Rhodium boryl complexes in the catalytic, terminal functionalization of alkanes

Published on Web 02/03/2005

Rhodium Boryl Complexes in the Catalytic, Terminal

Functionalization of Alkanes

John F. Hartwig,*^,†Kevin S. Cook,^†Marko Hapke,^†Christopher D. Incarvito,^†

Yubo Fan,^‡Charles Edwin Webster,^‡and Michael B. Hall*^,‡

Contribution from the Departments of Chemistry, Yale UniVersity, P.O. Box 208107,

New HaVen, Connecticut 06520-8107, and Texas A&M UniVersity, College Station,

Texas 77843-3255

Received August 15, 2004; E-mail: john.hartwig@yale.edu

Abstract: A series of studies have been conducted by experimental and theoretical methods on the

synthesis, structures, and reactions of Cp*Rh boryl complexes that are likely intermediates in the rhodium-

catalyzed regioselective, terminal functionalization of alkanes. The photochemical reaction of Cp*Rh(η⁶-

C₆Me₆) with pinacolborane (HBpin) generates the bisboryl complex Cp*Rh(H)₂(Bpin)₂(2), which reacts

with neat HBpin to generate Cp*Rh(H)(Bpin)₃(3). X-ray diffraction, density functional theory (DFT)

calculations, and NMR spectroscopy suggest a weak, but measurable, B-H bonding interaction. Both 2

and 3 dissociate HBpin and coordinate PEt₃or P(p-Tol)₃to generate the conventional rhodium(III) species

Cp*Rh(PEt₃)(H)(Bpin) (4) and Cp*Rh[P(p-tol)₃](Bpin)₂(5). Compounds 2 and 3 also react with alkanes and

arenes to form alkyl- and arylboronate esters at temperatures similar to or below those of the catalytic

borylation of alkanes and arenes. Further, these compounds were observed directly in catalytic reactions.

The enthalpies and free energies for generation of the 16-electron intermediate and for the C-H bond

cleavage and B-C bond formation have been calculated with DFT. These results strongly suggest that

the C-H bond cleavage process occurs by a metal-assisted σ-bond metathesis mechanism to generate a

borane complex that isomerizes if necessary to place the alkyl group cis to the boryl group. This complex

with cis boryl and alkyl groups then undergoes B-C bond formation by a second σ-bond metathesis to

generate the final functionalized product.

terminal C-H bonds as a result of shape selectivity,⁵but systems

Introduction

that are suitable for synthetic applications, because they lack

cofactors⁶or can be expressed in Escherichia coli,⁷have begun

to be developed only recently. Moreover, the terminal selectivity

depends on the length of the alkane.⁵

We reported that rhodium complexes generated in situ from

Cp*Rh(η⁴-C₆Me₆), Cp*Rh(C₂H₄)₂, or [Cp*RhCl₂]₂and either

pinacolborane (HBpin) or bispinacol diborane(4) (B₂pin₂)

(eq 1) catalyze the uniquely regioselective conversion of alkanes

The development of selective transformations of alkanes

would allow the generation of useful reagents from completely

unfunctionalized materials.¹The selective cleavage of the

primary C-H bonds of alkanes by several types of systems has

been reported,^2,3but the extension of this reaction to a catalytic

process has been challenging. New elementary processes that

cleave with high selectivity one type of C-H bond over another

and deliver a functional group to the position at which the C-H

bond is cleaved would create systems capable of selective

catalytic functionalization of unactivated C-H bonds.

Oxidation of alkanes by metal oxo complexes occurs with

many catalysts, but these reactions tend to cleave the weaker

C-H bonds of alkanes.⁴Preferential reaction at a secondary

C-H bond leads to product mixtures, unless reactions are

conducted with symmetric cyclic alkanes. Enzymes can cleave

to terminal alkylboronate esters.⁸The observation of C-H

activation with boryl compounds originated from studies aimed

at probing the potential of the unoccupied p-orbital of a covalent

ligand to create new reaction chemistry.^9,10A boryl group is

^†Yale University.

^‡Texas A&M University.

(5) Fisher, M. B.; Zheng, Y.-M.; Rettie, A. E. Biochem. Biophys. Res. Commun.

1998, 248, 352.

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9

2538

J. AM. CHEM. SOC. 2005, 127, 2538-2552

Rhodium Boryl Complexes in Alkane Functionalization

A R T I C L E S

strongly electron donating.¹¹Yet, it may also display some

Lewis acidity.^12,13Further, the coordination number of a boryl

group may expand in a fashion similar to the way a transition-

metal center expands its coordination number during many

reactions.

We have sought to identify the rhodium complexes that are

involved in the functionalization of alkanes. This information

would allow concrete conclusions about the elementary steps

that comprise the catalytic cycle and would reveal the origin of

the unusually high reactivity of boryl complexes toward the

cleavage of alkane C-H bonds. Yet, to date, isolation of

potential intermediates in these reactions in pure form has not

been reported.^8,14Identification of the rhodium complexes in

these catalytic reactions could lead to detailed information on

the mechanism of the C-H bond cleavage and functionalization

steps and the potential role of unsaturation at the boryl ligand.

We report a combination of experimental and theoretical

studies on the synthesis, structures, and reactivity of two

rhodium boryl complexes that are likely intermediates in the

catalytic borylation of alkanes. Both compounds react with

alkanes and arenes to generate alkyl- and arylboronate esters,

(Bpin)₃, (3). Other thermal chemistry was less well defined.

Reaction of 1 with an excess of B₂pin₂generated trisboryl 3 as

the predominant product, but this material was accompanied

by other Cp* complexes, and 3 could not be isolated from these

mixtures. Reactions of the bisethylene complex Cp*Ru(C₂H₄)₂

and HBpin led to detectable amounts of 2 and 3, but we were

unable to accumulate a significant quantity of these compounds

before they decomposed. The Rh(III) complex [Cp*RhCl₂]₂is

a convenient catalyst precursor for the borylation of alkanes,^20,21

but it is poorly soluble in alkanes. Although it reacts to generate

soluble species that catalyze the functionalization process, the

insolubility of this material in cyclohexane prevented its direct

conversion to the desired boryl compounds in high yield.

2. Photochemical Reactions of Cp*Rh Complexes with

HBpin and B₂pin₂. To facilitate the dissociation of ligands from

the Cp*Rh precursors at temperatures below those of the

catalytic borylation of alkanes and arenes, we conducted

reactions of hexamethylbenzene complex 1 with HBpin and B₂-

pin₂under photochemical conditions. In contrast to heating of

1 and HBpin, irradiation of a mixture of 1 and 30 equiv of

HBpin in cyclohexane with a medium-pressure mercury arc

lamp generated bisboryl 2 in 95% yield by NMR spectroscopy

(Scheme 1). After C₆Me₆was removed by sublimation, bisboryl

2 was isolated by standard crystallization from pentane in 86%

yield.

3. Synthesis of Trisboryl Complex 3. The thermal lability

of bisboryl complex 2 was greater than that of the bissilyl

compound or the olefin complexes and allowed for thermal

conversion of the bisboryl 2 to the trisboryl 3. To avoid reactions

of the bisboryl 2 with alkane C-H bonds, we conducted the

conversion of 2 to 3 in neat borane. The reaction of bisboryl 2

in neat HBpin for 3.5 h at 105 °C generated a 5:1 ratio of

trisboryl 3 and starting 2 (Scheme 1). Evaporation of HBpin

and recrystallizations from pentane generated pure trisboryl 3

in 51% yield.

Evidence for an equilibrium between the reactants HBpin and

2 and the products H₂and 3 was obtained. Although we did

not determine quantitatively the equilibrium constant between

the combination of 2 and HBpin and the combination of 3 and

H₂, the reaction of trisboryl 3 with hydrogen for 36 h at 40 °C

in cyclohexane regenerated HBpin and the bisboryl complex

in a 9:1 ratio with 3. Further, the reactions of 2 with neat HBpin

were conducted in sealed vessels, and the conversion of 2 to 3

depended on the volume of the vessel. High yields were obtained

in reaction vessels with a large volume, but not those with a

small volume, presumably because of the need to prevent the

accumulation of a significant pressure of H₂.

1

and each complex has been observed directly by H NMR

spectroscopy during the catalytic process. These studies provide

evidence for a new mechanism for C-H bond cleavage in a

catalytic process and the ability of the boryl ligand to couple

rapidly with an alkyl intermediate to form functionalized

products.

Results and Discussion

Isolation of rhodium boryl complexes that could be interme-

diates in the regioselective borylation of alkanes catalyzed by

Cp*Rh(η⁴-C₆Me₆) (1) or related Cp*Rh complexes required a

method to generate the boryl complexes faster than they react

with solvent C-H bonds. We followed two strategies to generate

these compounds. First, we heated 1,^15,16Cp*Rh(C₂H₄)₂, Cp*Rh-

(SiEt₃)₂H₂,¹⁷Cp*Rh(C₂H₃SiMe₃)₂,¹⁸and [Cp*RhCl₂]₂with

19

HBpin and with B₂pin₂in cycloalkanes that lack primary C-H

bonds at temperatures similar to or below those of the catalytic

process. Second, we irradiated 1 with a mercury arc lamp at

room temperature or below in the presence of pinacolborane

and bispinacol diborane(4).

1. Thermal Reactions of Cp*Rh Complexes with HBpin

and B₂pin₂. As summarized in eq 2, reaction of hexamethyl-

benzene complex 1^15,16with HBpin in octane at 95 °C generated

two major products. We were unable to separate the two, but

several spectral features that will be presented in detail below

suggested that these complexes were the bisboryl complex trans-

Cp*Rh(H)₂(Bpin)₂(2) and the trisboryl complex Cp*Rh(H)-

(10) (a) Waltz, K. M.; He, X.; Muhoro, C. N.; Hartwig, J. F. J. Am. Chem. Soc.

1995, 117, 11357. (b)Waltz, K. M.; Hartwig, J. F. Science 1997, 277, 211.

(11) Lam, K. C.; Lam, W. H.; Lin, Z. Y.; Marder, T. B.; Norman, N. C. Inorg.

Chem. 2004, 43, 2541.

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4776. (b) Cundari, T. R.; Zhao, Y. Inorg. Chim. Acta 2003, 345, 70.

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(16) Bowyer, W. J.; Merkert, J. W.; Geiger, W. E. Organometallics 1989, 8,

191.

4. NMR Spectral Data for Bisboryl 2 and Trisboryl 3.

The bisboryl and trisboryl complexes 2 and 3 could adopt

structures that reflect a rhodium(V) oxidation state with one

Cp* and four conventional terminal ligands, they could adopt

one of the structures in Figure 1 that are more consistent with

(17) (a) Fernandez, M. J.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1984,

2063. (b) Fernandez, M. J.; Bailey, P. M.; Bentz, P. O.; Ricci, J. S.; Koetzle,

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4385.

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9

J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005 2539

A R T I C L E S

Hartwig et al.

structure for 2 in solution and is inconsistent with a rigid cis

structure. Although it is possible that 2 undergoes fast site

exchange in solution, we did not observe broadening of the

pinacol methyl resonances at low temperature. Thus, the

combination of NMR and X-ray diffraction data and the steric

demands of the pinacolate group make a trans structure for 2

in solution most likely.

Solution NMR spectroscopic data for 2 and 3, which

contained direct scalar B-H coupling, suggested that their

structures contained B-H bonding interactions. A resonance

at δ 40.4 ppm was observed in the ¹¹B NMR spectrum of 2.

This resonance is consistent with a boryl group bound to the

metal but does not distinguish between a transition metal boryl

species or a borane complex,³⁰and splitting from ¹H-¹¹B

coupling was not resolved in the broad ¹¹B NMR signal.

Figure 1. Various possible geometries and valence bond descriptions of

bisboryl complex 2 and trisboryl complex 3.

Scheme 1

1

However, the H NMR spectrum of complex 2 contained a

doublet resonance for the hydrides at δ -11.9 ppm (¹J_Rh-H

)

40.5 Hz) and a single pinacolate resonance. These data are

consistent with a mutually trans arrangement of the boryl groups,

and this geometry precludes a strong H-H interaction. The ¹H

resonance for the hydride of 2 provided evidence for direct scalar

¹¹B-¹H coupling. The resonance for the hydride of 2 was broad,

and the 40 Hz width at half-height decreased to 10 Hz upon

¹¹B decoupling. Consistent with broad lines from scalar

a rhodium(III) metal center, or they could adopt structures that

are most consistent with a Rh(I) metal center. The rhodium-

(III) structures contain two terminal ligands and a borane

coordinated through the B-H σ-bond, one terminal boryl group

and a unit resembling a hydridoborate with partial bonding

between two hydrides, two boryl ligands with partial B-B

bonding, or, in the case of compound 2, two hydrides with

partial H-H bonding. The rhodium(I) structure for 2 would

contain two coordinated boranes. Each of these structures should

be considered because borane complexes have been character-

ized by X-ray diffraction and spectroscopic methods,^22-25

hydridoborate complexes are common²⁶and have weak B-H

bonds when the boron contains alkoxo groups,^13,27short B-B

distances have been measured by X-ray diffraction in some

bisboryl complexes,^11,28and dihydrogen complexes are now

common.²⁹

1

coupling, cross-peaks were observed in the H-¹¹B HMQC

spectrum of 2.

1

The H NMR spectra of complex 3 also contained spectral

1

features that suggested a B-H bonding interaction. The H

NMR spectrum of 3 contained a broad hydride signal at δ -11.3

that possessed a line width at half-height of 32 Hz (298 K).

Like the hydride resonance of 2, this resonance sharpened to

10 Hz upon ¹¹B decoupling.

1

The signals for the hydride ligands in the H NMR spectra

of many complexes with hydride and boryl groups are

sharp.^20,30,31For example, the hydride resonance in the ¹H NMR

spectrum of the closely related compound Cp*Ir(H)₂(Bpin)₂

was sharp.¹⁴We have now obtained a ¹H-¹¹B HMQC spectrum

of Cp*Ir(H)₂(Bpin)₂, and in contrast to those of 2 and 3, this

spectrum did not contain any cross-peaks. The difference

between the NMR spectra of this complex and those of the

rhodium analogues is consistent with a greater stability of

classical structures for metals of the third row than for those of

the second row.³²

The X-ray diffraction data described below show that the

bisboryl compound 2 adopts the trans structure as a solid. The

¹H NMR spectra of 2 contained a single set of pinacolmethyl

groups. This spectroscopic feature is consistent with a trans

(22) Muhoro, C. N.; He, X. M.; Hartwig, J. F. J. Am. Chem. Soc. 1999, 121,

5033.

(23) (a) Muhoro, C. N.; Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1997, 36,

1510. (b) Hartwig, J. F.; He, X.; Muhoro, C. N.; Eisenstein, O.; Bosque,

R.; Maseras, F. J. Am. Chem. Soc. 1996, 118, 10936.

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Chaudret, B. J. Am. Chem. Soc. 2002, 124, 5624.

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(27) (a) Lantero, D. R.; Motry, D. H.; Ward, D. L.; Smith, M. R., III. J. Am.

Chem. Soc. 1994, 116, 10811. (b) Lantero, D. R.; Ward, D. L.; Smith, M.

R., III. J. Am. Chem. Soc. 1997, 119, 9699.

(28) (a) Clegg, W.; Lawlor, F. J.; Marder, T. B.; Nguyen, P.; Norman, N. C.;

Orpen, A. G.; Quayle, M. J.; Rice, C. R.; Robins, E. G.; Scott, A. J.; Souza,

F. E. S.; Stringer, G.; Whittell, G. R. J. Chem. Soc. 1998, 301. (b) Clegg,

W.; Lawlor, F. J.; Lesley, G.; Marder, T. B.; Norman, N. C.; Orpen, A.

G.; Quayle, M. J.; Rice, C. R.; Scott, A. J.; Souza, F. E. S. J. Organomet.

Chem. 1998, 550, 183. (c) Dai, C.; Stringer, G.; Corrigan, J. F.; Taylor, N.

J.; Marder, T. B.; Norman, N. C. J. Organomet. Chem. 1996, 513, 273. (d)

Lesley, G.; Nguyen, P.; Taylor, N. J.; Marder, T. B.; Scott, A. J.; Clegg,

W.; Norman, N. C. Organometallics 1996, 15, 5137.

The NMR data for 3 also indicated that this complex was

stereochemically nonrigid. A four-legged piano-stool geometry

with one boryl group located trans to the hydride and two boryl

groups trans to each other would be expected for the trisboryl

complex. However, the ¹¹B NMR spectrum contained a single

1

boryl resonance at δ 39.9 ppm, and the H NMR spectrum

contained a single set of pinacol methyl groups. Thus, the

spectral data for 3 indicate that the boryl groups exchange

positions within this four-legged piano stool geometry. This type

(30) Irvine, G. J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.;

Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem. ReV.

1998, 98, 2685.

(31) The analogous iridium(III) boryl hydrides are very stable: Iverson, C. N.;

Smith, M. R., III. J. Am. Chem. Soc. 1999, 121, 7696.

(32) (a) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am. Chem.

Soc. 1991, 113, 4173. (b) Luo, X.-L.; Crabtree, R. H. J. Am. Chem. Soc.

1990, 112, 4813.

(29) (a) Heinekey, D. M.; Oldham, W. J. Chem. ReV. 1993, 93, 913. (b) Kubas,

G. J. Metal-dihydrogen and sigma-bond complexes: structure, theory, and

reactiVity; Kluwer Academic: New York, 2001.

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2540 J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005

Rhodium Boryl Complexes in Alkane Functionalization

A R T I C L E S

Table 1. Structural Data for Compounds 2-5 by X-ray Diffraction and DFT Methods

a

2

′

3

′

4

′

5

5′

Bond Distances

2.119

Rh-B(1)

Rh-B(2)

Rh-B(3)

Rh-H(1)

Rh-H(2)

B(1)-H(1)

B(2)-H(1)

B(3)-H(1)

B(1)-H(2)

B(2)-H(2)

Rh-P(1)

2.055(7)

2.069(6)

2.081(6)

2.079(6)

2.050

2.096

2.056(3)

2.059(3)

2.071(3)

2.065(3)

2.059(3)

2.078(3)

1.47(3)

2.029(4)

2.033

2.0282(13)

2.0308(14)

2.0282(13)

2.0332(14)

2.044

2.046

2.067

2.054

1.619

1.41(5)

1.47(6)

1.49(5)

1.59(6)

1.65(4)

1.70(6)

2.20(4)

2.30(6)

1.565

1.610

1.896

2.259

1.42(3)

2.23(3)

1.565

2.142

1.45(3)

1.69(3)

1.53(3)

1.459

2.269

1.99(3)

2.06(3)

2.13(5)

2.21(6)

1.57(5)

1.70(6)

2.331

1.501

2.2306(11)

78.5(12)

2.267

71.7

2.2414(6)

2.2359(5)

2.273

Angles with Rh-H

H(1)-Rh(1)-H(2)

H(1)-Rh(1)-B(1)

H(1)-Rh(1)-B(2)

H(2)-Rh(1)-B(1)

H(2)-Rh(1)-B(2)

H(1)-Rh(1)-B(3)

81(3)

94(3)

53.1(17)

54(2)

75.6(17)

79(2)

71.7(19)

73(2)

48.8(18)

53(2)

95.6

61.6

74.6

78.1

45.4

54.3(10)

43.4

69.4(11)

104.9(11)

100.8(11)

100.4

66.4(10)

47.5(11)

75.3

Angles with Rh-Bpin

B(1)-Rh(1)-B(2)

B(1)-Rh(1)-B(3)

B(3)-Rh(1)-B(2)

105.7(2)

104.0(2)

102.6

73.63(12)

76.8

105.3

77.9

76.37(5)

76.48(6)

77.5

78.47(13)

103.40(12)

102.27(13)

80.34(13)

74.79(12)

Angles with Rh-PR₃

H(1)-Rh(1)-P(1)

B(1)-Rh(1)-P(1)

83.7(12)

85.5

88.1

92.82(5)

91.03(4)

92.49(5)

92.26(4)

88.1

89.5

B(2)-Rh(1)-P(1)

^aCompounds with truncated structures calculated with DFT are named 2′, 3′, 4′, and 5′.

of stereochemical nonrigidity for compounds with such piano-

stool structures is well-known.³³

was deduced from NMR spectroscopy. The B-Rh-B angles

in the two independent molecules are 104.0(2)° and 105.7(2)°.

The shorter rhodium-boron bond distances in the two structures

are 2.055(7) and 2.069(6) Å, and the longer rhodium-boron

distances are 2.081(6) and 2.079(6) Å. These distances are

longer than the 2.047(7) Å distance of the closely related iridium

complex Cp*Ir(H)₃(Bpin)¹⁴and are longer than the 1.907-2.055

Å distances³⁴of other rhodium dioxaborolanyl complexes.

Most striking, each hydride ligand is located closer to one of

the boryl groups than to the other, and each boryl group is

similarly located closer to one of the hydrides than to the other.

The shorter B-H distances range from 1.57(5) to 1.70(6) Å,

and the longer distances range from 2.13(5) to 2.30(6) Å. The

two H-Rh-B angles also reflect the unsymmetrical geometry.

5. Structural Data for Boryl Complexes 2 and 3 from

X-ray Diffraction and Theoretical Methods. a. Structure of

Bisboryl 2 Determined by X-ray Diffraction. The structure

of compound 2 was determined by X-ray diffraction. This

complex crystallized with two independent molecules in the

asymmetric unit. An ORTEP diagram of 2 is provided in Figure

2, and selected distance and angle data are provided in Table

1, along with data on other experimental and theoretical

compounds described in this paper. The hydride ligands were

located and refined isotropically. Compound 2 adopts the same

trans, four-legged piano-stool structure in the solid state that

(33) The isoelectronic complexes CpMoL(CO)₂R are known to be stereo-

chemically nonrigid: Faller, J. W.; Anderson, A. S. J. Am. Chem. Soc.

1970, 92, 5852.

(34) CSD v5.25, November 2003.

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J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005 2541

A R T I C L E S

Hartwig et al.

Scheme 2. Exchange of the Hydrides and the Boryl Groups in

CpRh(H)₂(B(O₂C₂H₄))₂(2′) Calculated with DFT^a

^aEnergies are in units of kcal mol^-1. [Rh] ) CpRh, and BR₂

)

(BO₂C₂H₄). For clarity, the Cp ligand is not shown and is located behind

the rhodium, and all other ligands are located in front of the rhodium.

Figure 2. ORTEP drawing of one of two independent molecules of Cp*Rh-

(H)₂(Bpin)₂(2) in the asymmetric unit with 30% thermal ellipsoids.

Figure 3. Overlay of experimental structure Cp*Rh(H)₂(Bpin)₂(2) (in blue)

and calculated structure CpRh(H)₂(B(O₂C₂H₄))₂(2′) (in yellow) with the

Cp* and Cp groups removed for clarity. The calculated B-H distances are

given in angstroms.

The angles that include the hydride and the nearest boron atom

are between 49(2)° and 54(2)° in the two molecules, while the

angles that include the hydride and the more distant cis boron

atoms are between 72(2)° and 79(2)°. Thus, we conclude that

the structure of complex 2 contains significant B-H bonding

and that the bonding between each boryl group and one of its

cis hydrides is stronger than the bonding between the boryl

group and its other cis hydride.

b. Structure of 2′ Deduced by Theory. Because of the

ambiguity of the metrical parameters involving hydride ligands,

we conducted a computational study of the structure of this

compound. Throughout this work, structures were calculated

for truncated models in which Cp* was replaced by Cp and

Bpin was replaced by BO₂C₂H₄. These compounds are labeled

with a prime to distinguish them from the experimental

structures; i.e., compound 2 is the experimental species Cp*Rh-

(H)₂(Bpin)₂, and 2′ is the computed species CpRh(H)₂-

(BO₂C₂H₄)₂.

An overlay of the bottom portion of the experimental and

theoretical structures of 2 and 2′ is provided in Figure 3. Clearly,

the structure determined by theoretical methods corroborates

the unsymmetrical orientation of the hydride ligands deduced

by X-ray diffraction. As summarized in Table 1, the structure

deduced from theory contained one B-H distance that is shorter

than the other B-H distance. Further, one H-Rh-B angle for

the hydride involved in the B-H interaction is 45.5°, while the

other angle is 78.1°. The two H-Rh-B angles involving the

other hydride ligand are also unsymmetrical, 61.6° and 74.6°.

This trend toward an unsymmetrical positioning of the hydride

between the two boryl ligands and the boryl ligands between

the two hydrides in both experimental and theoretical data

Figure 4. ORTEP drawing of one of two independent molecules of Cp*Rh-

(H)(Bpin)₃(3) in the asymmetric unit with 30% thermal ellipsoids.

supports the B-H bonding deduced from ¹H NMR spectroscopy

and X-ray diffraction.

The computational work also shows that the barrier for

shortening and lengthening the B-H bonds is low. As shown

in Scheme 2, a concerted process by which one of the shorter

B-H bond distances lengthens while the other B-H distance

shortens occurs with a free energy barrier of only 0.8 kcal mol^-1

.

Therefore, the two hydrides and two boryl groups are indistin-

guishable in solution on the NMR time scale. It is reasonable

to assume that the barrier for interconversion of the hydrogens

is larger in the solid phase than it is in the gas phase because

this conversion involves movement of the Bpin group, and this

movement would be restricted by packing forces in the crystal.

c. Structure of Trisboryl 3 Determined by X-ray Diffrac-

tion. The structure of trisboryl complex 3 was also determined

by X-ray diffraction. Like 2, this compound crystallized with

two independent molecules in the asymmetric unit. An ORTEP

diagram of 3 is provided in Figure 4, and selected distance and

angle data are provided in Table 1. The hydride ligand of

trisboryl 3 was located and refined isotropically. Like the

hydrides of 2, the hydride in 3 appeared by X-ray diffraction to

be located unsymmetrically between the two nearest boryl

groups. One of the B-H distances in 3 was found to be 1.53-

(3) and 1.69(3) Å in the two molecules, and the other 1.99(3)

9

2542 J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005

Rhodium Boryl Complexes in Alkane Functionalization

A R T I C L E S

Scheme 4

Therefore, in solution, the cis and trans boryl groups undergo

rapid exchange on the NMR time scale.

The closest B-H distances in either 2 or 3 are significantly

longer than the 1.24-1.31 B-H distances in manganese borane

complexes (MeCp)Mn(CO)₂(HBR₂).²⁴Consistent with this

difference in distances, the B-H coupling in the manganese

compound was stronger. A 1:1:1:1 quartet was observed for

the hydride in the ¹H NMR spectrum of the manganese

compound, and a doublet was observed for the boryl group in

the ¹¹B NMR spectrum. The B-H distances in 2 and 3 are also

much longer than the 1.25-1.35 Å B-H distances in two

titanocene borane complexes^22,23and the 1.35 Å distance in a

ruthenium σ-borane complex.²⁵However, this distance is shorter

than the B-H distances of 2.013(5) and 2.004(10) Å in

[(PⁱPr₃)₂RhHCl(Bpin)] and [(PⁱPr₃)₂RhHCl(Bcat)], which were

determined by neutron diffraction and used to suggest modest

H-B bonding.³⁵Thus, we conclude that compounds 2 and 3

should be regarded as elongated σ-borane complexes, much like

dihydrogen complexes with long H-H distances are termed

“elongated” dihydrogen complexes.

Figure 5. Overlay of experimental structure Cp*Rh(H)(Bpin)₃(3) (in blue)

and calculated structure CpRh(H)(B(O₂C₂H₄))₃(3′) (in yellow) with the

Cp* and Cp groups removed for clarity. The calculated B-H distances are

given in angstroms.

Scheme 3. Energies for the Exchange of Boryl Groups in

CpRh(H)[B(O₂C₂H₄)]₃(3′) Calculated with DFT^a

^aEnergies are in units of kcal mol^-1. [Rh] ) CpRh, and BR₂

)

(BO₂C₂H₄). For clarity, the Cp ligand is not shown and is located behind

the rhodium, and all other ligands are located in front of the rhodium.

and 2.06(3) Å. The smaller H-Rh-B angles in the two

independent molecules were found to be 47.5(8)° and 54.3-

(10)°, and the larger H-Rh-B angles were 66.4(10)° and 69.4-

(11)°. The Rh-B distances for the two boryl groups that are

mutually trans are indistinguishable from the Rh-B distance

of the boryl group that is trans to the hydride. Thus, the overall

geometry of this species is that of a four-legged piano stool

with B-H bonding of the hydride to one of its cis boryl groups

that is stronger than the B-H bonding to its other cis boryl

group.

d. Structure of 3′ Deduced by Theory. The structure of

complex 3 was also evaluated by computational methods. An

overlay of the experimental and computed structures 3 and 3′

is provided in Figure 5, and the computed distances and angles

are included in Table 1. Again, the computational data support

the unsymmetrical structure with partial B-H bonding indicated

by X-ray diffraction. The two B-H distances deduced from

theory are significantly different, 1.46 and 2.27 Å, and the two

H-Rh-B angles are much different, 43.4° and 75.3°. Thus,

we conclude that complex 3 tends toward a structure with two

terminal boryl groups and one borane ligand.

6. Reactivity of Complexes 2 and 3. a. Dissociation of

Borane. Boryl compounds 2 and 3 could dissociate H₂, HBpin,

or B₂pin₂by H-H, H-B, or B-B bond formation. Consistent

with the apparent partial B-H bonding in the ground-state

structures, complexes 2 and 3 underwent dissociation of HBpin

faster than they underwent dissociation of H₂or B₂pin₂, as

summarized in Scheme 4.

Reaction of bisboryl 2 with P(p-tol)₃for 4 h at 85 °C formed

Cp*Rh[P(p-tol)₃](H)(Bpin) (4) in 68% yield by NMR spectros-

copy and 51% isolated yield. A second rhodium species, whose

NMR spectra are consistent with the formulation Cp*Rh(Bpin)₂-

[P(p-tol)₃], was formed in 17% yield. This minor compound

would form by elimination of H₂from 2. Reaction of this

complex 2 with PEt₃, instead of P(p-tol)₃, occurred similarly,

but the boryl hydride complex generated from reaction of 2 with

this trialkylphosphine was less crystalline. Assuming the

elimination of hydrogen would be irreversible, these product

ratios imply that the overall rate of the extrusion of HBpin from

2 is at least 4 times greater than the rate of extrusion of H₂.

However, the elimination of H₂from 2 would clearly require a

rearrangement of the ground-state structure of 2 to place the

hydrides cis to each other. Thus, the product ratios do not reflect

the relative rate constants for the elementary steps of B-H vs

H-H bond formation. Instead, they reflect the relative rate

constants for elimination of HBpin vs the combination of

rearrangement and elimination of H₂.

A minimum on the energy surface was also found for a C_s

symmetric, Rh(V) species, 3′b, with equivalent and long B-H

bond distances. In this C_sstructure, the Rh-H bond length is

1.549 Å, the B-H distances are 2.028 Å, and the corresponding

H-Rh-B angles are 66.4°. However, this species lies 1.3 kcal

mol^-1uphill of the unsymmetrical complex 3′ (Scheme 3). The

free energy barriers for interconversion of the different structures

of 3 appear to be small. The free energy barrier between 3′ and

3′b is only 2.6 kcal mol^-1, and the free energy barrier for

rotation of the borane ligand in 3′ through a structure with the

The rate of reaction of 2 in the presence of P(p-tol)₃was

faster than the rate of decomposition of 2 at this temperature in

(35) Lam, W. H.; Shimada, S.; Batsanov, A. S.; Lin, Z. Y.; Marder, T. B.;

Cowan, J. A.; Howard, J. A. K.; Mason, S. A.; McIntyre, G. J.

Organometallics 2003, 22, 4557.

hydride close to the remote boryl group is only 4.2 kcal mol^-1

.

9

J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005 2543

A R T I C L E S

Hartwig et al.

Figure 7. ORTEP drawing of Cp*Rh(PEt₃)(Bpin)₂(5) with 30% ellipsoids.

Figure 6. ORTEP drawing of Cp*Rh[P(p-Tol)₃](H)(Bpin) (4) with 30%

ellipsoids.

Scheme 5

cyclohexane in the absence of added phosphine. After heating

of 2 with P(p-tol)₃for 24 h at 45 °C, only 5% of the bisboryl

complex remained, but after heating of bisboryl 2 without

phosphine for the same time and temperature, 60-70% re-

mained. Complex 2 is an 18-electron species, and previous

replacements of boranes in 18-electron complexes with phos-

phine have occurred by dissociative paths.^22,24Considering this

information, the faster reaction of 2 in the presence of added

ligand than in the absence of added ligand implies that

dissociation of HBpin from 2 occurs reversibly when it is heated

alone in cyclohexane.

Trisboryl complex 3 also underwent dissociation of HBpin.

Heating of trisboryl 3 at 85 °C with 1 equiv of PEt₃for 1 h

formed Cp*Rh(Bpin)₂(PEt₃) (5) in 77% isolated yield. (Reaction

with P(p-tol)₃occurred in a similar fashion, but the substitution

product was isolated in only 51-54% yield, and the isolated

material was not analytically pure.) Thus, dissociation of HBpin

from trisboryl 3 is much faster than dissociation of B₂pin₂, even

though this complex could undergo dissociation of the diborane-

(4) species without the need for isomerization. One could

envision that the combination of dissociation of the diborane-

(4) species and coordination of PEt₃is not observed because it

is thermodynamically unfavorable. However, such a proposal

would be inconsistent with a large thermodynamic driving force

calculated with DFT for the elimination of B₂(OR)₄and

subsequent coordination of phosphine (vide infra).

Dissociation of borane from 3 was consistently faster than

dissociation of borane from 2. The approximate half-life for

decay of 3 at 55 °C was 40 min, while the approximate half-

life for decay of 2 at 55 °C was 127 min, as determined by

monitoring the reactions periodically by ¹H NMR spectroscopy

with added PEt₃and dodecahydrotriphenylene as internal

standard. These rates are in agreement with the calculated

energies for dissociation (vide infra).

more classical than those of 2 and 3. The B-H distance in

compound 4 is much longer (X-ray = 2.23(3) Å; computed for

4′, 2.14 Å) than the shorter of the B-H distances in compound

2, and the Rh-B distance is measurably shorter. Thus, the X-ray

diffraction data and DFT calculations imply that compound 4

contains terminal hydride and boryl ligands without any residual

B-H bonding. This difference in the bonding of 2 and 4 implies

a lower formal oxidation state of 4 and would result from a

lower coordination number of 4 and strong electron donation

from a phosphine ligand.

An ORTEP diagram of 5 is shown in Figure 7, and metrical

parameters are included in Table 1. This complex crystallized

with two independent molecules in the asymmetric unit. Some

complexes with two cis boryl groups contain a B-B interac-

tion,^11,28but the structure of 5 contains a long B-B distance

and no evidence for such partial B-B bonding. The Rh-B

distances in 5 are similar to that in 4 and are shorter than the

Rh-B distances in 2 and 3. The shorter M-B distances in

compound 5, relative to those in 2 and 3, are likely to stem

from the presence of two simple terminal boryl groups in a

conventional rhodium(III) species. This structure would again

be favored by the lower coordination number in 5 than in 3

and by the strong electron donation from the phosphine ligand.

c. Reactions of 2 and 3 with Hydrocarbons. The reactions

of boryl complexes 2 and 3 with hydrocarbons are summarized

in Scheme 5. Both complexes reacted with linear alkanes and

with benzene to form octyl- and phenylboronate esters, respec-

tively. Reactions with alkanes were slower than reactions with

b. Structures of the Ligand Substitution Products. The

structures of the rhodium(III) boryl complexes 4 and 5 resulting

from ligand substitution were determined. An ORTEP diagram

of compound 4 is provided in Figure 6, and metrical parameters

are included in Table 1. Compound 4 possesses a three-legged

piano-stool geometry. Consistent with a sharp ¹H NMR

resonance for the hydride in 4, the structure of compound 4 is

9

2544 J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005

Rhodium Boryl Complexes in Alkane Functionalization

A R T I C L E S

Scheme 6

Scheme 7

arenes, and the reactions of bisboryl complex 2 were slower

than those of trisboryl 3. The relative rates for reaction of 2

and 3 with hydrocarbons parallel the relative rates for dissocia-

tion of borane from 2 and 3.

Heating of 2 at 125 °C in octane for 14 h formed 2 equiv of

octylboronate ester in 70% yield, as determined by GC (Scheme

5). The product boronate ester has been generated previously

by catalytic studies⁸and by independent synthesis. Heating of

2 at 80 °C in C₆D₆for 14 h produced 2 equiv of PhBpin-d₅in

80% yield. By stoichiometry, the rhodium product from these

reactions would be Cp*RhH₄. However, this complex has never

been prepared and is presumably unstable because a variety of

bridging hydrides are formed upon reduction of Cp*RhX₂

compounds.³⁶Thus, a series of rhodium complexes are generated

from the stoichiometric reactions of 2 with hydrocarbons, and

we have not been able to characterize them. In a catalytic

system, the product rhodium fragment would likely be trapped

with the borane reagent to regenerate 2 or 3.

Reaction of trisboryl complex 3 with octane at 125 °C for

14 h formed 3 equiv of octylboronate ester in 72% yield, as

determined by GC methods. Reaction of 3 with C₆D₆at 80 °C

for 14 h produced 3 equiv of PhBpin-d₅in 85% yield. The

rhodium hydride product from reaction of 3 was also unstable,

and several signals for Cp*-rhodium complexes were observed

at the end of the reactions of 3 with octane and C₆D₆. At 125

°C, we were unable to observe any accumulation of bisboryl 2

from reaction of 3 with octane. However, at 90 °C 35-40% of

bisboryl complex 2 was observed. Along with 2, octylBpin was

generated in a 1.26:1 ratio to the amount of starting trisboryl 3.

After 3 h, however, the solution contained only 10% of the

bisboryl complex because this complex also reacts with octane.

We conducted reactions of boryl complexes 2 and 3 with

hydrocarbons at lower temperatures to distinguish their relative

reactivities. These experiments, summarized in Scheme 6,

revealed that trisboryl 3 reacted faster than bisboryl 2. After 90

min at 55 °C, trisboryl 3 underwent complete reaction in C₆D₆

to form 2 equiv of PhBpin-d₅in greater than 95% yield. This

reaction also generated 1 equiv of a mixture of HBpin and

DBpin in a roughly 1:1 ratio in 86% yield, as determined by

¹H and ¹¹B NMR spectroscopy. The 2 equiv of PhBpin-d₅and

1 equiv of the combination of HBpin and DBpin account for

all three boryl groups of the starting complex 3. Under the same

reaction conditions, more than 90% of bisboryl 2 remained.

The stoichiometry of the reactions of the trisboryl complex

with alkanes was less well defined. However, less than 5% of

the trisboryl 3 remained after heating for 60 min at 90 °C in

octane, while 35-40% bisboryl 2 remained after heating in

octane under identical conditions. Thus, the reaction of the

trisboryl 3 with octane is also faster than the reaction of the

bisboryl 2 with octane.

d. Mechanism of the Reactions of 2 and 3 with Hydro-

carbons. One could envision that heating of 2 and 3 would

generate small concentrations of free B₂pin₂and a rhodium

complex that could catalyze the reaction of alkane with the

released B₂pin₂by a pathway that does not involve 2 or 3

directly. To test this possibility, complexes 2 and 3 were heated

in C₆D₆in the presence of added B₂pin₂-d₂₄. If boryl complexes

2 and 3 react with alkanes directly, then the initial alkylboronate

ester product would lack the deuterium label, but if the alkyl-

and arylboronate ester products form by a catalytic reaction with

the extruded diborane(4), then the initial product would possess

the deuterium label.

The results of this labeling study are summarized in Scheme

7. The reaction of 2 with C₆D₆in the presence of 1.1 equiv of

B₂pin₂-d₂₄formed PhBpin-d₅and PhBpin-d₁₇in a ratio of 72:

28 after 50% conversion of 2. Only 14% of B₂pin₂-d₂₄was

consumed under these conditions in which a low 36 mM

concentration of B₂pin₂-d₂₄was present. The reaction of 3 with

C₆D₆in the presence of 1.1 equiv of B₂pin₂-d₂₄formed PhBpin-

d₅and PhBpin-d₁₇in an 86:14 ratio after 55% conversion of 3.

Again only 17% of the B₂pin₂-d₂₄was consumed. Similarly,

reactions of 2 and 3 with octane with 1 equiv of added B₂pin₂-

d₂₄produced octylBpin and octylBpin-d₁₂in ratios near 85:15

after 55% conversion of 2 and 80% conversion of 3.

Although one might expect that larger amounts of labeled

arylboronate and alkylboronate esters would be observed due

to a catalytic conversion of B₂pin₂-d₂₄to PhBpin-d₁₇, a large

ratio of unlabeled to labeled arylboronate and alkylboronate ester

is observed because the reaction of the rhodium product with

B₂pin₂-d₂₄to regenerate the starting boryl complexes is not

efficient at the low concentration of diborane(4) reagent. Most

of the B₂pin₂-d₂₄is unreacted. This set of labeling experiments

shows definitively that the arylboronate ester and alkylboronate

ester generated from thermolysis of 2 and 3 in benzene and

octane is generated from an intact boryl complex generated from

2 and 3. The combination of this result and our studies on the

dissociation of HBpin from these complexes (vide supra) implies

that the intermediate boryl complex that reacts with the alkane

or arene is the 16-electron species generated by dissociation of

HBpin from 2 and 3.

(36) (a) White, C.; Gill, D. S.; Kang, J. W.; Lee, H. B.; Maitlis, P. M. J. Chem.

Soc., Chem. Commun. 1971, 734. (b) Gill, D. S.; Maitlis, P. M. J.

Organomet. Chem. 1975, 87, 359. (c) White, C.; Oliver, A. J.; Maitlis, P.

M. J. Chem. Soc., Dalton Trans. 1973, 1901. (d) Churchill, M. R.; Ni, S.

W. J. Am. Chem. Soc. 1973, 95, 2150.

Because the reactions of 2 and 3 with alkanes are slower than

those with arenes, the rate-determining step of the reactions of

alkanes must involve the hydrocarbon and cannot be dissociation

9

J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005 2545

A R T I C L E S

Hartwig et al.

of borane. In this case, the reaction of the 16-electron intermedi-

ate with an alkane would be preceded by reversible dissociation

of borane. This assertion is consistent with the conclusion that

dissociation of borane from 2 occurs reversibly upon heating

in cyclohexane (vide supra). Moreover, this assertion predicts

that added HBpin would inhibit the reactions of these complexes

with alkanes.

°C for 90 min in the presence of 30 mol % trisboryl 3 led to

roughly 50% conversion of the diboron compound in the

absence of added HBpin but to only 20% conversion in the

presence of 11 equiv of added HBpin.

The effect of HBpin on the reactions of arenes is less

straightforward. The reaction of trisboryl 3 with benzene occurs

at roughly the same rate as the reaction of 3 with phosphines.

These similar rates imply that dissociation of borane may be

rate limiting, and consistent with this assertion, the reaction of

3 with C₆D₆appeared to occur at the same rate with or without

added HBpin. If so, then the accumulation of HBpin would not

affect the catalytic reactions of arenes, and the reactions of

arenes with HBpin would occur at the same rate as the reactions

of arenes with B₂pin₂. The similarities and differences between

the reactions of arenes with B₂pin₂and HBpin catalyzed by

Cp*Rh complexes and detailed kinetic studies on the reactions

of 3 with added HBpin will be the subject of future studies.

To test this proposal, we conducted reactions of the trisboryl

complex 3 with octane in the presence of added HBpin and

compared the qualitative rates in the presence and absence of

added HBpin. We did not conduct quantitative kinetic studies

on these reactions because a series of rhodium products are

formed. Further, the conditions of reactions in the presence of

added HBpin are those of a catalytic borylation of an alkane or

arene with HBpin as reagent, not a simple stoichiometric

process. Because the reaction conditions are those of the catalytic

borylation by HBpin, a difference in rates for the decay of 3 in

the presence and absence of added HBpin could result from

regeneration of 3 in the presence of HBpin, instead of consump-

tion of 3 by reaction of all three boryl groups, as occurs in the

absence of added borane. However, the catalytic reactions with

HBpin are much slower than those with B₂pin₂and require high

temperatures. Therefore, it is likely that a slower reaction of 3

with added HBpin would be due to an effect of the added HBpin

on the preequilibrium and not on regeneration of the trisboryl.

The reaction of 3 with octane was slower when conducted

with added HBpin than when conducted without added HBpin.

In one representative experiment, heating of a 13 mM solution

of trisboryl 3 in octane at 100 °C for 1 h without added HBpin

led to the generation of HBpin and a 15.7 mM solution of

octylBpin. This quantity of octylBpin corresponds to a 60% yield

for a reaction that generates 1 equiv of HBpin and converts the

remaining two boryl groups to octylBpin. In contrast, reaction

of an identical solution of 3 with 10 equiv (130 mM) of HBpin

for the same time and temperature led to only a 5.6 mM

concentration of octylBpin. Therefore, our data are consistent

with a pathway involving reversible dissociation of borane and

subsequent reaction of the resulting 16-electron boryl intermedi-

ate with alkane or arene, but conclusions from these experiments

must be drawn tentatively.

The relationship between the inhibition of the reactions of 3

with hydrocarbons by HBpin and the mechanism of the catalytic

borylation of alkanes in the presence of Cp*Rh(η⁴-C₆Me₆) is

significant. The stoichiometry of reactions of B₂pin₂with alkanes

to form R-Bpin leads to the formation of HBpin as a byproduct.

The mechanistic data presented here suggest that the HBpin

generated as byproduct may inhibit the borylation of alkanes.

Moreover, these data imply that reactions of alkanes initiated

with HBpin as reagent could be slower than those initiated with

B₂pin₂because the system contains a high concentration of

HBpin at the beginning of the reaction.

Indeed, we have obtained data in previous work and in the

current work that are consistent with these conclusions. First,

the reactions of HBpin with alkanes occurred more slowly and

in lower yields than reactions of B₂pin₂.⁸Second, reactions of

octane with B₂pin₂catalyzed by 2 and 3 were slower when

conducted with added HBpin. For example, the reaction of

octane with B₂pin₂for 90 min at 105 °C in the presence of 30

mol % 2 led to 73% conversion of the diboron compound in

the absence of HBpin, but to 31% conversion in the presence

of 11 equiv of HBpin. The reaction of B₂pin₂with octane at 90

An inverse kinetic order in a reactant is unusual. In this case,

the reaction order results from the need to dissociate HBpin

from the metal prior to the rate-limiting step. A similar effect

of added HBpin was observed during a titanium-catalyzed

hydroboration that involved borane complexes as intermediates

and dissociation of borane prior to reaction with olefin or

alkyne.³⁷In other contexts, this kinetic behavior has been

observed during studies of carbonylations and hydroaminations.

Under some conditions carbonylation reactions are inhibited by

CO, even though it is a reagent, because CO is a ligand that

must dissociate to allow coordination and insertion of olefin.³⁸

An inverse order in amine was observed under some conditions

during zirconocene-catalyzed hydroamination of alkynes because

free amine and the reactive zirconocene arylimido intermediate

are generated reversibly from a bisarylamido complex prior to

the turnover-limiting reaction of the imido complex with

alkyne.³⁹

7. Direct Observation of 2 and 3 in Catalytic Reactions.

In addition to demonstrating that the boryl compounds 2 and 3

react with alkanes and arenes to form products that are analogous

to those formed from the catalytic cycle, we have shown that

these species are present in catalytic reactions. Reactions

initiated with hexamethylbenzene precatalyst 1 or boryl com-

plexes 2 and 3 in pentane were run to partial conversion, and

¹H NMR spectra were obtained on the reaction solutions. The

amounts of complexes 2 and 3 were approximated by the

intensity of the Cp* signals of 2 and 3, relative to the signal of

dodecahydrotriphenylene internal standard, in ¹H NMR spectra

obtained after evaporation of pentane and dissolution of the

residue in C₆D₆. In addition, the conversion of B₂pin₂to

pentylBpin was determined by GC analysis of an aliquot of the

pentane solution.

Reactions initiated with precatalyst 1 were run at 125 °C to

40% conversion of B₂pin₂. After this time, complexes 2 and 3

1

were observed, but the sum of the intensities of the H NMR

signals of these two complexes accounted for only 10% of the

starting rhodium. The same reaction was conducted at 105 °C

with isolated 2 as catalyst and was analyzed in a similar fashion.

(37) Hartwig, J. F.; Muhoro, C. N. Organometallics 2000, 19, 30.

(38) Ikatchenko, I. In ComprehensiVe Organometallic Chemistry; Wilkinson,

G., Stone, F. G. A., Abel, E., Eds.; Pergamon: New York, 1982; Vol. 8,

p 140.

(39) Walsh, P. J.; Baranger, A. M.; Bergman, R. G. J. Am. Chem. Soc. 1992,

114, 1708.

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Rhodium Boryl Complexes in Alkane Functionalization

A R T I C L E S

Scheme 8. Energetics for Generation of the 16-Electron

a. Calculations on the Dissociation of H₂, HBpin, and

B₂pin₂. The experimental data presented in preceding sections

and computational data presented in this section of this paper

are consistent with generation of a 16-electron reactive inter-

mediate from 2 and 3 by dissociation of borane. The free energy

of dissociation of borane from bisboryl 2′ to give the 16-electron

boryl hydride complex 6′ is computed to be lower than the free

energy of dissociation of dihydrogen. The calculated difference

in the activation free energy of these two dissociation paths at

standard states is small (∆G^qfrom 2′ to TS_2′-7′- ∆G^qfrom 2′

to TS_2′-6′) 26.68 - 25.58 ) 0.9 kcal mol^-1). This value is

relatively independent of temperature, and the calculated

∆(∆G^q) at 85 °C of 1.3 kcal mol^-1is remarkably close to the

0.9 kcal mol^-1value that would give rise to the experimentally

observed 4:1 ratio of products from replacement of borane and

dihydrogen by phosphine.

Intermediate Deduced by DFT^a

The barrier for dissociation of borane from trisboryl 3′ to

give unsaturated bisboryl intermediate 7′ was smaller than that

for dissociation of borane from the bisboryl 2′ to give monoboryl

intermediate 6′. The difference in barriers (∼3 kcal mol^-1) is

larger than that which would give rise to the roughly 5-10-

fold greater rate constant for the reaction of the trisboryl 3 than

for reaction of the bisboryl 2, but the calculated trend in this

study matches that from the experiment. Significant for under-

standing the chemistry of the trisboryl complex, the barrier for

elimination of B₂pin₂to generate the boryl hydride intermediate

6′ is roughly 8 kcal mol^-1higher than the barrier for elimination

of borane. For this reason, no phosphine-ligated boryl hydride

complex 4 was obtained when trisboryl complex 3 was heated

in the presence of added phosphine. Furthermore, the origin of

the lack of formation of 4 from this reaction is kinetic and not

thermodynamic. The enthalpy (∆H) for coordination of phos-

phine to 6′ is 34 kcal mol^-1, and this large exothermicity would

make the dissociation of B₂pin₂and coordination of phosphine

to the resulting intermediate overall exoergic.

b. Calculation of the Mechanism of the C-H Bond

Cleavage and B-C Bond Formation. Our experimental data

provided no information on the intimate mechanism of the C-H

bond cleavage pathway. Thus, we conducted theoretical studies

to provide information on the pathway by which the 16-electron

intermediates react with a model alkane, methane.⁴¹Several

calculations of the pathways of C-H activation by metal boryl

compounds have now been reported by other researchers.^42-44

In addition, we have reported computational studies⁴⁵on the

formation of a methylboronate ester from methane and CpM-

^aEnergies are in units of kcal mol^-1([Rh] ) CpRh, BR₂) (BO₂C₂H₄)).

A superscript Roman “a” following “TS” indicates that the energies are

determined using the method described in the Computational Details and

in Figure 10.

In this case, the sum of the resonances of 2 and 3 accounted

for about 40% of the starting rhodium after 40% conversion of

B₂pin₂. Finally, the same reaction was conducted at 90 °C with

complex 3 as catalyst. In this case, the sum of the resonances

of 2 and 3 accounted for 80% of the starting rhodium.

Although the observation of a complex in a catalytic system

does not alone imply that it is an intermediate in the catalytic

cycle, several pieces of data do imply that these species are

intimately connected with the catalytic cycle. First, the system

that leads to reaction at the lowest temperaturesthe reaction

catalyzed by 3scontains the largest amount of the two boryl

complexes. Second, both complexes 2 and 3 are chemically and

kinetically competent to be intermediates in reactions conducted

with C₆Me₆complex 1 as precatalyst. Third, the most reactive

catalyst was also the complex that reacted with hydrocarbons

at the lowest temperatures during studies of the stoichiometric

chemistry. The lower rate of the reaction catalyzed by 1 indicates

that the rate and yield of the conversion of precatalyst 1 to the

active catalyst limit the concentration of the active catalyst, limit

the turnover numbers for reactions initiated with 1, and influence

the reaction rate.

(40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.

A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,

R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,

K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,

R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;

Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;

Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,

J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;

Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,

C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;

Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,

M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revisions A.7 and A.11.3;

Gaussian, Inc.: Pittsburgh, PA, 1998.

8. Theoretical Studies of the Reactivity of 2′ and 3′. Several

mechanistic issues that were unresolved by the experimental

data were addressed by computational studies. DFT calcula-

tions⁴⁰were conducted on the dissociation of H₂, borane, and

diborane from 2′ and 3′, on the C-H bond cleavage by the

resulting 16-electron intermediates, and on the B-C bond-

forming step.

The results from calculations of the reactions of slightly

truncated models of the bis- and trisboryl complexes with

methane are summarized in Schemes 8 and 9. Scheme 8 shows

the reactions that generate the 16-electron intermediates, and

Scheme 9 shows the relative energies of the species involved

in the reactions of the 16-electron intermediates. In the computed

structures, Cp* was replaced by Cp, and the pinacolboryl group

was replaced by an unsubstituted dioxaborolanyl unit.

(41) The barrier for primary C-H bond activation of propane (∆H^q) 11.9

kcal mol^-1) is very similar to the barrier for C-H bond activation of

methane (∆H^q) 11.1 kcal mol^-1).

(42) Tamura, H.; Yamazaki, H.; Sato, H.; Sakaki, S. J. Am. Chem. Soc. 2003,

125, 16114.

(43) (a) Lam, W. H.; Lin, Z. Y. Organometallics 2003, 22, 473. (b) Lam, W.

H.; Lam, K. C.; Lin, Z.; Shimada, S.; Perutz, R. N.; Marder, T. B. Dalton

Trans. 2004, 1556.

(44) Wan, X.; Wang, X.; Luo, Y.; Takami, S.; Kubo, M.; Miyamoto, A.

Organometallics 2002, 21, 3703.

9

J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005 2547

A R T I C L E S

Hartwig et al.

Scheme 9. Mechanism for the C-H Bond Cleavage and B-C Bond Formation Steps Deduced by DFT^a

^aEnergies are in units of kcal mol^-1. [Rh] ) CpRh, and BR₂) (BO₂C₂H₄). For clarity, the Cp ligand is not shown and is located behind the rhodium,

and all other ligands are located in front of the rhodium.

(CO)_nB(O₂R) (M ) Fe, n ) 2; M ) W, n ) 3; R ) -CH₂-

CH₂-). The barriers for a pathway involving a metal-assisted

σ-bond metathesis were lower than barriers for pathways

involving a sequence of oxidative addition and reductive

elimination calculated by others.^43a

Results on the steps of C-H cleavage and B-C bond

formation for the current system are summarized in Scheme 9.

Again, the theoretical data favor reaction of methane with the

16-electron intermediates 6′ and 7′ by pathways related to those

in a previous study⁴⁵that involve the unoccupied p-orbital on

boron. The structures of 10′ to 14′ that are computed to be

intermediates and transition states in this process are shown in

Figure 8. After coordination of methane to either the monoboryl

hydride intermediate 6′ or the bisboryl intermediate 7′ to

generate structures 10′ and 16′ in Scheme 9, a single transition

state with no intermediates leads to the methyl hydride and

methyl boryl complexes 12′ and 18′, both of which contain a

coordinated borane. Two conformational isomers, 12′ and 12′b,

can form from C-H bond cleavage of methane by 6′. Isomer

12′ contains mutually cis methyl and boryl groups, and isomer

12′b contains mutually trans methyl and boryl groups. Isomer

12′b is more stable than isomer 12′ by more than 3 kcal mol^-1

.

However, a low-energy transition state for rotation of the borane

ligand (6 kcal mol^-1higher in energy than 12′) connects these

two isomers.

After C-H bond cleavage and rotation of the borane ligand,

if necessary, intermediates 12′ and 18′ undergo B-C bond

formation through multicentered transition states 13′-TS and

19′-TS to generate complexes 14′ and 20′. These complexes

contain the product coordinated through a C-H bond of the

B-methyl group. Dissociation of product is then uphill, but

reaction of the 16-electron fragment resulting from dissociation

of product with borane or diboron reagent would return the

system to the stable 18-electron rhodium complexes 2′ and 3′.

Following dissociation of borane to form the active species

6′ and 7′, the individual C-H activation steps have the highest

enthalpic barriers, roughly 8 kcal mol^-1for reaction of

coordinated methane in the boryl hydride intermediate 10′

Figure 8. Structures of the intermediates and transition states in the reaction

between 6′ and CH₄. Distances are in angstroms. The unlabeled gray and

small white circles are carbons and hydrogens, respectively. B-H distances

listed for 10′, 11′-TS, and 13′-TS are for those between the boron and the

Rh-bound hydrogen, which is hidden behind B in the figure.

(generated initially from 2′) and 10 kcal mol^-1for reaction of

the coordinated methane in the bisboryl intermediate 16′

(generated initially from 3′). Although the difference in barriers

is small, these data suggest that the barrier for the elementary

C-H bond cleavage step by the more reactive starting complex,

trisboryl complex 3, is higher than that for the elementary C-H

(45) Webster, C. E.; Fan, Y.; Hall, M. B.; Kunz, D.; Hartwig, J. F. J. Am. Chem.

Soc. 2003, 125, 858.

9

2548 J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005

Rhodium Boryl Complexes in Alkane Functionalization

A R T I C L E S

Scheme 10

of the current system with a dioxaborolanyl group results from

the difference in the barrier for the isomerization. An intermedi-

ate with a methyl group trans to the boryl group must isomerize

to an intermediate with the methyl group cis to the boryl group

for B-C bond formation to occur. The barrier for C-H bond

cleavage of methane to generate a product with a hydride cis

to the BH₂group was calculated to be lower than that for C-H

bond cleavage to generate a product with a methyl group cis to

the BH₂group. However, the barrier for isomerization of the

kinetic addition product to the productive intermediate that

contains a methyl group cis to the boryl group was calculated

to be too high to occur during the reaction. Because of this

high barrier, the authors discounted this pathway and concluded

that the reaction occurs by an oxidative addition step that places

the methyl group cis to the boryl group.

The barriers for isomerization of the BH₂and B(OR)₂

complexes are different, and the origin of this difference is

depicted in Scheme 10. The complex with the hydride cis to a

BH₂group adopts a tetrahydridoborate structure. The strength

of the B-H interactions in hydridoborate complexes depends

on the substituents at boron. The B-H interaction in a

hydridoborate structure with π-donating alkoxide groups on

boron is much weaker than the B-H interaction in a hydri-

doborate structure lacking a π-donor group on boron.^27,46In

fact, the experimental and theoretical studies on the structure

of complex 2 imply that there is a significant interaction of the

dialkoxyboryl group with only one of the two hydride ligands.

As a result of the weak B-H interaction involving a second

hydride and a boryl group with alkoxo substituents, the barrier

for isomerization of the complex with mutually trans methyl

and dialkoxyboryl groups to a complex with mutually cis methyl

and dialkoxyboryl groups is low. This low barrier for isomer-

ization allows the product from a low-energy σ-bond metathesis

pathway to readily rearrange to an isomer that would form the

B-C bond in the final product.

Figure 9. Relative free energies (∆G) for the reaction of 2′ (blue solid

curve) and 3′ (red dashed curve) with CH₄. For clarity, dissociated molecules

and methane are omitted.

bond cleavage step by the less reactive starting complex,

bisboryl 2. If so, then the more favorable dissociation of borane

from the trisboryl complex 3 would lead to the higher overall

reactivity of 3 with alkanes (Figure 9), not a lower barrier for

cleavage of the C-H bond.

The transition state with the highest free energy on the

functionalization pathway is the transition state for B-C bond

formation (13′-TS and 19′-TS in Figure 9). Again, the barriers

for the individual B-C bond formation steps are calculated to

be higher for the more stable intermediate 7′ generated from

the trisboryl 3′. The barrier from 12′ to 13′-TS for reaction of

the intermediate generated from the bisboryl 2′ is 2.6 kcal mol^-1

lower than that from 18′ to 19′-TS for reaction of the

intermediate generated from the trisboryl 3′. Therefore, the

overall barrier for reaction of trisboryl 3′ is lower than that from

bisboryl 2′ because the dissociation of borane from 3′ is more

favorable than dissociation of borane from 2′ by an energy that

is larger than the difference in free energies of activation for

the overall C-H bond cleavage and B-C bond forming process

from the intermediates resulting from dissociation of borane.

c. Comparison with Other Pathways and Ligand Systems.

Recently, theoretical studies on the mechanism of the C-H

activation by rhodium boryl complexes similar to those de-

scribed in this paper were reported.⁴⁴These studies computed

pathways involving a trihydride monoboryl complex as the

starting species and dissociation of H₂to generate a boryl

hydride intermediate. Our experimental data imply that the

monoboryl trihydride complex is not present in the system and

that the monoboryl trihydride complex is less thermodynami-

cally stable than either 2 or 3, but the intermediate generated

from the monoboryl complex studied in this previous compu-

tational work would, nevertheless, contain one hydride and one

boryl ligand, as does the intermediate generated from 2. The

authors concluded that the process occurred by an oxidative

addition and reductive elimination pathway, but the presence

or absence of an intermediate from oxidative addition on the

pathway to C-H bond cleavage and B-C bond formation

depended on the computational method. Addition of polarization

functions to metal-ligated atoms produced different results.⁶⁶

In addition, the computations were conducted with BH₂, not

B(OR)₂, as the boryl ligand, and this difference in substituent

leads to a difference in mechanism, as described below.

The difference in the mechanism for reaction of the hypo-

thetical system with BH₂as the boryl group⁴⁴and for reaction

Thus, the choice of boron substituents for the calculation has

a large effect on the reaction pathway. Because the experimental

work is conducted with dioxaborolanyl groups, and complexes

with dialkylboryl ligands do not undergo the C-H activation

chemistry,⁴⁷the computational work that is most relevant to

the catalytic system is that conducted with dialkoxyboryl ligands.

Conclusion: A New Pathway for C-H Functionalization

Compounds that react through transition-metal boryl com-

plexes are, thus far, unique as small-molecule catalysts for the

(46) (a) Hartwig, J. F.; De Gala, S. R. J. Am. Chem. Soc. 1994, 116, 3661. (b)

Pandey, K. K. Inorg. Chem. 2001, 40, 5092.

(47) Waltz, K. M.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 11358.

9

J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005 2549

A R T I C L E S

Hartwig et al.

regiospecific, terminal functionalization of alkanes.^8,48Further,

several catalysts for the functionalization of arenes and benzylic

C-H bonds under mild conditions have been reported that react

through boryl complexes.^20,31,49Thus, the reactivity of transition-

metal boryl complexes toward C-H bonds is much different

from that of transition-metal alkyl or aryl complexes.

for C-H bond cleavage, stabilizes the product from C-H bond

cleavage, and likely facilitates the dissociation of borane to form

the species that undergoes C-H bond cleavage.

(3) The coupling to form a boron-carbon bond is rapid. From

the computational studies in this work, the elementary step of

B-C bond formation occurs with a low barrier, and previous

experimental work on the reactivity of transition-metal boryl

complexes implies that this reaction is fast.⁵³Because B-C bond

formation is rapid, the functionalization process rapidly follows

the C-H bond cleavage event. Yet, the computational work

suggests that the transition state for B-C bond formation lies

slightly above the transition state for C-H bond cleavage.

(4) The alkoxo substituents of the pinacolboryl groups appear

to modulate the strength of the B-H interactions. Judging from

previous computational work,⁴⁴stable hydridoborate structures

are adopted when the B-H interactions between a boryl group

and cis hydrides are strong. In this case, the barrier for

isomerization becomes large, and the geometry necessary for

B-C bond formation is not accessed by the intermediate

generated from σ-bond metathesis. Thus, the alkoxo groups

create electronic properties that allow for rapid isomerization

of the intermediate generated from σ-bond metathesis to a

second intermediate that contains mutually cis alkyl and boryl

groups.

Overall, our studies on rhodium boryl complexes in the

catalytic borylation of alkanes support a mechanism for C-H

activation first implied by recent data on stoichiometric reactions

of alkanes with iron and tungsten boryl complexes.^10,45,47,54Two

properties that lead to this chemistry in the catalytic borylation

of alkanes and arenes are an unoccupied orbital on boron and

measurable, but weak, interactions of the boryl ligand with

hydrides located cis to them. The formation of a strong B-C

bond provides the thermodynamic driving force,⁵⁵and the

unoccupied orbital creates fast rates for C-H bond cleavage

and formation of the functionalized product.

Our data suggest that the origin of this difference in reactivity

results from a new pathway for the reactions of boryl complexes

with the C-H bonds of alkanes and arenes. The computational

results imply that the transition-metal boryl complexes cleave

the C-H bonds of alkanes with the participation of the boron

p-orbital, either directly (as in 17′-TS or 11b-TS) or indirectly

(as in the changes from 11′-TS to 13′-TS). The transition state

and product of the C-H bond cleavage process are stabilized

by partial B-H bonding. This stabilization by partial X-H

bonding would be weaker during the activation of alkanes by

metal silyl complexes, and it would be absent during the

cleavage of C-H bonds by transition-metal alkyl complexes.

The σ-bond metathesis or [2 + 2] mechanism for the C-H

bond cleavage does bear some resemblance to the C-H

activation by transition-metal imido and carbene complexes.^3,50,51

Many of these complexes with metal ligand multiple bonds

activate hydrocarbons through [2 + 2] pathways,^3,50and metal

boryl complexes are isolobal with metal carbene complexes.

Although the metal-to-ligand back-bonding is much weaker in

the boryl complexes than in carbenes,^12,52the orbital interactions

during the C-H bond cleavage by boryl and either carbene or

imido complexes may be related.

The current work reveals several additional features of the

structure and reactivity of metal boryl compounds that contribute

to the facility of the catalytic functionalization.

(1) The dissociation of borane from a hydrido boryl complex

to generate a 16-electron intermediate occurs under mild

conditions. This dissociation is faster than the dissociation of

H₂from the bisboryl dihydride 2 or of B₂pin₂from the trisboryl

monohydride 3. The dissociation of borane from 2 also occurs

much faster than the dissociation of silane from the analogous

Cp*Rh(H)₂(SiEt₃)₂.¹⁷

Experimental Section

General Remarks. All manipulations were conducted under a

nitrogen atmosphere using standard Schlenk and glovebox techniques.

All NMR spectra were recorded on a Bruker AM-500 NMR spectrom-

eter. ¹H NMR chemical shifts are reported in parts per million relative

to the peak for residual protiated solvent as an internal reference. ¹¹B

NMR chemical shifts are reported relative to the peak for an external

standard of BF₃‚OEt₂. Pentane, octane, and cyclohexane were distilled

from sodium/benzophenone ketyl prior to use. Benzene-d₆and toluene-

d₈were dried over sodium/benzophenone ketyl and degassed prior to

use. All NMR spectra were recorded using benzene-d₆as solvent unless

otherwise noted. Cp*Rh(η⁴-C₆Me₆)¹⁶was prepared according to

literature procedures. B₂pin₂-d₂₄was prepared from BBr₃as published

for the preparation of B₂pin₂,⁵⁶except that the final step of the sequence

was conducted with pinacol-d₁₂. The pinacol-d₁₂was prepared by

reductive coupling of acetone-d₆as published for the reductive coupling

of acetone.⁵⁷All other chemicals were used as received from com-

mercial suppliers.

(2) The structures of the boryl complexes contain some B-H

bond character. This partial bonding stabilizes the transition state

(48) Chen, H.; Hartwig, J. F. Angew. Chem., Int. Ed. 1999, 38, 3391.

(49) (a) Cho, J. Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E.; Smith, M. R.

Science 2002, 295, 305. (b) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura,

N.; Anastasi, N.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390. (c)

Ishiyama, T.; Takagi, J.; Hartwig, J. F.; Miyaura, N. Angew. Chem., Int.

Ed. 2002, 41, 3056. (d) Ishiyama, T.; Nobuta, Y.; Hartwig, J. F.; Miyaura,

N. Chem. Commun. 2003, 2924. (e) Ishiyama, T.; Ishida, K.; Takagi, J.;

Miyaura, N. Chem. Lett. 2001, 1082.

(50) (a) Slaughter, L. M.; Wolczanski, P. T.; Klinckman, T. R.; Cundari, T. R.

J. Am. Chem. Soc. 2000, 122, 7953. (b) Schafer, D. F.; Wolczanski, P. T.

J. Am. Chem. Soc. 1998, 120, 4881. (c) Schaller, C. P.; Cummins, C. C.;

Wolczanski, P. T. J. Am. Chem. Soc. 1996, 118, 591. (d) Schaller, C. P.;

Wolczanski, P. T. Inorg. Chem. 1993, 32, 131. (e) Cummins, C. C.; Schaller,

C. P.; Vanduyne, G. D.; Wolczanski, P. T.; Chan, A. W. E.; Hoffmann, R.

J. Am. Chem. Soc. 1991, 113, 2985. (f) Cummins, C. C.; Baxter, S. M.;

Wolczanski, P. D. J. Am. Chem. Soc. 1988, 110, 8731. (g) Walsh, P. J.;

Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc. 1988, 110, 8729. (h)

Hoyt, H. M.; Michael, F. E.; Bergman, R. G. J. Am. Chem. Soc. 2004,

126, 1018. (i) Tran, E.; Legzdins, P. J. Am. Chem. Soc. 1997, 119, 5071.

(j) Adams, C. S.; Legzdins, P.; Tran, E. J. Am. Chem. Soc. 2000. (k)

Pamplin, C. B.; Legzdins, P. Acc. Chem. Res. 2003, 36, 223. (l) Wada, K.;

Pamplin, C. B.; Legzdins, P.; Patrick, B. O.; Tsyba, I.; Bau, R. J. Am.

Chem. Soc. 2003, 125, 7035.

Preparation of trans-Cp*Rh(H)₂(Bpin)₂(2). Cp*Rh(η⁴-C₆Me₆)

(250 mg, 0.625 mmol) was placed in a 20 mL scintillation vial and

(53) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. Angew.

Chem., Int. Ed. 1999, 38, 1110.

(54) Waltz, K. M.; Muhoro, C. N.; Hartwig, J. F. Organometallics 1999, 18,

3383.

(51) (a) Debad, J. D.; Legzdins, P.; Lumb, S. A.; Batchelor, R. J.; Einstein, F.

W. B. J. Am. Chem. Soc. 1995, 117, 3288. (b) Adams, C. S.; Legzdins, P.;

McNeil, W. S. Organometallics 2001, 20, 4939. (c) Ng, S. H. K.; Adams,

C. S.; Legzdins, P. J. Am. Chem. Soc. 2002, 124, 9380. (d) Wada, K.;

Pamplin, C. B.; Legzdins, P. J. Am. Chem. Soc. 2002, 124, 9681.

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2550 J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005

Rhodium Boryl Complexes in Alkane Functionalization

A R T I C L E S

dissolved in dry cyclohexane (5 mL). In a separate vial, HBpin (2.47

g, 19.3 mmol) was dissolved in cyclohexane (5 mL). The two solutions

were combined, transferred to a thin wall Pyrex bomb, and sealed under

1 atm of nitrogen. The red solution was irradiated at 10 °C for 24 h, or

until the red color of the starting solution had fully disappeared and a

clear gold solution was observed. Volatile materials were evaporated

under reduced pressure to leave a light tan powder, which was

transferred to a sublimation apparatus. Sublimation under dynamic

vacuum at 40 °C removed C₆Me₆and left 274 mg (0.554 mmol, 89%)

of a brown powder. The purity of the crude material (>95%)was

sufficient for further synthesis. Complex 2 was isolated in pure form

as light yellow crystals by the slow evaporation of a pentane solution

at -35 °C. Compound 2 is stable under a nitrogen atmosphere at -35

°C for weeks, but slowly decomposes at room temperature. Anal. Calcd

Hz), 11.4 (C₅(CH₃)₅), 9.1 (CH₃). ¹¹B NMR: δ 42.3 (br). ³¹P NMR: δ

41.3 (d, J_Rh-P) 168.8 Hz).

1

Typical Procedure for the Thermal Reaction of the Boryl

Complexes with Hydrocarbons. The metal boryl complexes (5-8 mg)

and dodecahydrotriphenylene (3-4 mg) were placed as solids in an

NMR sample tube equipped with an airtight valve and were dissolved

in 0.5 mL of neat hydrocarbon. Reactions in C₆D₆were heated at 90

°C, and reactions in alkanes were heated at 125 °C. The reactions were

1

followed by ¹¹B NMR and H NMR spectroscopy. Yields of product

1

were determined by either H NMR spectroscopy or GC. Response

factors for GC measurements were calculated from pure samples of

the octyl- and arylboronate esters.

Reaction of 2 and 3 with C₆D₆in the Presence of B₂pin₂-d₂₄. These

experiments were conducted as described for the reactions in the

absence of added B₂pin₂-d₂₄, except that B₂pin₂-d₂₄was added prior to

the solvent. A typical reaction contained complex 2 or 3 (9-10 mg),

B₂pin₂-d₂₄(5-6 mg, 1.1 equiv), and dodecahydrotriphenylene (4-5

mg) in 0.5 mL of neat C₆D₆. The ratio of labeled to unlabeled PhBpin

was determined from the areas of the two resolved peaks for PhBpin

and PhBpin-d₁₂in the GC trace. The assignments of these two peaks

were confirmed by GC/MS.

1

for C₂₂H₄₁B₂O₄Rh: C, 53.48; H, 8.36. Found: C, 53.21; H, 8.07. H

NMR: δ 2.04 (s, 15 H, C₅(CH₃)₅), 1.13 (s, 24 H, BO₂C₂(CH₃)₄), -11.9

1

(br d, 2H, J_Rh-H) 40.5 Hz, RhH). ¹³C NMR: δ 101.6 (C₅(CH₃)₅),

82.6 (BO₂C₂(CH₃)₄), 25.5 (BO₂C₂(CH₃)₄), 11.1 (C₅(CH₃)₅). ¹¹B NMR:

δ 40.4.

Preparation of Cp*Rh(H)(Bpin)₃(3). Complex 2 (226 mg, 0.457

mmol) was dissolved in neat HBpin (2.57 g, 20.1 mmol). The solution

was transferred to a 200 mL glass bomb, sealed under nitrogen, and

heated at 105 °C for 3.5 h. The volatile materials were evaporated from

the reaction mixture to leave a brown solid. The crude solid was

Reaction of 3 with Added HBpin. The reactions of 3 with benzene

and octane in the presence of HBpin were conducted as described for

the reactions in the absence of the additive, except that 10 equiv of

HBpin was added after the hydrocarbon. The conversion of the starting

1

analyzed by H NMR spectroscopy and found to contain 3 and 2 in a

1

boryl complex was determined by H NMR spectroscopy.

5:1 ratio. Following two crystallizations from a pentane solution at

-35 °C, 147 mg (0.234 mmol, 51%) of 3 was obtained as light yellow

crystals. Compound 3 is stable under a nitrogen atmosphere at -35 °C

for weeks but decomposes within hours at room temperature. Micro-

Typical Procedure for the Determination of the Conversion of

B₂pin₂and Concentration of Rhodium Boryl Complexes. B₂pin₂

(30.3 mg, 0.119 mmol), dodecahydrotriphenylene (11.6 mg, 0.048

mmol), and 16 mol % catalyst were placed in a glass vial and dissolved

in pentane (5.0 mL). Most of the solution (4.0 mL) was transferred to

a thick-walled glass bomb and sealed under N₂. The remaining solution

was divided into two portions for GC and ¹H NMR analysis. The bomb

was submerged in a circulating oil bath set at the appropriate

temperature (Cp*Rh(η⁴-C₆Me₆), 125 °C; 2, 105 °C; 3, 90 °C). At

intervals of 30 min, the bomb was removed from the oil bath and

quickly cooled by submersion in an ice-water bath. Two aliquots

(∼500 µL) of the solution were removed. The bomb was then resealed

under N₂and placed back into the oil bath. One aliquot was quenched

with 10 µL of H₂O, shaken vigorously, and allowed to sit for at least

10 min. This sample was then analyzed by GC, and the conversion of

B₂pin₂was determined by integration of the area of this material versus

dodecahydrotriphenylene internal standard. The pentane was evaporated

from the second aliquot, and the resulting solid was dissolved in C₆D₆.

The quantity of 2 and 3 was determined by ¹H NMR spectroscopy by

integration of the Cp* signals vs those of dodecahydrotriphenylene.

Reaction of Trisboryl 3 with H₂. Complex 3 (7.0 mg) was dissolved

in 0.4 mL of C₆D₁₂, and the resulting solution was placed in an NMR

tube equipped with a screw cap. The solution was frozen, and the

sample tube was evacuated and refilled twice with 1 atm of H₂. The

tube was sealed and heated in an oil bath at 40 °C. The ratio of trisboryl

1

analysis was not obtained due to the instability of the complex. H

NMR: δ 2.08 (s, 15 H, C₅(CH₃)₅), 1.19 (s, 36 H, BO₂C₂(CH₃)₄), -11.3

1

(br d, 2H, J_Rh-H) 39.0 Hz, RhH). ¹³C NMR: δ 102.9 (C₅(CH₃)₅),

82.3 (BO₂C₂(CH₃)₄), 25.7 (BO₂C₂(CH₃)₄), 11.0 (C₅(CH₃)₅). ¹¹B NMR:

δ 39.9.

Preparation of Cp*RhH(Bpin)P(p-C₆H₄Me)₃(4). Complex 2 (76.1

mg, 0.154 mmol) and P(p-C₆H₄Me)₃(46.9 mg, 0.154 mmol) were

placed in separate vials and dissolved in equal amounts of C₆H₁₂(2

mL). The solutions were combined in a glass bomb, sealed under

nitrogen, and heated at reflux for 4 h. Liberation of HBpin was

confirmed by ¹¹B NMR spectroscopy. The volatile materials were

evaporated from the reaction mixture to generate a brown oil.

Crystallization by slow evaporation of pentane yielded 52.6 mg (0.078

mmol, 51%) of yellow crystals. Anal. Calcd for C₃₇H₄₉BO₂PRh: C,

1

66.38; H, 7.37. Found: C, 66.69; H, 7.67. H NMR: δ 7.74 (m, 6H,

p-C₆H₄(CH₃)), 6.98 (m, 6H, p-C₆H₄(CH₃)), 2.05 (s, 9H, p-C₆H₄(CH₃)),

1.94 (d, ¹J_Rh-H) 1.1 Hz, 15H, C₅(CH₃)₅), 0.99 (s, 6H, BO₂C₂(CH₃)₄),

0.93 (s, 6H, BO₂C₂(CH₃)₄), -13.08 (t, ¹J_Rh-H) ²J_P-H) 33.8 Hz, 1H,

RhH). ¹³C NMR: δ 138.7 (d, p-C_aryl, ⁴J_C-P) 2.1 Hz), 135.2 (d, ipso-

1

2

C_aryl, J_C-P) 46.8 Hz), 134.5 (d, o-C_aryl, J_C-P) 11.8 Hz), 128.4 (d,

3

m-C_aryl, J_C-P) 10.8 Hz), 98.6 (C₅(CH₃)₅), 83.0 (BO₂C₂(CH₃)₄), 24.7

(BO₂C₂(CH₃)₄), 21.1 (C_arylCH₃), 11.0 (C₅(CH₃)₅). ¹¹B NMR: δ 42.3

1

3 to bisboryl 2 was measured by H NMR spectroscopy.

(br). ³¹P NMR: δ 58.7 (d, J_Rh-P) 179.1 Hz).

1

General Procedure for Structural Determination by X-ray

Diffraction. All data were collected using a Bruker-Nonius KappaCCD

diffractometer with graphite-monochromated Mo KR radiation

(λ ) 0.71073 Å). The structures were solved by direct methods and

refined by full-matrix least squares on F²using SHELXTL v. 6.12.⁵⁸

Non-hydrogen atoms were refined anisotropically. Hydrogen atoms,

with the exception of terminal hydrides, were treated as idealized

contributions. Full data collection parameters and structural data are

available as Supporting Information.

Preparation of Cp*Rh(Bpin)₂PEt₃(5). Complex 3 (75.1 mg, 0.120

mmol) and PEt₃(25.2 mg, 0.213 mmol) were placed in separate vials

and dissolved in equal amounts of C₆H₁₂(2 mL). The solutions were

transferred to a glass bomb, sealed under nitrogen, and heated at 70

°C for 2 h. Liberation of HBpin was confirmed by ¹¹B NMR

spectroscopy. The volatile materials were evaporated from the reaction

mixture to leave 5 as a yellow solid. Crystallization from pentane

solution at -35 °C yielded 56.4 mg (0.092 mmol, 77%) of yellow

crystals. Anal. Calcd for C₂₈H₅₄B₂O₄PRh: C, 55.11; H, 8.92. Found:

C, 55.08; H, 8.65. ¹H NMR: δ 2.06 (s, 15H, C₅(CH₃)₅), 1.78 (m, 6H,

CH₂), 1.19 (s, 12H, BO₂C₂(CH₃)₄), 1.17 (s, 12H, BO₂C₂(CH₃)₄), 1.03

(m, 9H, CH₃). ¹³C NMR: δ 100.0 (C₅(CH₃)₅), 81.0 (BO₂C₂(CH₃)₄),

Computational Details. In the calculations, the pentamethylcyclo-

pentadienyl (Cp*) is modeled by cyclopentadienyl (Cp), and Bpin is

truncated to the unsubstituted parent dioxaborolanyl ligand. All

1

26.3 (BO₂C₂(CH₃)₄), 25.9 (BO₂C₂(CH₃)₄), 19.3 (d, CH₂, J_C-P) 26.4

(58) Bruker AXS, Inc., Madison, WI 53719, 2001.

9

J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005 2551

A R T I C L E S

Hartwig et al.

gens, and all oxygens, while D95⁶⁵was used for all other carbons and

hydrogens. All structures were fully optimized, and analytical frequency

calculations were performed on each structure to ensure a minimum

or transition state was achieved. Zero-point energies were calculated,

and thermodynamic functions were computed for 298.15 K and 1 atm,

unless otherwise noted. Moreover, the activation free energies for

dissociation and association processes were determined as shown in

Figure 10.

Acknowledgment. We thank the National Science Foundation

for support of this work (Grant Nos. CHE 03-01907, CHE 02-

16275, and CHE 98-00184). K.S.C. thanks the NSERC for a

postdoctoral fellowship. M.H. thanks the Deutsche Forschungs-

gemeinschaft (DFG) for a postdoctoral fellowship. Work at

Texas A&M University was also supported by The Welch

Foundation (Grant A-648). The authors also thank Josiah

Manson for a Cerius²SDK implementation (in-house code

written by J. Manson, TAMU, 2002) of an interface between

Cerius²and POV-Ray (a ray tracer: POV-Team co/ Hallam

Oaks P/L, P.O. Box 407, Williamstown, Victoria, 3016 Aus-

tralia; http://www.povray.org) used in the production of Figures

3 and 5.

Figure 10. Determination of activation free energies for dissociation and

association processes.

calculations have been carried out using the Gaussian 98⁴⁰implementa-

tion of B3LYP [Becke three-parameter exchange functional (B3)⁵⁹and

the Lee-Yang-Parr correlation functional (LYP)⁶⁰]. A modified

version of Hay and Wadt’s LANL2DZ⁶¹was used for rhodium, where

the two outermost p functions have been replaced by a reoptimized

set, a (41) split.⁶²One d polarization and one p diffuse function⁶³were

added to LANL2DZ⁶¹for phosphorus. Dunning’s cc-pVDZ basis sets⁶⁴

were employed for all boron atoms, metal-bound carbons and hydro-

Supporting Information Available: Detailed procedures,

data, and CIF files for the structural determination of compounds

2-5. This material is available free of charge via the Internet

at http://pubs.acs.org.

JA045090C

(64) (a) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007. (b) Davidson, E. R.

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(65) Dunning, T. H.; Hay, P. J. Modern Theoretical Chemistry, Vol 3: Methods

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(66) For example, addition of polarization functions to metal-ligated atoms for

B3LYP calculations makes the geometry of 8 in ref 44 a stable species

and drastically changes many of the metric parameters for the MP2

calculated geometries. Fan, Y.; Webster, C. E.; Hall, M. B., unpublished

results.

(59) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.

(60) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.

(61) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt, W.

R.; Hay, P. J. J. Chem. Phys. 1985, 82, 299.

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9

2552 J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005

Article Doi

Rhodium boryl complexes in the catalytic, terminal functionalization of alkanes

DOI: 10.1021/ja045090c

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