Isolation of [Ru(IPr)2(CO)H]+ (IPr = 1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and Reactivity toward E-H (E = H, B) Bonds

Article abstract of DOI:10.1021/acs.organomet.6b00173

Halide abstraction from the ruthenium N-heterocyclic carbene complex Ru(IPr)₂(CO)HCl (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with NaBAr₄^F (BAr₄^F = B{C₆H₃(3,5-CF₃)₂}₄) gave the salt [Ru(IPr)₂(CO)H]BAr₄^F (2), which was shown through a combined X-ray/neutron structure refinement and quantum theory of atoms in molecules (QTAIM) study to contain a bifurcated Ru···η³-H₂C ξ-agostic interaction involving one ⁱPr substituent of the IPr ligand. This system complements the previously reported [Ru(IMes)₂(CO)H]⁺ cation (IMes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), where a non-agostic form is favored. Treatment of 2 with CO, H₂, and the amine-boranes H₃B·NR₂H (R = Me, H) gave [Ru(IPr)₂(CO)₃H]BAr₄^F (3), [Ru(IPr)₂(CO)(η²-H₂)H]BAr₄^F (4), and [Ru(IPr)₂(CO)(κ²-H₂BH·NR₂H)H]BAr₄^F (R = Me, 5; R = H, 6), respectively. Heating 5 in the presence of Me₃SiCH-CH₂ led to alkene hydroboration and formation of the C-H activated product [Ru(IPr)(IPr)′(CO)]BAr₄^F (7). X-ray characterization of 3 and 5-7 was complemented by DFT calculations, and the mechanism of H₂/H exchange in 4 was also elucidated. Treatment of 2 with HBcat resulted in Ru-H abstraction to form the boryl complex [Ru(IPr)₂(CO)(Bcat)] BAr₄^F (8), which proved to be competent in the catalytic hydroboration of 1-hexene. In 8, a combined X-ray/neutron structure refinement and QTAIM analysis suggested the presence of a single Ru···η²-HC ξ-agostic interaction.

Full text of DOI:10.1021/acs.organomet.6b00173

Article
pubs.acs.org/Organometallics
Isolation of [Ru(IPr)2(CO)H]+ (IPr = 1,3-Bis(2,6-
diisopropylphenyl)imidazol-2-ylidene) and Reactivity toward E−H (E
= H, B) Bonds
Ian M. Riddlestone,† David McKay,‡,⊥ Matthias J. Gutmann,§ Stuart A. Macgregor,*,‡ Mary F. Mahon,*,†
Hazel A. Sparkes,*,∥ and Michael K. Whittlesey*,†
†Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
‡Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
§ISIS Facility, STFC-Rutherford Appleton Laboratory, Didcot OX11 OQX, U.K.
∥School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
S
* Supporting Information
ABSTRACT: Halide abstraction from the ruthenium N-
heterocyclic carbene complex Ru(IPr)2(CO)HCl (IPr = 1,3-
F
bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with NaBAr4
F
(BAr4 = B{C6H3(3,5-CF3)2}4) gave the salt [Ru(IPr)2(CO)-
H]BAr4 (2), which was shown through a combined X-ray/
F
neutron structure refinement and quantum theory of atoms in
molecules (QTAIM) study to contain a bifurcated Ru···η3-H2C
i
ξ-agostic interaction involving one Pr substituent of the IPr
ligand. This system complements the previously reported [Ru(IMes)2(CO)H]+ cation (IMes =1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene), where a non-agostic form is favored. Treatment of 2 with CO, H2, and the amine−boranes H3B·NR2H (R =
Me, H) gave [Ru(IPr)2(CO)3H]BAr4F (3), [Ru(IPr)2(CO)(η2-H2)H]BAr4F (4), and [Ru(IPr)2(CO)(κ2-H2BH·NR2H)H]BAr4
F
(R = Me, 5; R = H, 6), respectively. Heating 5 in the presence of Me3SiCHCH2 led to alkene hydroboration and formation of
F
the C−H activated product [Ru(IPr)(IPr)′(CO)]BAr4 (7). X-ray characterization of 3 and 5−7 was complemented by DFT
calculations, and the mechanism of H2/H exchange in 4 was also elucidated. Treatment of 2 with HBcat resulted in Ru−H
F
abstraction to form the boryl complex [Ru(IPr)2(CO)(Bcat)] BAr4 (8), which proved to be competent in the catalytic
hydroboration of 1-hexene. In 8, a combined X-ray/neutron structure refinement and QTAIM analysis suggested the presence of
a single Ru···η2-HC ξ-agostic interaction.
Arguably, the preparation of four-coordinate RuIIL4 species is
an even greater synthetic challenge, on the grounds of their
greater electron deficiency: i.e., a 14-electron count. Such
INTRODUCTION
■
The preparation of coordinatively unsaturated transition-metal
complexes is a widespread pursuit for practitioners of
species are therefore, unsurprisingly, rare (Chart 1). The
organometallic chemistry with an eye to developing new or
chelate complexes Ru(PNP)Cl (A) and Ru(PO)2 (B) adopt
improved reactivity of organic substrates. In the case of
ruthenium, efforts to generate low-coordinate Ru(0) species
date from the mid 1960s with Chatt’s attempted synthesis of
triplet ground states, which appear to be enough to reduce their
Lewis acid character.6 Upon changing N(SiMe2CH2PtBu2)2 for
N(CH2CH2PtBu2)2, Ru(PNP)Cl (C) displays a square-planar
the 16-electron chelating phosphine complex Ru(dmpe)2
structure and a singlet ground state due to the combination of
(dmpe = 1,2-bis(dimethylphosphino)ethane),1 which was
employed in some of the earliest attempts to bring about
Chart 1
intra- and intermolecular C−H bond activation.2 It is now
known that this species is far too reactive to exist as anything
other than a transient intermediate that can only be detected at
very low temperature in inert-gas matrices or in solution on
very short, pico- to nanosecond time scales.3 However, some 30
years after Chatt’s studies, Caulton4 and Werner5 demonstrated
that Ru0L4 species could indeed be isolated (and even
structurally characterized) given the appropriate choice of L
ligands; namely, bulky phosphines in combination with π-
Received: March 1, 2016
accepting carbonyl or nitrosyl groups.
© XXXX American Chemical Society
A
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high ligand sterics and strong N→Ru π-donation.7 This same
combination of steric and electronic donor properties also
appears to help rationalize the stability of (Cy-PSiP)RuOtBu
(D).8 In other species, such as [Ru(PtBu2Me)2(CO)R]+ (R =
Ph, H)9−11 and Ru(PPh2{2,6-C6Me2H3})2Cl2 (Chart 2),12
RESULTS AND DISCUSSION
■
Synthesis and Characterization of [Ru(IPr)2(CO)H]+.
The BAr4 (B{C6H3(3,5-CF3)2}4) salt of [Ru(IPr)2(CO)H]+
(2) was isolated in high yield (80%) as a highly air and
moisture sensitive dark orange solid upon chloride abstraction
F
from Ru(IPr)2(CO)HCl19 with NaBArF in C6H5F at room
4
Chart 2
temperature over 12 h (Scheme 2).
Scheme 2. Synthesis of the BAr4F Salt of [Ru(IPr)2(CO)H]+
a
(2)
stabilization benefits from the presence of Ru···H−C agostic
interactions to afford complexes which react as latent 14-
electron species.12−15 Thus, the X-ray structures of both
[Ru(PtBu2Me)2(CO)Ph]+ and [Ru(PtBu2Me)2(CO)H]+ exhib-
it sawhorse configurations, in which both of the remaining
vacant coordination sites at ruthenium are occupied by agostic
a
Both here and in later graphics, the dotted contact between Ru and
i
an Pr methyl group represents the likelihood that some H3C···Ru
agostic interaction is retained in solution.
t
interactions from the phosphine Bu groups. In the case of
An X-ray structure determination on crystals of the
compound isolated from fluorobenzene/hexane revealed two
components, which in each case showed the presence of an ξ-
agostic interaction between the metal and one of the iPr methyl
substituents. This agostic C−H interaction lies trans to the CO
group, with the hydride ligand disordered over the remaining
two coordination sites, trans to each other, in the equatorial
plane. To examine this in more detail, we combined neutron
diffraction data with those from X-ray measurements in a joint
refinement. The cation of the major (55%) component (2a) is
shown in Figure 1. Interestingly, the presence of two similar,
Ru(PPh2{2,6-C6Me2H3})2Cl2, neutron diffraction reveals an
even more unusual stabilizing effect involving two sets of
bifurcated agostic Ru···η3-H2C interactions.16
Our interest in RuIIL4 species was raised by the report of
Gunnoe and co-workers from a number of years ago which
identified the cationic N-heterocyclic carbene (NHC) deriva-
tive [Ru(IMes)2(CO)H]+ (1, Scheme 1; IMes = 1,3-bis(2,4,6-
Scheme 1. Gunnoe’s Reported Synthesis of
17
F
[Ru(IMes)2(CO)H]BAr4 (1)
trimethylphenyl)imidazol-2-ylidene) as a true four-coordinate
Ru(II) species devoid of any agostic stabilization.17 All attempts
to isolate 1 for structural verification proved unsuccessful,
unfortunately, and hence characterization was based upon DFT
calculations and chemical trapping experiments. Given that
variations of NHC N substituents can often be used to bring
about significant changes in the structure/reactivity of
coordinatively unsaturated M(NHC)x complexes,18 we have
employed the bulkier IPr (1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene) ligand for the generation of [Ru-
(IPr)2(CO)H]+ (2). Structural methods (neutron/X-ray
diffraction) and DFT calculations have shown that 2 is
stabilized by a symmetric bifurcated Ru···η3-H2C ξ-agostic
Figure 1. Combined neutron/X-ray structure of the cation in
[Ru(IPr)2(CO)H]BAr4 (major component, 2a). Ellipsoids are
F
shown at the 30% probability level with all hydrogen atoms (except
Ru−H and those on the agostic methyl group) removed for clarity.
Selected bond lengths (Å) and angles (deg): Ru(1)−C(2) 2.102(3),
Ru(1)−C(29) 2.091(4), Ru(1)−C(1) 1.796(4), Ru(1)···C(51)
2.589(3), Ru(1)···H(51A) 2.21(2), Ru(1)···H(51B) 2.14(2),
C(51)−H(51A) 1.09(2), C(51)−H(51B) 1.13(2), C(1)−O(1)
1.160(5); C(2)−Ru(1)−C(29) 176.51(13).
i
interaction involving an Pr methyl group. In solution, 2
undergoes facile coordination of neutral donor ligands (CO,
H3B·NR2H (R = Me, H)) and B−H activation of a borane as
well as intramolecular C−H activation of an IPr ligand.
B
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short Ru···H−C contacts (Ru(1)···H(51A) 2.21(2) Å, Ru(1)···
H(51B) 2.14(2) Å, Ru(1)···H(51c) 3.27(2) Å)) supported the
presence of a bifurcated Ru···η3-H2C agostic interaction far
more symmetric in nature than that seen in Ru(PPh2{2,6-
C6Me2H3})2Cl2, where the Ru···H−C distances ranged from
2.113(10) to 2.507(11) Å.16 Conejero has reported that the
C−H activated NHC complex [Pt(IPr)(IPr)′]SbF620 exhibits a
single ξ-agostic interaction to the nonactivated IPr ligand with
Pt···H and Pt···C distances of 2.017(6) and 2.8760(1) Å,
respectively, and a Pt···H−C angle of 145°. In 2a, the Ru(1)···
C(51) distance is considerably shorter (2.589(3) Å), with Ru···
H−C angles (Ru(1)···H(51A)−C(51)/Ru(1)···H(51B)−
C(51)) of 97.4(11) and 100.2(11)°.
associated BCP has a low ρ(r) value of only 0.012 au (see
computational details in the Supporting Information).
We were unable to affirm that the Ru···H−C interactions
persisted in solution, as the four doublets and two septets of the
iPr groups observed by 1H NMR spectroscopy at room
temperature simply broadened rather than separated upon
cooling to 194 K.24 Low-temperature (200 K) 13C{1H} and 1H-
coupled 13C NMR spectra showed neither any low-frequency-
1
shifted methyl resonance nor any reduced JCH coupling
constant (Supporting Information). The low frequency of the
hydride chemical shift (δ −23.9 at 298 K) was similar to those
of both Ru(IPr)2(CO)HCl and 1 as a result of the vacant trans
coordination site. Notably, NMR measurements of 2 (including
13C spectra accumulated overnight) could be recorded in
CD2Cl2 and gave spectra nearly identical with those recorded in
fluorobenzene, revealing that, unlike [Ru(PtBu2Me)2(CO)H]+,
there was no binding of dichloromethane.11,25 Presumably, the
Lewis acidity of 2 is lowered by the presence of the two
strongly σ donating NHC ligands which, in combination with
their steric bulk, disfavor interaction with a poor base such as
CH2Cl2. A small amount of decomposition of 2 was evident by
NMR spectroscopy (only after several days) in chlorinated
solvents or upon warming to 343 K in C6H5F, although there
was no evidence to suggest that this involved dehydrogenation
of the carbene N substituent as seen for [Ir(IPr)2H2]+.26
Experimental and Computational Comparison of
[Ru(IPr)2(CO)H]+ and [Ru(IMes)2(CO)H]+. In Gunnoe’s
attempts to prepare 1, benzene was used as the solvent for
Further insight into the nature of the agostic interaction in 2a
was obtained from a quantum theory of atoms in molecules
(QTAIM)21 study, where the experimental structure of 2a was
used directly in the QTAIM analysis (Figure 2). This highlights
F
the attempted NaBAr4 abstraction. Upon turning to C6H5F,
we found no discernible change in color of the solution but did
observe a change in the hydride region of the proton NMR
spectrum, the signal for 1 at δ −25.4 being replaced by a new
resonance at δ −29.9 within the time of mixing Ru-
F
(IMes)2(CO)HCl and NaBAr4 . The species responsible for
this new signal proved to be stable for at least 48 h.
F
Comparison with Aldridge’s studies on NaBAr4 abstraction
of chloride from M(IMes)2H2Cl (M = Rh, Ir),26 in particular
the shift of the hydride signal to lower frequency, led us to
propose the formation of the sodium inclusion complex
Figure 2. QTAIM molecular graph of the cation of the major
component 2a, focusing on the Ru1···H51a/H51b interactions.
Calculations were based on the experimental X-ray/neutron structure
and used the BP86 functional. Bond critical points (BCPs) and ring
critical points (RCPs) are shown as green and magenta spheres,
respectively. Selected ρ(r) values (au): BCPs, Ru1···H51b 0.038 and
Ru1···H51a 0.033; RCP, Ru1···H51b−C51-H51a 0.033. See the
Supporting Information for full QTAIM metrics.
F
[Ru(IMes)2(CO)HCl(Na)]BAr4 , in which the sodium cation
is intercalated between the mesityl rings of the NHC. All efforts
to isolate this species with the aim of confirming this
assignment were unsuccessful. Similar behavior was found
upon re-examining the Ru(IPr)2(CO)HCl/NaBAr4F reaction. A
1H NMR spectrum recorded 15 min after mixing the reagents
showed loss of the starting Ru−H resonance (δ −24.5) and
formation of new signals at both higher (δ −23.9) and lower (δ
−28.2) frequencies, assigned to 2 and [Ru(IPr)2(CO)HCl-
curved bond paths associated with both the Ru···H51a and
Ru1···H51b contacts, indicative of bonding interactions and so
consistent with a bifurcated Ru···η3-H2C structure. This is
further confirmed by the presence of a ring critical point (RCP)
enclosed by the {Ru1···H51b−C51−H51a} unit. The com-
puted BCP electron densities, ρ(r), are relatively low at ca.
0.035 au and suggest that, despite the short Ru···H51a/H51b
and Ru···C51 distances, the resultant agostic interactions are
relatively weak.22
F
(Na)]BAr4 , respectively. After 48 h, only the hydride signal for
2 remained, consistent with the inclusion complex being an
intermediate on the pathway to full metathesis. Quite why the
IMes derivative is so much longer lived than the IPr derivative
is unclear. Different behavior was also apparent using
F
[Et3Si(toluene)]BAr4 for halide abstraction instead of
F
NaBAr4 . Ru(IPr)2(CO)HCl was now converted instantly
2b, the cation within the second component present in the
combined neutron/X-ray structure of 2,23 shows a geometry
around Ru1 very similar to that of 2a, with Ru···H51a and
Ru1···H51b contacts of 2.23(2) and 2.16(2) Å, respectively,
and a short Ru−C(51) contact of 2.590(3) Å. QTAIM
calculations also confirm a bifurcated structure. In addition, a
and cleanly through to 2, whereas with Ru(IMes)2(CO)HCl,
there was no clear evidence for the formation of a hydride-
containing product at all.
As a structural comparison of 1 and 2 was not possible
experimentally, DFT calculations were employed to probe the
differences between these two systems. Geometries were now
fully optimized with the BP86 functional: for 2a,b input
i
third Ru···η2-HC contact of 2.44(2) Å to an Pr substituent
located trans to the hydride ligand is seen, although the
C
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geometries were based on the X-ray/neutron structures, and
these structures were adapted to produce input geometries for
their IMes analogues 1a,b. The reported free energies include
corrections for dispersion (D3 parameter set) and C6H5F
solution (PCM approach). For [Ru(IPr)2(CO)H]+, the
optimized structures of 2a,b gave good agreement in the
heavy-atom positions but showed rotation around the C(50)−
C(51) bond such that the bifurcated Ru···η3-H2C agostic
interactions were replaced by a single Ru···η2-H(51a)−C(51)
agostic interaction (2a, Ru(1)···H(51a) = 2.01 Å, Ru(1)···
H(51b) = 2.54 Å; 2b, Ru(2)···H(51a) = 1.96 Å, Ru(2)···
H(51b) = 2.63 Å). In addition, for 2b, the short Ru···H contact
trans to hydride noted experimentally shortens to 2.14 Å in the
calculated structure, which therefore features two single Ru···η2-
H−C agostic interactions, one trans to each of the CO and H
ligands. In the course of these studies an alternative conformer
bereft of any agostic interaction (2c) was also located in which
the closest Ru···H contact was 3.87 Å. Of these three forms, 2b
is computed to be the most stable in C6H5F solution with 2a,c
respectively 0.8 and 3.0 kcal/mol higher in energy.
hydride singlet was indicative of the coordinative saturation.13a
Use of 13CO led to signal enhancement of just the two lowest
frequencies of the three 13C{1H} NMR carbonyl resonances at
δ 173, 190, and 193, consistent with the initial Ru−CO group
being inert to substitution. The cis-13C-labeled CO ligands
2
(Scheme 3) showed the expected small (4 Hz) JCC splitting.
Both signals coupled to the Ru−H resonance, to generate a
2
doublet of doublets signal, with JHC couplings of 26.1 (trans)
and 6.7 Hz (cis).
Of note in the X-ray structure of 3 (Figure 3) were the
distortions of the three distinctly nonlinear Ru−C−O bonds.
Three equivalent structures were also located for [Ru-
(IMes)2(CO)H]+, but now the nonagostic form 1c was the
most stable in C6H5F solvent (cf. 1a at +1.8 kcal/mol and 1b at
+2.1 kcal/mol). Although these computed differences are small,
the tendency to form agostic interactions is clearly greater in
[Ru(IPr)2(CO)H]+ in comparison to [Ru(IMes)2(CO)H]+.27
This reflects the greater ability of the iPr substituents to interact
with the Ru center without undue deformation of the NHC
ligand. For example, in 2a the angle between the plane of the
central imidazol-2-ylidene ring and that of the aryl group of the
2,6-iPr2C6H3 substituent engaged in the agostic interaction is
75.3°, whereas the equivalent angle with the mesityl substituent
in 1a is 55.8°.
Further evidence for 2 retaining an agostic interaction in
solution comes from the different colors observed for solutions
of 1 (brick red)17 and 2 (orange). TDDFT calculations
(CAMB3LYP(C6H5F)//BP86) indicate that the lowest-lying
absorption is dominated by a d−d transition between the
HOMO and LUMO of the system and show that this is blue-
shifted in the presence of an agostic interaction (1a, 440 nm;
1b, 432 nm; 1c, 477 nm; 2a, 399 nm; 2b, 390 nm; 2c, 486 nm).
This reflects the interaction of the C−H bond with the
{Ru(NHC)2(CO)H}+ fragment (NHC = IMes, IPr) in the
agostic structures 1a,b and 2a,b, which has the effect of
destabilizing the LUMO. Orbital plots are provided in the
Supporting Information.
Figure 3. Molecular structure of the cation in [Ru(IPr)2(CO)3H]-
BAr4 (3). Ellipsoids are shown at the 30% probability level with all
F
hydrogen atoms (except Ru−H) removed for clarity. Selected bond
lengths (Å) and angles (deg): Ru(1)−C(1) 2.140(2), Ru(1)−C(28)
2.129(3), Ru(1)−C(55) 1.976(3), Ru(1)−C(56) 1.922(3), Ru(1)−
C(57) 1.945(3); C(1)−Ru(1)−C(28) 171.64(10), C(55)−Ru(1)−
C(56) 91.81(12), C(56)−Ru(1)−C(57) 166.09(12).
The 81° angle between the two mean planes (each containing
the atoms of an NHC ring) revealed that the carbene ligands
are disposed at the upper limit of a staggered arrangement.
Moreover, the three carbonyl ligands about the equatorial girdle
of the cation were each seen to lie atop an IPr phenyl ring
(C55/O1 above the ring based on C16; C56/O2 above the
ring based on C43, and C57/O3 above the ring based on C31).
The ensuing steric factors have combined such that the CO
ligands are each bent away from the face of the aromatic ring
above which each is located. These features are retained in the
BP86-optimized structure of 3 but lost in the less congested
model species [Ru(IMe)2(CO)3H]+ (3′, IMe = 1,3-dimethyl-
imidazol-2-ylidene), confirming their steric origin (similar
deviations from linearity can also arise from electronic
effects4a). The carbonyl oxygens appear to have borne the
maximum brunt of these distortions away from the plane of the
proximate aromatic rings (Ru(1)−C(55)−O(1) 171.9(2)°,
Ru(1)−C(56)−O(2) 171.6(2), Ru(1)−C(57)−O(3)
169.1(2)) away from the plane of the proximate aromatic
ring. These compare to the values of 177.6(5), 176.9(5), and
175.1(5)° found in the cationic phosphine derivative [Ru-
(PPh3)2(CO)3H]+.28 Ultimately, “bowing” of the two trans
Coordination of CO, H2, and B−H Bonds to [Ru-
(IPr)2(CO)H]+. Addition of 1 atm of CO to a fluorobenzene
solution of 2 resulted in displacement of the agostic bonding
and coordination of two additional CO ligands to yield the 18-
F
electron tricarbonyl compound [Ru(IPr)2(CO)3H]BAr4 (3,
Scheme 3). The presence of a high-frequency-shifted (δ −6.81)
Scheme 3
D
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Scheme 4
carbonyl groups in 3 is evidenced by the C(56)−Ru(1)−C(57)
angle of 166.09(12)°. The trans influence of the hydride ligand
manifests itself in the elongation of the Ru(1)−C(55) distance
(1.976(3) Å) relative to the other two Ru−CO bond lengths
(1.945(3) and 1.922(3) Å).
Introduction of H2 (1 atm) into a CD2Cl2 solution of 2
brought about an immediate color change from orange to
yellow resulting from the formation of the dihydrogen hydride
F
complex [Ru(IPr)2(CO)(η2-H2)H]BAr4 (4, Scheme 4). At
room temperature, this showed a single, broad hydride
resonance at δ −4.95 of relative integral 3, suggestive of
rapidly exchanging Ru−H/(η2-H2) ligands. Even at 182 K, the
exchange could not be frozen out, an observation that is in line
with other ruthenium complexes containing a cis arrangement
of dihydrogen and hydride ligands.29,30 Freeze−pump−thaw
degassing failed to completely remove the η2-H2 ligand, and the
resonance at δ −4.95 could still be seen even after 10 degassing
cycles.31 When the solution of 4 was reduced to complete
dryness, 2 was regenerated.
Figure 4. (a) Isomers of [Ru(IPr)2(CO)(η2-H2)H]+ (4), with the
shortest agostic Ru···H contact indicated. (b) Computed structures of
4b and Ru−H/(η2-H2) exchange transition state TS(4b-4b) with
selected distances in Å. Nonparticipating H atoms are omitted for
clarity. All free energies (kcal/mol) are at the BP86-D3(CH2Cl2) level
and are quoted relative to 4c, set to 0.0 kcal/mol.
DFT calculations were employed to provide structural insight
into 4, and three local minima were again located, two of which
feature a single agostic interaction, either trans to CO (4a) or
H (4b), and a third, nonagostic form (4c). All three isomers are
within 0.9 kcal/mol of each other when computed at the BP86-
D3(CH2Cl2) level (Figure 4a). A transition state for Ru−H/
(η2-H2) exchange, TS(4b-4b), was also located. This process
involves H transfer from the original η2-H2 ligand in 4b (labeled
Ha-Hb, Figure 4b) onto the neighboring hydride (Hc).
Concomitant rotation of this new η2-Hc-Hb moiety then
delivers Hc back onto Ha to complete the exchange. In TS(4b-
4b), the agostic interaction shortens significantly (Ru···Hd =
1.91 Å; cf. 2.06 Å in 4b), reflecting the lower trans influence of
the η2-H2 moiety in comparison to a hydride. The overall
barrier (relative to the lowest energy form 4c) is 13.3 kcal/mol,
consistent with rapid exchange on the NMR time scale.
The amine−borane complexes32 [Ru(IPr)2(CO)(κ2-H2BH·
frequency region of the 1H NMR spectra, sharp hydride signals
(5, δ −15.61; 6, δ −15.86) were present in a 1:3 ratio with very
broad B−H resonances (5, δ −2.3; 6, δ −2.1). Upon cooling to
190 K, exchange of the bound and terminal B−H groups was
frozen out to give two distinct, single-integral Ru−H−B singlets
(5, δ −5.83, −3.94; 6, δ −5.63, −4.13), which sharpened upon
1
11B decoupling. In the case of 5, H{11B} NOESY studies
showed that the remaining, unbound B−H signal was hidden
underneath resonances from the IPr groups. The X-ray
structures of both 5 and 6 (Figure 5) revealed distorted-
octahedral geometries comprised of a trans arrangement of IPr
ligands with the CO and hydride then mutually cis and,
therefore, trans to the two metal-bound B−H groups of the
amine−borane ligands. The Ru···B distances of 2.293(4) and
2.333(2) Å were similar to the values in the large number of
known rhodium κ2-bound derivatives (e.g., [Rh(PiBu3)2(κ2-
F
NMe2H)H]BAr4 (5) and [Ru(IPr)2(CO)(κ2-H2BH·NH3)H]-
BAr4F (6) were prepared as alternative examples involving σ E−
H bond coordination to 2 (Scheme 4). 5 and 6 were identified
in the first instance by the appearance of 11B NMR signals at δ
4.5 and −2.4, respectively, characteristically downfield from
those of the free substrates (δ −13.4, −21.6).33,34 In the low-
E
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Article
F
Figure 5. Molecular structure of the cations in (left) [Ru(IPr)2(CO)(κ2-H2BH·NMe2H)H]BAr4 (5) and (right) [Ru(IPr)2(CO)(κ2-H2BH·
F
NH3)H]BAr4 (6). Ellipsoids are shown at the 30% probability level with all hydrogen atoms (except Ru−H and those on B or N) removed for
clarity. Selected bond lengths (Å) and angles (deg) in 5: Ru(1)−C(1) 2.136(3), Ru(1)−C(28) 2.107(3), Ru(1)−C(55) 1.805(4), Ru(1)−B(1)
2.293(4); C(1)−Ru(1)−C(28) 173.11(13), C(55)−Ru(1)−B(1) 142.06(16). Selected bond lengths (Å) and angles (deg) in 6: Ru(1)−C(1)
2.1170(16), Ru(1)−C(28) 2.0950(16), Ru(1)−C(55) 1.813(2), Ru(1)−B(1) 2.333(2); C(1)−Ru(1)−C(28) 176.78(6), C(55)−Ru(1)−B(1)
162.34(10).
H2BH·NMe2H)H2]+ (2.318(8) Å),33,35 [Rh(IMes)2(κ2-H2BH·
NtBuH2)H2]+ (2.305(4) Å)36) although (unsurprisingly)
significantly shorter than those in the κ1-bound ruthenium
complexes [Ru(xantphos)(PPh3)(κ1-HBH2·NH3)H]+
(2.939(3) Å)37 and [Cp′Ru(PMe3)2(κ1-HBH2·NMe3)]+
(2.648(3) Å).38
alkene hydrogenation must occur as well as hydroboration.
Continued heating led to the slow disappearance of the hydride
signal for 2 (15 days at 323 K in C6H5F), alongside a change in
color of the solution from orange to red. Spectroscopic
identification of the product(s) proved to be a thankless task,
due to extensive overlap of signals in both the methyl and
methine regions of the proton NMR spectrum.
The stability of 5 in solution proved to be solvent dependent.
Thus, the complex decomposed in CD2Cl2 over ca. 6 h at room
temperature but was stable for over 1 week in C6H5F. However,
warming to 343 K in C6H5F resulted in dehydrocoupling of the
amine−borane ligand to afford [Me2N-BH2]2 and the
dihydrogen hydride complex 4. Coordination of H2 eliminated
upon dehydrocoupling was also found37 for [Ru(xantphos)-
(PPh3)(κ1-HBH2·NtBuH2)H]+ whereas, in contrast, amino−
borane products of the type [ML2(κ2-H2B-NR2)H2]+ arise
upon the dehydrocoupling of Rh and Ir amine−borane
derivatives.39 This difference is not simply due to Ru vs Rh/
Ir, since Ru(PCy3)2(η2-H2)2H2 has also been shown to form the
amino−borane product Ru(PCy3)2(κ2-H2B-NR2)H2 upon
direct addition of H3B·NR2H (R = H, Me).40 Extension of
the bonding analysis performed by Alcaraz et al. on the
isoelectronic and isostructural complexes [M(PCy3)2(κ2-H2B−
NiPr2)H2]n+ (M = Ru, n = 0; M = Rh, Ir, n = 1) suggests that
the inability of cationic 2 to coordinate an amino−borane
ligand may be connected to poor overlap between the
contracted metal d orbitals and empty BN π* orbital.41
In an attempt to promote H2B−NMe2 coordination, 5 was
heated with an excess of Me3SiCHCH2 as a hydrogen
acceptor. This led, instead, to formation of the hydroboration
product Me3SiCH2CH2BH2NMe2H, which was identified by
comparison of the 11B NMR chemical shift to those of
Fortuitous isolation of a very small number of diffraction-
quality red-orange crystals proved possible. These were
characterized by X-ray crystallography (Figure 6) as the C−H
F
activated IPr complex [Ru(IPr)(IPr)′(CO)]BAr4 (7). The
sawhorse structure (C(1)−Ru(1)−C(29) 175.67(9)°; C(28)−
Ru(1)−C(12) 96.40(12)°) shows an agostic interaction trans
to the activated arm of the IPr ligand (Ru(1)···H(51C) 2.23(2)
Å, Ru(1)···C(51) 3.163(3) Å, Ru(1)−H(51C)−C(51)
158(2)°). This was confirmed by a QTAIM calculation based
on the heavy-atom positions of 7 that showed a Ru(1)···H51c
bond path with ρ(r) = 0.035 au (see Figure S19 in the
Supporting Information). The metalated C−Ru distance of
2.071(2) Å is much shorter than those in either [Ir-
(IPr)′(IPr)″H]+ (2.117(7) Å)43 or [Pt(IPr)(IPr)′]+ (2.226(6)
Å)20, which to the best of our knowledge are the only other
known examples of C−H activated IPr complexes.
B−H Activation by 2. The electrophilic nature of the Ru−
H bond in 2 was demonstrated by the reaction with HBcat,
which generated a rare example of a cationic boryl complex,44
F
[Ru(IPr)2(CO)(Bcat)]BAr4 (8, Scheme 4). The formation of
a boryl ligand was implied in the first instance by a signal at ca.
δ 42 in the 11B NMR spectrum, which is indicative of three-
coordinate boron.45 Free rotation about the Ru−B bond (on
the basis of the appearance of two proton and three 13C
catechol signals) could be frozen out at 213 K, while lowering
the temperature further (to 182 K) resolved the methine
42
t
RCH2CH2BH2NMe3 (R = Bu, Me(CH2)3). The initial
organometallic product of the reaction was 2, implying that
F
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Article
Figure 6. Molecular structure of the cation in [Ru(IPr)(IPr)′(CO)]-
F
BAr4 (7). Ellipsoids are shown at the 30% probability level with all
Figure 7. Combined X-ray/neutron structure of the cation in
[Ru(IPr)2(CO)(Bcat)]BAr4 (8). Ellipsoids are shown at the 30%
F
hydrogen atoms (except those on the agostic methyl group) removed
for clarity. Selected bond lengths (Å) and angles (deg): Ru(1)−C(1)
2.106(2), Ru(1)−C(29) 2.113(2), Ru(1)−C(28) 1.788(3), Ru(1)−
C(12) 2.071(2); C(1)−Ru(1)−C(29) 175.67(9), C(12)−Ru(1)−
C(28) 96.40(12).
probability level with all hydrogen atoms (except those on the agostic
methyl group) removed for clarity. Selected bond lengths (Å) and
angles (deg): Ru(1)−C(1) 2.141(4), Ru(1)−C(28) 2.138(4), Ru(1)−
C(55) 1.834(5), Ru(1)−B(1) 2.030(4); C(1)−Ru(1)−C(28)
172.41(15), C(55)−Ru(1)−B(1) 84.1(2).
protons of the IPr ligands into eight multiplets, each of integral
1. The methyl resonances remained partially overlapping,
although one doublet was shifted to low frequency to δ −0.34,
consistent with agostic bonding.
This was investigated in the solid-state by a joint X-ray/
neutron structure determination and QTAIM study. The
former (Figure 7) revealed metrics similar to those seen in 2,
although with somewhat greater asymmetry in the closest Ru···
H contacts (Ru(1)···C(27) 2.572(4) Å, Ru(1)···H(27b)
2.02(3) Å, Ru(1)···H(27a) 2.46(3) Å, Ru(1)···H(27b)−
C(27) 109(2)°). The associated QTAIM molecular graph
(Figure 8) this time indicates a single Ru···η2-HCiPr ξ-agostic
interaction, with no bond path evident between Ru1 and H27a
and, hence, no RCP that would be indicative of the bifurcated
Ru···η3-H2C form. The strong trans influence boryl ligand46,47
occupied the apical site of the square-pyramidal structure, with
an Ru−B distance (2.030(4) Å) much shorter than those found
in other Ru or Os boryl complexes.48 The catechol substituent
provided the optimal motif for coordination to Ru, since no
reaction at all was observed upon treatment of 2 with HBpin.
The reasons bifurcated Ru···η3-H2C structures are seen in
2a,b while a Ru···η2-HC interaction is preferred in 8 are
presently not clear to us. Our DFT calculations on the isolated
cations of 2a,b indicate that structures with different (or indeed
no) agostic interactions can be very close in energy. Moreover,
a second-order perturbation analysis based on the computed
natural bond orbitals (NBO) suggests that the overall strength
of the agostic interaction does not reflect the binding mode.
Thus, the total σ-donation from the C51−H51a and C51−
H51b σ-BMOs is strongest in 2a (21.2 kcal/mol), weakest in
2b (12.4 kcal/mol), and intermediate from the C27−H27a and
C27−H27b σ-BMOs in 8 (18.6 kcal/mol). See Figures S21 and
S22 in the Supporting Information for full details.
Figure 8. QTAIM molecular graph of 8 focusing on the Ru1···H27b
interaction. Calculations were based on the experimental X-ray/
neutron structure and used the BP86 functional. BCPs and RCPs are
shown as green and magenta spheres, respectively. ρ(r) for the Ru1···
H51b BCP is 0.042 au. See the Supporting Information for full
QTAIM metrics.
Catalytic Hydroboration of Alkenes with 8. Upon
exposure of 8 to 1 atm of H2, elimination of HBcat took place
in the time of mixing with concomitant formation of the
dihydrogen hydride complex 4. The reversible coordination of
the boryl ligand therefore prompted a preliminary study on the
use of 8 as a precursor for catalytic alkene hydroboration.
Rhodium, particularly with phosphine ligands,49 is typically the
element of choice for this transformation, with only a handful
of reports detailing the activity of ruthenium complexes.50
G
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3
298 K): δ 7.74 (s, 8H, o-Ar-H), 7.58 (s, 4H, p-Ar-H), 7.45 (t, JHH
=
Catalytic experiments with 1-hexene showed that 8 gave mainly
the linear hydroboration product, with a small amount of
hexane also generated through competitive alkene hydro-
genation (Table 1). The hydride complex 2 gave an identical
7.7 Hz, 4H, p-Ar-H), 7.18−7.21 (overlapping d, 8H, m-Ar-H), 7.06 (s,
3
4H, NCH), 2.39 (sept, JHH = 7.0 Hz, 4H, CH(CH3)2), 2.32 (sept,
3JHH = 6.8 Hz, 4H, CH(CH3)2), 1.09 (d, JHH = 6.8 Hz, 12H,
3
CH(CH3)2), 1.05 (d, 3JHH = 7.0 Hz, 12H, CH(CH3)2), 0.82 (d, 3JHH
=
3
6.8 Hz, 12H, CH(CH3)2), 0.73 (d, JHH = 7.0 Hz, 12H, CH(CH3)2),
−23.69 (s, 1H, Ru-H). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K): δ
200.2 (s, Ru-CO), 185.4 (s, Ru-CNHC), 162.2 (q, 1JCB = 50 Hz, i-ArC),
145.8 (s, o-ArC), 145.7 (s, o-ArC), 135.3 (s, o-ArC), 135.2 (s, NArC),
131.1 (s, p-ArC), 129.4 (qq, JCF = 32.2 Hz, JCF = 3.1 Hz), m-ArC),
125.8 (s, NCH), 125.1 (q, JCF = 270 Hz, CF3), 125.0 (s, m-ArC),
a
Table 1. Hydroboration of 1-Hexene
2
4
1
3
124.7 (s, m-ArC), 117.9 (sept, JCF = 4 Hz, p-ArC), 29.2 (s,
b
product ratio
CH(CH3)2), 29.2 (s, CH(CH3)2), 24.6 (s, CH(CH3)2), 24.4 (s,
CH(CH3)2), 23.8 (s, CH(CH3)2), 22.3 (s, CH(CH3)2). IR (CH2Cl2,
cm−1): 1964 (νCO). Anal. Calcd for C87H85BN4OF24Ru: C, 59.02; H,
4.84; N, 3.16. Found: C, 58.91; H, 5.00; N, 3.29.
entry
Ru precursor
branched
linear
hexane
1
2
8
2
14
18
80
76
6
6
F
[Ru(IPr)2(CO)3H]BAr4 (3). A J. Young resealable NMR tube was
a
Conditions: 20 equiv of alkene, 40 equiv of HBcat in C6H5F, 298 K
charged with a solution of 2 (0.043 g, 0.025 mmol) in C6H5F (0.5
mL), degassed via three freeze−pump−thaw cycles, and exposed to 1
atm of CO. After 3 h, the pale yellow solution was layered with hexane
to afford pale yellow crystals of 3. Yield: 0.016 g (36%). 1H NMR (500
MHz, CD2Cl2, 298 K): δ 7.73 (s, 8H, o-ArH), 7.56 (s, 4H, p-ArH),
7.51 (t, JHH = 8.1 Hz, 4H, p-ArH), 7.28 (d, JHH = 8.1 Hz, 8H, m-
ArH), 7.16 (s, 4H, NCH), 2.21 (sept, 3JHH = 7.0 Hz, 8H, CH(CH3)2),
1.09 (d, 3JHH = 7.0 Hz, 24H, CH(CH3)2), 1.01 (d, 3JHH = 7.0 Hz, 24H,
CH(CH3)2), −6.81 (s, 1H, RuH). 13C{1H} NMR (126 MHz, CD2Cl2,
298 K): δ 193.1 (s, Ru-CO), 189.6 (s, Ru-CO), 173.1 (s, Ru-CNHC),
b
for 24 h, average of two runs. Products and ratio determined by GC-
MS and GC.
product composition, suggesting that it is converted to 8 under
the catalytic conditions and that it is the boryl complex which
then propagates the subsequent chemistry.47
3
3
SUMMARY AND CONCLUSIONS
■
1
The latent four-coordinate Ru(II) complex [Ru(IPr)2(CO)H]-
162.1 (q, JCB = 51 Hz, i-ArC), 146.4 (s, o-ArC), 136.6 (s, NArC),
2
4
F
135.2 (s, o-ArC), 132.0 (s, p-ArC), 129.2 (qq, JCF = 32 Hz, JCF = 3
Hz, m-ArC), 126.8 (s, NCH), 125.1 (s, m-ArC), 125.0 (q, 1JCF = 271.1
Hz, CF3), 117.8 (m, p-ArC), 29.1 (s, CH(CH3)2), 26.3 (s, CH(CH3)2),
22.6 (s, CH(CH3)2). IR (KBr, cm−1): 2040 (νCO), 2025 (νCO). Anal.
Calcd for C8713C2H85BN4O3F24Ru: C, 58.56; H, 4.69; N, 3.06. Found:
C, 58.39; H, 4.60; N, 3.00.
BAr4 (2) has been prepared and shown by a combination of
structural and computational methods to contain a bifurcated
i
Ru···η3-H2C agostic interaction at one of the carbene Pr
substituents. The agostic bonding appears to play a central role
in allowing 2 to be isolated and structurally characterized, in
contrast to the case for the non-agostic IMes derivative. In
terms of reactivity, 2 behaves like a coordinatively unsaturated
fragment, readily coordinating H2, CO, and amine−boranes.
Treatment with catecholborane highlights the electrophilic
nature of the Ru−H bond, which results in the formation of the
[Ru(IPr)2(CO)(η2-H2)H]BAr4F (4). A J. Young resealable NMR tube
was charged with a solution of 2 (0.010 g, 0.005 mmol) in CD2Cl2 (0.5
mL), degassed via three freeze−pump−thaw cycles, and exposed to 1
atm of H2. After it was shaken, the tube was placed into the NMR
1
spectrometer for characterization. H NMR (400 MHz, CD2Cl2, 182
K): δ 7.72 (s, 8H, o-ArH), 7.53 (s, 4H, p-ArH), 7.44 (t, 3JHH = 7.5 Hz,
F
boryl derivative [Ru(IPr)2(CO)(Bcat)]BAr4 , featuring a
3
Ru···η2-HC interaction. This mode of reactivity, whereby
substrates E−H (E = B, H) can add over the Ru−H bond,
appears to be especially promising as a route to new Ru−E-
containing products and is something we will report more on in
due course.
4H, p-ArH), 7.14 (d, JHH = 7.5 Hz 8H, m-ArH), 7.11 (s, 4H, NCH),
1.95 (m, 8H, CH(CH3)2), 0.89 (d, JHH = 5.4 Hz, 24H, CH(CH3)2),
0.82 (d, JHH = 5.4 Hz, 24H, CH(CH3)2), −4.95 (br s, 3H, RuH +
3
3
Ru(η2-H2)).
F
[Ru(IPr)2(CO)(κ2-H2BH·NMe2H)H]BAr4 (5). H3B·NMe2H (6 μL
of 1.7 M solution in C6H5F, 0.01 mmol) was added to a solution of 2
(0.019 g, 0.01 mmol) in C6H5F (0.5 mL). After 2 h, the solvent was
removed in vacuo, and the residue was washed with hexane (3 × 0.4
mL) and then dried under vacuum. Layering the residue in
fluorobenzene/hexane afforded pale yellow crystals of 5. Yield: 0.017
g (78%). 1H NMR (400 MHz, CD2Cl2, 298 K): δ 7.74 (s, 8H, o-ArH),
EXPERIMENTAL SECTION
■
All manipulations were carried out using standard Schlenk, high-
vacuum, and glovebox techniques using dried and degassed solvents,
unless otherwise stated. NMR spectra were recorded on Bruker
Avance 400 and 500 MHz NMR spectrometers and referenced to
residual solvent signals for 1H and 13C spectra for C6D6 (δ 7.15, 128.0)
and CD2Cl2 (δ 5.32, 54.0). Unlocked samples in fluorobenzene were
referenced to the center of the downfield multiplet at δ 7.11. 11B
spectra were referenced externally to BF3·OEt2 at δ 0.0. All complexes
7.57 (s, 4H, p-ArH), 7.48 (t, 3JHH = 7.8 Hz, 4H, ArH), 7.23 (d, 3JHH
=
3
7.8 Hz, 4H, ArH), 7.20 (d, JHH = 7.8 Hz, 4H, ArH), 7.00 (s, 4H,
NCH), 2.75 (br s, 5H, NH + CH(CH3)2), 2.50 (br s, 4H, CH(CH3)2),
2.03/2.02 (s, 6H, N(CH3)2), 1.05 (d, JHH = 6.2 Hz, 24H,
3
3
CH(CH3)2), 0.92 (d, JHH = 6.7 Hz, 24H, CH(CH3)2), −2.23 (br s,
1
F
3H, RuHB), −15.72 (s, 1H, RuH). H NMR (500 MHz, C6H5F, 298
exhibited a singlet at δ −6.6 for the BAr4 anion. IR spectra were
K): δ 8.37 (s, 8H, o-ArH), 7.68 (s, 4H, p-ArH), 2.81 (br s, 4H,
CH(CH3)2), 2.68 (s, 1H, NH), 2.53 (br s, 4H, CH(CH3)2), 1.86 (s,
recorded on a Nicolet Nexus spectrometer. Elemental analyses were
performed by Elemental Microanalysis Ltd., Okehampton, Devon,
U.K. GC-MS data were collected on an Agilent Technologies 5975C
using an HP-5 column (GC data were collected on the same type of
column). Ru(IPr)2(CO)HCl was prepared according to the
literature.19
3
6H, N(CH3)2), 1.04 (d, JHH = 5.8 Hz, 12H, CH(CH3)2), 0.99 (d,
3JHH = 6.6 Hz, 12H, CH(CH3)2), 0.95 (br s, 12H, CH(CH3)2), 0.89
(d, 3JHH = 6.6 Hz, 12H, CH(CH3)2), −2.26 (br s, 3H, RuHB), −15.61
(s, 1H, RuH). Selected low temperature H{11B} NMR (400 MHz,
1
F
[Ru(IPr)2(CO)H]BAr4 (2). A C6H5F (8 mL) solution of Ru-
CD2Cl2, 190 K): δ −3.94 (s, 1H, RuHB), −5.83 (s, 1H, RuHB),
−15.33 (s, 1H, RuH). 11B NMR (161 MHz, C6H5F, 298 K): δ 4.5 (br
s, RuHB). IR (KBr, cm−1): 1991 (νRuH), 1953 (νCO). Anal. Calcd for
C89H95B2N5OF24Ru: C, 58.42; H, 5.23; N, 3.83. Found: C, 58.35; H,
5.02; N, 3.87.
F
(IPr)2(CO)HCl (0.21 g, 0.21 mmol) was added to a slurry of NaBAr4
(0.192 g, 0.22 mmol) in C6H5F (2 mL), and the suspension was
stirred for 12 h. After filtration, the reaction mixture was concentrated
to ca. 3 mL and layered with hexane to afford dark orange crystals of 2,
which were manually separated by hand from colorless crystals of
F
[Ru(IPr)2(CO)(κ2-H2BH·NH3)H]BAr4 (6). H3B·NH3 (0.0004 g,
F
residual NaBAr4 . Yield: 0.290 g (80%). 1H NMR (500 MHz, CD2Cl2,
0.01 mmol) was added to a solution of 2 (0.021 g, 0.01 mmol) in
H
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Article
C6H5F (0.5 mL). After 2 h, the solvent was removed in vacuo and the
residue was washed with hexane (3 × 0.4 mL) and dried under
vacuum. Recrystallization from fluorobenzene/hexane gave pale yellow
crystals of 6. Yield: 0.013 g (61%). 1H NMR (500 MHz, CD2Cl2, 298
K): δ 7.73 (s, 8H, o-ArH), 7.57 (s, 4H, p-ArH), 7.46 (t, 3JHH = 7.7 Hz,
diffractometer and on the SXD time-of-flight Laue single-crystal
diffractometer instrument at the ISIS spallation neutron source,52
respectively. The neutron experiment for 2 was carried out using two
single crystals that were mounted in random orientations relative to
each other inside a sealed vanadium container filled with argon gas.53
The vanadium can was loaded into a top-loading closed-cycle
refrigerator, and data were collected at three different orientations. A
Nonius kappaCCD instrument was also employed for the data
collection of 3, while those for 5 and 6 were effected using an Agilent
Xcalibur (Mo Kα) diffractometer and that for 7 was completed using
an Agilent SuperNova (Cu Kα) diffractometer. The structure of 8 was
refined using a combination of X-ray data garnered using Cu Kα
radiation and an Agilent SuperNova diffractometer plus neutron data
on the SXD instrument at ISIS. In the latter experiment, one crystal
was sealed inside a vanadium container under argon and placed into a
top-loading closed-cycle refrigerator with data collected at five
different orientations. All diffraction measurements were made at
150 K.
3
3
4H, ArH), 7.20 (d, JHH = 7.8 Hz, 4H, ArH), 7.18 (d, JHH = 8.0 Hz,
4H, ArH), 6.99 (s, 4H, NCH), 2.89 (br s, 3H, NH3), 2.56 (sept, 3JHH
=
3
6.6 Hz, 4H, CH(CH3)2), 2.47 (sept, JHH = 7.0 Hz, 4H, CH(CH3)2),
1.03 (d, 3JHH = 7.0 Hz, 12H, CH(CH3)2), 1.00 (d, 3JHH = 6.6 Hz, 12H,
CH(CH3)2), 0.93 (d, 3JHH = 7.0 Hz, 12H, CH(CH3)2), 0.88 (d, 3JHH
=
6.6 Hz, 12H, CH(CH3)2), −2.15 (br s, 3H, RuHB), −15.86 (s, 1H,
1
RuH). Selected low-temperature H{11B} NMR (400 MHz, CD2Cl2,
190 K): δ −4.13 (s, 1H, RuHB), −5.63 (s, 1H, RuHB), −14.95 (s,
1
RuH). H NMR (500 MHz, C6H5F, 298 K): δ 8.36 (s, 8H, o-ArH),
3
7.67 (s, 4H, p-ArH), 2.88 (br s, 3H NH3), 2.67 (sept, JHH = 6.9 Hz,
3
4H, CH(CH3)2), 2.55 (sept, JHH = 7.0 Hz, 4H, CH(CH3)2), 1.03 (d,
3
3JHH = 6.9 Hz, 12H, CH(CH3)2), 0.98 (d, JHH = 6.9 Hz, 12H,
CH(CH3)2), 0.94 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2), 0.90 (d, 3JHH
=
All of the X-ray refinements were carried out using SHELXL.54
With the exception of 6, the asymmetric unit in all structures
6.9 Hz, 12H, CH(CH3)2), −2.05 (br s, 3H, RuHB), −15.88 (s, 1H,
RuH). 11B NMR (161 MHz, CD2Cl2, 298 K): δ −2.4 (br s, RuHB). IR
(KBr, cm−1): 1948 (νCO). Anal. Calcd for C87H90N5B2N5OF24Ru·
C6H5F: C, 58.90; H, 5.10; N, 3.69. Found: C, 58.35; H, 5.02; N, 3.87.
F
comprises one cation and one BAr4 anion. Hydrides, where present,
were located and refined at a distance of 1.6 Å from the metal center in
the case of the X-ray-only refinements for 3, 5, and 6. Disorder of the
fluorine atoms in some of the anion CF3 groups was not uncommon.
In such instances, C−F and F···F distance restraints were included, and
if merited, ADP restraints were added for affected fractional occupancy
fluorine atoms. Convergence was reasonably straightforward with the
exception of the pertinent details, many of which pertain to disorder,
that follow.
F
[Ru(IPr)2(CO)(Bcat)]BAr4 (8). HBcat (0.003 g, 0.025 mmol) was
added to a solution of 2 (0.041 g, 0.023 mmol) in C6H5F (0.5 mL),
and the reaction mixture was allowed to stand for 1 h. The solvent was
removed under vacuum to yield a pale brown solid, which was washed
with hexane (3 × 0.8 mL) and then redissolved in fluorobenzene/
1
hexane to afford 8 as pale yellow crystals. Yield: 0.034 g (78%). H
NMR (500 MHz, CD2Cl2, 298 K): δ 7.73 (s, 8H, o-ArH), 7.57 (s, 4H,
3
The model in 2, which was solved and refined using X-ray data,
revealed that two of the isopropyl groups in the cation were
disordered, with the positions of C54/C55 and the carbon atoms
attached to C41 (C42/C43) each being split over two sites in a 55:45
ratio. Some C−C distance restraints were employed to help
convergence to a chemically sensible finale. The hydrogen atoms
attached to C51 were located and freely refined, subject to being
located 0.98 Å from the parent atom. The hydride ligand was seen to
be disordered over two trans sites (55:45 ratio), and each fraction was
p-ArH), 7.27 (m, 4H, ArH), 7.21 (m, 4H, ArH), 7.02 (d, JHH = 7.4
Hz, 4H, ArH), 6.97 (s, 4H, NCH), 6.72 (dd, 3JHH = 5.4 Hz, 3JHH = 3.6
3
3
Hz, 2H, ArH), 6.35 (dd, JHH = 5.4 Hz, JHH = 3.6 Hz, ArH), 2.48
3
3
(sept, JHH = 6.8 Hz, 4H, CH(CH3)2), 2.31 (sept, JHH = 6.8 Hz, 4H,
CH(CH3)2), 1.06 (d, 3JHH = 6.8 Hz, 24H, CH(CH3)2), 0.88 (d, 3JHH
=
3
6.8 Hz, 12H, CH(CH3)2), 0.75 (d, JHH = 6.8 Hz, 12H, CH(CH3)2).
1
Selected low-temperature H NMR (400 MHz, CD2Cl2, 182 K): δ
2.70 (sept, 3JHH = 5.1 Hz, 1H, CH(CH3)2), 2.50 (sept, 3JHH = 6.7 Hz,
3
1H, CH(CH3)2), 2.42 (sept, JHH = 6.2 Hz, 1H, CH(CH3)2), 2.35
F
(sept, 3JHH = 6.0 Hz, 1H, CH(CH3)2), 2.23 (m, 1H, CH(CH3)2), 2.13
refined at a distance of 1.6 Å from Ru1. In the BAr4 anion, the
3
3
fluorines attached to C79, C86, and C87 each exhibited disorder over
two sites in respective ratios of 70:30, 60:40, and 50:50. The arising
converged X-ray model was used as the basis for the results presented
here, which were obtained using Jana200655 and a combination of X-
ray and neutron data. With the exception of H51A, H51B, and H51C,
the disordered hydride (H1/H1A) hydrogens were initially refined in
four groups: namely, those confined to the anion and, in the cation,
primary hydrogens, tertiary hydrogens, and aromatic hydrogens. The
arising refined C−H distances were used as the basis for the rigid
groups with which these noncontentious hydrogens were ultimately
included. The disordered hydride was modeled subject to both
components being equidistant from the ruthenium center. The agostic
hydrogens attached to C51 were refined freely. All hydrogen atoms
were treated isotropically. Disordered fluorine atoms were refined with
ADP restraints and with restrained C−F and F···F distances of
1.330(5) and 2.14(3) Å, respectively.
Halide disorder was seen to bedevil many of the CF3 groups within
the anion in 3. In particular, the fluorine atoms attached to C64, C65,
C72, C80, C81, and C89 exhibited respective disorders of 65:35,
50:50, 70:30, 50:50, 80:20, and 55:45. C−F distances were restrained
to being similar within each affected functionality. The isopropyl
carbons, C23/C24, belonging to the cation in 6 were modeled as being
disordered over two sites in a 55:45 ratio. The hydrogen atom attached
to C22 was included at a calculated position on the basis of the major
fractional occupancy components of C23/C24. H5 (attached to N5)
was located and refined, subject to being located at a distance of 0.98 Å
from the parent atom. The hydrogen atoms attached to the boron
center, B1, were located and refined without restraints. Disorder was
also evident in some of the anion CF3 groups. In particular, the
fluorine atoms attached to C64, C72, and C73 were each modeled
over two proximate sites in disorder ratios of 50:50, 60:40, and 60:40,
(sept, JHH = 5.7 Hz, 1H, CH(CH3)2), 1.97 (sept, JHH = 5.8 Hz, 1H,
CH(CH3)2), 1.68 (sept, JHH = 6.5 Hz, 1H, CH(CH3)2), 1.38 (br s,
3
3H, CH(CH3)2), 1.26 (br s, 6H, CH(CH3)2), 1.20 (br s, 3H,
CH(CH3)2), 1.15 (br s, 3H, CH(CH3)2), 1.10 (br s, 3H, CH(CH3)2),
0.97 (br s, 3H, CH(CH3)2), 0.88 (br s, 3H, CH(CH3)2), 0.81 (d, 3JHH
3
= 6.0 Hz, 3H, CH(CH3)2), 0.76 (d, JHH = 5.1 Hz, 6H, CH(CH3)2),
3
3
0.62 (d, JHH = 5.7 Hz, 3H, CH(CH3)2), 0.49 (d, JHH = 6.5 Hz, 3H,
CH(CH3)2), 0.32 (br s, 6H, CH(CH3)2), −0.34 (d, 3JHH = 6.0 Hz, 3H,
CH(CH3)2). 13C{1H} NMR (101 MHz, CD2Cl2, 298 K): δ 199.4 (s,
Ru-CO), 182.2 (s, Ru-CNHC), 162.2 (q, 1JCB = 50 Hz, i-ArC), 149.5 (s,
OC), 145.2 (s, o-ArC), 135.3 (s, NArC), 135.2 (s, o-ArC), 130.8 (s, p-
2
4
ArC), 129.0 (qq, JCF = 32 Hz, JCF = 3 Hz, m-ArC), 126.9 (s,
1
NCH),125.8 (s, m-ArC), 125.0 (q, JCF = 271 Hz, CF3), 124.6 (s, m-
ArC), 120.9 (s, ArC), 117.8 (sept, 3JCF = 4 Hz, p-ArC), 112.1 (s, ArC),
29.8 (s, CH(CH3)2), 29.1 (s, CH(CH3)2), 25.1 (s, CH(CH3)2), 24.8
(s, CH(CH3)2), 22.4 (s, CH(CH3)2), 21.2 (s, CH(CH3)2). 11B NMR
(161 MHz, CD2Cl2, 298 K): δ 41.6 (br s, RuB). IR (CD2Cl2, cm−1):
1981 (νCO). Anal. Calcd for C93H88B2N4O3F24Ru: C, 59.14; H, 4.70;
N, 2.97. Found: C, 59.01; H, 4.55; N, 3.08.
Catalytic Hydroboration. To a solution of 2 (0.004 g, 0.0022
mmol) in C6H5F (0.5 mL) in a vial in the glovebox were added 1-
hexene (0.004 g, 0.0440 mmol) and HBcat (0.011 g, 0.088 mmol), and
the reaction mixture was stirred for 24 h. At this time, 1H NMR
spectroscopy showed no resonances attributable to any remaining 1-
hexene. The composition of the reaction mixture was analyzed by GC-
MS; assignment of the linear product (and, by default, therefore the
branched product) was made by comparison of retention time to a
sample comprising ca. 99% of linear isomer prepared via the
hydroboration of 1-hexene using Rh(PPh3)3Cl.51
Crystallography. Data for the combined X-ray (Mo Kα) and
neutron refinement of 2 were collected using a Nonius kappaCCD
I
DOI: 10.1021/acs.organomet.6b00173
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
respectively. In 6, the hydrogen atoms attached to B1 and N5 were
readily located and freely refined, without any restraints. There may be
some “wagging” disorder associated with the carbonyl ligand.
However, efforts to model this did not improve the refinement;
hence, these were abandoned. Only one CF3 group in the anion was
modeled for disorder, with the fluorines attached to C62 being treated
as located across two sites in a 75:25 ratio. There was also one
disordered molecule of fluorobenzene in the asymmetric unit of this
structure. This was ultimately treated using PLATON SQUEEZE, as
the solvent was disordered over two proximate sites and, in each of
these, the fractional fluorine was additionally disordered.
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.organomet.6b00173.
Multinuclear NMR spectra of 2−6 and 8, computational
data including full QTAIM data for BCPs and RCPs
associated with Ru···H agostic interactions, details of
TDDFT calculations, optimized geometries and energies
and geometries used in QTAIM calculations, and an
NBO analysis of the agostic interactions in 2a,b and 8
(PDF)
Cartesian coordinates of all calculated geometries (XYZ)
The asymmetric unit in 7 comprises one cation, one anion, half of
an ordered molecule of C6H5F, and a region of diffuse solvent. C88,
C91, H91, and F26 in the ordered solvent moiety are coincident with a
crystallographic 2-fold rotation axis which serves to generate the
remainder of the molecule. The disordered region exhibited some
evidence for the presence of one fluorobenzene molecule, but this was
not accessible to any sensible model and hence was treated via
PLATON SQUEEZE. On the basis of the results from this algorithm,
the empirical formula (as presented herein) contains one additional
formula unit of C6H5F, to account for the SQUEEZED solvent. The
hydrogen atoms attached to C51 were located and refined at a distance
of 0.98 Å from the parent atom and subjected to being equidistant
from each other. In the anion, F16−F18 were refined as being
disordered over two proximate sites in a 65:35 ratio.
Crystallographic data for 2, 3, and 5−8 (CIF)
AUTHOR INFORMATION
Corresponding Authors
*S.A.M.: e-mail, s.a.macgregor@hw.ac.uk.
*M.F.M.: e-mail, M.F.Mahon@bath.ac.uk.
*H.A.S.: e-mail, hazel.sparkes@bristol.ac.uk.
*M.K.W.: e-mail, m.k.whittlesey@bath.ac.uk; tel, 44 1225
383748.
■
Present Address
⊥School of Chemistry, University of St Andrews, St Andrews,
Fife KY16 9ST, U.K.
As for 2, the structure of 8 was solved to convergence using X-ray
data and the arising model then used as the basis for a combined
refinement55 using both X-ray and neutron data. In the X-ray-only
model, the hydrogens attached to C27 were located and refined at a
distance of 0.98 Å from the parent atom and with a common Uiso
value. Additionally, the hydrogen atoms attached to C12 were included
at calculated positions but, again, with a common Uiso value. Two of
the CF3 groups in the anion were modeled for disorder (55:45 and
60:40 ratios for fluorine atoms attached to C69 and C76, respectively).
The combined X-ray and neutron refinement for this structure, with
particular emphasis on the treatment of noncontentious hydrogen
atoms, was similar to the strategy adopted for 2. Ultimately, in this
instance, the hydrogens attached to C27 were refined without
restraints.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the experimental assistance of Drs. John Lowe
(NMR), Matthew Jones (GC-MS), and Mark Hutchby (GC).
Prof. Vaclav Petricek is thanked for useful discussions on the
Jana refinements. Experiments at the ISIS Pulsed Neutron and
Muon Source were supported by a beamtime allocation
(Experiment 1510181) from the Science and Technology
Facilities Council (STFC). The EPSRC (grants EP/J009962/1
and EP/J010677/1) is thanked for financial support.
Crystallographic data for compounds 2, 3, and 5−8 have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication nos. CCDC 1435594−1435599. Copies
of the data can be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. (fax (+44) 1223 336033, e-
mail deposit@ccdc.cam.ac.uk).
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