Inorganic Chemistry
Article
−OCH2 of diethyl ether), 2.50 (d, 2 H, 3JHH = 7.6 Hz, CH2, cis to
methine), 2.28 (m, 2H, γ-CH2 equatorial), 1.67 (br dd, 2JHH ≈ 9 Hz,
3JHH ≈ 9 Hz, 2H, γ-CH2 axial), 1.57 (br, 6H, β-Me axial), 1.37 (s, 6H,
β-Me equatorial), 0.78 (br, ∼6H, −CH3 of diethyl ether), ∼0.84 (br,
2H, α-CH2 axial, overlapping with the methyl group of diethyl ether),
In the final least-squares cycle, independent anisotropic displacement
factors were refined for the non-hydrogen atoms. The C−O bond
lengths of the disordered crown ether were constrained to an ideal
value (1.425 Å) using the DFIX restraint. Hydrogen atoms on the α
carbon and the olefinic carbons were located in the difference maps
and refined independently. The other hydrogen atoms were placed in
idealized positions; the methyl groups were allowed to rotate about
the C−C axis to find the best least-squares positions. The
displacement parameters for methylene and methine hydrogens
were set equal to 1.2Ueq for the attached carbon; those for methyl
hydrogens were set to 1.5Ueq. An isotropic extinction parameter was
refined to a final value of x = 3.55(8) × 10−6, where Fc is multiplied by
the factor k[1 + Fc2xλ3/sin 2θ]−1/4, with k being the overall scale
factor. Successful convergence was indicated by the maximum shift/
error of 0.003 for the last cycle. Final refinement parameters are given
in Table 1. The largest peak in the final Fourier difference map (0.64
eÅ−3) was located 1.66 Å from Li2. A final analysis of variance
between observed and calculated structure factors showed no
apparent errors.
2
0.68 (d, JHH = 8 Hz, 2H, α-CH2 equatorial). 13C{1H} NMR (126
MHz, C7D8, −40 °C): δ 66.32 (s, −OCH23of diethyl ether), 50.87
(br, γ-CH2), 48.95 (vb, β-C), 34.35 (s, JPtC = 99.6 Hz, β-Me
equatorial), 33.12 (br, β-Me axial), 14.47 (s, −CH3 of diethyl ether).
The resonances corresponding to the α-CH2 and olefinic carbon
atoms are too broad to be observed at this temperature.
Bis(12-crown-4)lithium Bis[cis-bis(η1,η2-2,2-dimethylpent-4-
en-1-yl)rhodate(I)]lithium (2). To a suspension of [Rh2Cl2(CH2
CH2)4] (0.15g, 0.38 mmol) in diethyl ether (20 mL) at −78 °C was
added dropwise a solution of (2,2-dimethylpent-4-en-1-yl)lithium
(0.17 g, 1.6 mmol) in pentane (20 mL). The mixture, which turned
bright orange in 5 min, was stirred at −78 °C for 2 h and then
warmed to −20 °C and stirred at this temperature for 2 h. The
reaction flask was evacuated at −20 °C for 5 min to remove the
evolved ethylene. The mixture was cooled to −78 °C and filtered into
a cooled (−78 °C) receiver; the filtrate was treated with a cooled
(−20 °C) solution of 12-crown-4 (0.18 g, 1.0 mmol) in diethyl ether
(10 mL) with stirring. An orange precipitate formed immediately. The
mixture was warmed to −20 °C and filtered, and the orange solid was
extracted with diethyl ether (3 × 10 mL). The filtrates and extracts
were combined and slowly evaporated at −20 °C to afford the
product as orange prisms. Anal. Calcd for C22H42Li2O4RhCl: C, 50.5;
H, 8.10. Found: C, 48.6; H, 8.18. Note that we have been unable to
synthesize 2 reproducibly owing to its low thermal stability and high
chemical sensitivity (it decomposes when mixed with hydrocarbons
such as toluene and Nujol); these properties prevented further
characterization by IR or NMR spectroscopy.
NOE NMR Experiments. NOE experiments were performed at
−40 °C on solutions of 1 (∼0.7 M) in C7D8 placed in flame-sealed
NMR tubes. The resonance due to the equatorial α-CH2 protons,
which is well separated from other resonances, shows a relatively large
NOE of 1.5%. The NOEs for the other resonances (most of which are
smaller) were determined by a comparison of the difference peak
7
integral with that of the equatorial α-CH2 proton. The Li−1H NOE
difference spectrum was obtained by subtracting a standard 1H
spectrum from the 1H spectrum collected after saturating the 7Li
resonance for 5 s, with a recycle delay of 2.0 s.
Computational Details. Structural optimizations were performed
with the Gaussian 09 program package87 with density functional
theory (DFT), Grimme’s D3 empirical dispersion correction,88 and
the Becke−Johnson damping function.89 A full list of density
The def2-TZVP90 basis set was used for all atoms except Rh. For Rh,
an additional polarization function was added (def2-TZVPP90 basis
set), and the core electrons were treated with the SDD effective core
potential.91 The minimum-energy structures calculated with the PBE-
D3(BJ) and PBE0 functionals showed the greatest geometrical
similarity of the Rh−Li−Rh unit to that seen in the crystal structure of
2. Because of the large steric bulk of the surrounding ligands, the
dispersion corrections at the PBE level provide increased accuracy.
For all optimized structures with bond lengths within 0.02 Å of the
crystal structure, a frequency calculation was performed to ensure that
the optimized geometry was at a minimum rather than a saddle point
on the potential energy surface.
Extended-transition-state natural orbitals for chemical valence
(ETS-NOCV)92 were computed with the ORCA 4.2.0 package93
using the def2-QZVP90 basis set and SDD electron core potentials91
for the rhodium atoms and def2-TZVP for all other atoms. The
NOCV method partitions the interaction energy into meaningful
contributions and provides a qualitative picture of electron donations
and back-donations. Two different fragmentation schemes for 2 were
considered in the ETS-NOCV computations. The first scheme
considers the Li atom to be the first fragment (subsystem) whereas all
of the other atoms constitute the second fragment. The second
scheme considers one Rh-ligand complex to be the first fragment,
whereas the Li atom together with the second Rh-ligand complex is
the second fragment. A detailed analysis is provided in the Supporting
Information. All ETS-NOCV calculations were performed with the
range-separated hybrid density functional ωB97x-D3(BJ).94−96
Natural energy decomposition analysis (NEDA)81 calculations
were performed as implemented in the NBO7.097 software and
interfaced with Gaussian 16.98 NEDA calculations were performed
with a natural bond orbital (NBO) basis, the long-range ωB97XD
density functional, and the def2-TZVP basis set. The NEDA method
was used to study 2 as well as two model molecular compounds with
dative bonds (NH3BH3 and N2BH3).
Crystallographic Studies. Single crystals of 2 were mounted on
glass fibers with Paratone-N oil (Exxon) and immediately cooled to
−100 °C in a cold nitrogen gas stream on the diffractometer. Standard
peak search and indexing procedures gave rough cell dimensions, and
least-squares refinement using 63 318 reflections yielded the cell
dimensions given in Table 1.
Data were collected with an area detector by using the
measurement parameters listed in Table 1. The monoclinic lattice
and systematic absences uniquely suggested space group P21/c, which
was confirmed by the success of the subsequent refinement. The
measured intensities were reduced to structure factor amplitudes and
their estimated standard deviations by correction for background, scan
speed, and Lorentz and polarization effects. No corrections for crystal
decay were necessary, but a face-indexed absorption correction was
applied, with the minimum and maximum transmission factors being
0.9142 and 0.9695. Systematically absent reflections were deleted and
symmetry-equivalent reflections were averaged to yield the set of
unique data. Two reflections (1 0 2, 0 2 0) were obscured by the
beam stop and were deleted; the remaining 10 418 unique reflections
were used in the least-squares refinement.
Intensity data were collected on a Bruker D8 Venture kappa
diffractometer equipped with a Photon 100 CMOS detector. An Iμs
microfocus source provided the Mo Kα radiation (λ = 0.71073 Å),
which was monochromated with multilayer mirrors. The collection,
cell refinement, and integration of intensity data were carried out with
the APEX3 software.84 Face-indexed absorption corrections were
performed numerically with SADABS.85 The initial structure solution
was solved by direct methods and refined with full-matrix least-
squares program SHELXL.86 Correct positions for the non-hydrogen
atoms were deduced from an E-map and subsequent least-squares
refinement and difference Fourier calculations. One of the 12-crown-4
rings in the bis(12-crown-4)lithium cation is disordered over two
conformations; a site occupancy factor for the major conformer
refined to 0.55. The quantity minimized by the least-squares program
2
was Σw(Fo2 − Fc2)2, where w = {[σ(Fo )]2 + (0.0084P)2 + 4.8998P}−1
2
and P = (Fo + 2Fc2)/3. The analytical approximations to the
scattering factors were used, and all structure factors were corrected
for both the real and imaginary components of anomalous dispersion.
8797
Inorg. Chem. 2021, 60, 8790−8801
Liu, Sumeng
