solvation free energies predicted by continuum

models do not separate the electrostatic stabilization of solutes

(

94101.

107) Tannor, D. J.; Marten, B.; Murphy, R.; Friesner, R. A.; Sitkoff,

(

enthalpy) from the solvent reorganization and changes in dynamics

upon solvation (entropy).

88) Heaton, B. Mechanisms in homogeneous catalysis: a spectroscopic

approach; Wiley-VCH: Weinheim, 2006.

89) Pidcock, A. Coupling and “virtual ” coupling in the nuclear

magnetic resonance spectra of phosphine complexes. Chem. Commun.

London) 1968, 92.

90) Liou, S.-Y.; Gozin, M.; Milstein, D. Directly observed oxidative

D.; Nicholls, A.; Honig, B.; Ringnalda, M.; Goddard, W. A., III

Accurate first principles calculation of molecular charge distributions

and solvation energies from ab initio quantum mechanics and

continuum dielectric theory. J. Am. Chem. Soc. 1994, 116, 11875−

(

1882.

108) Martin, J. M.; Sundermann, A. Correlation consistent valence

(

basis sets for use with the Stuttgart −Dresden −Bonn relativistic

effective core potentials: The atoms Ga −Kr and In −Xe. J. Chem. Phys.

2001, 114, 3408−3420.

(109) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-

Salvetti correlation-energy formula into a functional of the electron

density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988 , 37 , 785 −789.

(110) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-Consistent

Molecular-Orbital Methods: 12. Further Extensions of Gaussian-Type

Basis Sets for Use in Molecular-Orbital Studies of Organic Molecules.

J. Chem. Phys. 1972, 56, 2257−2261.

addition of a strong carbon-carbon bond to a soluble metal complex. J.

Am. Chem. Soc. 1995, 117, 9774−9775.

(91) Okazaki, M.; Tobita, H.; Kawano, Y.; Inomata, S.; Ogino, H.

Thermal reactions of alkyl(hydrido)(hydrosilyl)iridium(III) com-

plexes: generation of a hydrido(silylene)iridium(I) species via the

reductive elimination of alkane and 1,2-H-shift from the silicon atom

to the Ir(I) metal center. J. Organomet. Chem. 1998, 553, 1−13.

(92) Montag, M.; Leitus, G.; Shimon, L. J. W.; Ben-David, Y.;

Milstein, D. Solvent-Dependent Interconversions between RhI, RhII,

(111) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.;

Gordon, M. S.; DeFrees, D. J.; Pople, J. A. Self-consistent molecular

orbital methods. XXIII. A polarization-type basis set for second-row

elements. J. Chem. Phys. 1982, 77, 3654−3665.

and RhIII Complexes of an Aryl −Monophosphine Ligand. Chem. Eur.

J. 2007, 13, 9043−9055.

93) Lipke, M. C.; Neumeyer, F.; Tilley, T. D. Interconversion of η -

(112) Bochevarov, A. D.; Harder, E.; Hughes, T. F.; Greenwood, J.

H SiRR ′ σ -Complexes and 16-Electron Silylene Complexes via

R.; Braden, D. A.; Philipp, D. M.; Rinaldo, D.; Halls, M. D.; Zhang, J.;

Friesner, R. A. Jaguar: A high-performance quantum chemistry

software program with strengths in life and materials sciences. Int. J.

Quantum Chem. 2013, 113, 2110−2142.

Reversible H −H or C −H Elimination. J. Am. Chem. Soc. 2014, 136,

94) Zhou, M.; Goldman, A. S. Chlorination of (Phebox)Ir-

092−6102.

114) Sheldrick, G. SHELXT - Integrated space-group and crystal-

mesityl)(OAc) by Thionyl Chloride. Molecules 2015, 20, 10122−

(

113) Saint, SADABS, APEX3, Bruker AXS Inc.: Madison, WI, USA,

012.

structure determination. Acta Crystallogr., Sect. A: Found. Adv. 2015,

1, 3−8.

115) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A.

0130.

(95) Muller, G.; Klinga, M.; Leskela, M.; Rieger, B. Iron and Cobalt

Complexes of a Series of Tridentate P, N, P Ligands  Synthesis,

Characterization, and Application in Ethene Polymerization Reac-

tions. Z. Anorg. Allg. Chem. 2002, 628, 2839−2846.

(

(96) Ammann, C.; Meier, P.; Merbach, A. E. A simple multinuclear

K.; Puschmann, H. OLEX2: a complete structure solution, refinement

NMR thermometer. J. Magn. Reson. 1982 , 46 , 319 −321.

and analysis program. J. Appl. Crystallogr. 2009 , 42 , 339 −341.

(97) Shih, W.-C.; Ozerov, O. V. One-Pot Synthesis of 1,3-

(116) Spek, A. PLATON SQUEEZE: a tool for the calculation of

Bis(phosphinomethyl)arene PCP/PNP Pincer Ligands and Their

the disordered solvent contribution to the calculated structure factors.

Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 9−18.

Nickel Complexes. Organometallics 2015, 34, 4591−4597.

(98) Neumann, J.; Elangovan, S.; Spannenberg, A.; Junge, K.; Beller,

M. Improved and General Manganese-Catalyzed N-Methylation of

Aromatic Amines Using Methanol. Chem. Eur. J. 2017, 23, 5410−

5413.