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

Article abstract of DOI:10.1021/jacs.8b09364

Solid/gas single-crystal to single-crystal (SC-SC) hydrogenation of appropriate diene precursors forms the corresponding σ-alkane complexes [Rh(Cy₂P(CH₂)_nPCy₂)(L)][BAr^F₄] (n = 3, 4) and [RhH(Cy₂P(CH₂)₂(CH)(CH₂)₂PCy₂)(L)][BAr^F₄] (n = 5, L = norbornane, NBA; cyclooctane, COA). Their structures, as determined by single-crystal X-ray diffraction, have cations exhibiting Rh···H-C σ-interactions which are modulated by both the chelating ligand and the identity of the alkane, while all sit in an octahedral anion microenvironment. These range from chelating η²,η² Rh···H-C (e.g., [Rh(Cy₂P(CH₂)_nPCy₂)(η²η²-NBA)][BAr^F₄], n = 3 and 4), through to more weakly bound η¹ Rh···H-C in which C-H activation of the chelate backbone has also occurred (e.g., [RhH(Cy₂P(CH₂)₂(CH)(CH₂)₂PCy₂)(η¹-COA)][BAr^F₄]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the supporting anion microenvironment, [Rh(Cy₂P(CH₂)₃PCy₂)][CO?BAr^F₄], in which the metal center instead forms two intramolecular agostic η¹ Rh···H-C interactions with the phosphine cyclohexyl groups. CH₂Cl₂ adducts formed by displacement of the η¹-alkanes in solution (n = 5; L = NBA, COA), [RhH(Cy₂P(CH₂)₂(CH)(CH₂)₂PCy₂)(κ¹-ClCH₂Cl)][BAr^F₄], are characterized crystallographically. Analyses via periodic DFT, QTAIM, NBO, and NCI calculations, alongside variable temperature solid-state NMR spectroscopy, provide snapshots marking the onset of Rh···alkane interactions along a C-H activation trajectory. These are negligible in [Rh(Cy₂P(CH₂)₃PCy₂)][CO?BAr^F₄]; in [RhH(Cy₂P(CH₂)₂(CH)(CH₂)₂PCy₂)(η¹-COA)][BAr^F₄], σ_C-H → Rh σ-donation is supported by Rh → σ?_C-H pregostic donation, and in [Rh(Cy₂P(CH₂)_nPCy₂)(η²η²-NBA)][BAr^F₄] (n = 2-4), σ-donation dominates, supported by classical Rh(dπ) → σ?_C-H π-back-donation. Dispersive interactions with the [BAr^F₄]^- anions and Cy substituents further stabilize the alkanes within the binding pocket.

Full text of DOI:10.1021/jacs.8b09364

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Article
Modulation of #-Alkane Interactions in [Rh(L2)(alkane)]+ Solid-
State Molecular Organometallic (SMOM) Systems by Variation of
the Chelating Phosphine and Alkane: Access to #2,#2-#-Alkane
Rh(I), #1-#-Alkane Rh(III) Complexes, and Alkane Encapsulation.
Antonio J. Martinez-Martinez, Bengt E. Tegner, Alasdair I. McKay, Alexander J. Bukvic, Nicholas H
Rees, Graham J. Tizzard, Simon J. Coles, Mark R. Warren, Stuart A. Macgregor, and Andrew S. Weller
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09364 • Publication Date (Web): 10 Oct 2018
Downloaded from http://pubs.acs.org on October 10, 2018
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Modulation of -Alkane Interactions in [Rh(L2)(alkane)]+ Solid-State
Molecular Organometallic (SMOM) Systems by Variation of the
Chelating Phosphine and Alkane: Access to 2,2--Alkane Rh(I), 1-
-Alkane Rh(III) Complexes, and Alkane Encapsulation.
Antonio J. Martínez-Martínez,a Bengt E. Tegner,b Alasdair I. McKay,a Alexander J. Bukvic,a
Nicholas H. Rees,a Graham J. Tizzard,c Simon J. Coles,c Mark R. Warren,d Stuart A. Macgregor*b
and Andrew S. Weller*a
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a Chemistry Research Laboratories, University of Oxford, Oxford OX1 3TA, United Kingdom
b Institute of Chemical Sciences, Heriot Watt University, Edinburgh EH14 4AS, United Kingdom
c UK National Crystallography Service, Chemistry, Faculty of Natural and Environmental Sciences, University of
Southampton. Southampton, SO17 1BJ, United Kingdom
d Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot, OX11 0DE. United Kingdom
ABSTRACT: Solid/gas Single–Crystal to Single–Crystal (SC–SC) hydrogenation of appropriate diene precursors forms the
corresponding
–alkane
complexes
[Rh(Cy2P(CH2)nPCy2)(L)][BArF ]
(n
=
3,
4)
and
4
[RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(L)][BArF ] (n = 5, L = norbornane, NBA; cyclooctane, COA). Their structures, as
4
determined by single–crystal X-ray diffraction, have cations exhibiting Rh···H–C –interactions which are modulated by
both the chelating ligand and the identity of the alkane, while all sit in an octahedral anion–microenvironment. These range
from chelating 2,2 Rh···H–C (e.g. [Rh(Cy2P(CH2)nPCy2)(22–NBA)][BArF ], n = 3 and 4), through to more weakly bound
4
1 Rh··H–C in which C–H activation of the chelate backbone has also occurred (e.g. [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(–
COA)][BArF ]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the
4
supporting anion microenvironment – [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF ] – in which the metal center instead forms two
4
intramolecular agostic 1 Rh···H–C interactions with the phosphine cyclohexyl groups. CH2Cl2 adducts formed by
displacement of the 1–alkanes in solution (n = 5; L = NBA, COA), [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(1–ClCH2Cl)][BArF ],
4
are characterized crystallographically. Analyses via periodic DFT, QTAIM, NBO and NCI calculations, alongside variable
temperature solid–state NMR spectroscopy, provide snapshots marking the onset of Rhalkane interactions along a CH
activation trajectory. These are negligible in [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF ]; in [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(–
4
COA)][BArF ] CHRh -donation is supported by RhCH ‘pregostic’ donation; and in [Rh(Cy2P(CH2)nPCy2)(22-
4
NBA)][BArF ] (n = 2-4) -donation dominates, supported by classical Rh(d)CH -back donation. Dispersive
4
interactions with the [BArF ]- anions and Cy substituents further stabilize the alkanes within the binding pocket.
4
1. Introduction. The ability to tune the local environment
around a metal center by variation of supporting ligands is an
important concept widely used in homogenous
organometallic synthesis and catalysis.1,2 A well–documented
example of this comes from bidentate phosphine ML2–type
complexes,3,4 as by altering the L–M–L bite angle and
substitution at phosphine the resulting steric (e.g. the solid–
cone angle, Θ5), or electronic (e.g. oxidation state6 or degree
of bond activation7), changes ultimately can provide the
ability to control structure, speciation and, through the
energetics of elementary reaction steps in catalysis, activity
and selectivity. Fig 1A.
Extending such concepts to heterogeneous systems is difficult
given the resulting challenges associated with precisely
defining single–site active centers and their extended
coordination environments.8 While chelating phosphine
complexes supported by metal organic frameworks,9 porous
coordination polymers,10 nanoparticles,11 mesoporous–hosts12
and silica–surfaces13 have been reported, the role of the
chelating ligand in determining structure and reactivity is less
well–developed. Nevertheless, precise control of well–defined,
and reactive, metal centers in heterogeneous systems could
lead to enhanced activity and selectivity in catalysis, as
frequently demonstrated in homogenous processes.1,4
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from chelating 2,2 Rh···H–C, through to more weakly bound
We have recently shown that single–crystal to single–crystal
1 Rh··H–C and ultimately to systems where the alkane is not
ligated with the metal center, but sits encapsulated in the
anion framework.
(SC–SC) solid/gas reactions between H2 and the appropriate
[RhL2(diene)]+ precursors forms well–defined but reactive –
alkane14-16 complexes directly in the solid–state, e.g.
[Rh(Cy2P(CH2)2PCy2)(η2η2–NBA)][BArF ],
[1–NBA][BArF ]
4
4
Scheme 1. Precursor diene complexes used in this
study and SC-SC SMOM–synthesis of –complexes.
(NBA = norbornane, ArF = 3,5–(CF3)2C6H3), Fig. 1B).17-20 Such
–complexes contain 3–center 2–electron (3c-2e) Rh···H–C
bonds,21,22 and are of general interest from the fundamental
challenges presented by their synthesis and characterization,
9
14,15
as well as their central role as intermediates in C–H
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activation processes.23-26 When prepared in this way these –
alkane complexes show remarkable relative stability
compared with species prepared by solution routes; the latter
are generally characterized in situ using NMR spectroscopy,
on small scale (2–20 mg) at very low temperature and have
limited lifetimes even under these relatively constrained
conditions.27-31
[BArF
]
(B)
(A)
Variation of
chelating
ligands
R
4
R
R
Cy2
H
+ H2
P
P
[1–NBD][BArF
]
H
Rh
4
P
SC–SC
298 K
β
leads to
H
M
Ln
Cy2
n
H
changes in
structure &
reactivity
[1–NBA][BArF
]
P
4
R
SC–SC = single–crystal to single–crystal
SMOM
(C)
2. Results and Discussion
2.1 Precursor diene complexes.
Solid–state
Molecular
Precursor
complexes
of
the
4
general
formula
OrganoMetallic
Chemistry and
Catalysis
[Rh(Cy2P(CH2)nPCy2)(diene)][BArF ] (diene = NBD or COD)
were prepared in which the phosphine and diene are
Molecular
systematically varied: n = 3, [2–diene][BArF ]; n = 4, [3–
Anion Oh–microenvironment
Organometallic
4
diene][BArF ]; n = 5, [4–diene][BArF ] (Scheme 2). These are
4
4
conveniently prepared by reaction of [Rh(COD)2][BArF ] with
Figure 1. (A) Control of local environment in {ML2}
complexes: Bite angle = β. (B) Synthesis of the –alkane
4
Cy2P(CH2)nPCy2 to give [Rh(Cy2P(CH2)nPCy2)(COD)][BArF ],
4
complex [1-NBA][BArF ]; NBD = norbornadiene. (C) The
followed by addition of H2/1,2–F2C6H4/NBD to give
4
[Rh(Cy2P(CH2)nPCy2)(NBD)][BArF ] via in situ formation of
SMOM concept.
4
difluorobenzene-bound38
intermediates
This stability in the solid–state originates from the [BArF ]–
4
[Rh(Cy2P(CH2)nPCy2)(1,2–F2C6H4)][BArF ]. Both COD and
4
anions providing
a
robust, octahedral, crystalline
NBD precursors were isolated in good (~80%) yield after
microenviroment,32 that allows for isolation, characterization
and onward reactivity of the encapsulated organometallic
cation to be studied in detail (Fig. 1C).26,33 These so–called33
Solid–state Molecular Organometallic (SMOM) systems are
related to Supported Organometallic Catalysts (SOMC),13
Single–Site Heterogeneous Catalysts (SSHC),34 and MOF–
functionalized organometallics,9,35-37 but, in contrast to these,
are not supported by a platform material. Moreover, SMOM
systems have the desirable properties of being readily studied
at the molecular–level by single–crystal X–ray diffraction,
Solid–State NMR (SSNMR) spectroscopy and computational
techniques such as periodic DFT.
recrystallization from CH2Cl2/pentane. Complexes [1–
diene][BArF ] (n = 2) have previously been reported.18
4
These precursor complexes have been characterized by
solution NMR spectroscopy and single–crystal X-ray
diffraction, which show an Oh arrangement of [BArF ]– anions
4
surrounding the organometallic cations, as observed for [1–
NBA][BArF ]18 and related complexes.17,20 The Supporting
4
Materials detail their structures. For the NBD precursors, this
homologous series allows for the bite–angle () of the various
diphosphines in this environment to be compared.
Unsurprisingly,4  becomes progressively larger with
increasing number of methylene units in the chelate backbone
(Scheme 2B). The same trend, albeit interestingly with slightly
smaller –angles, is apparent for the COD precursors. With
these complexes in hand a systematic study of solid/gas
hydrogenation was undertaken.
We now report that systematic variation of the P–Rh–P bite
angle coupled with the identity of the diene in SMOM systems
based upon precursors [Rh(Cy2P(CH2)nPCy2)(diene)][BArF ]
4
(n = 3 to 5, diene = norbornadiene, NBD, or 1,5–cyclooctadiene,
COD, Scheme 1) results in significant changes in structure and
reactivity on addition of H2 in SC–SC reactions. This results in
crystallographically characterized –alkane complexes that
show markedly different degrees of Rh···H–C interaction in
response to the changes in both phosphine and the precursor
diene, while stabilized in the microenvironment provided by
Scheme 2. (A) Synthesis of diene precursors and (B)
representative P–Rh–P bite angles () taken from
diene structures.17,39
the octahedral arrangement of [BArF ]– anions: these range
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[BArF
]
[BArF
]
(B)
[BArF
]
(A)
(A)
4
4
4
F
F
Cy2
Rh
Cy2
Cy2
Rh
[BArF
]
H2
NBD
P
P
4
P
Cy2
Rh
Rh
P
P
P
n
n
n
P
1,2–F2C6H4
– COA
Cy2
Cy2
Cy2
4
P
Cy2
5
6
(B) [1–diene][BArF
]
4
[2–diene][BArF
]
[3–diene][BArF
]
4
[4–diene][BArF
]
4
SC–SC
5 min
H2A
4
C20
C2
P2
C22
+ H2
Cy Cy
Cy
Cy
Cy
Rh
Cy
Cy
Cy
Cy
7
8
9
C1
Rh1
P
Rh
P
H1A
P
P
[BArF
]
P1
4
Cy2
Rh
Rh
P
H
H
P
Rh
P
P
P
P
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H
H
Cy2
Cy
Cy
Cy
Cy
Cy Cy
Cy
[2–NBA]+
β = 85.2(1)º
β = 84.5(1)º
β = 93.9(1)º
β = 91.9(1)º
β = 98.9(1)º
β = 96.5(1)º
β = 105.1(1)º
β = 99.6(1)º
NBD
COD
[2–NBA][BArF
]
4
Figure 2. (A) Synthesis of [2–NBA][BArF ]. (B) Cation of [2–
4
NBA][BArF ]. 50% displacement ellipsoids. Selected bond
2.2 Hydrogenation of {Rh(Cy2P(CH2)2PCy2)}+/COD.
4
lengths (Å) and angles (º): Rh1···C1 2.408(2); Rh1···C2, 2.402(2);
C1–C2, 1.554(3); Rh1–P1, 2.2065(4); Rh1–P2, 2.2049(4); P1–Rh1–
P2 93.91(2)º, Rh1–H1A–C1, 105.1(2); Rh1–H2A–C2, 106.0(2); ∠
P1P2Rh/Rh1C1C2, 5.15(7)º.
As previously reported,18 hydrogenation of [1–NBD][BArF ]
4
gives [1–NBA][BArF ] in a rapid (less than 10 mins) SC–SC
4
transformation (see Figure 1B). Here, use of [1–COD][BArF ]
4
40,41
results a slower
reaction with H2 (3 hrs) and loss of
The 31P{1H} SSNMR spectrum of [2–NBA][BArF ] shows two
crystallinity. Dissolving the resulting solid in CD2Cl2 afforded
the previously–reported zwitterion [Rh(Cy2P(CH2)2PCy2){(η-
C6H3(CF3)2)BArF }], [1–BArF ] and free COA.18
4
relatively sharp environments at  = 50.6, 51.6 [J(RhP) ~ 170
Hz], while the 13C{1H} NMR spectrum is featureless between
 110 and 50, demonstrating hydrogenation of the NBD. A
1H/13C Frequency Switched Lee-Goldburg (FSLG) HETCOR
experiment at 298 K, which has been used to characterize E–
H···M interactions in the solid–state (E = Si, C),18,26,43 shows
two strong correlations between (13C) ~26 and (1H) –2 which
are consistent with the crystallographically inequivalent
Rh···H–C interactions observed in the solid–state (Figure S13).
An additional correlation [(13C) 42/(1H) –0.6]44 is assigned to
3
4
2.3 {Rh(Cy2P(CH2)3PCy2)}+/NBA: an 2, 2 –alkane
complex. Addition of H2 (1 bar, 298 K) to single–crystals of
[Rh(Cy2P(CH2)3PCy2)(NBD)][BArF ],
[2–NBD][BArF ],
4
4
resulted in the rapid (~ 5 mins) formation of the –alkane
complex
[Rh(Cy2P(CH2)3PCy2)(η2η2–NBA)][BArF ],
[2–
4
NBA][BArF ], in a SC–SC transformation, as shown by single–
4
crystal X-ray diffraction (Fig. 2) and 31P{1H}/13C{1H} SSNMR
spectroscopies. The octahedral arrangement of [BArF ]–
4
the CH2 bridge on the norbornane, which is affected by [BArF ]
anions is retained in [2–NBA][BArF ]39 while the central
4
4
ring current effects as described for [1–NBA][BArF ] and
cation is a pseudo–square planar {RhL2}+ fragment bound with
the alkane NBA through two endo Rh···H–C sigma interactions
showing relatively short Rh···C distances [Rh···C
2.408(2)/2.402(2) Å] and rather acute ∠RhHC, e.g. Rh1–H1A–
C1 105.1(2)º. These H–atoms were located and freely refined.
These data suggest an 2,2 chelating Rh···H–C motif,14,42 as
corroborated by computational studies (see later). Despite the
P–Rh–P bite–angle increasing compared with [1–
4
related complexes.18,19,26
Dissolving crystalline material of [2–NBA][BArF ] in CD2Cl2 at
4
183 K results in free NBA being observed by 1H NMR
spectroscopy and a 31P{1H} NMR spectrum that is suggestive of
a
solvent-coordinated
complex,
[Rh(Cy2P(CH2)3PCy2)(ClCH2Cl)n][BArF ],  47.4 [J(RhP) = 198
4
Hz],
similar
to
that
reported
for
NBA][BArF ]18 (e.g. Scheme 2B) the Rh···C distances are not
[Rh(iPr2P(CH2)3PiPr2)(ClCH2CH2Cl)][BArF ].18 On warming,
4
4
different within error [cf. 2.389(3)/2.400(3) Å] and the
structures are very similar, suggesting that the NBA ligand fits
comfortably into the ligand pocket defined by the Cy–groups.
This similarity is not strongly influenced by the anion
microenvironment as evidenced by calculations on isolated
cations (Section 2.7 and Supporting Materials). In the solid–
state there are a number of weak C–H···F interactions between
decomposition occurs via C–Cl activation to give a mixture of
partially soluble chloride–bridged hydride dimers, e.g.
[Rh(Cy2P(CH2)3PCy2)H(–Cl)]2[BArF ] , that precipitate from
4 2
solution and are best identified by ESI–MS. The formation of
the [BArF ]– coordinated zwitterion was not observed,45 in
4
contrast to [1–NBA][BArF ] that forms [1–BArF ] in CD2Cl2.18
4
4
We suggest this is a consequence of the increased steric profile
of the chelating phosphine Cy2P(CH2)3PCy2 versus
Cy2P(CH2)2PCy2, disfavoring coordination of the, local to the
the NBA ligand and the [BArF ]– anion, Figure S73.
4
metal center, planar and bulky [BArF ]– anion, coupled with
4
the wider bite–angle phosphines encouraging oxidative
addition at Rh(I).6
2.4 {Rh(Cy2P(CH2)3PCy2)}+/COA:
A 12–electron Rh(I)
complex supported by agostic interactions with an
encapsulated non–bonding alkane.
Exposing single crystals of [2–COD][BArF ] to H2 for 3 hours
4
results in, slower,40,41 SC–SC hydrogenation and expulsion of
cyclooctane (COA) from the metal–center to form formally 12–
electron “naked”46,47 [Rh(Cy2P(CH2)3PCy2)]+, [2]+. Remarkably
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the free alkane is retained inside the anion octahedral cavity
to give [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF ], [2][COA⊂BArF ],
Scheme 3.
agostic interactions, a view supported by calculations (see
Section 2.7). This movement of the metal center keeps the
phosphine ligand in essentially the same environment in the
anion cavity, and maximizes the Rh···H–C agostic interactions,
with two cyclohexyl group also enfolding the metal center.51 It
also retains the square planar geometry around Rh, with
angles around the metal center = 360º. A minor, disordered,
agostomeric52 component is also present (~10%) in which the
Rh–center swings up to interact with the chemically
equivalent C15–H and C21–H bonds (Fig. 3C). The Rh–P
distances in the major component [2.166(2) and 2.171(2) Å] are
4
4
Scheme 3. Synthesis of [2][COA⊂BArF ].
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59
60
~0.04 Å shorter than in [2–NBA][BArF ]. Whether this reflects
4
The solid–state structure of the cation [2]+ (Fig. 3A) shows two
–agostic48-50 Rh···H–C interactions from the cyclohexyl rings
[Rh···C3 2.91(1), Rh···C9 2.87(1) Å] that form in response to the
unsaturation now at the Rh(I) metal center. The Rh1–P1–C1
angle reflects this, for example being more acute [108.9(3)º]
than Rh1–P1–C13 [119.8(3)º]. The Rh–center also moves
towards the C–H bonds involved in these agostic interactions,
a
weaker trans–influence of the bis–agostic Rh···H–C
interactions in [2][COA⊂BArF ] compared with the
4
intermolecular interactions in [2–NBA][BArF ], or is
a
4
consequence of the metal’s unusual disposition, is not clear.
Although formally 2–coordinate group 9 complexes are
known,53,54 albeit rare, as far as we are aware the cation in
[2][COA⊂BArF ] is the first example reported with a chelating
4
phosphine.
as shown by the angle between the planes
∠
The expelled alkane, COA, sits encapsulated55,56 within the
anion–framework (Fig. 3D), essentially equally disordered
between two boat–chair conformations.57 The closest Rh···C
distance is 3.74(1) Å (C28), approaching the Rh–center above
the square plane, suggesting no significant, and at best very
weak, interaction with the metal center. This distance is
similar to that measured in [U(ArO)3tacn(c-C6H11Me)]
[3.864(7) Å, albeit U versus Rh] where a bonding interaction
was suggested,58 although studies on related uranium
complexes suggest such as distance reflects a non–bonding
interaction.59
P1P2C25C27/Rh1P1P2 = 42.6(2)º (Fig. 3B). The relatively long
Rh···C distances, coupled with more open ∠RhHC angles, e.g.
Rh1–H9A–C9 147.9(6)º, albeit the Hs–placed in calculated
positions, suggests an 1 Rh···H–C bonding motif for the
(A)
(B)
C15
C21
43º
C14
C13
C25
P1
C27
P2
P1
C2
C1
C25
Rh1
C3
In the 158 K 31P{1H} SSNMR spectrum of [2][COA⊂BArF ] only
4
Rh1
H3A
H9A
two broad (fwhm ~ 400 Hz) environments are observed at 
51.3 and  48.1 in an approximate 1:1 ratio. Coupling to 103Rh
was not resolved. These do not vary significantly in chemical
shift or relative intensity when measured at 298 K. The
observation of only 2 signals suggests that the disorder
observed in the cation could be static (with coincident signals)
or dynamic60 (and fast61) on the NMR timescale, but that no
change is observed on changing the temperature suggests the
former. 13C{1H} SSNMR spectrum shows a featureless region
between  115 and 43, indicating that the COD has been
hydrogenated to COA, which is observed as a sharp signal at 
25.7. This assignment is supported by a Non-Quaternary
Suppression experiment (NQS) which, as well as detecting
quaternary carbons, identifies CHn groups that experience
motion in a frequency range similar to, or greater than the 1H–
13C dipolar coupling.62,63 In the 158 K NQS spectrum a single
prominent signal remains at  25.7 in the alkyl region,
compared with the 13C{1H} SSNMR spectrum (Figure 4A). This
peak remains at 298 K.39 These data are consistent with the
encapsulated COA undergoing a low energy site–exchange
within the cavity (Figure 4B), suggested to be a combination
of 1,2–jumps and/or exchange between the two disordered
COA components.64 To calibrate our observations, a variable
C3
C9
[2]+
(C)
(D)
C8H16
C(H2)28
Rh1A
Rh1
[Rh(P2C3)]+
Rh1
[2][COA∠ BArF
]
4
Figure 3. (A) Cation of [2][COA⊂BArF ]. 50% displacement
4
ellipsoids, only the major disordered component shown.
Selected bond lengths (Å) and angles (º): Rh1···C3 2.912(10);
Rh1···C9, 2.873(10); Rh1–P1, 2.171(2); Rh1–P2, 2.166(2); P1–Rh1–
P2 91.21(8)º;
∠
P1P2C25C27/Rh1P1P2
=
42.6(2)º. (B)
Displacement of the Rh–center in [2]+ towards the agostic C–
H bonds in the major disordered component. (C) Ball-and–
stick representation of the two disordered components in [2]+.
temperature NQS experiment on [1–NBA][BArF ] showed a
4
NBA fragment undergoing motion at 298 K which is halted at
158 K – fully consistent with previous variable temperature–
(D) Packing diagram of [2][COA⊂BArF ] showing Oh
4
arrangement of [BArF ]–, COA (C = red) shown at van der
4
SSNMR studies (Figure S3).26 [2][COA⊂BArF ] is not stable in
4
Waals radii and only one disordered component shown;
Rh1···C28, 3.74(1) Å.
solution. Vacuum transfer of CD2Cl2 onto solid
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[2][COA⊂BArF ] and warming to 183 K resulted in a
Dissolution of [3–NBA][BArF ] in CD2Cl2 resulted in a yellow
precipitate, identified by ESI–MS, again, as a mixture of
dimeric hydrido/chlorides.
4
4
2
3
4
5
precipitate that was persistent on warming to room
temperatures. ESI–MS shows this to contain multiple dimeric
hydrido–chlorides.
[BArF
]
4
(A)
(B)
13C{1H} SSNMR (10 kHz)
(A)
(B)
Cy2
Rh
B
6
P
P
7
8
9
Cy2
+ H2
Cy2
SC–SC
5 min
C1
C2
Rh1
P2
[BArF
]
10
11
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51
52
53
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59
60
4
P1
B
B
B
B
H
H
Rh
P
Cy
Cy
P
Cy
Cy
Rh
NQS
P
P
 25.7
H
Cy2
H
*
*
[3–NBA][BArF
]
[3–NBA]+
4
B
Figure 6. (A) SMOM synthesis of [3–NBA][BArF ]. (B) Cation
4
of [3–NBA][BArF ] (50% displacement ellipsoids): Rh1···C1
4
Figure 4. (A) 13C{1H} SSNMR spectra (10 kHz, 158 K) and NQS
2.399(2); Rh1···C2, 2.396(2) Å; Rh1–P1, 2.2154(4); Rh1–P2,
2.2215(4); P1–Rh1–P2 97.87(2)º.
spectra of [2][COA⊂BArF ] (10 kHz, 158 K). * = spinning–side
4
band. (B) Proposed fluxional processes (Ⓑ = [BArF ])
4
Increasing the bite angle of the phosphine promoted different
reactivity in the resulting –alkane complex. Surprisingly
given that the structural metrics have not changed
significantly from the smaller bite–angle congeners, [3–
The structural changes evident between [2–NBA][BArF ] – a
4
22 –alkane complex – and [2][COA⊂BArF ] – with a non–
4
bonding alkane – can be traced back to the change in
hydrocarbon, as the {ML2}+ fragment is the same.
Consideration of the van der Waals surfaces of NBA versus
COA, Figure 5, shows that the latter presents a larger steric
profile to the metal center. We suggest that this, alongside
possible conformational preferences for metal binding of the
alkane and non–covalent F···H–C interactions in the
microenvironment (see Figure S73 and S76), are drivers for the
different structural motifs observed.
NBA][BArF ] is not stable when exposed to a moderate
4
vacuum for
3 days. Crystallinity is lost and SSNMR
spectroscopy shows the formation of multiple species – as yet
unidentified. Thus, although the binding of the NBA ligand
appears to not be influenced significantly by the increase in
bite angle of the chelating phosphine, in the measured
ground–state structure steric pressure and/or enhanced
stability of any decomposition products as driven by the
change in phosphine appear to promote reactivity towards
loss of NBA. Hydrogenation of single–crystals of [3–
COA
NBA
COD][BArF ] in a solid/gas reaction resulted in loss of
4
crystallinity.39 We have not characterized the product of this
further.
2.6 {Rh(Cy2P(CH2)5PCy2)}+/COA and NBA: Phosphine
8.15 Å
6.49 Å
ligand
backbone
C–H
activation,
structural
reorganization with retention of crystallinity and Rh(III)
1-–alkane complexes.
Addition of H2 to crystalline [4–NBD][BArF ] resulted in a
4
6.48 Å
7.29 Å
rapid (~5 minutes as measured by 31P{1H} SSNMR
spectroscopy) SC–SC reaction. Analysis of the product formed
using single–crystal X–ray diffraction was hampered by long
range disorder, which is also present in the starting material.
The structure was modelled using a supercell (Z’ = 2) which
gave a satisfactory solution (R = 15.7%) that allowed for the
gross structure of the cation to be determined, Figure 7, but
does not allow for detailed metrics to be discussed. Hydrogen
atoms were not located, and the NBA fragments formed by
hydrogenation were necessarily modelled as rigid bodies.
There are two chemically very similar, but crystallographically
independent, cations in this supercell in which each has a
disordered NBA over two conformations (Figure 7C). There is
no crystallographically imposed local symmetry. The Oh
arrangement of anions in relation to each cation is retained
(Figure S83–S88).
Figure 5. van der Waals65 surfaces for NBA and COA.
2.5
{Rh(Cy2P(CH2)4PCy2)}+/NBA: 2,2––alkane
A
complex.
Addition
of
H2
to
single–crystals
of
4
[Rh(Cy2P(CH2)4PCy2)(NBD)][BArF ],
[3–NBD][BArF ],
4
resulted in a fast (~5 mins as measured by 31P{1H} SSNMR
spectroscopy) SC–SC transformation to give the
corresponding –alkane complex [Rh(Cy2P(CH2)4PCy2)(η2η2–
NBA)][BArF ], [3–NBA][BArF ], Figure 6. The solid–state
4
4
structure of [3–NBA][BArF ] reveals a –bound NBA ligand,
4
with two Rh···H–C interactions using the endo C–H bonds (H–
atoms located). Despite the bite–angle increasing further
compared with [1–NBA][BArF ] and [2–NBA][BArF ] the key–
4
4
metrics associated with this interaction remain essentially
unchanged: Rh1–C1 2.399(2) Å, Rh1–C2 2.396(2) Å. 31P{1H},
1
13C{1H} SSNMR and H/13C FLSG HETCOR data (Figure S33–
S36) are fully consistent with the solid–state structure, i.e.
31P{1H}  66.7 [J(RhP) = 201 Hz], 49.4 [J(RhP) = 196 Hz].
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[BArF
]
4
[BArF
]
cage (Figure S54). Such a low energy process is consistent with
(A)
Cy2
P
4
H
Cy2
Rh
H
the disorder of the NBA fragment modelled in the solid–state.
In the 158 K FSLG HETCOR spectrum cross peaks between
these aliphatic signals in the 13C SSNMR spectrum and low–
+ H2
P
Rh
P
H
SC–SC
5 min
Cy2
H
P
Cy2
[4–NBA][BArF
1
[4–NBD][BArF
]
field peaks in the H projection ( –1.8 to –2.6) are observed.
]
4
4
We assign these to the Rh···H–C interactions, although we
cannot discount ring–current effects from the proximal ArF
groups causing such a high–field shift in other C–H bonds.18,26
(C)
(B)
P
P
H
H
Rh
Rh
+
9
H
H
P
P
(A)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(B)
[BArF
]
C10
4
Cy2
P
Rh···C 2.75 Å
Rh···C 3.10 Å
H
C1
P
H
Rh
H
Rh
P
H
Cy2
P
H
P
H1
P
+
H
Rh
Rh
H
[4–NBA][BArF
]
H
C3
4
P
P
CH2Cl2 – NBA
P2
Rh1
P1
Rh···C 2.75 Å
Rh···C 2.85 Å
H3
Cl1
[BArF
Cy2
]
4
Figure 7. (A) Synthesis of [4–NBA][BArF ]; (B) Ball and stick
H
P
4
Cl2
representation of the one of the crystallographically–
independent cations in the solid–state showing one
disordered NBA component, Rh–C10, ~2.04 Å; P–Rh–P, ~167º;
(C) Representation of the four disordered NBA fragments at
the two crystallographically–independent cations with
Rh···C(alkane) distances. Hydride positions were not
determined, Cy–groups not shown.
Rh Cl
H
P
Cl
Cy2
[4–CH2Cl2][BArF
]
4
[4–CH2Cl2]+
Figure 8. (A) Synthesis of [4–CH2Cl2][BArF ]; (B) Cation of
4
[4–CH2Cl2][BArF ] (50% displacement ellipsoids): Rh1–C3,
4
2.074(2) Å; Rh1–P1, 2.3182(5); Rh1–P2, 2.3099(5); Rh1–Cl1,
2.6032(5); Rh1–H1, 1.65(4); P1–Rh1–P2 167.04(2)º.
Despite the challenges associated with structural
identification, immediately apparent is that the phosphine
pentamethylene backbone has undergone a C–H activation in
the solid–state to form a wide bite angle trans–spanning
phosphine PCP pincer complex. Such intramolecular C–H
activation with a R2P(CH2)5PR2 ligand has precedent in
solution studies, either to form a hydrido–alkyl complex66-68
or through a further –elimination to give a diphosphino–
carbene complex.69 Solution trapping experiments (vide infra)
show that the former has occurred in [4–NBA]+, and thus a
formulation of [Rh(Cy2P(CH2)2(CH)(CH2)2PCy2)H(–NBA)]
[BArF ], [4–NBA][BArF ] is proposed. The disordered NBA
Unlike for [2–NBA][BArF ] or [3–NBA][BArF ], dissolving [4–
4
4
NBA][BArF ] in CD2Cl2 gave a stable complex that could be
4
characterized by solution NMR spectroscopy and single–
crystal
X-ray
diffraction
(crystals
grown
from
CH2Cl2/pentane) as [Rh(Cy2P(CH2)2(CH)(CH2)2PCy2)H(1–
ClCH2Cl)][BArF ], [4–CH2Cl2][BArF ], Figure 8, in which the
4
4
NBA ligand has been replaced by a CH2Cl2 ligand. Vacuum
transfer of the volatiles demonstrates NBA is liberated,
consistent with the initial solid/gas hydrogenation of the NBD
to form [4–NBA][BArF ]. The molecular structure of [4–
4
CH2Cl2][BArF ], Figure 8B, does not suffer from disorder and
4
4
4
ligand shows a range of Rh···C distances to the Rh(III) center
[2.75 – 3.10 Å] and interacts via either the basal or bridge
methylene groups. These distances reflect weak (at best) –
clearly shows the trans–spanning PCP–pincer motif suggested
for [4–NBA][BArF ]. The Rh–hydride (H1) was located, sitting
4
trans to a vacant site, and anti to the remaining hydrogen
associated with the C–H activated methylene group (H3). This
stereochemistry is as expected for an intramolecular C–H
activation.66,68 A CH2Cl2 molecule has displaced the labile
NBA fragment. Displacement of a weakly bound –alkane
ligand by halogenated solvent is well–established,18,28,30 and
the structure is consistent with other crystallographically
interactions compared to, e.g., [2–NBA][BArF ];49 while this
4
spread suggests that the NBA fragment finds a better spatial
fit with the {RhPCP}+ fragment for some conformations over
others. The data, clearly, do not allow the precise binding
mode of the alkane (1–HC, 3–H2C70) to be determined. A
related SC–SC N–C oxidative addition at a Rh(I) center has
been reported in Rh–PNP pincer complexes.71 [4–NBA][BArF ]
is stable indefinitely in a Ar-filled glovebox or under vacuum.
The 298 K 13C{1H} SSNMR spectrum of [4–NBA][BArF ] shows
complexes.49,73,74
[4–
4
characterized
Rh–ClCH2Cl
CH2Cl2][BArF ] is a 16–electron Rh(III) complex, but there is
4
no evidence for any significant supporting agostic interaction
from the cyclohexyl groups: closest Rh···C = 3.261(3) Å.
Although this is not a SC–SC transformation, the Oh
arrangement of anions is retained in the extended solid–state
structure of recrystallized matrial.
4
a featureless region between  116 and 59, demonstrating
hydrogenation of the diene. The 298 K 31P{1H} SSNMR shows
a tightly–coupled ABX system,72 which is less well resolved at
158 K, consistent with inequivalent trans–phosphines that are
in chemically very similar environments bound to a Rh(III)
center:  64.1, 63.4 [J(RhP) ~ 105, J(PP) 325 Hz] (Figure S49–
S50). The 13C NQS spectrum at 158 K shows at least four signals
grouped between  38–35 and  29–27, in the region associated
with aliphatic C–H groups, that indicate a low–energy
molecular motion of the NBA ligand within the cavity of the
Solution NMR data for [4–CH2Cl2][BArF ] at 298 K (CD2Cl2)
4
reveal a fluxional process is occurring. In the 1H NMR
spectrum relatively broad signals are observed for the
phosphine ligand, and no characteristic signal due to the C–H
activated methylene (ca. 3 ppm68) or Rh–H were observed. The
31P{1H} NMR spectrum showed a relatively sharp doublet at 
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[4–COA]+
66 [J(RhP) 121 Hz]. Cooling to 243 K results in a sharpening of
1
the aliphatic region in the H NMR spectrum, and two new
signals at  2.71 and –27.2 [d, J(RhH) = 55 Hz, br, fwhm ~ 40
Hz] appear which integrate to 1 H each. These are assigned to
the C–H activated methylene (i.e. C3) and Rh–H respectively,
the latter with a chemical shift that places it trans to a vacant
site.66,75,76 A spin–saturation experiment at this temperature
shows that these two signals are undergoing slow mutual
exchange. The 31P{1H} NMR spectrum is still a doublet but
shifted slightly to lower field [ 64.4]. These low temperature
data are fully consistent with the solid–state structure of [4–
C35a
C8a
P2a
Rh1a
C1a
H1A
P1a
C3a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
C2a
CH2Cl2][BArF ]. An exchange process that involves reversible
4
reductive C–H bond forming/C–H oxidative addition is
proposed – as suggested for closely related systems66 – and
DFT calculations confirm this is favoured (G‡
= 18.7
calc
kcal/mol) over an alternative –elimination process via a
Figure 9. Cation of [4–COA][BArF ] (30% displacement
diphosphino–carbene intermediate (G‡ = 27.4 kcal/mol,
4
calc
ellipsoids), H atoms not located, disordered [4–COD][BArF ]
see Figure S105, Supporting Materials).69 It is also likely that
the bound CH2Cl2 molecule is undergoing rapid exchange at
the metal center with the solvent.77
4
not shown: Rh1a···C1a, 2.90(3) Å; Rh1–C35a 2.08(3) Å, C1a–C2a,
1.477(2) Å; C1a–C8a, 1.45(2); P1a–Rh1a–P2a 168.5(3)º; C35a–
Rh1a···C1a, 174.6(9); Rh1a–H1A–C1a 140.7(2).
Scheme 4. Synthesis of [4–COA][BArF ]
4
found in [2][COA⊂BArF ] (where we propose a minimal
4
interaction at best), and is of a similar distance to the weak
agostic interaction in trans-[Rh(2,2’-biphenyl)(PiBu3)2][BArF ]
4
[2.979(4) Å], as also interrogated by QTAIM analysis.49 This
distance, combined with a rather open Rh1a–H1A–C1a (albeit
the H–placed in a calculated position) of 140.7(2)º, points to a
1 Rh···H–C interaction. The hydrogen atoms were not located
in [4–COA]+, but as dissolution of the crystalline material in
CD2Cl2 formed [4–CD2Cl2][BArF ]/free COA alongside [4–
4
COD][BArF ] in a 30:70 ratio, Scheme 4, the same Rh–H, C–H
4
activated motif as for [4–NBA][BArF ] is proposed. As for the
4
other –alkane structures reported here, there are multiple
stabilizing C–H···F interactions between the alkane and
proximal CF3 groups in the lattice (Figure S91).
Complex [4–COA][BArF ] is a –alkane complex in which only
4
If [4–COD][BArF ] is subjected to H2 in the solid–state, after
4
one Rh···H–C interaction is present, being related to
structures characterized using solution and DFT techniques,
such as [Re(η6–C6Me6)(CO)2(C5H10)][Al(ORf)4]31 or Rh(η5–
C5H5)(CO)(C8H16)]79 respectively. However, as far as we are
aware this is the first structurally determined example of an
alkane interacting 1– with a transition metal center. Such
motifs have been probed in the ps–timescale using fast time–
resolved infrared spectroscopy (TRIR) combined with DFT
calculations, and are early intermediates in the oxidative
addition of alkanes to coordinatively unsaturated metal
centers.80-82 Valence isoelectronic amine–borane and silane
complexes with 1–M···H–E coordination have been reported
30 mins crystallinity is lost. However, if the reaction is stopped
after only 10 minutes and the resulting single crystals quickly
transferred to an X-ray diffractometer and cooled to 150 K, the
resulting analysis shows that a new complex is formed in 30%
yield in a SC–SC process, with the remainder being unreacted
[4–COD][BArF ]. These data showed this new complex to be
4
[4–COA][BArF ],
[Rh(Cy2P(CH2)2(CH)(CH2)2PCy2)H(–
4
COA)][BArF ]. Although the single–crystal refinement showed
4
a superpositional mixture of [4–COD][BArF ] (70%) and [4–
4
COA][BArF ] (30%) in the Oh–anionic cage, refinement of the
4
constituent components gave a reliable and robust solution (R
= 9.6%).
and show comparable structural metrics to [4–COA][BArF ],
4
The molecular cation of [4–COA]+ is shown in Figure 9. This
shows a C–H activated, trans–spanning, diphosphine “PCP”
pincer ligand with a cyclooctane located in close proximity to
the Rh(III) metal center [Rh1a···C1a 2.90(3) Å]. The COA ligand
is not disordered and shows that all the C–C bonds are single
[1.46(2) – 1.48(2) Å]. The Rh···C distance is very long compared
with other Rh···H–C –alkane complexes, cf [1–
such as wide M–H–E angles and long M···E distances:
83
[Ir(POCOP)H(1-HSiEt3)][B(C6F5)4], A,
(POCOP = 2,6-
[OPtBu2]2C6H3)
and
[Rh(3-Xantphos)(H)2(1-
Chart 1. Of course the increased
H3B·NMe3)][BArF ], B,
84
4
polarity of the E–H bond in these analogs will make a
significant contribution to bonding and thus they are
sufficiently stable to observed using solution NMR
techniques, unlike [4–COA][BArF ].
pentane][BArF ], 2.522(5)Å in which the pentane acts as a
4
4
bidentate ligand.78 However, it is considerably shorter than
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tBu2
P
Atoms in Molecules (QTAIM) and Natural Bond Orbital
(NBO) 2nd order perturbation donor-acceptor interaction
H
O
Ph
P
2
Me
Me
B
A
Rh
H
O
Ir
Si
Et3
analyses performed on the isolated cations shown in Figure 10,
while non-covalent interaction (NCI) plots were run on ion-
H
H
H
P
H
O
P
H
B
Ph2
tBu2
pairs featuring the nearest-neighbour [BArF ]– anion.
4
NMe3
Chart 1. Examples of 1–coordinated –complexes. Anions are
not shown.
If hydrogenation of [4–COD][BArF ] is continued for a total of
4
9
30 minutes decomposition to an, as yet unidentified,
product(s) is observed upon dissolving in CD2Cl2, from which
a featureless 31P{1H} NMR spectrum and a very broad 1H NMR
spectrum are observed. We speculate that this signals the
formation of a paramagnetic Rh(II) dimeric complex on
dissolution,85 but the identity of this species remains to be
resolved. Attempts to obtain meaningful SSNMR data for [4–
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COA][BArF ] were hampered by temporal and temperature
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sensitivity that was amplified by the requirement to use finely
crushed material for analysis by SSNMR that meant that
crystallinity is lost much faster than for larger samples. In the
31P{1H} SSNMR spectrum at 158 K a major, broad, peak at  64
Figure 10. Computed distances (Å) involving key H atoms in
[X-NBA][BArF ] (X = 1, 2 and 3; n = 0, 1 and 2 respectively),
4
[2][COA⊂BArF ] and [4-COD][BArF ]. aendo-CH bonds
4
4
indicated: all exo-CH bond distances fall between 1.094 and
is observed (being similar to that seen for [4–NBA][BArF ]),
1.100 Å bBased on optimization of the major component.
4
alongside [4-COD][BArF ] and decomposition products. The
4
2.7.1 [X-NBA][BArF ] (X = 1, 2 and 3). Computed structures
4
sensitivity of [4–COA][BArF ] contrasts with the relative
4
for all three species show short RhH11/H21 contacts of
around 1.88 Å and elongation of the C1-H11/C2-H21 bonds to
ca. 1.15 Å consistent with the presence of two 3c-2e C–HRh -
interactions in each case. QTAIM and NBO analyses on the [2-
NBA]+ cation confirm this (Figure 11A-C), with the presence of
Rh–H11 and Rh–H21 bond paths and reduced bond critical
point (BCP) metrics for C1–H11 and C2–H21 compared to the
spectator C1–H21/C2–H22 bonds. NBO calculations highlight
the dominance of C–HRh -donation into the trans-*Rh–P
orbital, reinforced by -back-donation from a Rh lone pair (d
orbital) and the cis-Rh-P bonding orbital. Similar results were
computed for [1-NBA]+ and [3-NBA]+ (see Supporting
Materials) indicating that changing the bite angle has a
minimal effect on the Rh–NBA interaction.
stability of [4–NBA][BArF ] or [2][COA⊂BArF ] and perhaps
4
4
reflects the combination of the different steric profile of the
alkane in the former (NBA versus COA) combined with the
stabilizing influence of agostic interactions in the latter.
[4–NBA][BArF ] and [4–COA][BArF ] are shown to have very
4
4
similar structures, with the former’s stability allowing for a
more detailed characterization by SSNMR spectroscopy and
the latter providing a good structural solution by X-ray
crystallography. Combined they thus provide a convincing
analysis as a weakly bound –alkane complex bound 1– at a
Rh(III) center. We believe they are the first Rh(III) –alkane
complexes isolated, or observed, by any method.
2.7 Computational Studies
Figure 11D shows the NCI plot of the [2-NBA][BArF ] ion-pair
Periodic DFT calculations were performed on the extended
4
solid state structures of [X-NBA][BArF ] (X = 1,86 2 and 3),
4
and reveals a broad curved feature running roughly parallel to
the H11C1C2H21 bonds. This reflects the chelating nature
of the NBA ligand and is predominantly stabilising (blue) in
[2][COA⊂BArF ] and [4-COD][BArF ] to assess the structure
4
4
and bonding of these species (see Supporting Materials for
details and tables of computed structures). For the [X-
character, while also exhibiting
a central destabilising
NBA][BArF ] series geometries optimised with the PBE-D3
4
(orange/red) region that is consistent with the presence of the
ring critical point (RCP) in the QTAIM study. Some
destabilising character is also seen between Rh and the center
of the two CH bonds (see detail in Figure 11E). This suggests
two cyclic {RhCH} features in the electron density topology
that are consistent with an 2-interaction and the significant
contribution of classical Rh(d) to *CH -back donation
identified in the NBO analysis. This also highlights how the
NCI approach can amplify the insight gained from the local
QTAIM critical points.87-89 These features contrast with the
more localised 1-interactions associated with C–HRh
bonding in [2][COA⊂BArF ] and [4-COA][BArF ] (see below).
approach provided good agreement with the experimental
structures. For [2][COA⊂BArF ] both the major and minor
4
components were computed and structural metrics around
the Rh centres were again well reproduced. However, more
movement of the COA was seen in these calculations, perhaps
reflecting the absence of significant bonded interactions
between Rh and the COA (see below). For [4–COA][BArF ] the
4
calculations slightly underestimate the Rh1···C1 distance (calc:
2.80 Å; exp: 2.90(3) Å). Given the variations between
computed and experimental structures across the range of
systems under consideration, the subsequent analyses were
based on computed structures in which the Rh, C, B and P
positions were taken from the crystallographic studies and the
H and F atoms were optimised with the PBE-D3 approach.
Selected distances involving key H atom positions optimized
on this basis are shown in Figure 10. A more quantitative
analysis of bonding was then provided by Quantum Theory of
4
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The NCI plot also highlights broad swathes of green that
indicate weak, dispersive stabilization between the NBA
ligand and (i) the aryl groups of the borate anions and (ii) the
cyclohexyl substituents of the cation. Thus intermolecular
dispersion interactions play a crucial role in stabilizing these
alkane ligands within the binding ‘pocket’.90 Similar NCI plots
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were obtained for [1-NBA][BArF ] and [3-NBA][BArF ] and a
side-by-side comparison is provided in Figure S100 in the
Supporting Materials.
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Figure 12. (A) Molecular graph of the [2]+ cation; density
contours plotted in the Rh(H3B)(H9A) plane with BCPs
(green) and RCPs (red); (B) Key BCP metrics and (C) donor-
acceptor interactions associated with the Rh–H11C1
Figure 11. (A) Molecular graph of the [2-NBA]+ cation; density
contours plotted in the RhH11H21 plane with BCPs (green) and
RCPs (red); (B) Key BCP metrics, and (C) donor-acceptor
interactions associated with the Rh–H11C1 interaction; (D)
interaction; (D) NCI plot of the [2][COA⊂BArF ] ion-pair;
4
isosurface generated for s = 0.3 au and -0.07 <  < 0.07 au; (E)
Detail highlighting the region around Rh (the narrow disk
within the circled region arises from an interaction between
cyclohexyl hydrogens and sits well above the coordination
plane). a similar data are obtained for the Rh–H9A and C9–
H9A interactions. b COA omitted from the molecular graph for
clarity, see Supporting Materials.
NCI plot of the [2-NBA][BArF ] ion-pair; isosurface generated
4
for s = 0.3 au and -0.07 <  < 0.07 au; (E) detail of the {Rh-
a
NBA} interaction. similar data are obtained for the Rh–H21,
C2–H21 and C2–H22 interactions.
2.7.2. [2][COA⊂BArF ]. QTAIM and NBO data for the [2]+
The closest Rh…COA contact in [2][COA⊂BArF ] is via H21
4
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cation are displayed in Figure 12A-C. RhH3B and RhH9A
bond paths signal the presence of intramolecular agostic
interactions. The associated Rh…H contacts are longer (ca. 1.94
Å), and the corresponding CH distances shorter (ca. 1.14 Å),
than in the [X-NBA]+ series, suggesting weaker interactions in
[2]+. This is confirmed by reduced (r) values at the RhH
BCPs and lower C-H to trans-*Rh–P -donation via NBO.
NBO, however, also suggests a similar degree of back donation
as [2-NBA]+, although this is not classical Rh(d) to *CH -
back donation but rather involves contributions from both the
cis- and trans-RhP bonds (see also the discussion of [4-COA]+
below). This suggests an 1-C-HRh interaction and is
supported by the NCI plot which highlights these stabilising
agostic interactions with well-defined, localized blue disks
(Figures 12D and E).
with a computed distance of 2.844 Å. This is well within the
sum of the van der Waals radii of Rh and H (3.64 Å)65 and a
weak BCP is computed between these centers ((r) = 0.011eÅ-
3).91 No equivalent donor-acceptor interaction is computed
with NBO, although the NCI plot does suggest a weak,
stabilizing feature between Rh and H21 (see Figures 12D and
E). This is part of a broad area of weakly stabilizing
interactions between the COA and the [2]+ cation, suggesting
any direct covalent RhH21 interaction is at best very weak, if
it exists at all. The COA is further stabilized within the cavity
by dispersive interactions with the two proximate aryl
substituents of the [BArF ]– anion.92
4
2.7.3. [4-COA][BArF ]. The computed structure of [4-
4
COA][BArF ] shows a RhH11 distance of 2.002 Å and an
4
elongated C1H11 distance of 1.13 Å (Figure 10). These, along
with the computed Rh…H11 BCP metrics (Figure 13) suggest a
somewhat weaker -interaction than in the [2]+ cation,
although NBO indicates a similar degree of -donation and, if
anything, greater back donation in this case. A blue/green disk
in the NCI plot confirms this 1 C1H11Rh -interaction, as
well as highlighting broad areas of stabilising dispersion
interactions with the cyclohexyl substituents and the [BArF ]-
4
aryl groups.
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C interactions that would be very difficult to probe in –
alkane complexes using solution techniques due to the
instability of such systems combined with fluxionality
between different M···H–C bonds that often results in a
time–averaged structures in solution, even at very low
tempertaures.14 Thus with the NBA alkane ligand and
simple, chelating, phosphines relativitely strong bidentate
chelating 22 motifs are observed, e.g. [2–NBA][BArF ].
4
9
With a trans spanning C–H activated PCP pincer ligand,
reduced access to the metal center results in
unprecedented 1 Rh(III)···H–C motifs being observed in
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the solid–state, e.g. [4–COA][BArF ], a model for the early
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stages of C–H activation at metal centers. Finally, when the
steric requirement of the phosphine and the alkane
combine with the ability for the phosphine to engage in
stabilizing intramolecular agostic interactions the alkane is
expelled from the metal center, but remarkably stays
encapsulated within the anion framework – providing
baseline experimental structural data for a close – but
essentially nonbonding – approach of an alkane with a
Figure 13. (A) Molecular graph of the [4-COA]+ cation; density
contours plotted in the RhH11C1 plane, BCPs (green) and RCPs
(red); (B) Key BCP metrics and (C) donor-acceptor
interactions associated with the Rh–H11C1 interaction; (D)
metal center: viz. [2][COA⊂BArF ]. Underpinning these
4
NCI plot of the [4-COA][BArF ] ion-pair; isosurface generated
4
remarkable structures is the stabilizing effect of the anion
microenvironment, and in particular the role that
intermolecular dispersion interactions play in stabilizing
these alkane ligands within the binding pocket. Reflecting
this, none of the complexes reported are stable in solution
even at low temperature. The SMOM technique thus
complements elegant low temperature in situ solution
techniques for the synthesis and characterization of –
alkane complexes.29,31,81
for s = 0.3 au and -0.07 <  < than 0.07 au; (E) Detail
highlighting the region around Rh.
An interesting aspect of the NBO analysis of both agostic [2]+
and [4-COA]+ is the degree of donation into *CH from the
trans-Rh-L bonding orbitals (L = P2 or C32 respectively). In [4-
COA]+ this is supported by donation from the cis-Rh–H2
bonding orbital (see Figure 14). These interactions reflect the
1-orientation of the CH bonds in these species ([2]+:
RhHCcalc(ave) = 138; [4-COA]+: RhHCcalc = 133). Similar -
donation has been identified with the onset of the CH…Rh
(‘pregostic’) interaction93,94 and are consistent with the end-
on approach of CH4 in Bürgi-Dunitz trajectories95 and of H2 in
oxidative addition reactions.96,97
22 –alkane
1 –alkane
encapsulated alkane
C
H
C
H
Rh
Rh
Rh
C
H
Rh···C
2.4 Å
2.9 Å
141º
3.7 Å
141º
106º
∠ Rh···H-C
Figure 14. NBO donor-acceptor pairs highlighting -donation
from (a) the trans-RhC32 -bond and (b) the cis-RhH2 -
bond into *CH. NBO occupancies are also indicated.
QTAIM
(r) Rh-H
0.06
0.05
0.01
dominant
significant
*Rh-L
*Rh-L
C-H
C-H
negligible
NBO
*C-H
*C-H
Rh-L
Rhdp
3. CONCLUSIONS
classical
pregostic
localized
weak
stabilizing
NCI
Rh···HC
delocalized
stabilizing
The studies reported herein provide a demonstration of the
power of Solid–State Molecular Organometallic Chemistry
(SMOM–chem), and in particular single–crystal to single–
crystal transformations to provide access to and, when
combined with periodic DFT calculations, characterize a
wide range of different sigma–alkane M···H–C
coordination motifs by systematic variation of the ligand
set. Figure 15 highlights these, alongside selected structural
and computationally–determined bonding parameters for
the corresponding Rh···alkane interaction. The complexes
described present snapshots along a continuum of M···H–
stabilizing
plus
Dispersive stabilization with the [BArF4]– aryl groups and Cy substituents
Figure 15. Snapshots along
a continuum of M···H–C
interactions provided by the complexes reported in this paper,
with key, selected, structural and computationally–
determined bonding parameters for the Rh···alkane
interaction.
That such wide variations of alkane binding modes and
structures are observed while a well–defined molecular
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Bohnsack, A. M.; Ibarra, I. A.; Bakhmutov, V. I.; Lynch,
environment is maintained in the solid–state provides an
exemplar of potentially tuneable, molecular
heterogeneous system that can be precisely characterized.
By using core metal–ligand fragment, i.e.
V. M.; Humphrey, S. M. Rational Design of Porous Coordination
Polymers Based on Bis(phosphine)MCl2 Complexes That Exhibit
High-Temperature H2 Sorption and Chemical Reactivity. J. Am.
Chem. Soc. 2013, 135, 16038.
a
4
a
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{Rh(diphosphine)}+, it also offers a wide range of potential
opportunities for transformations where variation of
metal–ligand interactions are likely to influence rate,
stability and selectivity in catalysis.98 It will be interesting
to see if this can be translated to productive C–H activation
reactions of hydrocarbons using SMOM systems, and our
efforts are currently focussed in this direction.
(11)
González-Gálvez, D.; Nolis, P.; Philippot, K.; Chaudret,
B.; van Leeuwen, P. W. N. M. Phosphine-Stabilized Ruthenium
Nanoparticles: The Effect of the Nature of the Ligand in Catalysis.
ACS Catal. 2012, 2, 317.
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(12)
Thomas, J. M.; Raja, R. Exploiting Nanospace for
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Asymmetric Catalysis: Confinement of Immobilized, Single-Site
Chiral Catalysts Enhances Enantioselectivity. Acc. Chem. Res.
2008, 41, 708.
(13)
Copéret, C.; Comas-Vives, A.; Conley, M. P.; Estes, D. P.;
ASSOCIATED CONTENT
Fedorov, A.; Mougel, V.; Nagae, H.; Núñez-Zarur, F.; Zhizhko, P.
A. Surface Organometallic and Coordination Chemistry toward
Single-Site Heterogeneous Catalysts: Strategies, Methods,
Structures, and Activities. Chem. Rev. 2016, 116, 323.
Supporting Information
Supporting information detailing the synthesis, structural
characterization, solution and solid-state NMR experiments
and computational studies. The Supporting Information is
available free of charge on the ACS Publications website.
(14)
Weller, A. S.; Chadwick, F. M.; McKay, A. I. Transition
Metal Alkane-Sigma Complexes: Synthesis, Characterization, and
Reactivity. Adv. Organomet. Chem. 2016, 66, 223.
(15)
Hall, C.; Perutz, R. N. Transition Metal Alkane
AUTHOR INFORMATION
Corresponding Author
Complexes. Chem. Rev. 1996, 96, 3125.
(16)
Complexes. Chem. Eur. J. 2014, 20, 12704.
(17)
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.
Young, R. D. Characterisation of Alkane σ ‐
Andrew Weller. andrew.weller@chem.ox.ac.uk
Stuart Macgregor , S.A.Macgregor@hw.ac.uk
Pike, S. D.; Thompson, A. L.; Algarra, A. G.; Apperley, D.
ACKNOWLEDGMENT
(18)
Pike, S. D.; Chadwick, F. M.; Rees, N. H.; Scott, M. P.;
SCG Chemicals Co. Ltd the EPSRC EP/M024210/1 and the
Leverhulme Trust (RPG-2015-447). This work used the
Weller, A. S.; Krämer, T.; Macgregor, S. A. Solid-State Synthesis
and Characterization of σ-Alkane Complexes, [Rh(L2)(η2,η2-
ARCHER
UK
National
Supercomputing
Service
C7H12)][BArF ] (L2 = Bidentate Chelating Phosphine). J. Am. Chem.
4
(http://www.archer.ac.uk); Professors J. Brown (Oxford) and
D. Heller (Leibniz-Institut für Katalyse) for helpful
discussions
Soc. 2015, 137, 820.
(19)
Christensen, K. E.; Macgregor, S. A.; Weller, A. S. Formation of a
McKay, A. I.; Krämer, T.; Rees, N. H.; Thompson, A. L.;
σ-alkane Complex and a Molecular Rearrangement in the Solid-
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Additional weak Rh···H–C interactions with similar
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4
9
anion. Stabilizing non-covalent interactions are also seen to the
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Changing the phosphine & hydrocarbon
Variation in

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encapsulated alkane
Periodic DFT,
QTAIM, NBO
1 –alkane
22 –alkane
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