A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane

pubs.acs.org/JACS

Article

A Series of Crystallographically Characterized Linear and Branched

σ‑Alkane Complexes of Rhodium: From Propane to

3‑Methylpentane

Alexander J. Bukvic,^#Arron L. Burnage,^#Graham J. Tizzard, Antonio J. Mart í nez-Mart í nez,

Alasdair I. McKay, Nicholas H. Rees, Bengt E. Tegner, Tobias Kr ä mer, Heather Fish, Mark R. Warren,

Simon J. Coles, Stuart A. Macgregor,* and Andrew S. Weller *

Cite This: J. Am. Chem. Soc. 2021, 143, 5106 −5120

Read Online

sı

*

Metrics & More

Article Recommendations

Supporting Information

ACCESS

ABSTRACT: Using solid-state molecular organometallic (SMOM)

techniques, in particular solid/gas single-crystal to single-crystal

reactivity, a series of σ-alkane complexes of the general formula

[Rh(Cy₂PCH₂CH₂PCy₂)(ηⁿ:η^m-alkane)][BAr^F] have been prepared

4

(alkane = propane, 2-methylbutane, hexane, 3-methylpentane; Ar^F=

3,5-(CF₃)₂C₆H₃). These new complexes have been characterized

using single crystal X-ray diﬀraction, solid-state NMR spectroscopy

and DFT computational techniques and present a variety of Rh(I)···

H−C binding motifs at the metal coordination site: 1,2-η²:η²(2-

methylbutane), 1,3-η²:η²(propane), 2,4-η²:η²(hexane), and 1,4-η¹:η²

(3-methylpentane). For the linear alkanes propane and hexane, some

additional Rh(I)···H−C interactions with the geminal C−H bonds are also evident. The stability of these complexes with respect to

alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane

that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in

a binding pocket deﬁned by the {Rh(Cy₂PCH₂CH₂PCy₂)}⁺fragment and the surrounding array of [BAr^F]⁻anions. For the propane

4

complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates

with the ﬂeeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the

complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically,

there therefore appears to be an optimal ﬁt between the size and shape of the alkane and overall stability. Such observations are

related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in

metalloenzymes.

necessarily coupled with the generation of a vacant site on

the metal center using ligand photoejection or protonation of a

metal−alkyl bond. Using these methodologies, σ-alkane

complexes from methane to dodecane have been gener-

ated.^13,14Chart 1 shows examples characterized using NMR

spectroscopy where methane,¹⁵propane,¹⁶cyclopentane,¹⁷

pentane,¹⁸2-methylbutane,¹⁹and 2,2-dimethylbutane²⁰act as

ligands.

1. INTRODUCTION

The coordination of alkanes with metal centers and their

subsequent C−H activation are central to chemical trans-

formations that add value to these simple feedstocks,¹such as

dehydrogenation,^2,3C−H functionalization,^4,5and isotopic

enrichment through H/D exchange.⁶Coordination with a

metal center prior to C−H bond cleavage occurs through 3c-2e

[M]···H−C interactions,⁷forming so-called σ-complexes.⁸For

alkanes, such complexes are challenging to generate and

observe under standard laboratory conditions. This is because

the strong, nonpolar, and relatively sterically congested C−H

bonds make alkanes very poor ligandsbinding to metal

centers often with bond enthalpies of 15 kcal/mol or less.⁹In

solution such complexes have only been observed using low-

temperature in situ NMR spectroscopy (lifetimes of

minutes),¹⁰or on very short time scales (lifetimes of

microseconds to seconds) using time-resolved infrared

(TRIR)¹¹or XAFS techniques.¹²These analyses are

These solution-based techniques provide unequivocal

evidence for alkane coordination at a metal center. However,

their use for the subsequent isolation of a crystalline material

Received: January 22, 2021

Published: March 26, 2021

American Chemical Society

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

5106

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

stability, providing an approximately octahedral microenviron-

ment around the cation, which supports the weak alkane

binding with the metal center through multiple noncovalent

interactions. This crystalline²⁷nanoreactor²⁸environment

allows for long-range order to be retained, local coordinate

ﬂexibility at the reactive site, and hydrophobic pathways

through the lattice from the CF₃groups.²⁹The retention of

crystallinity also allows for detailed characterization by single-

crystal X-ray diﬀraction and solid-state NMR spectroscopy

(SSNMR).

Chart 1. Examples of σ-Alkane Complexes Characterized

Using In Situ NMR Techniques

a

When these structural and spectroscopic data are combined

with an analysis of the electronic structures and noncovalent

environment using periodic-DFT techniques,³⁰a detailed

description of the bonding in these complexes is possible.

For example, σ-alkane complexes have been characterized in

which the alkane (e.g., isobutane) engages in two diﬀerent

η²:η²-C−H interactions with a Rh(I) center,³¹η¹:η¹-NBA at a

³{Co(I)} center,³²or η¹-cyclooctane with Rh(III) (Scheme

1B).³³The mobility and reactivity of the alkane ligand can also

be studied using combined experimental and computational

techniques: for example in H/D exchange,^31,34acceptorless

a

Anions are not shown.

that allows for detailed structural characterization using single-

crystal X-ray diﬀraction, or onward reactivity studies, has yet to

be realized.^21,22This is because rapid alkane displacement by a

solvent or a photogenerated ligand leads to lifetimes unsuitable

for solution-based crystallization techniques, a situation

compounded by the low temperatures used and less than

100% photoconversations achieved.

In response to this challenge, we have developed techniques

where competing ligands (solvent or otherwise) and photo-

generation of a vacant site are eliminated by using single-crystal

to single-crystal (SC-SC²³) solid/gas reactivity on molecular

organometallic precursors.²⁴We term this solid-state molecular

organometallic chemistry (SMOM).²⁵

dehydrogenation,³¹and ligand substitution processes.²⁵

A

systematic variation of the ligand and anion^33,35,36can lead to

systems that promote solid/gas SMOM catalysis: e.g., 1-butene

isomerization under a continuous ﬂow.³⁷

Despite these advances, a fundamental question is what are

the limits of the SMOM methodology in terms of the smallest

and largest alkane fragment that can be incorporated into the

solid-state microenvironment provided by the [BAr^F]⁻

4

anions? Exploring this chemical space would provide structural

data for the broadest set of σ-alkane complexes yet and also

probe comparative reactivity and stability proﬁles. In this

contribution we report the synthesis, structures, bonding, and

reactivity of four new σ-alkane complexes of the [Rh-

(Cy₂PCH₂CH₂PCy₂)]⁺fragment, ranging from propane to 3-

methylpentane (Scheme 2). For one, a 2-methylbutane

complex, a quantitative SC-SC acceptorless dehydrogenation

occurs at room temperaturean endothermic process that

normally requires high temperatures.³⁸

This approach is exempliﬁed by addition of H₂to the simple

precursor [Rh(Cy₂PCH₂CH₂PCy₂)(NBD)][BAr^F] ([1-

4

NBD][BAr^F]), which results in the quantitative formation

4

of [Rh(Cy₂PCH₂CH₂PCy₂)(NBA)][BAr^F] ([1-NBA]-

4

[BAr^F], NBD = norbornadiene, NBA = norbornane, Ar^F=

4

3,5-(CF₃)₂C₆H₃),²⁶(Scheme 1A). This σ-alkane complex is

remarkably stable, surviving months at 298 K under an Ar

atmosphere. The [BAr^F]⁻anions play a key role in this

4

2. RESULTS AND DISCUSSION

2.1. Synthesis and Solid-State Structures Using SC-SC

Techniques. 2.1.1. General Methodology. Our synthetic

methodology is one where 40−50 mg of the appropriate

crystalline monoalkene or diene precursor [Rh-

a

Scheme 1. (A) The SMOM Synthetic Route and (B)

Examples of Crystallography Characterized σ-Alkane

b

Complexes (B)

(CyPCH₂CH₂PCy₂)(alkene)][BAr^F] is treated with H₂(1

4

Scheme 2. σ-Complexes Prepared in This Contribution

Using SMOM Techniques

a

b

SC-SC = single crystal to single crystal. [BAr^F]⁻anions not shown.

4

5107

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

Figure 1. Solid-state molecular structures of the new cationic σ-alkane complexes (insets highlight Rh−alkane coordination) and synthetic details:

(A) [1-propane][BAr^F]; (B) [1-(2-methylbutane)][BAr^F]; (C) [1-hexane][BAr^F]; (D) [1-(3-methylpentane)][BAr^F] (L = 2-methyl-1,3-

4

pentadiene, 3-ethylbutadiene). Displacement ellipsoids are shown at the 30% probability level. Minor disordered components are not shown (see

the Supporting Information). Table 1 gives selected bond lengths and angles.

Table 1. Selected Bond Lengths and Angles for the New σ-Alkane Complexes

a

b

complex

Rh···C/Å

Rh−P/Å

C−C/Å

C−Rh−C/deg

PPRh−RhCC/deg

[1-propane][BAr^F]

2.46(2), 2.45(2)

2.348(9), 2.39(1)

2.527(3), 2.549(4)

2.430(4), 2.788(6)

2.206(4), 2.226(6)

2.187(2), 2.185(2)

2.2002(6), 2.1910(6)

1.54(2), 1.52(2)

1.60(1), 1.51(2)

1.511(5), 1.528(6)

1.504(7), 1.475(8)

61.3(8)

39.4(3)

58.5(1)

71.0(2)

3.7 (81.4³⁹

5.3 (73.1)

2.2 (83.7)

16.5 (75.7)

)

4

[1-(2-methylbutane)][BAr^F]

4

[1-hexane][BAr^F

]

4

[1-(3-methylpentane)][BAr^F

]

2.1959(7), 2.1965(7)

4

a

b

C−C distances associated with the hydrogenated alkene groups (Figure 1). Angle between planes deﬁned by P1P2Rh and RhCC (σ-interaction).

Angles in parentheses are for the equivalent measurement in the precursor alkene complexes.

atm) between 3 and 25 min at 298 K (optimized). This results

in formation of the alkane complex and a color change from

orange (alkene) or cherry red (diene) to plum red (alkane

complex). The rapid transfer of selected crystals to a precooled

diﬀractometer allows for structural analysis at 150 K. In

addition, an analysis of the bulk reaction sample was performed

by ³¹P{¹H} or ¹³C{¹H} solid-state NMR spectroscopy

(SSNMR) (section 2.2). This technique also allows for the

relative stability toward decomposition by loss of alkane, or

onward reactivity, of the σ-alkane complexes to be assessed at

298 K, by monitoring the evolution of the system with time.

For complexes that are particularly sensitive to alkane loss and

decomposition, synchrotron radiation at the Diamond Light

Source (Beamline I19) was combined with a bespoke gas cell

that allows for addition of H₂to a selected single crystal with

concurrent cooling (see the Supporting Information). Figure 1

shows the structurally characterized σ-alkane complexes and

the precursor alkene complexes used (which are fully described

in the Supporting Information). Table 1 gives selected

structural metrics.

2.1.2. Ethane. With selected single crystals of the bis-ethene

complex [Rh(Cy₂PCH₂CH₂PCy₂)(η²-H₂CCH₂)₂][BAr^F]

4

5108

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

as the starting material,²⁵addition of H₂with concurrent

cooling from 298 to 150 K in situ on the I19 Beamline resulted

in loss of diﬀraction. While the ethene is likely hydrogenated to

ethane under these conditions (vide infra), the subsequent σ-

alkane complex is not suﬃciently stable to allow for a structural

determination. This suggests a lower size limit for σ-alkane

complex formation using this metal/ligand/anion combination.

The interaction of ethane with metal centers has been

described using in situ solution NMR spectroscopy for

Mn(η⁵-C₅H₅)(CO)₂(ethane) (135 K)¹⁹and [Rh(PONOP)-

interaction) to one where the ligand now lies in the square

plane of the Rh(I) center, ligated to the metal through two

Rh···H−C σ-interactions. The C−C distances in the hydro-

carbon are consistent with single bonds, and the Rh···C

distances are ∼0.2 Å longer than in the starting propene

complex (Table 1).³⁹The propane binds in a 1,3-motif: Rh···C

2.46(2), 2.45(2) Å (calculated 2.50 and 2.51 Å, section 2.3),

and the central carbon (C2) is considerably farther away, being

nonbonding (Rh···C2 2.99(3) Å), further signaling a change

from the π-bound propene complex. Given the quality of the

data, we cannot rule out that the propane binds slightly

asymmetrically, as suggested by the DFT-calculated distances.

With this caveat, the Rh···C1 and Rh···C3 distances sit in the

range of those measured for other σ-alkane complexes (Table

(ethane)][BAr^F] (123 K)⁴⁰and in situ powder neutron

4

diﬀraction for the MOF M₂(dobdc) (M = Fe, Co; 4−10

K).⁴¹⁻⁴³In the bulk at 298 K the pale yellow anion-

coordinated zwitterion [1-BAr^F]²⁶(Chart 2) is formed

4

S2). For example, they are longer than in [1-NBA][BAr^F]

immediately on addition of H₂, as measured by ³¹P{¹H}

SSNMR. Ethane is also formed (gas-phase ¹H NMR

spectroscopy).

4

(2.389(3) and 2.400(3) Å²⁶) but shorter than in [1-

cyclohexane][BAr^F] (2.62(2) and 2.53(3) Å³¹), which have

4

1,2- and 1,3-alkane binding motifs, respectively. They are

considerably shorter than for d⁶-propane weakly bound to an

open Fe site in the MOF Fe₂(dobdc) (∼3 Å), as analyzed by

powder neutron diﬀraction at 4 K.⁴¹The 1,3-motif is similar to

that proposed for propane coordination to a PdO(101)

surface, albeit spanning two Pd sites.⁴⁵The propane σ-complex

Mn(η⁵-C₅H₅)(CO)₂(propane) has been characterized in

solution using in situ NMR spectroscopy at low temperature

(133 K).¹⁶Given the quality of the reﬁnement, hydrogen

atoms were not located, and so the hapticity of the Rh···H−C

Chart 2. Structure of [1-BAr^F]

4

2.1.3. Propane. The addition of H₂to the propene complex

[Rh(Cy₂PCH₂CH₂PCy₂)(η²:η²_C−H-H₂CCHCH₃)][BAr^F]

π

4

interaction in [1-propane][BAr^F] (e.g., η¹or η²) was

([1-propene][BAr^F])^25,39was studied on selected single

4

interrogated using computational techniques (section 2.3).

crystals in situ on the I19 Beamline. H₂was added while the

sample was being cooled from 298 to 150 K. This allowed for

rapid initial reaction with H₂and also slowed decomposition of

the alkane complex, once formed. The resulting complex,

The ∼O_henvironment of [BAr^F]⁻anions, which is

4

encoded³⁶in the propene starting complex,²⁵is retained in

[1-propane][BAr^F] (Figure S49) and is very similar to that

4

observed in [1-NBA][BAr^F] and [1-cyclohexane][BAr^F].

[Rh(Cy₂PCH₂CH₂PCy₂)(H₃CCH₂CH₃)][BAr^F] ([1-

4

propane][BAr^F]), is stable enough when it is formed under

However, these two alkane complexes are considerably more

stable than the propane congener with respect to the formation

4

these conditions to allow for a structural analysis at 150 K. In

of [1-BAr^F]. [1-NBA][BAr^F] is indeﬁnitely stable at 298 K,

the bulk [1-propane][BAr^F] decomposes rapidly (30 min) at

4

while [1-cyclohexane][BAr^F⁴] undergoes acceptorless dehy-

4

298 K to give [1-BAr^F], as measured by SSNMR (section

4

drogenation over 16 h to give [1-cyclohexadiene][BAr^F].

2.2)⁴⁴and shown visually by a color change from plum red to

pale yellow. Ex situ hydrogenation strategies thus result in

decomposition.

4

This change in stability, despite similar Rh···C distances, likely

reﬂects diﬀerences in the weak, multiple, stabilizing dispersive

interactions between the alkane and the surrounding micro-

environment provided in the solid state. These interactions are

explored in more detail in section 2.3.

Figure 1A shows the solid-state structure of [1-propane]-

[BAr^F]. The resulting structural reﬁnement gives a satisfactory

4

solution. While there is no disorder evident that would signal

2.1.4. Butane. Addition of H₂to the previously reported²⁵

the presence of unreacted [1-propene][BAr^F], we cannot rule

4

butadiene complex [1-butadiene][BAr^F] as a bulk single-

out the presence of a small amount of this still being present in

the unit cell, as indicated in the bulk by SSNMR (section 2.2).

4

crystalline material resulted in decomposition at 298 K to form

[1-BAr^F], as measured by ³¹P{¹H} SSNMR spectroscopy.⁴⁴

The formation of [1-propane][BAr^F] involves three consec-

4

While the in situ addition to a selected crystal of [1-

utive SC-SC transformations on bulk materials (40−50 mg)

butadiene][BAr^F] on the I19 Beamline with simultaneous

starting from [1-NBD][BAr^F]^25,26(eqs 1−3). This is

4

cooling to 150 K allowed for a structural reﬁnement of the

product, this was not of suﬃcient quality to unambiguously

conﬁrm whether a σ-alkane complex, or a partially hydro-

genated Rh(III) metallocyclopentane intermediate, is formed.

reﬂected in the relatively high residual (R(2σ) = 10.5%)

observeda consequence of the falloﬀ in high-angle data.

As found for [1-propane][BAr^F], the relative stability of the

4

targeted [1-butane][BAr^F] is considerably lower in compar-

4

ison with other σ-alkane complexes with the same {Rh(L₂)}⁺

fragment. This, again,^30,36hints at the importance of the

stabilizing noncovalent interactions between the alkane and the

secondary microenvironment, which is modiﬁed by changing

the shape and size of the alkane ligand. This hypothesis is

strengthened by noting that the previously reported branched

The generation of a σ-alkane complex in the single-

crystalline sample is signaled by a change in the binding

mode of the hydrocarbon: from propene (π-face/C−H agostic

isomer [1-isobutane][BAr^F]³¹(Scheme 1B) is stable at 298

4

K toward decomposition. Instead, this alkane complex

5109

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

undergoes acceptorless dehydrogenation over 4 h in an SC-SC

transformation. M(η⁵-C₅H₅)(CO)₂(butane) (M = Mn, Re)

complexes have been generated by in situ NMR photochemical

techniques in liquid butane at 136 K and have lifetimes of

minutes at this temperature.¹⁶

Scheme 3. Suggested SC-SC Reorganization Process for [1-

(2-methylbutane)][BAr^F] on Hydrogenation of [1-

4

a

isoprene][BAr^F]

4

2.1.5. 2-Methylbutane. Addition of H₂for 25 min to single

crystals of the precursor diene complex [1-isoprene][BAr^F]

4

forms [Rh(Cy₂PCH₂CH₂PCy₂){H₃CCH(CH₃)CH₂CH₃}]-

[BAr^F] ([1-(2-methylbutane)][BAr^F]) in a SC−SC trans-

4

formation (Figure 1B). While this complex is stable toward

decomposition at 298 K in the solid state, it slowly loses H₂(6

h) in an SC-SC acceptorless dehydrogenation under an Ar-

ﬂow, similar to the closely related [1-isobutane][BAr^F]³¹

4

(section 2.4).

a

The molecular structure of [1-(2-methylbutane)][BAr^F]

Computed relative energies for the two rotamers in the solid state

4

are given in kcal/mol.

demonstrates that the branched C5-alkane binds to the metal

center in a 1,2-motif, via methyl (C1) and methine (C2) Rh···

H−C interactions. All of the C−C bonds in the hydrocarbon

fragment are in the range associated with C−C single bonds

(Table 1 and Figure S51). The two Rh···C distances, 2.348(9)

and 2.39(1) Å (calculated 2.38 and 2.48 Å, section 2.3), sit at

the shorter end of the range observed with these Rh systems:

noncovalent interactions, which are provided by the ∼O_h

microenvironment of [BAr^F]⁻anions, play a signiﬁcant part

4

in this. When this inﬂuence of the secondary coordination

environment is removed, the relative stabilities of related σ-

complexes become leveled. For example Mn(η⁵-C₅H₅)-

(CO)₂(butane)¹⁶and Mn(η⁵-C₅H₅)(CO)₂(isobutane)¹⁹have

very similar lifetimes of ∼5−6 min at ∼135 K in liquid alkane

solvent.

e.g., [1-NBA][BAr^F]²⁶(2.389(3) and 2.400(3) Å) and [1-

4

isobutane][BAr^F]³¹(2.362(14) and 2.442(7) Å) (Table S2).

4

The C−C angles around C2 sum to 328.6°, supporting the

formation of an alkane ligand (sp³hybridization) on

hydrogenation. While the residual of R(2σ) = 9.6% may

reﬂect a small amount of superpositionally disordered alkene in

the unit cell, we were unable to sensibly model a secondary

alkene fragment being present (Figure 5 shows the

corresponding alkene structure that arises from acceptorless

2.1.6. Hexane. The precursor to a σ-complex of hexane is

the 2,4-hexadiene complex [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-

H₃CCHCHCHCHCH₃)][BAr^F] ([1-hexadiene]-

4

[BAr^F]) (Figure 1C and Figure S53). This complex is

4

isolated in single-crystal form as the symmetric 2,4-isomer but

in solution coexists in slow equilibrium with the 1,3-isomer

(see the Supporting Information). This likely occurs through

successive 1,3-hydride shifts⁴⁸via an allyl hydride intermediate.

dehydrogenation of [1-(2-methylbutane)][BAr^F]). Solution

4

trapping experiments (CD₂Cl₂) on the bulk sample immedi-

ately after hydrogenation recover 2-methylbutane with no

evidence for residual alkene. However, we cannot discount the

presence of a small amount of alkene complex in the unit cell

of the analyzed sample that may contribute to these apparently

shorter Rh···C distances.⁴⁶The hydrogen atoms were placed at

calculated positions, and a full discussion of the bonding with

the metal center is provided in section 2.3.

Addition of H₂to single crystals of [1-hexadiene][BAr^F]

4

results in hydrogenation of the diene in a SC-SC trans-

formation to form [Rh(Cy₂PCH₂CH₂PCy₂)(H₃C-

(CH₂)₄CH₃)][BAr^F] ([1-hexane][BAr^F]) (R(2σ) = 4.1%).

4

Figure 1C shows the resulting structure determined from a

single-crystal X-ray diﬀraction study. The hexane ligand binds

in a 2,4-motif (i.e., the Rh···H−C interactions are separated by

The 2-methylbutane ligand is not disordered, which is in

contrast with the precursor diene complex [1-isoprene]-

a methylene group as in [1-propane][BAr^F]), and all C−C

4

[BAr^F], which exists in the solid state as a 50:50 mixture of

distances in the hydrocarbon ligand are consistent with single

bonds (C−C range 1.460(8)−1.55(2) Å). The Rh····C

distances (2.527(3)/2.549(4) Å; calculated 2.54/2.62 Å,

section 2.3) are within the range observed for σ-alkane

complexes with {Rh(L₂)}⁺fragments (see Table S2). They are

4

superpositionality-imposed orientations of the diene that are

related by a noncrystallographically imposed C₂rotation

(Scheme 3 and Figure S52). ³¹P{¹H} SSNMR spectroscopy

of [1-(2-methylbutane)][BAr^F] also conﬁrms that a single

4

similar to those reported for [1-pentane][BAr^F], 2.514(4)

isomer is formed in the bulk sample (section 2.2). As

hydrogenation might be expected to initially form two diﬀerent

orientations of the bound 2-methylbutane ligand, we suggest

that a relatively low energy reorganization of the alkane ligand

is accessible to give the thermodynamically preferred

orientation, which is both observed and computed in the

solid state. This is likely a simple rotation. Low-energy

ﬂuxional processes for related σ-alkane complexes in the solid

state have reported for NBA,³⁴pentane,⁴⁷and cyclohexane

ligands.³¹We cannot discount alternative mechanisms in which

the stepwise hydrogenation accesses intermediates that result

in a single isomer being favored.

4

and 2.522(5) Å, which also binds in a 2,4-motif.⁴⁷The quality

of the data was suﬃcient to locate and reﬁne the hydrogen

atoms associated with the Rh···H−C interactions (R(2σ) =

4.1%). These data suggest that both methylene C−H groups

on each carbon are interacting with the metal center, although

to diﬀering degrees: i.e., Rh−H4B = 2.14(4) Å versus Rh−

H4A = 2.46(A) Å. These interactions are analyzed in the

computational section (section 2.3).

In the solid state the [BAr^F]⁻anions do not form an ∼O_h

4

arrangement, this being diﬀerent from the other complexes

discussed here: e.g., [1-(2-methylbutane)][BAr^F]. Instead, a

4

[1-(2-methylbutane)][BAr^F] (and the closely related [1-

bicapped square prism (BCSP) of anions accommodates two

4

isobutane][BAr^F]³¹) is stable toward alkane loss and

[Rh]⁺cations (Figure 2). This arrangement of anions has been

4

formation of [1-BAr^F], this being very diﬀerent from the

observed before for [1-pentane][BAr^F]⁴⁷and is encoded in

4

case for proposed [1-butane][BAr^F]. Again, we suggest that

the starting diene precursor (Figure S54).

4

5110

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

currently accessible single-crystalline examples of σ-alkane

complexes generated by SMOM techniques using this

combination of a metal fragment and anion.

2.2. Relative Stabilities in the Solid State as

Measured by Solid-State NMR Spectroscopy. In addition

to characterization of the new σ-alkane complexes using single-

crystal X-ray diﬀraction, ³¹P{¹H} and ¹³C{¹H} SSNMR

spectroscopy was used to characterize the reaction in the

bulk and assess their relative stabilities (Scheme 4). For all of

Scheme 4. Relative Stabilities of the σ-Alkane Complexes as

Bulk Solids (under Ambient Conditions)

Figure 2. Packing diagrams of [1-(2-methylbutane)][BAr^F] and [1-

4

hexane][BAr^F] (van der Waals radii) showing the diﬀerent

4

arrangements of the [BAr^F]⁻anions.

4

While a hexane σ-complex has not been directly

characterized in solution using NMR techniques, a related

heptane complex, W(CO)₅(heptane), has been observed using

time-resolved XAFS.¹²This allowed for the W···C distance to

be modeled at 3.07(6) Å for the σ-interaction. This is

the systems reported here a diagnostic downﬁeld shift is

observed in the ³¹P{¹H} SSNMR spectrum on formation of the

σ-alkane complex (δ 102−110) from the alkene precursor (δ

74−90). There is an increase in the J(RhP) coupling constant

on forming the σ-alkane complex, consistent with a more

weakly bound trans alkane ligand, i.e. 152−182 to 188−236

Hz, respectively. In the ¹³C{¹H} SSNMR spectrum signals due

to the coordinated alkene in the precursor (100−50 ppm)

disappear on hydrogenation. The decomposition product [1-

considerably longer than in [1-hexane][BAr^F], even given the

4

diﬀerence in covalent radii between W and Rh (0.2 Å).⁴⁹

2.1.7. 3-Methylpentane. The branched-hexane σ-alkane

complex [1-(3-methylpentane)][BAr^F] is formed from a

4

mixture of precursor isomeric alkene complexes [1-(3-methyl-

1,3-pentadiene)][BAr^F] and [1-(2-ethylbutadiene)][BAr^F]

4

(Figure 1D and Figure S55). In the solid state these

cocrystallize as a superpositionally disordered 50:50 mixture,

with an O_harrangement of [BAr^F]⁻anions. Addition of H₂to

BAr^F] is observed as a very broad signal, indicating loss of

4

these single crystals results in an SC−SC transformation and

the formation of a single isomer of the σ-complex [1-(3-

crystallinity, at δ ∼88.

The propane σ-complex is so unstable at room temperature

that on hydrogenation of the bulk precursor in situ only ∼30%

of the target alkane complexes is initially observed at 294 K. In

methylpentane)][BAr^F] (Figure 1E). The resulting structural

4

reﬁnement was of good quality (R(2σ) = 4.8%) and shows the

alkane to be interacting with the Rh center through methyl

(Rh···C1 2.430(4) Å) and methylene (Rh···C4 2.788(6) Å)

groups, in a 1,4-motif (calculated 2.46 and 2.89 Å, section 2.3).

On the basis of these distances, the former is likely a η²

interaction, while the latter is considerably longer, suggesting

η¹bonding.^33,50Hydrogen atoms were placed in calculated

positions. The ﬁne details of the bonding mode of the Rh···H−

C interactions are discussed in section 2.3. The single isomer

addition to [1-propane][BAr^F] a small amount of unreacted

4

[1-propene][BAr^F] is observed (∼10%), and the remainder is

4

[1-BAr^F]. After 30 min at 294 K there is no [1-propane]-

4

[BAr^F] observed. Thus, characterization rests on the in situ

4

analysis of a single crystal on the I19 beamline. [1-

hexane][BAr^F] is relatively more stable toward decomposi-

4

tion, being formed in ∼90% spectroscopic yield and

decomposing over 1 h to form [1-BAr^F]. This allows for

4

observed in [1-(3-methylpentane)][BAr^F] in the solid state

good ³¹P{¹H} and ¹³C{¹H} SSNMR NMR spectral data to be

4

suggests that a reorganization occurs on hydrogenation of the

recorded (Figures S28 and S29). [1-pentane][BAr^F]

4

two isomeric dienes present in the precursors. As for [1-(2-

decomposes over a comparable time scale (∼4 h).⁴⁷

methylbutane)][BAr^F], this can be explained by a simple

Once synthesized, [1-(2-methylbutane)][BAr^F] is stable

4

rotation of the bound alkane to form the thermodynamically

toward decomposition to form amorphous [1-BAr^F],

4

preferred isomer (ΔE_calc= +2.5 kcal/mol). The stability of [1-

although a small amount (∼10−20%) is formed during the

synthesis that arises from over-hydrogenation, as we have

commented on before.³¹Instead, an acceptorless SC-SC

(3-methylpentane)][BAr^F] is discussed in section 2.2.

4

2.1.8. Octane. The precursor to a potential octane σ-alkane

complex, [1-(octa-2,4-diene)][BAr^F], was synthesized and

dehydrogenation occurs to form [1-(methylbutenes)][BAr^F]

4

structurally characterized (Figure S57). This structural analysis

that is described in more detail in section 2.4. This means that

also showed that the [BAr^F]⁻anions adopt the same ∼O_h

a small amount (∼10%) of [1-(methylbutenes)][BAr^F] is

4

arrangement as for many of the other precursors discussed

here. However, addition of H₂resulted in an immediate loss of

also observed in the ﬁrst NMR spectrum that is taken after 10

min post H₂addition. In contrast, [1-(3-methylpentane)]-

diﬀraction and the formation of pale yellow [1-BAr^F],

[BAr^F] is not as stable in the solid state at 298 K. Although a

4

suggesting that octane is too large to support an SC-SC

σ-alkane complex is initially formed on hydrogenation (Figure

1D and Figure S38 and S39), this changes over 16 h (Figure

S40) to form a mixture of dehydrogenated methylpentene

transformation in this [BAr^F]⁻cavity. Thus, ethane and

4

octane deﬁne lower and upper bounds, respectively, for

5111

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

Figure 3. (A) Computed structure of the [1-alkane]⁺cations highlighting key distances (Å) and angles to H atoms. Non-alkane H atoms are

omitted for clarity. (B) QTAIM molecular graphs with bond critical bonds (BCPs) in green, ring critical points (RCPs) in pink, and selected BCP

electron densities, ρ(r), in au. Contour plots are in the plane containing Rh and the two H atoms involved in the σ-interactions. (C) Detail of the

NCI plots viewed from the Rh center looking down an axis passing through the center of the alkane moiety interacting with Rh. Isosurfaces are

generated for σ = 0.3 au and −0.07 < ρ < 0.07 au, and a key showing the color scheme employed is also provided. (D) Major donor−acceptor

interactions derived from a second-order perturbation NBO analysis (kcal/mol). Values are the sum of each type of donor−acceptor interaction

(e.g., there are up to four Rh_LP→σ*_C−Hand two σ_Rh−P→σ*_C−Hdonations; see the Supporting Information for full details).

complexes and [1-BAr^F] (section 2.4). Despite this, in

microenvironment, we turned to a computational analysis of

these new systems, as well as a comparison with those

previously reported. We initially discuss the primary

coordination sphere around the metal centers, which provides

a baseline for the subsequent analysis of the inﬂuence of the

wider environment.

4

comparison to [1-hexane][BAr^F], 1-(3-methylpentane)]-

4

[BAr^F] is considerably more stablelikely a consequence of

4

the branched alkane structure that modiﬁes interactions with

the anion microenvironment, and the diﬀerent motifs of anions

(Figure 2).

The stability of any particular alkane σ-complex toward

decomposition is likely to be strongly inﬂuenced by a

combination of the primary coordination sphere interactions

(i.e., the strength of the Rh···H−C bonds), stabilizing or

destabilizing interactions from the secondary microenviron-

ment, and diﬀerences in the tertiary, periodic, crystal structure.

To probe both the intimate interactions of the alkane with the

Rh(I) centers and the inﬂuence of the wider secondary

2.3. Computational Studies on the Primary and

Secondary Coordination Spheres. Further insights into

the structure and stability of the Rh σ-alkane complexes were

provided by periodic DFT calculations and electronic structure

analyses. The latter were based on the fully optimized solid-

state structures rather than the crystallographic data, and this

choice was prompted by the experimental uncertainties in

some of the alkane atom positions, notably in [1-propane]-

5112

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

[BAr^F]. For the other complexes the observed and fully

analysis suggests comparable σ-donation (13.8 kcal/mol), but

unusually the degree of back-donation (9.1 kcal/mol) now

approaches that of the σ-donation. The major Rh_LP→σ*_C−H

components exhibit π-character (Table S20), and this relatively

strong back-donation may be a feature of the wide bite angle of

the 1,4-alkane binding mode that is observed here for the ﬁrst

time as the thermodynamically preferred structure.⁴⁷σ-

Donation from the geminal C₁−H₁₂bonds is also somewhat

larger in [1-(3-methylpentane)]⁺(4.8 kcal/mol) than in [1-

propane]⁺(ca. 3.5 kcal/mol). In contrast, the C₄−H₄₁→Rh

interaction in [1-(3-methylpentane)]⁺is markedly diﬀerent

4

optimized structures provided very similar data, and both sets

of results are compared in the Supporting Information. In the

following discussion we ﬁrst assess the intramolecular Rh···H−

C σ-interactions, before probing the eﬀect of the extended

solid-state environment on the stability of both the Rh σ-

alkane complexes reported here and related complexes from

previous studies.

2.3.1. Computational Characterization of σ-Alkane

Hapticities. The alkane σ-complexes characterized here

present three diﬀerent binding motifs: 1,3-binding ([1-

propane][BAr^F]) and its equivalent 2,4-binding ([1-hexane]-

and exhibits an η¹

binding mode. This is most evident in

4

C−H

[BAr^F]), 1,4-binding ([1-(3-methylpentane)][BAr^F]), and

the NCI plot, which shows a localized blue disk along the Rh···

4

1,2-binding ([1-(2-methylbutane)][BAr^F]). The computed

H₄₁vector. The degree of σ-donation is close to that of the

4

structures of the [1-propane]⁺, [1-hexane]⁺, [1-(3-methyl-

pentane)]⁺and [1-(2-methylbutane)]⁺cations are shown in

Figure 3A along with the results of the quantum theory of

atoms in molecules (QTAIM), noncovalent interaction (NCI)

and natural bond orbital (NBO) analyses in Figure 3B−D,

respectively.

η²

interaction (12.6 kcal/mol), and a similar degree of

C1−H11

back-donation is also found (12.0 kcal/mol). However, in this

case back-donation is dominated by σ-donation from the

occupied σ_Rh−Porbitals into σ*_C−Hand this reﬂects a more

end-on approach of the C₄−H₄₁bond to the Rh center. The

diﬀerent η²

and η¹

binding modes are reﬂected in

C−H

The computed structure of [1-propane]⁺shows two short

Rh···H contacts (Rh···H₁₁2.03 Å; Rh···H₃₁2.00 Å) and

slightly elongated C₁−H₁₁and C₃−H₃₁bonds (1.13/1.14 Å)

that are indicative of two Rh→H−C σ-interactions. These are

conﬁrmed by the presence of Rh···H₁₁and Rh···H₃₁bond

paths in the QTAIM analysis that feature bond critical point

(BCP) electron densities, ρ(r), of 0.046 and 0.048 au,

respectively. The C₁−H₁₁/C₃−H₃₁BCPs also exhibit reduced

ρ(r) values of ca. 0.246 au, consistent with σ-donation to Rh

(cf. the spectator C₁−H₁₃/C₃−H₃₃bonds 1.10 Å; ρ(r) = 0.272

au). We have previously found NCI plots to be a good

indicator of C−H bond hapticity.³³In this case the stabilizing

blue features between the Rh center and the alkane that span

both the C₁−H₁₁and C₃−H₃₁bonds suggest an η²_C−Hbinding

mode. This is conﬁrmed by NBO calculations that quantify σ-

donation from the C₁−H₁₁and C₃−H₃₁bonds at 14.1 and

13.0 kcal/mol, respectively. This σ-donation is supported by

total back-donations of 8.7 and 7.6 kcal/mol, respectively. An

inspection of the Rh_LP→σ*_C−Hback-donation conﬁrms that

this is dominated by π-character (Figures S64 and S66). An

η²_C−Hbinding mode is also consistent with Rh−H−C angles of

ca. 102°:^33,50i.e., a “closed” M···H−C interaction.⁵¹

RhC₁H₁₁and RhC₄H₄₁angles of 101 and 139°, respectively.

The 1,2-bound alkane ligand in [1-(2-methylbutane)]-

[BAr^F] exhibits two chemically distinct C−H→Rh σ-

4

interactions involving 1° and 3° C−H bonds. Both exhibit

an η²_C−Hhapticity with NBO indicating that the 1° C₂−H₂₁→

Rh interaction is marginally the stronger of the two. In this

case the orientation of the C₂−H₂₂bond rules out any

additional geminal stabilization and this is reﬂected in the NCI

plot, where the blue stabilizing region runs parallel to the C₂−

H₂₁bond without extending toward H₂₂.

2.3.2. Comparison with Related σ-Alkane Complexes.

Previously, we have reported the structures of [1-isobutane]-

[BAr^F],³¹[1-NBA][BAr^F],²⁶[1-pentane][BAr^F],⁴⁷and [1-

4

cyclohexane][BAr^F].³¹The alkane ligands in the [1-

4

isobutane]⁺and [1-NBA]⁺cations both exhibit a 1,2-binding

mode that closely resembles that of the 2-methylbutane ligand

in [1-(2-methylbutane)]⁺. Moreover the 2,4-binding mode of

pentane in [1-pentane]⁺has features similar to those of the

alkane ligands in [1-propane]⁺and [1-hexane]⁺. These last

three linear alkanes all lie parallel to the {RhP₂} coordination

plane. In contrast, the cyclohexane ligand in [1-cyclohexane]⁺

sits perpendicular to this plane and this results in a binding

mode that is best described as intermediate between η²_C−Hand

η¹_C−H. The diﬀerent orientation of the cyclohexane also rules

out any stabilization from the geminal C−H bonds that was a

feature of the linear alkanes (see Figure S97).

In addition to these η²

interactions, some contribution

C−H

from the geminal C₁−H₁₂and C₃−H₃₂bonds is also evident.

Rh···H₁₂and Rh···H₃₂contacts of 2.39 and 2.43 Å are

computed, as well as C−H distances and BCP ρ(r) values that

are intermediate between those of the C₁−H₁₁/C₃−H₃₁and

C₁−H₁₃/C₃−H₃₃pairs. Although QTAIM does not identify

Rh···H₁₂or Rh···H₃₂bond paths, the NCI plot does show

extension of the stabilizing blue features over the C₁−H₁₂and

C₃−H₃₂bonds.⁵²An NBO analysis also identiﬁes σ-donation

from each bond (C₁−H₁₂→Rh = 3.3 kcal/mol; C₃−H₃₂→Rh =

3.6 kcal/mol) supported by weak back-donation in each case

of ca. 1.0 kcal/mol. Overall, these data suggest that the

2.3.3. Stability of the σ-Alkane Complexes. Scheme 4

summarizes the room-temperature stabilities of the σ-alkane

complexes reported here. In this context “stability” refers to the

lifetime of the σ-alkane complex before either (i) loss of the

alkane to give the [1-BAr^F] zwitterion and/or (ii)

4

dehydrogenation to an alkene complex. [1-propane][BAr^F]

4

and [1-hexane][BAr^F] fall into the ﬁrst category, forming the

4

zwitterion in 30−60 min. In contrast, [1-(2-methylbutane)]-

dominant η²

and η²

σ-interactions are supported

[BAr^F] and [1-(3-methylpentane)][BAr^F] undergo accept-

C1−H11

C3−H31

4

by additional stabilization from the geminal C₁−H₁₂and C₃−

orless dehydrogenation, implying a greater stability toward

H₃₂bonds. Thus, propane binds to Rh through two {CH₂}→

alkane lossalthough zwitterion formation is a competitive

Rh interactions that lie along the η²

to η²:η²

process with [1-(3-methylpentane)][BAr^F].

C−H

4

continuum.^13,50,53An analysis of the [1-hexane]⁺cation

indicates that a very similar situation pertains, although the

supporting geminal interactions are now somewhat weaker.

The C₁−H₁₁→Rh σ-interactions in the [1-(3-methylpen-

tane)]⁺cation can be interpreted in a similar way. An NBO

To probe these diﬀering, empirically determined behaviors,

we have computed ΔE₁, the normalized lattice energy (i.e.,

taking into account the number of formula units per unit cell),

and ΔE₂, the energy required to remove one alkane from the

unit cell. These provide a direct measure of the stability of the

5113

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

crystal lattice and of the strength of alkane binding within that

lattice, respectively. ΔE₃quantiﬁes the interaction energy

between the alkane ligand and [Rh(Cy₂PCH₂CH₂PCy₂)]⁺in

the isolated cation. Scheme 5 illustrates these terms for [1-

Table 2. Computed Normalized Lattice Energy (ΔE₁),

Energy Required to Remove on Alkane from the Unit Cell

(ΔE₂), and Interaction Energy between Alkane and Metal

a

Fragment (ΔE₃)

propane][BAr^F].

b

4

complex/microenvironment

ΔE₁

ΔE₂

ΔE₃

ΔE₄

[1-propane]⁺/O_h

110.4

118.0

119.1

120.0

118.9

119.7

121.5

119.4

34.0

40.6

45.4

39.2

c

47.1

45.6

48.3

25.7

28.2

31.7

27.8

30.7

33.1

30.7

31.4

8.3

Scheme 5. Energy Interaction Terms Computed to Assess

the Stability of the σ-Alkane Complexes

[1-cyclohexane]⁺/O_h

[1-(3-methylpentane)]⁺/O_h

[1-isobutane]⁺/O_h

12.5

13.6

11.4

c

14.0

14.9

16.8

[1-(2-methylbutane)]⁺/O_h

[1-NBA]⁺/O_h

[1-pentane]⁺/BCSA

[1-hexane]⁺/BCSA

d

a

All energies are in kcal/mol. [BAr^F] anions are not shown in the

4

b

c

formula. ΔE₄= ΔE₂−ΔE₃; The SCF energy of the apo-alkane unit

cell did not converge. BCSA denotes a bicapped square antiprism.

d

that this factor can be substantial. For example the molecular

binding energy of propane (ΔE₃= 25.7 kcal/mol) is enhanced

by almost 33% through intermolecular stabilization (ΔE₄= 8.3

kcal/mol). Incorporating such environmental eﬀects (be these

due to the solid state or solvent) is therefore essential in order

to provide a full picture of the factors aﬀecting σ-complex

stability.

Similar trends are seen on comparison of [1-isobutane]-

[BAr^F] and [1-NBA][BAr^F] (both 1,2-binding motifs) and

4

[1-pentane][BAr^F] vs [1-hexane][BAr^F] (2,4-binding mo-

4

tifs within a bicapped square antiprism of anions). In both

cases larger values of ΔE₂are computed for the larger alkane,

reﬂecting increased values of both ΔE₃and ΔE₄. In [1-

For each energy term all geometries were ﬁxed at those

hexane][BAr^F] the intermolecular stabilization now rises to

4

found in the fully optimized structures. In addition to the four

above 50% of the intramolecular binding energy. However,

these factors do not now translate into a larger lattice energy.

σ-complexes characterized here, [1-cyclohexane][BAr^F], [1-

4

isobutane][BAr^F], [1-pentane][BAr^F], and [1-NBA]-

More generally, although [1-propane][BAr^F] has the lowest

4

[BAr^F] have been added to the analysis. Like its hexane

values of ΔE₁and ΔE₂and this seemingly correlates with its

susceptibility to alkane loss and zwitterion formation, for the

larger alkanes no such relation is seen. Instead, these show

remarkably little variation in ΔE₁(120 2 kcal/mol), while

4

congener, the pentane complex loses the alkane to form the

zwitterion, whereas the cyclohexane and isobutane complexes

undergo room-temperature dehydrogenation. In contrast to all

the other σ-alkane complexes [1-NBA][BAr^F] is essentially

the ΔE₂values are comparable for [1-hexane][BAr^F] and [1-

4

indeﬁnitely stable when it is maintained under an inert

atmosphere. Thus, taken collectively, these σ-alkane complexes

provide a good basis to compare the underlying factors that

might inﬂuence stability in the solid state.

NBA][BAr^F] despite the much greater stability of the latter.

4

This lack of correlation reﬂects the diﬃculties in comparing

structures with diﬀerent anion arrangements in the lattice.

However, it may also point to the possibility that diﬀerential σ-

alkane complex stabilities are kinetic in origin rather than

thermodynamic.

Computed data are presented in Table 2 and are organized

by alkane binding mode and anion environment to bring

together the most directly comparable structures with the same

tertiary, periodic structure of anions. Some evidence for

increased stability with a larger alkane can be seen in the higher

2.3.4. Anion Microenvironment Eﬀects. We have pre-

viously commented on the role of nonclassical C−H^δ+···F^δ−−C

H-bonds in stabilizing σ-alkane complexes in the solid

state.^26,36Figure 4 highlights short contacts (at or below the

sum of the van der Waals radii⁵⁵) of this type, as well as C−

H···C contacts between the alkane H atoms and the

surrounding anions in the computed structures. For [1-

values of ΔE₁and ΔE₂computed for [1-cyclohexane][BAr^F]

4

vs [1-propane][BAr^F] (both 1,3-binding motifs). These

4

diﬀerences arise from a greater interaction not only within

the cation (ΔE₃) but also, more signiﬁcantly, with the

surrounding microenvironment, the latter being quantiﬁed by

ΔE₄(=ΔE₂− ΔE₃). Although, as discussed, there are some

subtle variations in the C−H→Rh σ-interactions, an additional

factor is likely to be the presence of stabilizing dispersive

interactions between the alkane ligand and both the cyclohexyl

substituents of the chelating phosphine and the surrounding

anionic framework. Intramolecular dispersive eﬀects have been

highlighted as playing a key role in σ-complex stability;⁵⁴

however, our study clearly highlights the role of the solid-state

environment in providing additional stabilization and the fact

propane][BAr^F] only two C−H···C contacts are present. In

4

[1-(2-methylbutane)][BAr^F] four C−H···C and three C−

4

H···F contacts are seen and these increase in number to ﬁve

and six, respectively, in [1-(3-methylpentane)][BAr^F]. The

4

C−H···F contacts are also apparent in NCI plots of the

proximal ion pairs (Supporting Information). Although it is

diﬃcult to quantitively compare these noncovalent inter-

actions, the paucity of such contacts in [1-propane][BAr^F]

4

does correlate with the instability of this system. However, the

presence of several C−H^δ+···F^δ−−C contacts is not a suﬃcient

5114

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

1

metal center, and an analysis by H NMR spectroscopy shows

that 2-methylbut-1-ene and 2-methylbut-2-ene are formed in a

1.4:1 ratio under these conditions.

Following the overall process from the initial formation of

[1-(2-methylbutane)][BAr^F] to the products of dehydrogen-

4

ation using ³¹P{¹H} SSNMR spectroscopy shows that the

reaction is remarkably clean (Figure 5A). At room temperature

Figure 4. Details of the alkane binding pocket in (A) [1-

propane][BAr^F], (B) [1-(2-methylbutane)][BAr^F], and (C) [1-

4

(3-methylpentane)][BAr^F] highlighting alkane···⁴anion C−H···C

4

and C−H···F contacts below the sum of the van der Waals radii.⁵⁵

(D) Key and the total number of each short contacts for each species.

condition for stability; thus, [1-(2-methylbutane)][BAr^F] is

4

stable to decomposition and undergoes acceptorless dehydro-

genation, whereas [1-(3-methylpentane)][BAr^F] (with twice

4

the number of C−H···F contacts) is susceptible to

decomposition.

Overall, several factors appear to be at play in controlling σ-

alkane complex stability in the solid state: the strength of the

intramolecular Rh···H−C interactions, the extent of inter-

molecular interactions in the microenvironment, and the ﬁt of

the alkane to the binding pocket. In addition, the kinetics of

Figure 5. (A) ³¹P{¹H} SSNMR spectra of the sequential SC−SC

transformation of [1-isoprene][BAr^F] to [1-(2-methylbutane)]-

alkane displacement by the incoming [BAr^F]⁻anion may also

4

[BAr^F] and [1-(2-methylbut-1-ene)][BAr^F]/[1-(2-methylbut-2-

4

be a factor and this will itself also be related to the

microenvironment and the periodic, tertiary structure of the

anion framework.

ene)][BAr^F]. Asterisks indicate an unidentiﬁed product. (B) Solid-

4

state molecular structures of the dehydrogenation products. Both

50:50 disordered components are shown, with 30% displacement

ellipsoids. Selected bond distances (Å) and angle (deg) for [1-(2-

2.4. Acceptorless SC-SC Dehydrogenation in the

methylbut-1-ene)][BAr^F]: Rh−C1, 2.23(2); Rh−C2, 2.12(2); Rh−

Solid State of 2-Methylbutane and 3-Methylpentane.

4

In the solid state [1-(2-methylbutane)][BAr^F] undergoes an

4

C3, 2.36(2); C1−C2, 1.39(3); C2−C5, 1.53(3); C2−C3, 1.52(3); ∑

SC-SC acceptorless dehydrogenation to form a mixture of the

angles around C2, 359.9. Selected bond distances (Å) and angle (deg)

two monoalkene isomers [1-(2-methylbut-1-ene)][BAr^F]

for 1-(2-methylbut-2-ene)][BAr^F]: Rh−C2A, 2.11(2); Rh−C3A,

4

and [1-(2-methylbut-2-ene)][BAr^F] (Scheme 6). This

2.3(2); Rh−C1A, 2.41(3); C1A-C2A, 1.54(3); C2A-C5A, 1.55(4);

C2A-C3A, 1.37(3); ∑ angles around C2A, 359.9°.

4

occurs under an Ar ﬂow (6 h, unoptimized) or vacuum (2 ×

10⁻²mbar, 4 h) to remove H₂. Addition of CO at 298 K to the

resulting single crystals liberates the bound alkenes from the

the ¹³C{¹H} SSNMR spectrum of the dehydrogenation

products is featureless in the alkene region (100−50 ppm).

However, cooling to 158 K reveals three (1 + 1 + 2) alkene

environments. This suggests that in the solid state there is a

low-energy isomerization process occurring, as described for

Scheme 6. Acceptorless Dehydrogenation of [1-(2-

methylbutane)][BAr^F] and Trapping of Alkene Products

4

[1-propene][BAr^F], likely operating via an allyl/hydride

4

intermediate.^25,36

This dehydrogenation is an SC-SC process, and the resulting

structural reﬁnement of the alkene products (R(2σ) = 8.6%) is

of suﬃcient quality to unambiguously determine the formation

of a 50:50 mixture of superpositional isomers of [1-(2-

methylbut-1-ene)][BAr^F] and [1-(methylbut-2-ene)]-

4

[BAr^F] (Figure 5B) at 150 K. The formation of an alkene

4

ligand is signposted by coordination of a π-face of the ligand,

sp²geometries of the salient carbon atoms, and a

corresponding short C−C distance in each isomer. Each

monoene ligand also engages in a supporting Rh···H−C

5115

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

agostic interaction (Rh···C3, 2.36(2) Å; Rh···C1A, 2.41(3) Å).

These are revealed in the 183 K ¹H NMR spectrum in CD₂Cl₂

solution by the observation of resonances in the alkene region

and signals diagnostic of agostic interactions at δ −0.25 and

−1.23 (two overlapping signals); the latter are assigned to the

diastereotopic agostic interactions from methylene C3 that

results in two diﬀerent agostomers.⁵⁶The ³¹P{¹H} NMR

spectrum shows resonances due to three Rh(I) complexes,

each with inequivalent phosphines. At 298 K this becomes a

single environment with coupling to ¹⁰³Rh, and alkene/agostic

signals in the ¹H NMR spectrum are lost, indicating a ﬂuxional

process at this temperature, likely a rapid alkene isomerization,

as proposed in the solid state.

decomposition product [1-BAr^F] grow in considerably

4

(∼35%). For this reason a reaction progress analysis using

the JMAK approach was not appropriate. The presence of [1-

BAr^F] and other decomposition products also meant that the

4

solution characterization was not unambiguous. Dissolution of

the solid in C₆D₆/d₆-acetone liberated the bound alkenes from

the metal center by forming the corresponding benzene adduct

of Rh(I).³¹¹H NMR spectroscopy and ESI-MS data show that

these alkenes are a mixture of isomers of methylpentene

(Scheme 7)conﬁrming that an acceptorless dehydrogenation

in [1-(3-methylpentane)][BAr^F] has occurred.

4

Scheme 7. Acceptorless Dehydrogenation of [1-(3-

methylpentane)][BAr^F] and Trapping of Alkene Products

Kinetic data for this dehydrogenation process were collected

by running individual reactions using batches of ﬁnely ground

4

[1-(2-methylbutane)][BAr^F] and quenching by dissolving

4

them in CD₂Cl₂. The relative ratio of [1-BAr^F] (from

4

decomposition of [1-(2-methylbutane)][BAr^F]) versus [1-

4

(2-methylbut-1-ene)][BAr^F]/[1-(2-methylbut-2-ene)]-

4

[BAr^F] was measured using ³¹P{¹H} NMR spectroscopy

4

(with an internal reference) (Figure 6).⁵⁷These data were

The acceptorless dehydrogenation of alkanes to alkenes is an

important industrial process that requires high temperatures

and a heterogeneous catalyst, as it is an endothermic process.⁶³

Using molecular organometallic systems it can be driven

64

catalytically by removing H₂or working in the solid phase

under continuous-ﬂow conditions.^38,65We have recently

demonstrated that spontaneous, albeit stoichiometric, dehy-

drogenation and H₂loss occur in the well-deﬁned σ-alkane

Figure 6. Modiﬁed JMAK plot of conversion versus time for the

dehydrogenation of in situ prepared [1-(2-methylbutane)][BAr^F] to

4

F

●

[1-methylbutenes][BAr ]: (···) ﬁt; ( ) experimental data. k

complexes [1-isobutane][BAr^F] and [1-cyclohexane]-

4

denotes the growth rate constant and n the Avrami exponent.

[BAr^F] to form isobutene and cyclohexadiene complexes,

4

respectively.³¹This demonstrates that, if pre-equilibria for

alkane binding at a metal center is biased to the σ-alkane

complex, dehydrogenation is kinetically a rather straightfor-

ward process. While the coordination of the alkene product

makes these processes more thermodynamically favorable than

for the free alkane/alkene, calculations show that they are still

slightly endergonic.³¹The same concept operates here but is

now extended to methylbutane and methylpentane alkane

ligands. The mechanism for dehydrogenation (which is also

the microscopic reverse of hydrogenation), as described in

modeled using modiﬁed Johnson−Mehl−Avrami−Kologoro-

mov (JMAK) kinetics. This approach describes reaction

progress in the solid state in terms of a nucleation and growth

model, where k is the growth rate constant and n is the Avrami

exponent.⁵⁸JMAK analysis has been used to describe SC-SC

photoreactions in the solid state.⁵⁸⁻⁶¹For the process here k =

9.5 × 10⁻⁴( 6 × 10⁻⁵) s⁻¹and n = 0.50

0.02. Avrami

exponents close to n = 4, 3, and 2 are suggestive of 3-D, 2-D,

and 1-D growth, respectively, while n = 1 is indicative of a

noncooperative transformation that occurs throughout the

crystal. It has been suggested that noninteger Avrami

constants, such as those observed here, point to the kinetics

being diﬀusion controlled.⁶²This could be related to a reaction

front (i.e., H₂loss) that moves through the crystal from outside

to inside. A JMAK analysis of the dehydrogenation of [1-

detail for [1-cyclohexane][BAr^F],³¹likely proceeds via initial

4

C−H oxidative cleavage followed by β-H elimination and loss

of H₂, closely related to solution-based dehydrogenation

systems.²

CONCLUSIONS

■

isobutane][BAr^F] also has n ≈ 0.5,³¹suggesting that this may

By using the {Rh(Cy₂PCH₂CH₂PCy₂)}⁺metal fragment 1

4

be a more general observation for this type of reactivity in the

single crystal. Avrami exponents of n ≈ 0.5 have been

measured for other SC-SC processes.⁵⁹

with a supporting anionic framework of [BAr^F]⁻anions, we

4

have prepared a series of C₃−C₆linear and branched σ-alkane

complexes using single-crystal to single-crystal solid/gas

transformations from the corresponding alkene precursors. In

combination with our previous studies using the same metal/

ligand/anion combination, this provides structurally charac-

terized σ-alkane complexes of propane, isobutane,³¹pentane,⁴⁷

2-methylbutane, hexane, cyclohexane,³¹3-methylpentane, and

norbornane.^26,34These complexes display a wide variety of

A similar dehydrogenation process occurs for [1-(3-

methylpentane)][BAr^F] in the single crystal over the course

4

of 16 h under a dynamic vacuum (10⁻²mbar) to form a

mixture of methylpentene isomers bound to Rh(I): [1-

(methylpentenes)][BAr^F]. However, this is not an SC-SC

4

process; the crystallinity is lost, and signals due to the

5116

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

Rh(I)···H−C binding motifs at the metal coordination site:

1,2-η²:η², 1,3-η²:η², 1,4-η¹:η², and intermediate η¹/η². In

addition methyl, methylene, and methine C−H groups are

all shown to be involved in bonding to Rh. Detailed DFT,

QTAIM, NCI, and NBO computational studies on the isolated

cations reveal additional subtleties of the Rh···H−C binding

modes, including the contribution of weaker geminal C−H···

Rh interactions with the linear alkanes.

complexes are generated that then undergo remarkable

reactivity, such as the SC-SC acceptorless dehydrogenation

for [1-(2-methylbutane)][BAr^F] described here or elsewhere

4

for [1-isobutane][BAr^F],³¹and selective H/D exchange in

4

[1-NBA][BAr^F].³⁴This combination of primary active site

4

(alkane bonding), secondary structure (anion microenviron-

ment), and the tertiary, periodic, motif of the anions has

parallels with enzymatic systems, where equivalent environ-

ments and structures work holistically to control the reactivity,

selectivity, and stability.^68,69It will be interesting to see

whether ﬁne control of these elements in SMOM systems, by a

judicious choice of metal/ligand fragment, anion structure, and

packing motifs, encourages the binding of even more weakly

binding alkanes, such as ethane and methane.

The new alkane complexes show a range of stabilities with

respect to alkane loss in the solid state. Thus, [1-propane]-

[BAr^F] and [1-hexane][BAr^F] both lose the alkane within

4

30−60 min to form the zwitterionic complex [1-BAr^F]. In

4

contrast, [1-(2-methylbutane)][BAr^F] is stable toward alkane

4

loss and instead undergoes acceptorless alkane dehydrogen-

ation to form a bound alkene complex, while for [1-(3-

methylpentane)][BAr^F] this dehydrogenation dominates

ASSOCIATED CONTENT

4

■

with zwitterion formation being a competing process.

Computed alkane bonding energies show that the interaction

between the alkane ligand and the {Rh(Cy₂PCH₂CH₂PCy₂)}⁺

fragment within the isolated cation (ΔE₃) does not by itself

reﬂect the empirically observed relative stabilities toward loss

of the alkane in the solid state. For example, [1-propane]-

[BAr^F] and [1-cyclohexane][BAr^F] show dramatically

sı

* Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/jacs.1c00738.

Full details of experimental methods, characterization

data, details of computational methods and experiments,

and single-crystal X-ray diﬀraction collection and

reﬁnement data (PDF)

4

diﬀerent stabilities but rather similar values of ΔE₃. Thus,

while an analysis that is restricted to the primary coordination

sphere provides detailed insight into the bonding at the metal

center, it does not capture the stability of these σ-alkane

complexes in the solid state. The secondary coordination

sphere due to the microenvironment provided by the anionic

Computed Cartesian coordinates (XYZ)

Accession Codes

CCDC 2056857−2056860 and 2056862−2056866 contain

the supplementary crystallographic 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.

framework of [BAr^F]⁻counterions must also be taken into

4

account. This is shown to provide considerable additional

stabilization and hence signiﬁcantly increases the alkane

binding energy within the lattice (ΔE₂). A somewhat better

correlation between the energy associated with microenviron-

ment stabilization (ΔE₄) and empirical stability is seen. This

arises from both dispersive interactions and more localized C−

H···H−X (X = C, F) contacts, which are maximized with

larger, and branched, alkane ligands. However, there is also an

upper limit. Too large an alkane also results in decomposition

of the complex, despite a numerical increase in C−H···H−X

contacts. This suggests that there are ideal conditions for the ﬁt

of the alkane into the anion binding pocket. Highlighting this,

AUTHOR INFORMATION

■

Corresponding Authors

Stuart A. Macgregor − Institute of Chemical Sciences, Heriot-

Watt University, Edinburgh EH14 4AS, U.K.; orcid.org/

0000-0003-3454-6776; Email: s.a.macgregor@hw.ac.uk

Andrew S. Weller − Department of Chemistry, University of

York, York YO10 5DD, U.K.; orcid.org/0000-0003-

1646-8081; Email: andrew.weller@york.ac.uk

[1-NBA][BAr^F], [1-isobutane][BAr^F₄], and [1-(2-

4

methylbutane)][BAr^F] are remarkably stable, and these

4

alkanes clearly ﬁnd a good ﬁt in the binding pocket and

receive signiﬁcant stabilization from the microenvironment,

with ΔE₄contributing up to an additional 40% to the total

alkane binding energies in these cases. In some respects this is

related to the optimal guest volume to host volume ratio for

supramolecular systems in solution,⁶⁶where noncovalent

interactions are crucial in determining structure and

reactivity.⁶⁷

Authors

Alexander J. Bukvic − Department of Chemistry, University of

York, York YO10 5DD, U.K.; Department of Chemistry,

Chemistry Research Laboratories, University of Oxford,

Oxford OX1 3TA, U.K.; orcid.org/0000-0002-5717-

2474

Arron L. Burnage − Institute of Chemical Sciences, Heriot-

Watt University, Edinburgh EH14 4AS, U.K.; orcid.org/

0000-0001-7136-2402

Graham J. Tizzard − UK National Crystallography Service,

University of Southampton, Southampton SO17 1BJ, U.K.

Antonio J. Martínez-Martínez − Department of Chemistry,

Chemistry Research Laboratories, University of Oxford,

Oxford OX1 3TA, U.K.; orcid.org/0000-0002-0684-

1244

Alasdair I. McKay − Department of Chemistry, Chemistry

Research Laboratories, University of Oxford, Oxford OX1

3TA, U.K.; orcid.org/0000-0002-6859-172X

In addition, kinetic factors, which are even more challenging

to deconvolute in molecular solid-state systems, will no doubt

aﬀect the stability. While more speculative, a comparison

between ∼O_hand bicapped-square-prism (BCSP) arrange-

ments of anions is instructive here. For example, [1-

cyclohexane][BAr^F] and [1-hexane][BAr^F] both enjoy

4

signiﬁcant stabilization from the microenvironment, but the

latter has a BCSP tertiary structure of anions and is

signiﬁcantly less stable with regard to decomposition.

When the balance of these factors is just right (the

“Goldilocks” conditions), room-temperature stable σ-alkane

5117

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

Catalysis; Perez, P. J., Ed.; Spinger: Dordrecht, The Netherlands,

2012; Vol. 38, pp 1−15.

Nicholas H. Rees − Department of Chemistry, Chemistry

Research Laboratories, University of Oxford, Oxford OX1

3TA, U.K.

Bengt E. Tegner − Institute of Chemical Sciences, Heriot-Watt

University, Edinburgh EH14 4AS, U.K.; orcid.org/0000-

0002-6880-9741

Tobias Krämer − Institute of Chemical Sciences, Heriot-Watt

University, Edinburgh EH14 4AS, U.K.; orcid.org/0000-

0001-5842-9553

Heather Fish − Department of Chemistry, University of York,

York YO10 5DD, U.K.

Mark R. Warren − Diamond Light Source Ltd., Didcot OX11

0DE, U.K.

Simon J. Coles − UK National Crystallography Service,

University of Southampton, Southampton SO17 1BJ, U.K.;

orcid.org/0000-0001-8414-9272

(10) Ball, G. E. In Situ Photochemistry with NMR Detection of

Organometallic Complexes. In Spectroscopic Properties of Inorganic and

Organometallic Compounds: Techniques, Materials and Applications;

The Royal Society of Chemistry: 2010; Vol. 41, pp 262−287.

(11) Cowan, A. J.; George, M. W. Formation and Reactivity of

Organometallic Alkane Complexes. Coord. Chem. Rev. 2008, 252,

2504−2511.

(12) Bartlett, S. A.; Besley, N. A.; Dent, A. J.; Diaz-Moreno, S.;

Evans, J.; Hamilton, M. L.; Hanson-Heine, M. W. D.; Horvath, R.;

Manici, V.; Sun, X.-Z.; Towrie, M.; Wu, L.; Zhang, X.; George, M. W.

Monitoring the Formation and Reactivity of Organometallic Alkane

and Fluoroalkane Complexes with Silanes and Xe Using Time-

Resolved X-ray Absorption Fine Structure Spectroscopy. J. Am. Chem.

Soc. 2019, 141, 11471−11480.

(13) Hall, C.; Perutz, R. N. Transition Metal Alkane Complexes.

Chem. Rev. 1996, 96, 3125−3146.

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.1c00738

(14) Weller, A. S.; Chadwick, F. M.; McKay, A. I.; Pérez, P. J.

Transition Metal Alkane-Sigma Complexes: Synthesis, Character-

ization, and Reactivity. Adv. Organomet. Chem. 2016, 66, 223−276.

(15) Bernskoetter, W. H.; Schauer, C. K.; Goldberg, K. I.; Brookhart,

M. Characterization of a Rhodium(I)-Methane Complex in Solution.

Science 2009, 326, 553−556.

Author Contributions

^#A.J.B. and A.L.B. contributed equally to the experimental and

computational aspects, respectively. The manuscript was

written through contributions of all authors.

(16) Calladine, J. A.; Duckett, S. B.; George, M. W.; Matthews, S. L.;

Perutz, R. N.; Torres, O.; Vuong, K. Q. Manganese Alkane

Complexes: An IR and NMR Spectroscopic Investigation. J. Am.

Chem. Soc. 2011, 133, 2303−2310.

Notes

The authors declare no competing ﬁnancial interest.

(17) Geftakis, S.; Ball, G. E. Direct Observation of a Transition

Metal Alkane Complex, CpRe(CO)₂(cyclopentane), Using NMR

Spectroscopy. J. Am. Chem. Soc. 1998, 120, 9953−9954.

(18) Yau, H. M.; McKay, A. I.; Hesse, H.; Xu, R.; He, M.; Holt, C.

E.; Ball, G. E. Observation of Cationic Transition Metal-Alkane

Complexes with Moderate Stability in Hydrofluorocarbon Solution. J.

Am. Chem. Soc. 2016, 138, 281−288.

ACKNOWLEDGMENTS

■

This paper is dedicated to Professor Robin N. Perutz on the

occasion of his 70th birthday. We thank the EPSRC (EP/

M024210, and the UK National Crystallography Service), the

Leverhulme Trust (RPG-2015-447), and SGC Chemicals for

funding, T. M. Boyd (York) for experimental assistance and

useful discussions, and Dr. M. Chadwick (Imperial College)

(19) Torres, O.; Calladine, J. A.; Duckett, S. B.; George, M. W.;

Perutz, R. N. Detection of σ -Alkane Complexes of Manganese by

NMR and IR Spectroscopy in Solution: (η⁵-C₅H₅)Mn(CO)₂(ethane)

and (η⁵-C₅H₅)Mn(CO)₂(isopentane). Chem. Sci. 2015, 6, 418−424.

(20) Calladine, J. A.; Torres, O.; Anstey, M.; Ball, G. E.; Bergman, R.

G.; Curley, J.; Duckett, S. B.; George, M. W.; Gilson, A. I.; Lawes, D.

J.; Perutz, R. N.; Sun, X.-Z.; Vollhardt, K. P. C. Photoinduced N₂Loss

as a Route to Long-Lived Organometallic Alkane Complexes: A

Time-Resolved IR and NMR Study. Chem. Sci. 2010, 1, 622−630.

(21) The close approach of cyclohexane to a U(III) center (U···C

3.821(4) Å) in the complex ((^tBuArO)₃tacn)U(MeC₆H₁₁), which

was crystallized using solution techniques, has been determined by

experimentally calibrated DFT calculations on the isolated molecule

as being due to noncovalent, dispersive interactions between the

alkane and the tacn ligand and is not a σ-alkane complex. See: Jung, J.;

Löffer, S. T.; Langmann, J.; Heinemann, F. W.; Bill, E.; Bistoni, G.;

Scherer, W.; Atanasov, M.; Meyer, K.; Neese, F. Dispersion Forces

Drive the Formation of Uranium-Alkane Adducts. J. Am. Chem. Soc.

2020, 142, 1864−1870.

(22) Heptane has been located in the vicinity of a metal center using

single-crystal X-ray diﬀraction: Evans, D. R.; Drovetskaya, T.; Bau, R.;

Reed, C. A.; Boyd, P. D. W. Heptane Coordination to an Iron(II)

Porphyrin. J. Am. Chem. Soc. 1997, 119, 3633−3634.

(23) Young, R. J.; Huxley, M. T.; Pardo, E.; Champness, N. R.;

Sumby, C. J.; Doonan, C. J. Isolating Reactive Metal-Based Species in

Metal-Organic Frameworks - Viable Strategies and Opportunities.

Chem. Sci. 2020, 11, 4031−4050.

for the initial synthesis of [1-isoprene][BAr^F]. This work

4

used the ARCHER UK National Supercomputing Service

(http://www.archer.ac.uk) and the Cirrus UK National Tier-2

HPC Service at the EPCC (http://www.cirrus.ac.uk) funded

by the University of Edinburgh and the EPSRC (EP/P020267/

1).

REFERENCES

■

(1) Goldberg, K. I.; Goldman, A. S. Large-Scale Selective

Functionalization of Alkanes. Acc. Chem. Res. 2017, 50, 620−626.

(2) Kumar, A.; Bhatti, T. M.; Goldman, A. S. Dehydrogenation of

Alkanes and Aliphatic Groups by Pincer-Ligated Metal Complexes.

Chem. Rev. 2017, 117, 12357−12384.

(3) Sattler, J. J. H. B.; Ruiz-Martinez, J.; Santillan-Jimenez, E.;

Weckhuysen, B. M. Catalytic Dehydrogenation of Light Alkanes on

Metals and Metal Oxides. Chem. Rev. 2014, 114, 10613−10653.

(4) Hartwig, J. F.; Larsen, M. A. Undirected, Homogeneous C-H

Bond Functionalization: Challenges and Opportunities. ACS Cent. Sci.

2016, 2, 281−292.

(5) Mkhalid, I. A. I.; Barnard, J. H.; Marder, T. B.; Murphy, J. M.;

Hartwig, J. F. C-H Activation for the Construction of C-B Bonds.

Chem. Rev. 2010, 110, 890−931.

(6) Sattler, A. Hydrogen/Deuterium (H/D) Exchange Catalysis in

Alkanes. ACS Catal. 2018, 8, 2296−2312.

(7) Perutz, R. N.; Sabo-Etienne, S. The σ -CAM Mechanism: σ

Complexes as the Basis of σ -Bond Metathesis at Late-Transition-

Metal Centers. Angew. Chem., Int. Ed. 2007, 46, 2578−2592.

(8) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes; Kluwer:

New York, 2001.

(9) Gunnoe, T. B. Introduction: Alkane C-H Activation by Single-

Site Metal Catalysis. In Alkane C-H Activation by Single-Site Metal

(24) Pike, S. D.; Thompson, A. L.; Algarra, A. G.; Apperley, D. C.;

Macgregor, S. A.; Weller, A. S. Synthesis and Characterization of a

Rhodium(I) σ -Alkane Complex in the Solid State. Science 2012, 337,

1648−1651.

(25) Chadwick, F. M.; McKay, A. I.; Martinez-Martinez, A. J.; Rees,

N. H.; Kramer, T.; Macgregor, S. A.; Weller, A. S. Solid-State

Molecular Organometallic Chemistry. Single-Crystal to Single-Crystal

5118

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

Reactivity and Catalysis with Light Hydrocarbon Substrates. Chem.

Sci. 2017, 8, 6014−6029.

and Rhodium(I) σ -Ethane Complexes: Experimental and Computa-

tional Studies. J. Am. Chem. Soc. 2013, 135, 15933−15947.

(41) Bloch, E. D.; Queen, W. L.; Krishna, R.; Zadrozny, J. M.;

Brown, C. M.; Long, J. R. Hydrocarbon Separations in a Metal-

Organic Framework with Open Iron(II) Coordination Sites. Science

2012, 335, 1606−1610.

(26) Pike, S. D.; Chadwick, F. M.; Rees, N. H.; Scott, M. P.; Weller,

A. S.; Krämer, T.; Macgregor, S. A. Solid-State Synthesis and

Characterization of σ -Alkane Complexes, [Rh(L₂)(η²,η²-C₇H₁₂)]-

[BAr^F] (L₂= Bidentate Chelating Phosphine). J. Am. Chem. Soc.

4

(42) PONOP = 2,6-(^tBu₂PO)₂C₅H₃N; dobdc = 2,5-dioxido-1,4-

benzenedicarboxylate.

2015, 137, 820−833.

(27) Das, A.; Van Trieste, G. P.; Powers, D. C. Crystallography of

Reactive Intermediates. Comments Inorg. Chem. 2020, 40, 116−158.

(28) Petrosko, S. H.; Johnson, R.; White, H.; Mirkin, C. A.

Nanoreactors: Small Spaces, Big Implications in Chemistry. J. Am.

Chem. Soc. 2016, 138, 7443−7445.

(43) Geier, S. J.; Mason, J. A.; Bloch, E. D.; Queen, W. L.; Hudson,

M. R.; Brown, C. M.; Long, J. R. Selective Adsorption of Ethylene

over Ehane and Propylene over Propane in the Metal-Organic

Frameworks M₂(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn). Chem. Sci.

2013, 4, 2054−2061.

(29) Martínez-Martínez, A. J.; Rees, N. H.; Weller, A. S. Reversible

Encapsulation of Xenon and CH₂Cl₂in a Solid-State Molecular

Organometallic Framework (Guest@SMOM). Angew. Chem., Int. Ed.

2019, 58, 16873−16877.

(30) Algarra, A. G.; Burnage, A. L.; Iannuzzi, M.; Krämer, T.;

Macgregor, S. A.; Pirie, R. E. M.; Tegner, B.; Weller, A. S.

Computational Studies of the Solid-State Molecular Organometallic

(SMOM) Chemistry of Rh σ-Alkane Complexes. In 21st Century

Challenges in Chemical Crystallography II; Mingos, D., Raithby, P. R.,

Eds.; Springer: Berlin, Heidelberg, 2021; pp 183−228.

(31) McKay, A. I.; Bukvic, A. J.; Tegner, B. E.; Burnage, A. L.;

Martínez-Martínez, A. J.; Rees, N. H.; Macgregor, S. A.; Weller, A. S.

Room Temperature Acceptorless Alkane Dehydrogenation from

Molecular σ -Alkane Complexes. J. Am. Chem. Soc. 2019, 141,

11700−11712.

(32) Boyd, T. M.; Tegner, B. E.; Tizzard, G. J.; Martínez-Martínez,

A. J.; Neale, S. E.; Hayward, M. A.; Coles, S. J.; Macgregor, S. A.;

Weller, A. S. A Structurally Characterized Cobalt(I) σ -Alkane

Complex. Angew. Chem., Int. Ed. 2020, 59, 6177−6181.

(33) Martínez-Martínez, A. J.; Tegner, B. E.; McKay, A. I.; Bukvic, A.

J.; Rees, N. H.; Tizzard, G. J.; Coles, S. J.; Warren, M. R.; Macgregor,

S. A.; Weller, A. S. Modulation of σ -Alkane Interactions in

[Rh(L₂)(alkane)]⁺Solid-State Molecular Organometallic (SMOM)

Systems by Variation of the Chelating Phosphine and Alkane: Access

to η²,η²-σ -Alkane Rh(I), η¹-σ -Alkane Rh(III) Complexes, and Alkane

Encapsulation. J. Am. Chem. Soc. 2018, 140, 14958−14970.

(34) Chadwick, F. M.; Krämer, T.; Gutmann, T.; Rees, N. H.;

Thompson, A. L.; Edwards, A. J.; Buntkowsky, G.; Macgregor, S. A.;

Weller, A. S. Selective C-H Activation at a Molecular Rhodium Sigma-

Alkane Complex by Solid/Gas Single-Crystal to Single-Crystal H/D

Exchange. J. Am. Chem. Soc. 2016, 138, 13369−13378.

(35) McKay, A. I.; Martínez-Martínez, A. J.; Griffiths, H. J.; Rees, N.

H.; Waters, J. B.; Weller, A. S.; Krämer, T.; Macgregor, S. A.

Controlling Structure and Reactivity in Cationic Solid-State

Molecular Organometallic Systems Using Anion Templating. Organo-

metallics 2018, 37, 3524−3532.

(44) Due to rapid decomposition and the corresponding low relative

proportion of the σ-alkane complex relative to [1-BAr^F], ¹³C{¹H}

4

SSNMR spectrocopic data are best interpreted as simply showing that

the precursor diene has been hydrogenated by the alkene resonances

observed in the precursor not being observed. Spectroscopic

characterization rests on the ³¹P{¹H} SSNMR spectrum (section

2.2 and the Supporting Information).

(45) Zhang, F.; Pan, L.; Choi, J.; Mehar, V.; Diulus, J. T.; Asthagiri,

A.; Weaver, J. F. Propane σ -Complexes on PdO(101): Spectroscopic

Evidence of the Selective Coordination and Activation of Primary C-

H Bonds. Angew. Chem., Int. Ed. 2015, 54, 13907−13911.

(46) The alkane ligand in [1-(2-methylbutane)][BAr^F] was

4

modeled with geometrical and thermal restraints. Completely

removing these results in ﬁdelity of the alkane motif, but C1−C2

and C2−C3 are lengthened and shortened, respectively. C2 remains

tetrahedral, signaling an alkane ligand. We cannot discount, as

discussed in the main text, that a small amount of superpositionally

disordered alkene ligand is present in this particular single-crystal

sample that disproportionally aﬀects the reﬁned position of C2.

(47) Chadwick, F. M.; Rees, N. H.; Weller, A. S.; Kramer, T.;

Iannuzzi, M.; Macgregor, S. A. A Rhodium-Pentane Sigma-Alkane

Complex: Characterization in the Solid State by Experimental and

Computational Techniques. Angew. Chem. 2016, 128, 3741−3745.

(48) Sherbo, R. S.; Bindra, G. S.; Budzelaar, P. H. M. Square-Planar-

Tetrahedral Interconversion without Spin Flip in (β -diiminate)Rh-

(1,3-diene) Complexes. Organometallics 2016, 35, 2039−2048.

(49) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.;

Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Covalent Radii

Revisited. Dalton Trans. 2008, 2832−2838.

(50) Johnson, A.; Martínez-Martínez, A. J.; Macgregor, S. A.; Weller,

A. S. A d¹⁰Ag(I) Amine-Borane σ -Complex and Comparison with a

d⁸Rh(I) Analogue: Structures on the η¹to η²:η²continuum. Dalton

Trans. 2019, 48, 9776−9781.

(51) Green, M. L. H.; Parkin, G. The Covalent Bond Classiﬁcation

Method and Its Application to Compounds That Feature 3-Center 2-

Electron Bonds. In The Chemical Bond III: 100 years old and getting

stronger; Mingos, D. M. P., Ed.; Springer International Publishing:

Cham, Switzerland, 2017; pp 79−139.

(36) Furfari, S.; Tegner, B.; Burnage, A.; Doyle, L.; Bukvic, A.;

Macgregor, S.; Weller, A. S. Selectivity of Rh ···H-C Binding in a σ -

Alkane Complex Controlled by the Secondary Microenvironment in

the Solid-State. Chem. - Eur. J. 2021, 27, 3177−3183.

(52) Lane, J. R.; Contreras-García, J.; Piquemal, J.-P.; Miller, B. J.;

Kjaergaard, H. G. Are Bond Critical Points Really Critical for

Hydrogen Bonding? J. Chem. Theory Comput. 2013, 9, 3263−3266.

(53) Baratta, W.; Mealli, C.; Herdtweck, E.; Ienco, A.; Mason, S. A.;

Rigo, P. Nonclassical vs Classical Metal ···H₃C-C Interactions:

Accurate Characterization of a 14-Electron Ruthenium(II) System

by Neutron Diffraction, Database Analysis, Solution Dynamics, and

DFT Studies. J. Am. Chem. Soc. 2004, 126, 5549−5562.

(54) Lu, Q.; Neese, F.; Bistoni, G. London Dispersion Effects in the

Coordination and Activation of Alkanes in σ -Complexes: A Local

Energy Decomposition Study. Phys. Chem. Chem. Phys. 2019, 21,

11569−11577.

(55) Alvarez, S. A Cartography of the Van der Waals Territories.

Dalton Trans. 2013, 42, 8617−8636.

(56) van der Eide, E. F.; Yang, P.; Bullock, R. M. Isolation of Two

Agostic Isomers of an Organometallic Cation: Different Structures

and Colors. Angew. Chem., Int. Ed. 2013, 52, 10190−10194.

(37) Martínez-Martínez, A. J.; Royle, C. G.; Furfari, S. K.; Suriye, K.;

Weller, A. S. Solid-State Molecular Organometallic Catalysis in Gas/

Solid Flow (Flow-SMOM) as Demonstrated by Efficient Room

Temperature and Pressure 1-Butene Isomerization. ACS Catal. 2020,

10, 1984−1992.

(38) Sheludko, B.; Cunningham, M. T.; Goldman, A. S.; Celik, F. E.

Continuous-Flow Alkane Dehydrogenation by Supported Pincer-

Ligated Iridium Catalysts at Elevated Temperatures. ACS Catal. 2018,

8, 7828−7841.

(39) Bukvic, A. J.; Crivoi, D. G.; Garwood, H. G.; McKay, A. I.;

Chen, T. T. D.; Martínez-Martínez, A. J.; Weller, A. S. Tolerant to Air

σ -Alkane Complexes by Surface Modification of Single Crystalline

Solid-State Molecular Organometallics Using Vapour-Phase Cationic

Polymerisation: SMOM@polymer. Chem. Commun. 2020, 56, 4328−

4331.

(40) Walter, M. D.; White, P. S.; Schauer, C. K.; Brookhart, M.

Stability and Dynamic Processes in 16VE Iridium(III) Ethyl Hydride

5119

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Journal of the American Chemical Society

pubs.acs.org/JACS

Article

(57) The initial formation of 20% of [1-BAr^F] from the initial

4

hydrogenation of the starting material is accounted for in this analysis.

(58) Bertmer, M.; Nieuwendaal, R. C.; Barnes, A. B.; Hayes, S. E.

Solid-State Photodimerization Kinetics of α -trans-Cinnamic Acid to

α -Truxillic Acid Studied via Solid-State NMR. J. Phys. Chem. B 2006,

110, 6270−6273.

(59) Jarvis, A. G.; Sparkes, H. A.; Tallentire, S. E.; Hatcher, L. E.;

Warren, M. R.; Raithby, P. R.; Allan, D. R.; Whitwood, A. C.; Cockett,

M. C. R.; Duckett, S. B.; Clark, J. L.; Fairlamb, I. J. S. Photochemical-

mediated solid-state [2 + 2]-cycloaddition reactions of an unsym-

metrical dibenzylidene acetone (monothiophos-dba). CrystEngComm

2012, 14, 5564−5571.

(60) Benedict, J. B.; Coppens, P. Kinetics of the Single-Crystal to

Single-Crystal Two-Photon Photodimerization of α -trans-Cinnamic

Acid to α -Truxillic Acid. J. Phys. Chem. A 2009, 113, 3116−3120.

(61) Hatcher, L. E.; Skelton, J. M.; Warren, M. R.; Stubbs, C.; da

Silva, E. L.; Raithby, P. R. Monitoring photo-induced population

dynamics in metastable linkage isomer crystals: a crystallographic

kinetic study of [Pd(Bu₄dien)NO₂]BPh₄. Phys. Chem. Chem. Phys.

2018, 20, 5874−5886.

(62) Hulbert, S. F. Models for Solid-state Reactions in Powdered

Compacts: A Review. J. Br. Ceram. Soc. 1969, 6, 11−20.

(63) Searles, K.; Chan, K. W.; Mendes Burak, J. A.; Zemlyanov, D.;

Safonova, O.; Copéret, C. Highly Productive Propane Dehydrogen-

ation Catalyst Using Silica-Supported Ga-Pt Nanoparticles Generated

from Single-Sites. J. Am. Chem. Soc. 2018, 140, 11674−11679.

(64) Aoki, T.; Crabtree, R. H. Homogeneous Tungsten, Rhenium,

and Iridium Catalysts in Alkane Dehydrogenation Driven by Reflux of

Substrate or of Cosolvent or by Inert-Gas Flow. Organometallics 1993,

12, 294−298.

(65) Kumar, A.; Zhou, T.; Emge, T. J.; Mironov, O.; Saxton, R. J.;

Krogh-Jespersen, K.; Goldman, A. S. Dehydrogenation of n-Alkanes

by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α -

Olefin Product. J. Am. Chem. Soc. 2015, 137, 9894−9911.

(66) Mecozzi, S.; Rebek, J., Jr. The 55% Solution: A Formula for

Molecular Recognition in the Liquid State. Chem. - Eur. J. 1998, 4,

1016−1022.

(67) Leenders, S. H. A. M.; Gramage-Doria, R.; de Bruin, B.; Reek, J.

N. H. Transition metal catalysis in confined spaces. Chem. Soc. Rev.

2015, 44, 433−448.

(68) Ringe, D.; Petsko, G. A. How Enzymes Work. Science 2008,

320, 1428.

(69) Fasan, R.; Meharenna, Y. T.; Snow, C. D.; Poulos, T. L.;

Arnold, F. H. Evolutionary History of a Specialized P450 Propane

Monooxygenase. J. Mol. Biol. 2008, 383, 1069−1080.

5120

https://doi.org/10.1021/jacs.1c00738

J. Am. Chem. Soc. 2021, 143, 5106−5120

Article Doi

A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane

DOI: 10.1021/jacs.1c00738

Source and publish data:

Authors:

Article abstract of DOI:10.1021/jacs.1c00738

Full text of DOI:10.1021/jacs.1c00738

Products guided by the article

R&D Labs maybe for 13643-02-2

Relevant to this article

Hot Product