Wrapping Rhodium in a Borane Canopy: Implications for Hydride Formation and Transfer

Article abstract of DOI:10.1021/acs.organomet.1c00194

The unique reactivity of metal hydrides has encouraged considerable advances in both synthesis and catalysis. In this arena, ligand optimization has played a significant role, wherein metal-ligand cooperativity can be employed to guide reactivity. Of the

Full text of DOI:10.1021/acs.organomet.1c00194

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Wrapping Rhodium in a Borane Canopy: Implications for Hydride
Formation and Transfer
Joseph A. Zurakowski, Brady J. H. Austen, and Marcus W. Drover*
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ABSTRACT: The unique reactivity of metal hydrides has
encouraged considerable advances in both synthesis and catalysis.
In this arena, ligand optimization has played a significant role,
wherein metal−ligand cooperativity can be employed to guide
reactivity. Of the organometallic hydride donors, rhodium(I)
bis(diphosphine) compounds are known to be among the strongest
−
with hydricity (ΔGH ) values near that of conventionally used
−
reducing agents: e.g., trialkyl-substituted borohydrides (ΔGH ≈ 26
kcal mol−1 for [HBEt3]). Herein, we assess the thermodynamic site of hydride transfer using a rhodium(I) compound bearing a
boron-rich diphosphine ligand: [RhI(P2BCy4)2]BPh4 (P2BCy = 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane). The
4
divergent reactivity of [RhI(P2BCy4)2]BPh4 and its all-alkyl relative [RhI(dnppe)2]BPh4 (dnppe = 1,2-bis(di-n-propylphosphino)-
ethane) toward hydride donors is underscored, where for the former the preferred site of hydride transfer is the ligand scaffold and
not rhodium. This result is contrasted by facile generation of [RhI(dnppe)2(H)]a species having a value of ΔGH ostensibly similar
−
to that of [RhI(P2BCy4)2(H)] (which is not observed), underscoring a noninnocent and cooperative role for the boron-rich
secondary coordination sphere of the P2BCy4 scaffold. The use of the ligand to serve as a hydride source was accordingly examined.
−1
[Ni(dmpe)2(H)]+ has ΔGH = 49.9 kcal mol , whereas its 5d
−
INTRODUCTION
Well-defined metal hydrides facilitate a number of important
■
congener [Pt(dmpe)2(H)]+, a better hydride donor, has ΔGH
−
= 41.4 kcal mol−1.1a These examples highlight the breadth of
chemical transformations, cementing their importance across
myriad branches of chemistry.1 These moieties are active in
tunability achieved simply by the choice of ligand and/or
hydride (H−), proton (H+), and hydrogen atom (H•) transfer
metal.
As a current axis of our research program, we have begun
researching the behavior of late transition metals ligated by a
reactions; the thermodynamics of which are guided by metal
choice, metal oxidation state, and of course, ligand design.
new diphosphine that boasts four peripheral boranes, P2BCy
−
Insofar as hydride transfer is concerned, hydricity (ΔGH ) can
4
(P2 BC y
= 1,2-bis(di(3-dicyclohexylboraneyl)-
be used as a metric to predict transfer proclivity, with lower
4
propylphosphino)ethane).3 In this regard, we have studied a
terminal hydride complex of nickel(II), though this complex is
a weak donor and transfer to the surrounding boranes of the
values of ΔGH indicating favorable hydride loss (Chart 1A).1
−
In this vein, reactions of metal hydrides and acceptors (A+)
−
−
occur when ΔGH (M−H) is lower than that of the ΔGH (A−
P2BCy ligand was not observed.3b We thus wondered about
−
H) unit being formed. In cases where the ΔGH values of a
4
the outcome if the P2BCy ligand were installed at a metal
donor and acceptor are similar, an equilibrium can be
observed. As a germane example, [Rh(dmpe)2(H)] (dmpe =
1,2-bis(dimethylphosphino)ethane) is known to be one of the
4
−
whose ΔGH −(M−H) value was similar to that of
−
ΔGH ([HBR3] ) (Chart 1C). Such tautomeric behavior has
−
strongest organometallic hydride donors with ΔGH (Rh−H) =
been the subject of great interest, owing to a desire to develop
new bifunctional reagents for H−, H+, and H• transfer with
Cp*(Mn−H)/Cp*H(Mn−2) complexes being an example.4 In
the realm of ligand design, significant progress has also been
made with the aim of developing bifunctional H− delivery
26.6 kcal mol−1; this value is similar to that reported for
2
−1
−
−
triethylborohydride (ΔGH ([HBEt3] ) = 26 kcal mol ). As
such, the reaction of [Rh(dmpe)2]+ with [HBEt3]− produces
an equilibrium mixture of [Rh(dmpe)2(H)] and [Rh-
(dmpe)2]+/[HBEt3]−.2a
Hydricity values are tunable, providing a toolbox of M−H
units primed for selective transfer. Compare, for instance, two
[Rh(diphosphine)2(H)] compounds (Chart 1B), where the
use of depe (depe = 1,2-bis(diethylphosphino)ethane) instead
Received: March 30, 2021
Published: July 1, 2021
of dmpe gives [Rh(depe)2(H)]a slightly worse donor by
1a
about 2 kcal mol−1 (ΔGH = 28.3 kcal mol−1). These values
−
can also be compared to those for some group 10 d8 metals:
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Chart 1. Defining Hydricity (kcal mol−1) and Providing a
Hydricity Scale for Some d8 Metal Bis(diphosphine)
Compounds
Scheme 1. Accessing the Bis(diphosphine) Rh(I) Cations
[1]BPh4 and [2]BPh4
a
a
The inset shows the scXRD structures of [1]BPh4 and [2]BPh4
(30% ellipsoid probability and hydrogens omitted for clarity, except
for those on one hydrocarbyl arm).
Scheme 2. Accessing the Octaboraneyl Rh(I) Cation
a
[3]BPh4
reagents using tethered borohydride5 and other organic-based6
donors.
RESULTS AND DISCUSSION
■
Toward this goal, treatment of a THF solution of [RhI(η4-
NBD)(μ-Cl)]2 (NBD = norbornadiene) with 4 equiv of tape
(tape = tetraallylbisphosphinoethane) resulted in NBD
decoordination and the formation of a yellow solution with
an ample precipitate of [RhI(tape)2]Cl; this compound was
solubilized using Na[BPh4] and isolated as [RhI(tape)2]BPh4
([1]BPh4) in 68% yield (Scheme 1). The 31P{1H} NMR
spectrum (THF-d8) readily corroborated ligand coordination
with the presence of a rhodium-coupled doublet at δP 46.2
1
(1JRh,P = 127 Hz). The H NMR spectrum is additionally
a
The inset shows the DFT-optimized structure of [3Me]+ (hydrogens
consistent with the inclusion of two tape diphosphine ligands,
with signals appropriate for eight pendant allyl groups. Further
validating its molecular structure, layering a saturated THF
solution of [1]BPh4 with hexanes and cooling to −35 °C
overnight provided yellow blocks suitable for analysis by single-
crystal X-ray diffraction (Scheme 1). The structure featured a
square-planar Rh(I) center with eight allyl groups projecting
vertically from the [Rh(diphosphine)2]+ coordination plane.
The n-propyl variant [RhI(dnppe)2]BPh4 ([2]BPh4; dnppe =
1,2-bis(di-n-propylphosphine)ethane) was similarly prepared
and characterized, including by X-ray diffraction.
With the target starting material [1]BPh4 in hand, we next
probed hydrofunctionalization. Addition of [1]BPh4 to a THF
slurry of 8 equiv of HBCy27 provided a clear yellow solution in
<5 min (Scheme 2). Analysis by 31P{1H} NMR spectroscopy
(THF-d8) evidenced complete consumption of [1]+ and the
omitted for clarity).
formation of [RhI(P2BCy4)2]BPh4 ([3]BPh4) with a new
downfield-shifted doublet at δP 54.1 (1JRh,P = 125 Hz, ΔδP =
7.9 ppm; cf. [1]+). In terms of 1H NMR spectroscopy, a cluster
of signals attributable to a series of overlapping C(sp3)−H
resonances were apparent at the cost of the allyl resonances
noted previously for [1]+. Consistent with the presence of
peripheral boraneyl groups, the 11B NMR spectrum of [3]+
featured a broad signature at δB 84 (Δ1/2 = 1820 Hz). By
density functional theory (DFT), a truncated variant of [3Me]+
([RhI(P2BMe4)2]+) employing −BMe2 groups in place of
−BCy2 was optimized, revealing a square-planar rhodium(I)
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center surrounded by eight peripheral sp2-hybridized
(∑<(C−B−C) = 360°) boron centers (Scheme 2).8
spectrum featured two sharp signals at δB −6.5 and −16.3
ppmthe former ascribable to a Li[BPh4] and the latter
characteristic of a borate functionality,9 which we attribute to
the octabutylated product [RhI(P2BCy4(nBu)4)2]7− ([4]7−).
The 1H NMR spectrum of [4]7− also featured diagnostic
upfield-shifted signals associated with CH(Cy)/CH2 groups α
to the borate, [B(Cy)2(nPrP)(nBu)]− with an integration of
∼48 H (theoretical 48 H, in comparison to the aryl resonances
of lost [BPh4]−), consistent with eight alkylated borate
moieties (see the Supporting Information). Indeed, over-
alkylation of BF3 with n-BuLi has been reported to give
The neutral hydride [RhI(P2BCy4)2(H)] was next targeted.
As such, a THF solution of [3]BPh4 was treated with excess n-
BuLi to generate a Rh−H species via β-H elimination (Scheme
3). Multinuclear NMR spectroscopy, however, did not
Scheme 3. Reactions of [2]BPh4 and [3]BPh4 with (A) n-
a
BuLi and (B) K[HBEt3]
[BnBu4]−.10 The ability of the P2BCy ligand framework to
4
datively coordinate Lewis bases has been documented,3 though
this represents the first example of its modification using an
anionic nucleophile. In this instance, alkylation at boron, rather
than rhodium, is favored, protecting the metal site and
discouraging formation of a Rh−[nBu] species that is requisite
for β-H elimination.11
Further contrasting the reactivity of [RhI(P2BCy4)2]+ ([3]+)
with conventional alkyl-substituted [RhI(diphosphine)2]+
compounds, we elected to assess the reactivity of a model
where “BCy2 ” was condensed to “H”. Thus,
[RhI(dnppe)2]+([2]+) was reacted with n-BuLi, rapidly (<5
min) providing the neutral terminal hydride,
[RhI(dnppe)2(H)] (5) (Scheme 3). Akin to other members
of the [RhI(diphosphine)2(H)] family,2 hydride formation is
substantiated by a doublet of quintets at δH −12.2 ppm (1JRh,H
2
1
= 11.8 Hz, JH,P = 13.7 Hz) in the H NMR spectrum.
[RhI(P2BCy4)2(H)] was next targeted via treatment of
[3]BPh4 with excess K[HBEt3] (Scheme 3). In this vein, the
peripheral P2BCy ligand, rhodium, or both could serve as an
4
acceptor site. The 31P NMR spectrum confirmed the presence
of a single symmetrical rhodium(I) compound with δP 53.1
(1JRh,P = 122 Hz)close to the value observed for [3]+,
though the 1H NMR spectrum did not evidence formation of a
RhI−H species. The 11B NMR spectrum illustrated two major
resonances: a sharp signal at δB −6.5 (K[BPh4]) and a broad
signal at δB −3 ppm (Δ1/2 = 470 Hz). A signal for “free”
K[HBEt3] (doublet, δB −12.8; JB,H = 70 Hz)12 was not
1
observed, despite there being excess present in the reaction
mixture. These data suggest a network of rapidly exchanging μ-
Et3B−H−BR3 moieties, resulting from secondary-sphere
interactions between [HBEt3]− and the boron-dense P2BCy
4
ligand framework, a result that is in keeping with ostensibly
similar values of ΔGH between [HBEt3]− and
−
[HBCy2(CH2CH2CH2R)]−. The depiction of [6]7− in Scheme
3 is speculative and is provided to show only proposed
interactions. Indeed, fast exchange has been noted between
K[HBEt3] and BEt3 to give [μ-Et3B−H−BEt3]−.13 In the case
of [4]7− and [6]7−, the unusually low overall 7− charge is
worth noting. For [4]7−, this is supported by 1H NMR
spectroscopy by way of integration, though for [6]7−, this is
not the case. To probe whether fewer than 8 equiv are bound,
compound [3]+ was treated with 1, 2, 4, 8, and 50 equiv of
K[BEt3H] and monitored by NMR spectroscopy. By 11B NMR
spectroscopy, a gradual upfield shift was noted from 84 to 72
ppm (1 equiv), 66 ppm (2 equiv), 48 ppm (4 equiv), and 19
ppm (8 equiv), consistent with interactions between BCy2R
groups and added K[BEt3H]. By 31P NMR spectroscopy, the
observed doublet for [3]+ at δP 54.1 ppm broadens on reaction
with 1 and 2 equiv of K[BEt3H]. Only after 8 equiv of
K[BEt3H] has been added does the signal at δP 53.1 appear
this signal is maintained upon addition of 50 equiv of
a
The structure of [6]7− is shown to demonstrate proposed
interactions. [4]7− and [6]7− were not isolated.
conclude the formation of [RhI(P2BCy4)2(H)]no hydride
resonance was observed by 1H NMR spectroscopy, though the
31P NMR spectrum provided a new signature at δP 46.1 (1JRh,P
= 121 Hz), shifted upfield in comparison to [3]+, the
observation of one signal being consistent with a symmetrical
secondary coordination sphere. Strikingly, the 11B NMR
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K[BEt3H], lending credence to the proposed interactions. This
reactivity showcases the luring effect of the P2BCy4 framework,
encouraging the formation of a concentrated canopy of strong
hydride donors.14
the 31P NMR spectrum showed decoalescence of the
previously noted signal at δP 51.4 ppm to provide two
2
doublets of triplets at δP 53.3 (1JRh,P = 129 Hz, JP,P = 30 Hz)
and 50.9 (1JRh,P = 125 Hz, 2JP,P = 30 Hz). Together, these data
point to 7 as having a Cs-symmetric μ-H−B−H bridge rather
than a terminal C1-symmetric B−H binding mode; this result is
also in keeping with DFT calculations (vide infra). When the
temperature is lowered to 233 K, additional sets of doublets of
triplets are observed, suggesting the presence of multiple μ-H−
B−H isomers.16
To establish the feasibility of hydride transfer from a Rh−H
unit to the P2BCy ligand framework, a THF solution of
4
[RhI(dnppe)2]+ ([2]+) was reacted with 5 equiv of [HBEt3]−
and monitored by NMR spectroscopy (Scheme 3). This
reaction revealed a 1:1 mixture of [RhI(dnppe)2]+ and
[RhI(dnppe)2H], indicating similar values of ΔGH between
−
the two. Together, these data suggest that the unobserved
boraneyl-containing rhodium(I) hydride complex is sufficiently
hydridic to facilitate H− transfer to P2BCy4. Nevertheless, from
the equilibrium observed for [2]+, it might be expected that
some amount of Rh−H should be observedthis is not the
case.
To probe whether eight peripheral boranes are important to
drive H− transfer from 5 to [3]+, we reacted THF solutions of
n
5 with a model compound, BCy2 Oct (nOct = octyl). For
n
addition of 1 equiv of BCy2 Oct, the ratio of [Rh(dnppe)2]+:
[Rh(dnppe)2(H)] was found to be 0.1:1, whereas addition of 8
equiv provided [Rh(dnppe)2]+ as the sole species by 31P NMR
spectroscopy. These experiments suggest that borane concen-
tration, and hence the number of pendant boranes present on
To further benchmark the hydride affinity of P2BCy4, we next
evaluated transfer from a bona fide Rh−H species,
[RhI(dnppe)2(H)] (5), to [RhI(P2BCy4)2]BPh4 ([3]+)
(Scheme 4). The reaction of these two components in a 1:1
the P2BCy ligand scaffold, plays a pivotal role in encouraging
4
transfer. This ligand-based B−H unit can also be used as a
hydride transfer reagent; the reaction between 7 and 3-acetyl-
N-benzylpyridinium hexafluorophosphate ([BNAcP]PF6) pro-
vided 1,4-BNAcPH (Scheme 4).17,18
Scheme 4. (A) Reaction of [3]BPh4 with [Rh(dnppe)2(H)]
(5) to Give 7 and[2]BPh4, (B) 11B NMR Chemical Shifts on
Treatment of [3]BPh4 with 5, and (C) Hydride Transfer to
an [NAD]+ Analogue Using 7
To establish the thermodynamic site of hydride transfer,
density functional theory (DFT)8 was used (Scheme 5). Five
possible hydride transfer isomers, I−V, were calculated: the
hydride [RhI(P2BMe4)2(H)] (I), the terminal borohydride
[RhI(P2BMe4−BH)(P2BMe4)] (II), and the three isomeric
bridging borohydrides [RhI(P2BMe4-μ−B−H−B)(P2BMe4)]
(III−V) with the μ-B−H−B bridging unit being between the
same (III and IV) or different (V) P2BMe ligands. For
4
[RhI(P2BMe4)2(H)] (I), a trigonal-bipyramidal structure was
determined with a Rh−H distance of 1.62 Å, while for
[RhI(P2BMe4−H)(P2BMe4)] (II), a square-planar structure
similar to [3Me]+ having a sole peripheral borohydride unit
(∑∠(C−B−C) = 333°, dB−H = 1.24 Å) was obtained. For the μ-
H−B−H structures III and IV, Rh(I) bridging borohydrides
were optimized: ∑∠(C−B−C) = 340 and 342°, ∠(H−B−H) =
158°, and dB−H = 1.37 and 1.40 Å for III and ∑∠(C−B−C) = 343
and 341°, ∠(H−B−H) = 157°, and dB−H = 1.37 and 1.38 Å for
IV. For IV, Rh(I) was found to deviate from a square planar
geometry with the two planes made by each Rh(diphosphine)
unit differing by 21°(cf. 6° for III). For V, analogous bridging
borohydride metrics of ∑∠(C−B−C) = 336 and 337°, ∠(H−B−
H) = 151°, and dB−H = 1.42 and 1.43 Å were observed. When
the Rh−H isomer (I) was taken to have Grel = 0 kcal mol−1,
that of the terminal B−H variant (II) was found to be −8.7
kcal mol−1. For the bridging borohydrides III−V Grel values of
−20.2, −16.6, and −4.8 kcal mol−1 were obtained, respectively,
indicating III to be the most stable. We also explored the
thermodynamics of hydride transfer using [Rh(dnppe)2(H)]
(5) as a donor (Scheme 5b). Transfer of a hydride from 5 to
[Rh(P2BMe4)2]+ ([3Me]+) provided ΔG°(298 K) = +12.8
kcal mol−1 for the formation of I and ΔG°(298 K) = +4.1 kcal
mol−1 for the formation of II. For the μ-H−B−H-bridged
analogues, ΔG°(298 K) = −7.4, −3.8, and +8.0 kcal mol−1 for
the formation of III−V, respectively. Taken together, these
data indicate that a pendant borane group of P2BMe4 and not
rhodium(I) is the thermodynamic site of hydride transfer and,
further, that a bridging borohydride is favored.
ratio in THF at room temperature proceeded rapidly (<5 min)
as suggested by 31P NMR spectroscopy, where a doublet at δP
53.7 (1JRh,P = 126 Hz) for [RhI(dnppe)2]BPh4 ([2]+) indicated
complete transfer. Given its solubility in hydrocarbon solvent,
complex 7 can be conveniently separated from [RhI(dnppe)2]-
BPh4 ([2]+). Consistent with hydride transfer, the 31P NMR
spectrum provided a broad signal at δP 51.4, with no
discernible couplingwe ascribe this signal to a mixture of
zwitterions bearing bridging μ-B−H−B interactions,
[RhI(P2BCy4-μ-B−H−B)(P2BCy4)] (7).15 This assignment is
consistent with fluxional behavior, the absence of a resonance
attributable to a terminal Rh−H unit, and an upfield shift in
the 11B NMR spectrum for the eight averaged boranes from δB
1
84 to 71 ppm. The variable-temperature H and 31P NMR
spectra for 7 corroborate such dynamic behavior. At 298 K,
there are no upfield-shifted signals attributable to CH(Cy)/
CH2 groups α to a quaternary borate, [B(Cy)2(nPrP)(μ-H)]−;
however, on cooling, several new multiplets appear in the range
0.6 ⩾ δH ⩾ 0.1 ppm, similar to that noted for [4]7−. At 283 K,
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Scheme 5. (A) Relative Energies among Rh−H, Terminal
B−H, and μ-B−H−B Isomers and (B) DFT-Calculated
Thermodynamics of Hydride Transfers
CONCLUSION
■
In summary, we have exposed a unique role for the borane-rich
secondary coordination sphere of the P2BCy4 ligand scaffold.
The behavior of this system is derived from similarity in ΔGH
−
between Rh−H and [HBCy2(CH2CH2CH2R)]−. In this
regard, the secondary coordination sphere acts as a borane
storage site for reactive hydride equivalents. This observation is
supported by DFT, which shows that the borane-rich ligand
and not rhodium(I) is the thermodynamic site of hydride
transfer. Careful choice of model reactions using
[RhI(dnppe)2]+ shows that Rh−H formation should be feasible
using the P2BCy4 scaffold and that [RhI(dnppe)2(H)]/
[RhI(P2BCy4)2(H)] are sufficiently strong hydride donors to
generate borohydrides. Taking advantage of this mode of
reactivity, we show that [RhI(P2BCy4−μ-B-H-B)(P2BCy4)]
promotes secondary-sphere hydride transfer. Further work will
focus on the applications of such systems (and their stored
hydride equivalents) to unearth cooperative reactivity.
EXPERIMENTAL SECTION
■
General Considerations. All experiments were carried out
employing standard Schlenk techniques under an atmosphere of dry
nitrogen employing degassed, dried solvents in a solvent purification
system supplied by PPT, LLC. Nonhalogenated solvents were tested
with a standard purple solution of sodium benzophenone ketyl in
tetrahydrofuran in order to confirm effective moisture removal. d6-
Benzene was dried over molecular sieves and degassed by three
freeze−pump−thaw cycles. HBCy2,7 tape,3a and BCy2 Oct were
prepared using literature procedures. All other reagents were
purchased from commercial vendors and used without further
purification unless otherwise stated.
19
n
1
Physical Methods. H NMR spectra are reported in parts per
million (ppm) and are referenced to residual solvent e.g., 1H(C6D6) δ
7.16 and 13C(C6D6) 128.06; coupling constants are reported in Hz.
13C, 11B, and 31P NMR spectra were performed as proton-decoupled
experiments and are reported in ppm.
Preparation of Compounds. {[RhI(tape)2]BPh4} ([1]BPh4;
C52H68BP4Rh, MW = 930.7 g/mol). In the glovebox, [Rh(μ-Cl)NBD]2
(20 mg, 0.04 mmol, 1 equiv), tape (44 mg, 0.17 mmol, 4 equiv), and
Na[BPh4] (30 mg, 0.09 mmol, 2 equiv) were combined in a 20 mL
scintillation vial equipped with a stir bar. Approximately 4 mL of THF
was added, and the solution was stirred for 4 h at room temperature.
The resulting yellow solution was filtered through Celite to remove
NaCl, and volatiles were removed in vacuo. Recrystallization at −35
°C overnight from a hexanes-layered THF solution gave bright yellow
crystals of [1]BPh4 (55 mg, 68%). 1H NMR (500 MHz, THF-d8, 298
K): δ 7.29 (br, 8H; BPh4), 6.86 (t, 3JHH = 7.3 Hz, 8H; BPh4), 6.71 (t,
3JHH = 7.3 Hz, 4H; BPh4), 5.77 (m, 8H; CH(allyl)), 5.14 (m, 16H;
CH(allyl)), 2.76 (m, 16H; CH2(allyl)), 1.77 (m, 8H; P−CH2CH2
1
linker). 13C{1H} NMR (125 MHz, THF-d8, 298 K): δ 165.1 (q, JBC
= 50.9 Hz; BPh4), 137.1 (BPh4), 132.0 (allyl CH), 125.5 (BPh4),
121.6 (BPh4), 119.4 (allyl CH), 35.0 (m, allyl CH2), 24.2 (m, P−
CH2CH2 linker, obscured by THF-d8 and assigned by 1H−13C
HSQC). 31P{1H} NMR (203 MHz, THF-d8, 298 K): δ = 46.2 (d,
1JRhP = 126.8 Hz). 11B{1H} NMR (161 MHz, THF-d8, 298 K): δ
−6.54 (BPh4). Anal. Calcd for C52H68BP4Rh (930.7): C, 67.11; H,
7.36. Found: C, 66.94; H, 7.22.
{[RhI(dnppe)2]BPh4} ([2]BPh4; C52H84BP4Rh, MW = 946.5 g/mol).
In the glovebox, [Rh(μ-Cl)NBD]2 (20 mg, 0.04 mmol, 1 equiv),
dnppe (45 mg, 0.17 mmol, 4 equiv), and Na[BPh4] (30 mg, 0.09
mmol, 2 equiv) were combined in a 20 mL scintillation vial equipped
with a stir bar. Approximately 4 mL of THF was added, and the
solution was stirred for 4 h at room temperature. The resulting yellow
solution was filtered through Celite to remove NaCl, and volatiles
were removed in vacuo. Recrystallization at −35 °C overnight from a
hexanes-layered THF solution gave bright yellow crystals of [2]BPh4
(57 mg, 70%). 1H NMR (500 MHz, THF-d8, 298 K): δ 7.29 (br, 8H;
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1
BPh4), 6.86 (t, 3JHH = 7.3 Hz, 8H; BPh4), 6.71 (t, 3JHH = 7.3 Hz, 4H;
BPh4), 1.88 (m, 8H), 1.63 (m, 8H), 1.68 (m, 8H), 1.43 (m, 16H),
1.00 (t, 3JHH = 7.2 Hz, 24H). 13C{1H} NMR (125 MHz, THF-d8, 298
importantly, no Rh−H unit is observed. H NMR data are broad
and are presented in Figures S26 and S27. 31P{1H} NMR (203 MHz,
THF-d8, 298 K): δ = 53.1 (d, JRhP = 122 Hz). 11B{1H} NMR (161
1
1
MHz, THF-d8, 298 K): δ 19 (this shift is dependent on the amount of
K[HBEt3] addedsee Figure S31). Compound [6]7− was not
isolated, and thus, an elemental analysis was not performed.
K): δ 165.1 (q, JBC = 50.9 Hz; BPh4), 137.1 (BPh4), 125.5 (BPh4),
121.6 (BPh4), 33.2 (m), 26.6 (m), 20.3, 16.6. 31P{1H} NMR (203
1
MHz, THF-d8, 298 K): δ 53.7 (d, JRhP = 125.5 Hz). 11B{1H} NMR
[RhI(P2BCy4-μ-B−H−B)(P2BCy4)] (7; C124H233B8P4Rh, MW = 2037.5
g/mol). In the glovebox, [3]BPh4 (41 mg, 0.017 mmol, 1 equiv) was
placed in a 20 mL scintillation vial equipped with a stir bar.
Approximately 4 mL of THF was added, and [Rh(dnppe)2(H)] (5)
(11 mg, 0.017 mmol, 1 equiv) was added dropwise at room
temperaturethe reaction mixture was stirred for 30 min. The
resulting yellow solution was filtered through Celite, and volatiles
were removed in vacuo. Extraction of the residue into hexanes (to
remove [Rh(dnppe)2]BPh4 ([2]+)) and removal of solvent in vacuo
gave 7 as an orange oil (32 mg, 90%). 1H NMR (500 MHz, C6D6, 298
K): δ 2.30−1.10 (multiple broad overlapping C(sp3)−H resonances).
1H NMR (500 MHz, THF-d8, 243 K): δ 2.30−1.10 (multiple broad
(161 MHz, THF-d8, 298 K): δ −6.54 (BPh4). Anal. Calcd for
C52H84BP4Rh (946.5): C, 65.96; H, 8.94. Found: C, 66.17; H, 9.01.
{[RhI(P2BCy4)2]BPh4} ([3]BPh4; C148H252B9P4Rh, MW = 2355.7 g/
mol). In the glovebox, {[Rh(tape)2]BPh4} ([1]BPh4) (20 mg, 0.02
mmol, 1 equiv), and HBCy2 (31 mg, 0.17 mmol, 8 equivs) were
combined in a 20 mL scintillation vial equipped with a stir bar.
Approximately 4 mL of THF was added, and the solution was stirred
for 30 min at room temperature. The resulting yellow solution was
filtered through Celite, and the solvent was removed in vacuo to give
[3]BPh4 as a yellow foam (46 mg, 90%). 1H NMR (500 MHz, THF-
d8, 298 K): δ 7.29 (br, 8H; BPh4), 6.86 (t, 3JHH = 7.3 Hz, 8H; BPh4),
6.71 (t, 3JHH = 7.3 Hz, 4H; BPh4), 1.95 (m, 8H; P−CH2CH2 linker),
1.80−1.00 (multiple overlapping C(sp3)−H resonances). 13C{1H}
NMR (125 MHz, THF-d8, 298 K): δ 165.1 (q, 1JBC = 50.9 Hz; BPh4),
137.1 (BPh4), 125.5 (BPh4), 121.6 (BPh4), 36.5 (m), 33.5 (m), 28.8,
28.2, 27.9, 27.7, 27.6, 21.3. 31P{1H} NMR (203 MHz, THF-d8, 298
K): δ 54.1 (d, 1JRhP = 125.1 Hz). 11B{1H} NMR (161 MHz, THF-d8,
298 K): δ +83 (Δ1/2 = 2200 Hz), −6.54 (BPh4). Anal. Calcd for
C148H252B9P4Rh (2355.7): C, 75.46; H, 10.78. Best found: C, 74.00;
H, 10.79. These results are outside the range viewed as establishing
analytical purity but are provided to illustrate the best values obtained
to date. Clean NMR data indicate >95% purity.
overlapping C(sp3)−H resonances), 0.52 (m), 0.46 (m), 0.30 (m),
0.14 (m). 13C{1H} NMR (125 MHz, THF-d8, 298 K): δ 36.4 (v br),
35.3 (br), 29.7 (br), 28.1 (v br). 31P{1H} NMR (203 MHz, THF-d8,
298 K): δ 51.4 (v br). 31P{1H} NMR (203 MHz, THF-d8, 253 K): δ
53.3 (dt, 1JRh,P = 129 Hz, 2JPP = 30 Hz), 50.9 (dt, 1JRhP = 125 Hz, 2JPP
= 30 Hz). 11B{1H} NMR (161 MHz, THF-d8, 298 K): δ +94−58 (v
br); see Figure S45. 11B{1H} NMR (161 MHz, THF-d8, 193 K): no
signal.
Generation of a Model Compound, [Rh(dnppe)]2[HB(Cy)2(nOct)]
(8; C48H104BP4Rh, MW = 919 g/mol). In the glovebox,
n
[RhI(dnppe)2H] (5) (7 mg, 0.01 mmol, 1 equiv) and BCy2 Oct (7
[RhI(P2BCy4(nBu)4)2]7− ([4]7−). In the glovebox, [3]BPh4 (15 mg,
0.006 mmol, 1 equiv) was dissolved in ca. 500 μL of THF-d8 and the
mixture was transferred to a J. Young NMR tube. Subsequently, n-
BuLi (1.6 M in hexanes) (32 μL, 0.048 mmol, 8 equiv) was added and
the tube was shaken. The NMR tube was removed from the glovebox
and immediately analyzed by NMR spectroscopy. Efforts to isolate
[4]7− were not met with success due to poor solubility. By 31P NMR
mg, 0.024 mmol, 2 equiv) were combined in 500 μL of THF-d8. The
mixture was transferred to a J. Young NMR tube, removed from the
glovebox, and immediately analyzed by NMR spectroscopy, indicating
a 0.3:1 [RhI(dnppe)2H]:[RhI(dnppe)2]+ ratio. For the isolation of 8,
solvent was removed in vacuo and the resulting residue was washed
n
with hexanes (to remove unreacted 5 and BCy2 Oct), giving a yellow
n
1
spectroscopy, the conversion of [3]+ was >99%. H NMR data are
solid (9 mg, 88%). N.B.: despite there being 2 equiv of BCy2 Oct in
n
this reaction, the terminal borohydride [HBCy2 Oct]− and not the μ-
broad and are presented in Figures S16−S19; signals present between
0.20 and −0.20 ppm are consistent with alkyl groups adjacent a four-
coordinate borate: 0.15 ppm (m, 16H; CH2), 0 (m, 8H; CH), −0.06
(m, 16H; CH2), −0.14 (m, 8H; CH). Importantly, no Rh−H unit is
observed. 31P{1H} NMR (203 MHz, THF-d8, 298 K): δ 46.1 (d, 1JRh,P
= 121 Hz). 11B{1H} NMR (161 MHz, THF-d8, 298 K): δ −16.3
H−B−H bridged borohydride counteranion was observed in the
isolated product. However, an analysis of the reaction mixture (prior
n
1
to isolation and removal of BCy2 Oct) revealed different H and 11B
NMR spectra, suggesting rapid exchange between terminal and
bridged species (see the Supporting Information). 1H NMR (500
MHz, THF-d8, 298 K): δ 2.03 (m, 8H), 1.91 (m, 8H; P−CH2CH2
linker), 1.73 (m, 10H, obstructed by THF-d8), 1.63 (m, 10H), 1.51
n
([BCy2 BunPrP]−). Compound [4]7− was not isolated, and thus, an
elemental analysis was not performed.
3
[RhI(dnppe)2(H)] (5; C28H65P4Rh, MW = 628.6 g/mol). In the
glovebox, [2]BPh4 (20 mg, 0.02 mmol, 1 equiv) was placed in a 20
mL scintillation vial equipped with a stir bar. Approximately 4 mL of
THF was added, and n-BuLi (1.6 M in hexanes) (13 μL, 0.02 mmol, 1
equiv) was added at room temperaturethe reaction mixture was
stirred for 30 min. The resulting yellow solution was filtered through
Celite, and volatiles were removed in vacuo. Extraction of the residue
into hexanes and removal of the solvent in vacuo gave 5 as an orange
powder (11 mg, 82%). 1H NMR (500 MHz, THF-d8, 298 K): δ 1.50
(m, 16H), 1.40 (m, 16H), 1.39 (m, 8H; P−CH2CH2 linker), 0.95 (t,
3JHH = 7.1 Hz, 24H), − 12.2 (dq, 1JRhH = 11.8 Hz, 3JHP = 13.7 Hz, 1H;
Rh−H). 13C{1H} NMR (125 MHz, THF-d8, 298 K): δ 38.9 (m), 31.1
(m), 19.6, 16.5. 31P{1H} NMR (203 MHz, THF-d8, 298 K): δ 48.3
(m, 16H), 1.28 (m, 10H), 1.23−1.08 (m, 10H), 1.03 (t, JH,H = 7.3
3
Hz, 24H), 0.88 (t, JH,H = 7.2 Hz, 3H), 0.14 (m, 2H), 0.02 (m, 2H),
BH not observed. 31P{1H} NMR (203 MHz, THF-d8, 298 K): δ 54.0
(d, 1JRh,P = 125.3 Hz). 11B{1H} NMR (161 MHz, THF-d8, 298 K): δ
−4.6.20
Hydride Transfer from 7 to [BNAcP]PF6. In the glovebox,
[RhI(P2BCy4−H)(P2BCy4)] (7) (6 mg, 0.003 mmol, 1 equiv) and
[BNAcP]PF6 (1 mg, 0.003 mmol, 1 equiv) were combined in 500 μL
of THF-d8. The mixture was transferred to a J. Young NMR tube,
removed from the glovebox, and immediately analyzed by NMR
spectroscopy, which evidenced formation of 1,4-BNAcPH in 65%
conversion (Figure S58).
1
(d, JRh,P = 139.9 Hz). FT-IR (ATR, cm−1): 1875 (Rh−H). Anal.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00194.
■
sı
Calcd for C28H65P4Rh (628.6): C, 53.50; H, 10.42. Found: C, 53.59;
H, 11.18.
[RhI(P2BCy4(μ-HBEt3)4)2]7− ([6]7−). In the glovebox, [3]BPh4 (15
mg, 0.006 mmol, 1 equiv) was dissolved in ca. 500 μL of THF-d8 and
the mixture was transferred to a J. Young NMR tube. Subsequently,
K[HBEt3] (1 M in THF) (51 μL, 0.051 mmol, 8 equiv) was added
and the tube was shaken, giving a fluffy white solid and a yellow
solution. The NMR tube was removed from the glovebox and
immediately analyzed by NMR spectroscopy. Efforts to isolate [6]7−
were not met with success due to poor solubility. By 31P NMR
spectroscopy, the conversion of [3]+ was >99%. N.B.: the structure of
[6]7− is drawn to illustrate proposed interactions only. Most
1H, 13C{1H}, 31P{1H}, and 11B NMR spectra for all
complexes, crystallographic data, and computational
details (PDF)
XYZ coordinates for DFT calculations (XYZ)
Accession Codes
CCDC 2069807−2069808 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
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Organometallics 2021, 40, 2450−2457

Organometallics
pubs.acs.org/Organometallics
Article
(diphosphine)2]+ Complexes: Exploiting Phosphine Ligand Lability
for Hydride Transfer to an [NAD]+ Model. Inorg. Chem. 2021, 60, 37.
(4) (a) Pitman, C. L.; Finster, O. N. L.; Miller, A. J. M.
Cyclopentadiene-mediated hydride transfer from rhodium complexes.
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Potent C-H PCET Donors: Ligand-Induced Fe-to-Ring Proton
Migration from a Cp*FeIIIH Complex Demonstrates a Promising
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Nickel-Borane Unit as a Promising Strategy for Catalysis. J. Am. Chem.
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Homogeneous CO Hydrogenation: Dihydrogen Activation involves a
Frustrated Lewis Pair Instead of a Platinum Complex. J. Am. Chem.
Soc. 2010, 132, 3301. (c) Huang, S. − H.; Wang, X.; Richmond, M. G.
Phosphinoborane-induced fragmentation of the unsaturated hydride
H2Re2(CO)8: X-ray structure of HRe(CO)4(κB,P-Ph2PCH2CH2BR2)
(where BR2 = 9-borabicyclo[3.3.1]nonanyl) and DFT Evaluation of
hydride versus CO coordination by the ancillary borane. J. Organomet.
Chem. 2012, 700, 103.
(6) For a review, see: McSkimming, A.; Colbran, S. B. The
coordination chemistry of organo-hydride donors: new prospects for
efficient multi-electron reduction. Chem. Soc. Rev. 2013, 42, 5439.
(7) Abiko, A. Dicyclohexylboron Trifluoromethanesulfonate. Org.
Synth. 2003, 79, 103.
(8) All geometry optimizations and frequency calculations were
performed at the BP86-D3(BJ)/def2-TZVP level of theory using tight
optimization (TIGHTOPT) methods. Accurate electronic energies
were calculated using CCSD(T) at the DLPNO-CCSD(T)/def2-
TZVP level of theory and thermally corrected to 298 K using the
BP86-D3(BJ)/def2-TZVP energies. All calculations were performed
in the gas phase. More details on the computational methods can be
found in the Supporting Information.
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Marcus W. Drover − Department of Chemistry and
Biochemistry, The University of Windsor, Windsor, Ontario
N9B 3P4, Canada; orcid.org/0000-0002-2186-1040;
Email: mdrover@uwindsor.ca
Authors
Joseph A. Zurakowski − Department of Chemistry and
Biochemistry, The University of Windsor, Windsor, Ontario
N9B 3P4, Canada
Brady J. H. Austen − Department of Chemistry and
Biochemistry, The University of Windsor, Windsor, Ontario
N9B 3P4, Canada
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00194
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to the University of Windsor, the
Council of Ontario Universities for a John C. Polanyi award to
M.W.D., and the Natural Sciences and Engineering Research
Council of Canada (Discovery Grant, RGPIN-2020-04480,
Discovery Launch Supplement, DGECR-2020-00183, and
graduate award (CGS-M to J.A.Z.)) for funding. We also
acknowledge Dr. M. Revington (UWindsor NMR). This work
was also made possible by the facilities of the Shared
Hierarchical Academic Research Computing Network
(SHARCNET: www.sharcnet.ca) and Compute/Calcul Cana-
da.
(9) A signal at δB −16.3 ppm has been noted for [nBu4N]BEt4. See:
(a) Ermoshkin, A. A.; Nikolaeva, E. S.; Neckers, D. C.; Fedorov, A. V.
New Tetraalkyl Borate Initiators for Remote Polymerization.
Macromolecules 2008, 41, 9063. (b) Hubbard, J. L.; Kramer, G. W.
11B NMR studies: I. Reactions of dialkylboranes with organolithium
reagents. J. Organomet. Chem. 1978, 156, 81. (c) Fischbach, A.;
Bazinet, P. R.; Waterman, R.; Tilley, T. D. β-Phosphinoethylboranes
as Ambiphilic Ligands in Nickel-Methyl Complexes. Organometallics
2008, 27, 1135. (d) Braunschweig, H.; D’Andola, G.; Welton, T.;
White, A. J. P. Synthesis and structure of novel organocycloborates.
Chem. - Eur. J. 2006, 12, 600. (e) Saturnino, D. J.; Yamauchi, M.;
Clayton, R. W.; Nelson, R. W.; Shore, S. G. Singly Hydrogen-Bridged
Borane Anions Derived from the Hydroboration Products of 1,3-
Butadiene. An X-ray Structure Determination of [N(n-C4H9)+]-
[B2(C4H8)2H3]. J. Am. Chem. Soc. 1975, 97, 6063.
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n
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