Inorganic Chemistry
Communication
within 5 min, along with the formation of ≈1 equiv of NEt3
and a new singlet in the 1H NMR spectrum at δH = −14.3 ppm
for the hydride.12 By 31P NMR spectroscopy (THF-d8, 298 K),
a single resonance at δP = 40.8 ppm (cf. 37.1 ppm for 1 in
THF-d8) is observed as well as a very broad signal in the 11B
NMR spectrum for the eight peripheral boranes at δB = 82.9
ppm (Δ1/2 = 2900 Hz in THF-d8). High-resolution positive-
ion electrospray ionization mass spectrometry [HRESI(+)-
MS] also provided an [M]+ signal for [2]+ at m/z 1992.739
(calcd m/z 1992.737) of the appropriate isotope pattern. To
test whether complex [2]+ is fluxional in solution (possibly
giving a weakly bound σ-boraneyl [Ni]−H···B complex),13
Unlike in the case of the nickel P2BCy complexes where
4
crystallization proved futile, the P2BMes scaffold allowed for
4
isolation of a crystalline material of both 4 and its hydride, [5]+
(for [5]+, see Scheme 2). Both species are nearly isostructural
and feature a tetrahedral (4) or trigonal-bipyramidal ([5]+)
nickel center. For both structures, the nickel center is buried
deep within a hexadecamesityl cavity, providing rationale for
the slow protonation reactivity witnessed for 4. Indeed,
visualization of a space-filling diagram (Scheme 2) shows
complete obscuration of the nickel site. In terms of metrics, the
Ni−P distances in 4 elongate by ∼5% [2.179(3) and 2.237(3)
Å; cf. 2.138(1) Å for Ni(tape)2]9 following hydroboration.
Despite the structure’s size, the hydride ligand of [5]+ was also
located from the difference map and freely refined [Ni(1)−
H(1) = 1.531(16) Å] and is ≈4.5 Å from the nearest B-Mes2
group.
1
variable-temperature (VT) H/31P NMR spectroscopy studies
were undertaken. Cooling a THF-d8 solution of [2]+ from 298
to 193 K evidences broadening of the hydride and 31P NMR
resonances, although no dramatic shifting was observed (see
acidic secondary coordination sphere, treatment of [2]+ with
excess DMAP gave [3]+ (Scheme 1), which has hydride and
31P NMR chemical shifts nearly identical with those of [2]+
Next, we elected to benchmark the hydride transfer
propensity of [2]+ and [5]+ by studying their reaction with
−
substrates of known hydricity (ΔGH ). With this goal in mind
and to potentially distinguish the reactivity profile of [2]+ and
[5]+ from their 1,2-bis(dialkylphosphino)ethane cousins, the
nickel(II) hydride [Ni(H)(dnppe)2]BPh4 ([6]+), devoid of
octaboraneyl functionality, was prepared (δP = 39.5 ppm) as a
model compound (see the SI).
Encouraged by the ability to access [2]+, we wondered if
alteration of the secondary coordination sphere using other
“BR2” fragments might be feasible, and if so, how would “R”
influence accessibility to nickel? To probe this question, a
solution of [Ni(tape)2] [tape = 1,2-bis(diallylphosphino)-
As an initial foray, treatment of THF-d8 solutions15 of [2]+,
[3]+, [5]+, or [6]+ with CO2 did not result in the formation of
4
HCO2− (ΔGH = 44 kcal mol−1 in MeCN). For [3]+ and [6]+,
−
14
ethane] was reacted with 8 equiv of HBMes2 in toluene
which lack free boraneyl functionality, this result is
unsurprising given that, among the [Ni(H)(diphosphine)2]+
cations, [Ni(H)(dmpe)2]+ is the most hydridic, with larger
groups on phosphorus resulting in species for which hydride
(Scheme 2). Dimesitylborane was chosen because of its bulky
2,6-dimethyl substitution, discouraging appreciable interaction
of boron with Lewis bases, offering a contrast to [2]+, which
readily undergoes adduct formation. Monitoring the afore-
mentioned mixture by NMR spectroscopy evidenced the
formation of a deep-red solution and high conversion to
[Ni(P2BMes4)2] (4) after 4 days. By contrast, hydroboration of
[Ni(tape)2] using 8 equiv of HBCy2 gave 1 in <1 min,9
highlighting the marked difference between the Cy and Mes
groups. Despite its bulky secondary coordination sphere, the
31P{1H} NMR spectrum of 4 at 298 K features a single
absorption at δP = 38.0 ppm, indicating equivalent phosphorus
environments. Further, the 1H NMR spectrum features
absorbances at δH 6.78 (CHAr), 2.38 (CH3ortho), and 2.19
(CH3para) of integration 32:96:48 H. Complex 4 features 48
methyl groups; low-temperature 1H NMR spectroscopy
establishes a number of broadened features in the afore-
mentioned chemical shift regimes, consistent with the presence
of a number of rotational conformers (see the SI). At 193 K,
the 31P NMR spectrum consists of a single broad feature.
Complex 4 undergoes protonation using [HNEt3]BPh4,
albeit over a period of hours, not minutes, highlighting the
difference in accessing the electron-rich Ni0 center in 4.
Following 2 h of stirring and subsequent workup, [Ni(H)-
(P2BMes4)2]+ ([5]+) was isolated as a yellow solid (δH = −15.2
ppm for the hydride resonance and δP = 39.3 ppm).
HRESI(+)-MS also provided an [M]+ signal for [5]+ at m/z
transfer is less favorable [e.g., ΔGH = 56.0 kcal mol−1 in
−
MeCN for [Ni(H)(depe)2 ]+ ; depe
= 1,2-bis-
(diethylphosphino)ethane].16 Complexes [2]+ and [5]+ are
thus weaker hydride donors than formate in THF.
We next assessed the reactivity of these hydrides with an
[NAD]+ analogue (a better hydride acceptor compared to
CO2), 3-acetyl-N-benzylpyridinium bromide ([BNAcP]Br),
and its hexafluorophosphate salt ([BNAcP]PF6), which have
been previously used to benchmark hydricity based on the
known value of ΔGH = 60 2 kcal mol−1 for 1,4-[BNAcP]H
−
in MeCN.5 We first carried out reaction of [BNAcP]Br
1
(notably, [BNAcP]Br is completely insoluble in THF by H
NMR spectroscopy) with the octaboraneyl NiII−H cation, [2]+
(Scheme 3). Analysis of the 31P NMR spectrum evidenced
consumption of the signal for [2]+ and the presence of two
new signals at δP ∼ 14 and 74 ppm of integration 2:1.2. The
former of these is ascribed to free P2BCy4 ligand, and the latter
1
to Ni(P2BCy4)(Br)2 (7; see the SI). Analysis of the H NMR
spectrum also evidenced complete consumption of the hydride
starting material and formation of 1,4-[BNAcP]H in 57%
conversion. It is notable that this process occurs in THF;
previous reports reveal that related transfers occur in polar
solvents (e.g., MeCN) only.17 Highlighting a counteranion
effect, we also assessed the reactivity of [2]+ with [BNAcP]PF6
(which is THF-soluble); this reaction did not result in hydride
transfer, highlighting the importance of [Ni]−Br bond
formation to this process.
1
2569.738 (calcd m/z 2569.738). VT H NMR spectroscopy
experiments confirmed the static nature of the [Ni]−H group
(the hydride chemical shift is nearly invariant of temperature),
while VT 31P NMR spectroscopy showed two broad
nonequivalent phosphorus environments at 213 K, owing to
low symmetry. Consistent with the rationale discussed above,
complexes 4 and [5]+ do not bind DMAP, owing to protection
of the boraneyl sites by flanking methyl groups.
To test whether the boraneyl unit in [2]+ played a role in
this transformation, we next carried out reactions of [BNAcP]
Br and [BNAcP]PF6 with [6]+ in THF (Scheme 3). In both
cases, no reaction was observed. To understand this differential
outcome, we undertook a series of control experiments. First,
C
Inorg. Chem. XXXX, XXX, XXX−XXX