Organometallics
Article
intermediate, which is generated from dissociation of COE
(COE = cyclooctene) from the metal center, can activate the
arene substrate through oxidative addition or σ-bond meta-
thesis, with the former being more likely (Scheme 1). This
seven-coordinate Ir(V) adduct is then proposed to undergo
isomerization, followed by reductive elimination of the PhBPin
product (Scheme 1). The Ir(III)trisboryl is then regenerated
following reaction with B2Pin2 (Scheme 1). This has also been
supported computationally23,25,27,45,47 and is hypothesized to
be the mechanistic pathway with other organometallic iridium
complexes as well.27,34
Computational investigation of the borylation mechanism in
an Ir catalyst immobilized in a MOF has also been carried out.
Gagliardi and co-workers studied the mechanism of methane
borylation with UiO-67-mix-Ir to gain insight into the origin of
the chemoselectivity that was observed.48 It was found that the
Ir center inside the MOF had similar reactivity to the
homogeneous analogues, proceeding through an iridium-
trisboryl intermediate. The selectivity for monoborylated
methane originated from a larger barrier of diffusion (24.7
kcal/mol) for the diborylated product, which is 14.2 kcal/mol
higher than that for monoborylated methane. In addition, the
rate-determining step was found to be isomerization of the
seven coordinate Ir(V) rather than C−H bond cleavage via
oxidative addition as in the homogeneous reaction.48
The kinetic and mechanistic implications of MOF
immobilization of iridium borylation catalysts, however, have
not been thoroughly investigated, experimentally. Herein, we
examine the mechanism of borylation with UiO-67-mix-Ir
catalysts. Using benzene as a model substrate, for ease of
handling and higher activity, kinetic data was collected, and
catalyst speciation was studied. Vapor-phase sorption studies
were used to gain insight into affinity of the Ir catalyst for
benzene. Time course profiles revealed an induction period in
benzene borylation, which is effected by the concentration and
nature of arene that is present. A comparison of benzene and
toluene borylation indicated similar substrate selectivity to the
corresponding homogeneous reaction. Characterization of
UiO-67-mix-Ir postcatalysis also showed low boron content
present in the material, which may indicate little boron
accumulation in the catalyst in its resting state, representing a
departure from previously reported homogeneous systems.
Information for characterization).49 Exposure of 2 to ambient
atmosphere results in a color change from blue to orange,
generating the Ir(III) material, UiO-67-mix-Ir(III) (3), which
had previously shown competency for chemoselective methane
borylation.40 Following washing and soaks in acetone over-
night to remove residual water and activation under high
vacuum at 150 °C overnight, the N2 isotherm for 3 showed a
decrease in surface area from 2430 m2/g to 1550 m2/g, with a
corresponding decrease in quantity of N2 adsorbed (Figure S4
and S8), though the pore width distribution remained
relatively unchanged, centered at ∼1.0 nm. Powder X-ray
diffraction (PXRD) patterns confirmed crystallinity and phase
purity was maintained after postsynthetic metalation and
plasma-optical emission spectroscopy (ICP-OES) confirmed Ir
loading onto the MOFapproximately two Ir atoms per Zr6
node were present (∼13 wt % Ir, Table S2), which
corresponded to the approximately two PhenDC linkers per
Zr6 node.
While structural elucidation by single crystal X-ray
diffractometry of 2 and 3 was precluded due to the disordered
nature of the mixed-linker MOF, X-ray pair distribution
function (PDF)/difference envelope density (DED) analyses
(dPDF, Figure 3A), X-ray absorption spectroscopy (XAS), and
DRIFTS were used to probe the ligand environment around
the as-prepared iridium materials.
High resolution X-ray powder diffraction patterns were
collected for the native MOF 1, and the Ir(I) and Ir(III)
metalated MOFs, 2 and 3 respectively, after which PDF/DED
analyses were conducted. In all three samples, crystallinity was
maintained (Figure S3). DED analysis revealed electron
density centered around the linkers of samples 2 and 3,
consistent with Ir chelated by the phenanthroline linkers and
was localized to the octahedral pore of 1 as opposed to the
tetrahedral pore based on PDF/DED analysis, on average
UiO-67 framework in all samples, though slight distortion of
the Zr6 node was observed after postsynthetic modification of
the native MOF.
A comparison between 3 and authentic samples of
[Ir(COD)Cl]2 and (phen)Ir(COD)(Cl)49(a molecular ana-
logue to the proposed structure of 2, where Cl− is in the inner
coordination sphere) by X-ray absorption near edge structure
(XANES) indicates changes to the local coordination of
iridium upon metalation and air exposure. The increased white
line intensity suggests the presence of Ir(III) in 3 (Figure S19)
and the EXAFS are consistent with the first coordination
sphere consisting of light-scattering atoms (i.e., C, N, or O)
(fitting results in Table S8), both consistent with previously
Upon conducting differential pair distribution function
(dPDF) analysis, with 1 as a reference, local information
regarding the guest iridium and its interactions with the
frameworks in 2 and 3 could be obtained. Three atom−atom
distances in particular were observed in various magnitudes,
centered at ∼2.1, ∼2.4, and ∼3.0 Å (Figure 3A). On the basis
of previous structural characterization of organometallic
phenanthroline-iridium complexes, the distance at 2.1 Å
could be assigned to bonds such as Ir−N, Ir−O, or Ir−C
(consistent with EXAFS) and 3.0 Å to longer range
interactions such as that between Ir and aromatic carbon
atoms on the phenanthroline backbone (Figure 3A). A
distance at 2.4 Å is consistent with the presence of Ir−Cl in
RESULTS AND DISCUSSION
■
Synthesis and Characterization of UiO-67-mix and
UiO-67-mix-Ir Materials. UiO-67-mix (1) was synthesized
according to our reported sample preparation40 (for
spectroscopic characterization confirming catalyst integrity,
discussion and characterization). The MOF was then
metalated with iridium via solvothermal deposition in MOFs
(SIM) at room temperature under inert atmosphere, using a
modification of the previously reported preparation (see the
addition of [Ir(COD)Cl]2 and anhydrous tetrahydrofuran
(THF), the off-white MOF turned green (Figure S1),
consistent with chelation of the Ir(I) precursor by the 1,10-
phenanthroline linker.49 Upon shaking overnight, to facilitate
mass transport, under inert argon atmosphere, the suspension
changed from green to blue (Figure S1), likely due to the
coordination of Cl− to the Ir center, as observed by Colacot
and co-workers with homogeneous (phen)Ir complexes49 (vide
infra), forming UiO-67-mix-Ir(I) (2, see the Supporting
C
Organometallics XXXX, XXX, XXX−XXX