Nickel complexes of phosphine-appended benzannulated boron heterocycles

Article abstract of DOI:10.1016/j.tet.2019.02.047

We report the synthesis and characterization of two diphosphine nickel complexes containing 9-borafluorene (PBFlu, 9-(diisopropylphosphino)phenyl-9-borafluorene) and 9,10-dihydroboranthrene (B₂P₂, 9,10-bis(2-(diisopropylphosphino)phe

Full text of DOI:10.1016/j.tet.2019.02.047

Accepted Manuscript
Nickel complexes of phosphine-appended benzannulated boron heterocycles
Laura A. Essex, Jordan W. Taylor, W. Hill Harman
PII:
S0040-4020(19)30207-8
https://doi.org/10.1016/j.tet.2019.02.047
TET 30170
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 18 December 2018
Revised Date: 15 February 2019
Accepted Date: 22 February 2019
Please cite this article as: Essex LA, Taylor JW, Harman WH, Nickel complexes of phosphine-appended
benzannulated boron heterocycles, Tetrahedron (2019), doi: https://doi.org/10.1016/j.tet.2019.02.047.
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ACCEPTED MANUSCRIPT
Nickel complexes of phosphine-appended benzannulated boron
heterocycles
Laura A. Essex, Jordan W. Taylor, and W. Hill Harman*
Department of Chemistry, University of California, 501 Big Springs Rd. Riverside 92521
KEYWORDS: boranes, acceptor ligands, boron heterocycles
TOC Graphic:
Ni
P
P
B
η5-borafluorene
ABSTRACT: We report the synthesis and characterization of two diphosphine nickel complexes containing
9-borafluorene (PBFlu, 9-(diisopropylphosphino)phenyl-9-borafluorene) and 9,10-dihydroboranthrene
(B2P2, 9,10-bis(2-(diisopropylphosphino)phenyl)-9,10-dihydroboranthrene) cores. Metalation of PBFlu and
B2P2 with Ni(PPh3)4 leads to the monometallic complexes (PBFlu)Ni(PPh3) and (B2P2)Ni, respectively.
Cyclic voltammetry studies show a reversible redox event at ~ 0.1 V and a quasi-reversible event at ca. –3
V versus ferrocene/ferrocenium for (B2P2)Ni while (PBFlu)Ni(PPh3) features no reversible redox events.
Electronic structure calculations were performed to provide further insight into the bonding in these
complexes.
Introduction
Boron for carbon substitution is a powerful method for modulating the properties of polycylic aromatic
hydrocarbons (PAH), with applications in optoelectronic materials1,2 and catalysis.3 The empty p orbital on
boron can act as a Lewis acid4–7 or can introduce electron deficiency into the molecule, engendering facile
multi-electron redox chemistry in many cases.8 The boron substituted analogues of classic ligands such as
cyclopentadienyl9 and benzene10 have been used to prepare a range of metal complexes with interesting
electronic properties and chemical reactivity. They are of special interest given their potential to function as
acceptor or Z-type ligands.11–13
The five-membered monoboron heterocycle borole (C4H4BH) is isoelectronic to the cyclopentadienyl
cation and formally classified as an L2Z ligand. The antiaromatic character of borole engenders significant
reactivity in the molecule, and it has found applications in small molecule activation.14 For example,
perfluoropentaphenylborole is capable of cleaving dihydrogen15 or undergoing Diels-Alder or insertion
reactions with alkynes.16 Reduced boroles can be stabilized by N-heterocyclic carbenes, conferring apparent
boron-centered nucleophilicity.17,18 The doubly benzannulated analogue of borole, 9-borafluorene (BFlu)19
attenuates the anti-aromatic character of the central ring. BFlu derivatives have found applications as
activators for transition metals.20 Despite the rich chemistry of boron heterocycles as ligands, there are
relatively few metal complexes of BFlu. Piers reported an adduct between a perfluoro-9-phenyl-9-
borafluorene and Cp*Al,21 and Bourissou used a phosphine-buttressed ligand (PBFlu, 1) to prepare a BFlu
complex exhibiting monohapto coordination through the boron, the first Au borane complex (Figure 1).22
1

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Figure 1. Previously reported Au complexes of PBFlu and B2P2 ligands (top). Ni complexes of PBFlu and
B2P2 presented in this work (bottom).
The doubly benzannulated analogue of 9,10-dihydro-9,10-diborabenzene, 9,10-dihydro-9,10-
diboraanthracene (DBA), is an electrochemically rich platform with broad synthetic flexibility and
applications in organic electronic materials. Upon two electron reduction, Wagner has shown that DBA
variants can cleave dihydrogen,23 disproportionate CO2,24 break chalcogen-chalcogen bonds,25 and activate
sp C–H bonds.26 We recently reported that NHC-stabilized 9,10-diboraanthracene undergoes formal [4+2]
cycloaddition reactions at the boron centers with O2, CO2, and ethylene.27 A number of electronically
saturated metal complexes of DBA have been prepared, some of which contain multiple metal centers
bound to a single DBA unit.28 Due to the electron deficiency of DBA, oxidation of these complexes tends
result in irreversible de-coordination of the ligand.29
We are interested in the emergent redox properties and chemical reactivity of metal complexes with
heterocyclic boron-containing ligands. Recently, we reported the synthesis of a DBA-based ligand featuring
two tethered phosphine donors straddling the central C4B2 ring (9,10-bis(2-(diisopropylphosphino)phenyl)-
9,10-dihydroboranthrene, B2P2, 2). Metal complexes of B2P2 can undergo DBA-centered redox events, as in
the zwitterionic radical complexes (B2P2)Cu and (B2P2)Ag we recently described.30 A series of Au
complexes of B2P2 were also prepared in three states of charge, with the most reduced species, [(B2P2)Au]–
being the first example of a molecular boroauride, featuring very short interactions between the Au atom
and pyramidalized borons of the DBA core.31 Given the intriguing chemistry uncovered for d10 coinage
metal complexes with both PBFlu and B2P2, we investigated the related Ni complexes. Herein we report the
synthesis, structure, NMR, and electrochemical characterization of the neutral Ni complexes of both 1 and
2. Unlike their coinage metal analogues, these Ni complexes form penta- and hexahapto complexes,
respectively, with the central boron heterocycles (Figure 1). Although the BFlu and DBA heterocycles have
one and two borane centers, respectively, DFT calculations their Ni complexes are essentially isoelectronic
and best described as d10 ML4 Ni(0) complexes with a backbonding component (Z’) into an empty orbital
with boron character.
Scheme 1. Preparation of (PBFlu)Ni
Results and Discussion
The phosphine-appended BFlu ligand 1 was first prepared by Bourissou and metalated in situ with
(Me2S)AuCl without isolation of the ligand.22 In our hands, lithiation of 2-(
diisopropylphosphino)bromobenzene in toluene proceeds cleanly at –40 °C and the lithiated species is very
stable in this solvent, even at room temperature. Subsequent addition of 9-bromo-9-borafluorene at –60°C
results in a yellow solution that, upon warming to –20°C, changes to an intense red color, indicating near
complete decomposition of the desired ligand. However, by avoiding warming reaction mixtures past –20
2

°C, 1 can be isolated as a colorless crystalline solid in low yields (16%) (Scheme 1). Once isolated, 1 is
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stable at room temperature allowing its characterization by NMR spectroscopy. In solution, 1 exhibits a
broad resonance centered at 0.9 ppm in the 11B{1H} NMR spectrum. For comparison, the 11B resonance for
9-phenyl-9-borafluorene is 64.5 ppm, suggesting P–B interactions in 1, although no 31P–11B coupling is
observed (Scheme 1).32
A
B
Figure 2. 11B{1H} NMR spectra of (A) PBFlu (1, blue) and (PBFlu)Ni (3, red); (B) B2P2 (2, blue) and
(B2P2)Ni (4, red).
Metalation of 1 with Ni(PPh3)4 affords the nickel complex (PBFlu)Ni (3), denoted by a downfield shift
11
of the B resonance to 22.5 ppm (Figure 2). Single-crystal X-ray diffraction (XRD) of 1 revealed an η5-
BFlu moiety coordinated to Ni with a Ni–B distance of 2.135(2) Å and Ni–C distances ranging from
2.217(2) to 2.230(2) Å (Figure 3). In contrast, the previously reported η1-BFlu gold complex has a Au–B
distance of 2.663(1) Å with no appreciable interaction between the Au center and other atoms of the BFlu
unit (average dAu-Cα = 3.118(1) Å; dAu-Cβ = 3.750(1) Å) (Figure 3). While many η5-borole complexes have
been reported,33–35 complex 3 is the first example of pentahapto coordination of BFlu.
Ni
P
Ni
P
P
P
B
B
B
Figure 3. Labeled thermal ellipsoid plots (50%) of (PBFlu)Ni (3, left) and (B2P2)Ni (4, right). Unlabeled
ellipsoids correspond to carbon. Hydrogen atoms have been omitted for clarity.
The DBA-based ligand 2 allowed us to explore the effect of a second boron center in the coordination
sphere of Ni. Metalation of 2 to give (B2P2)Ni (4) can be achieved using either Ni(PPh3)4 or treating by a
11
mixture of 2 and NiBr2 with metallic Na (Scheme 2). In solution, 4 features a B{1H} resonance at 27.8
ppm that is shifted slightly upfield to that of the free ligand (34.1 ppm). Single-crystal XRD reveals η6
coordination of Ni to the DBA core with an average Ni-B distance of 2.196(2) Å (Figure 3). In contrast,
[(B2P2)Au]+ has Au–B distances of 2.610(2) and 2.678(2) Å and no Au–C contact shorter than 2.75 Å.31
These differences can be rationalized both by the prevalence of 14-electron Au(I) complexes as well as the
stronger backbonding ability of Ni(0). We note that in all of the Ni complexes reported her, the boron
centers are essentially planar (see Table 1).
Scheme 2. Synthesis of (B2P2)Ni
To further explore the electronic properties of these two complexes, we investigated their
electrochemistry by cyclic voltammetry (CV). The CV of 2 features a single irreversible reduction at –3.00
3

V vs. Fc/Fc+ with multiple irreversible events in the oxidative direction (Figure S17, S18). In contrast, 4
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displays a reversible reduction event at −2.99 V vs. Fc/Fc+ and an electrochemically slow but chemically
reversible oxidation at +0.06 V (Figure S19). It is instructive to compare the electrochemistry of 2 and 4 to
that of the corresponding free ligands 1 and 3. The CV of 1 displays a reversible reduction at –2.09 and an
irreversible reduction at –2.70 V. The irreversibility of the second reduction may be due to reductive
cleavage of the endocyclic B–C bond, a phenomenon that has been observed in boroles and related
molecules.16,36,37 In contrast, 2 undergoes reversible reductions at –2.18 and –2.71 V vs. Fc/Fc+. In both
cases, the free ligands are more easily reduced than the corresponding Ni complexes, consistent with strong
donation from a filled Ni d-orbital into the empty boron-based orbitals on the BFlu and DBA fragments.
These results are counter to what we have observed in the cases of the coinage metal complexes of 3
[(B2P2)M]+ (M = Cu, Ag, Au), all of which are easier to reduce than free 3 (Table 1). The positive charge
on the coinage metal complexes is likely a significant contributor to this observation.
DBA
BFlu
LUMO+1
LUMO+1
LUMO
LUMO
Figure 4. The two lowest energy unoccupied Kohn-Sham orbitals calculated for BFlu (left) and DBA
(right, see SI for computational details).
DFT calculations were carried out on truncated models38 of 2 and 4 as well as BFlu and DBA to shed
light on the interaction between the Ni center and the boron heterocycles. The two lowest energy
unoccupied orbitals for BFlu and DBA are shown in Figure 4 and reveal that in both cases, the LUMO has
significant boron character. In the case of DBA, this orbital is composed of the out-of-phase combination of
the two boron p orbitals. This orbital is analogous to the HOMO of anthracene and has overall bonding
character. The second unoccupied molecular orbital in DBA (LUMO+1) corresponds to the in-phase
combination of the empty boron p-orbitals. As this orbital is analogous to the LUMO of anthracene, it is
antibonding. Although there are in principle two relevant acceptor orbitals in DBA, the in-phase
combination of the boron p orbitals (LUMO+1) is significantly higher in energy than the out-of-phase
combination (LUMO) due to the antibonding nature of the former and bonding nature of the latter. The five
highest energy occupied molecular orbitals of 2 and 4 are shown in Figures 5 and 6, respectively. In both
cases, the five highest energy filled orbitals have significant if not predominantly d-character, consistent
with a d10 Ni(0) description. In 3, the Ni–B acceptor interaction serves to stabilize an orbital of d parentage
that is antibonding with respect to the Ni–PPh3 interaction (HOMO–2). In 4 the primary molecular orbital
featuring Ni–B bonding is also the HOMO–2, which consists of the in-phase combination of a Ni d orbital
and the LUMO of DBA described above. The HOMO–1 in 4 contains a small component of the in-phase
combination of the boron p orbitals, corresponding to the LUMO+1 of DBA. This interaction is less
pronounced, owing to the higher energy of the DBA LUMO+1 (in-phase boron p orbitals) relative to the
4

LUMO (out-of-phase boron p orbitals). For these reasons, we favor a description of both BFlu and DBA in
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2 and 4 as L2 ligands with a significant backbonding component associated with the boron p orbitals. In
both cases, the d-orbital splittings can be rationalized as perturbations on an approximately tetrahedral
ligand field.
-3.8
HOMO
-4.052
-4.0
-4.2
HOMO-1
-4.274
-4.4
-4.6
HOMO-2
-4.501
-4.8
-5.0
HOMO-3
-4.707
-5.2
-5.4
HOMO-4
-5.374
-5.6
Figure 5. The five highest energy occupied Kohn-Sham orbitals calculated for a truncated model of
(PBFlu)Ni (2).
-4.2
HOMO
-4.434
-4.4
-4.6
HOMO-1
-4.615
-4.8
HOMO-2
-4.634
-5
-5.2
HOMO-3
-4.950
-5.4
-5.6
HOMO-4
-5.757
-5.8
Figure 6. The five highest energy occupied Kohn-Sham orbitals calculated for a truncated model of
(B2P2)Ni (4).
Conclusion
In summary, we have synthesized two new nickel complexes 2 and 4 featuring strong interactions of the Ni
center with boron heterocycles. The interaction between Ni and the boron heterocycles in these complexes
is much more pronounced than in the analogous complexes of the coinage metal cations. Where the coinage
metal complexes are easier to reduce than the corresponding free ligands, 2 and 4 are significantly more
5

difficult, consistent with strong backbonding of the Ni center into the boron-based orbitals of the
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heterocycles. DFT calculations on 2 and 4 are consistent with this description. The five highest energy
orbitals in both 2 and 4 have significant d character, with the HOMO–2 in both cases possessing Ni–B
bonding interactions. Given the presence of the Ni–B bonding orbital in the middle of the d manifold, a d10
electron count is appropriate for these complexes, wherein the Ni–B interaction is best described as
backbonding.
Table 1. Selected bonds, angles, 11B NMR data and CV data for PBFlu, B2P2 and their transition metal
complexes
Compound
11B [ppm]
M–Bavg [Å]
M–Cavg [Å]
ΣBα [°]
E1/2 [V]
PBFlu
B2P2
0.90
34.1
22.5
27.8
47.2
29.2
32.0
11.1
–2.09b, –2.70c
–2.18, –2.71
–3.00c
(PPh3)Ni(PBFlu)
Ni(B2P2)
2.316
2.196
2.372
2.567
2.892
2.644
3.049
2.239
2.233, 2.216a
2.349
360.0
360.0
360.0
360.0
359.9
360.0
360.0
343.9
0.06, –2.99b
–1.66b
–1.56, –2.21c
[Cu(B2P2)][BArF ]
2.514
4
[Ag(B2P2)][BArF ]
2.730
2.985
2.832
3.189
4
Ag(B2P2)
[Au(B2P2)][BArF ]
–1.60, –2.05
4
Au(B2P2)
[Au(B2P2)][K(18-c-6)]
6

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aThe averages are taken between the two crystallographically distinct molecules of
(PPh3)Ni(PBFlu) in the unit cell and are listed in the order of alpha and beta to denote the slight
asymmetry about the Ni-BC4 unit
bThis process was quasi-reversible on the CV timescale (100 mV/s scan rate)
cThis process was irreversible on the CV timescale (100 mV/s scan rate)
Experimental Section
General considerations. Unless otherwise noted, all manipulations were carried out using
standard Schlenk or glovebox techniques under a N2 atmosphere. Hexanes, benzene, toluene, and
acetonitrile were dried and deoxygenated by argon sparge followed by passage through activated
alumina in a solvent purification system from JC Meyer Solvent Systems followed by storage
over 4 Å molecular sieves. THF and Et2O were distilled from sodium-benzophenone ketyl under
N2 followed by storage over 4Å molecular sieves for at least 24 hours prior to use. Non-
halogenated and non-nitrile containing solvents were tested with a standard purple solution of
sodium benzophenone ketyl in THF to confirm effective oxygen and moisture removal prior to
use. Hexamethyldisiloxane (HMDSO) was distilled from sodium metal and stored over 4Å
molecular sieves for 24 hours prior to use. All reagents were purchased from commercial
suppliers and used without further purification unless otherwise noted. 9-bromo-9-
borafluorene,39
2-(diisopropylphosphino)bromobenzene,22
and
9,10-bis(2-
(diisopropylphosphino)phenyl)-9,10-dihydroboranthrene (B2P2, 2)31 were synthesized according
to literature procedures. Elemental analyses were performed by Midwest Microlab, LLC,
Indianapolis, IN. Deuterated solvents were purchased from Cambridge Isotope Laboratories Inc.,
degassed, and dried over activated 3Å molecular sieves for at least 24 h prior to use. NMR
spectra were recorded on Bruker Neo 400 MHz, and Bruker Avance 600 MHz spectrometers. 1H
and 13C chemical shifts are reported in ppm relative to tetramethylsilane using residual solvent as
11
11
an internal standard. B chemical shifts are reported in ppm relative to BF3•Et2O. Original B
NMR spectra were processed using MestReNova 11.0.2 with a backwards-linear prediction
applied to eliminate background signal from the borosilicate NMR tube. For 11B NMR spectra
with peaks overlapping the borosilicate signal, a manual baseline correction was applied. UV-Vis
spectra were recorded using a Cary Bio 500 spectrometer using a 1 cm path length quartz cuvette
with a solvent background subtraction applied. X-ray diffraction studies were performed using a
Bruker-AXS diffractometer. Cyclic Voltammetry (CV) experiments were performed using a Pine
AFP1 potentiostat. The cell consisted of a glassy carbon working electrode, a Pt wire auxiliary
electrode and a Pt wire pseudo-reference electrode. All potentials are referenced vs. the Fc/Fc+
couple measured as an internal standard. Calculations were run using Orca v4.0.1 and visualized
with Avogadro v1.2.0.
9-(2-(diisopropylphosphino)phenyl)-9-borafluorene (1). Using a modified procedure from
Bourissou et al. n-BuLi (2.5 M in hexanes, 2.5 mL, 6.25 mmol) and 9-bromo-borafluorene (1.5
g, 6.2 mmol) were successively added at –40 °C and –60 °C to a solution of 1-bromo-2-
diisopropylphosphinobenzene (1.5 g, 5.5 mmol) in toluene (80 mL).22 The suspension was
pumped down to a pale orange solid and brought into the glovebox. The solid was re-suspended
in toluene (80 mL) that had been pre-chilled to –50°C and filtered through celite at the same
temperature. The filtrate was concentrated in vacuo and layered with hexanes, providing
1
colorless crystals. Yield: 0.311 g (16%). H NMR (400 MHz, C6D6) δ 7.87 (dd, J = 7.5, 0.9 Hz,
2H), 7.49 (dd, J = 7.2, 2.3 Hz, 2H), 7.36 (tdd, J = 7.5, 2.5, 1.3 Hz, 2H), 7.26 – 7.17 (m, 4H), 7.14
– 7.08 (m, 2H), 2.15 (dp, J = 9.3, 7.3 Hz, 2H), 0.80 (d, J = 7.1 Hz, 3H), 0.77 (d, J = 7.2 Hz, 3H),
7

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0.73 (d, J = 7.4 Hz, 3H), 0.69 (d, J = 7.4 Hz, 3H).13C{1H} NMR (101 MHz, C6D6) δ 164.8 (br),
153.2 (br), 149.7 (d, J = 4.7 Hz), 136.1 (d, J = 48.1 Hz), 132.3 (d, J = 2.6 Hz), 131.4 (d, J = 39.0
Hz), 131.3 (d, J = 3.8 Hz), 127.2 (d, J = 3.5 Hz), 127.1, 125.8 (d, J = 3.7 Hz), 119.7 (d, J = 2.4
Hz), 23.5 (d, J = 8.5 Hz), 18.4, 18.0. 11B{1H} NMR (128 MHz, C6D6) δ 0.9. 31P NMR (162
MHz, C6D6) δ 33.0. HRMS (ESI): m/z for C24H27BP [M+H]+ calcd.: 357.1938, found: 357.1978.
(PBFlu)Ni(PPh3) (3). A solution of 1 (69 mg, 0.19 mmol) in toluene (4 mL) was added to a
stirring solution of tetrakis(triphenylphosphine)nickel(0) (213 mg, 0.19 mmol) in toluene (10
mL). After 2 hours, the solution was concentrated in vacuo and layered with hexane (2 mL),
providing burgundy-brown crystals of 2 that were washed with hexanes (2 x 3 mL) and dried in
vacuo. Crystals suitable for XRD can be generated from layering hexane over toluene or ether
over THF. Yield: 0.120 g (91%). 1H NMR (400 MHz, C6D6) δ 7.97 (d, J = 7.3 Hz, 1H), 7.66 (d,
J = 7.6 Hz, 2H), 7.40 (tdd, J = 7.4, 2.5, 1.2 Hz, 1H), 7.21 (dddd, J = 9.5, 7.3, 4.9, 2.3 Hz, 7H),
7.14 – 7.07 (m, 3H), 7.07 – 6.96 (m, 11H), 6.51 (d, J = 8.0 Hz, 2H), 1.17 (h, J = 7.2 Hz, 2H),
0.85 (d, J = 7.0 Hz, 3H), 0.81 (d, J = 7.0 Hz, 3H), 0.53 (d, J = 7.1 Hz, 3H), 0.49 (d, J = 7.0 Hz,
3H). 13C{1H} NMR (101 MHz, C6D6) δ 155.8, 144.8 (dd, J = 40.4, 15.1 Hz), 134.8, 134.6, 134.5
(d, J = 9.7 Hz), 131.9 (dd, J = 24.1, 2.7 Hz), 131.2, 129.9, 129.3, 129.2 (d, J = 2.5 Hz), 127.7,
127.7, 126.2 (d, J = 2.9 Hz), 125.7 (d, J = 5.6 Hz), 123.6, 121.1, 119.0 (d, J = 2.8 Hz), 114.9,
31
25.6 (d, J = 19.3 Hz), 19.3 (d, J = 5.6 Hz), 17.8. 11B{1H} NMR (128 MHz, C6D6) δ 22.5. P
NMR (162 MHz, C6D6) δ 55.7 (d, J = 24.2 Hz), 36.7 (d, J = 24.2 Hz). UV-vis (Benzene) λmax
(nm) (εmax (M−1cm−1)) 560 (sh, 3.2 x 102), 417 (sh, 3.5 x 103). Anal. Calcd. for C36H44B2NiP2 (1 x
C4H10O): C, 73.73 H, 6.59. Found: C, 73.47 H, 5.95.
(B2P2)Ni (4).
Via NiBr2: A solution of B2P2 (0.200 g, 0.357 mmol) in THF (3 mL) was added to a slurry of
NiBr2 (0.080 g, 0.366 mmol) in THF (3 mL) and stirred 4 hours. The orange/red mixture had its
volatiles removed, was added Et2O (5 mL) and again had its volatiles removed. The remaining
orange/red foam was extracted with toluene (2 x 3 mL) and filtered through celite into a 20 mL
vial containing sodium (0.021 g, 0.893 mmol). The solution was stirred 10 hours during which
time a deep red solution formed. The mixture was filtered through celite and concentrated to ca.
2 mL before adding Et2O (5 mL). The mixture was filtered through a 1” pad of silica gel and
rinsed with toluene:Et2O (2:5, 10 mL). Removal of the volatiles gave the product as a red solid.
Overall yield: 0.157 g, 71%.
Via Ni(PPh3)4: A solution of B2P2 (0.200 g, 0.357 mmol) in THF (5 mL) was added to Ni(PPh3)4
(0.395 g, 0.357 mmol) in THF (3 mL) and the mixture stirred at 50 °C for 12 hours. The deep red
solution had its volatiles removed in vacuo before dissolving the residue in THF:Et2O (1:9, 10
mL) and passing it through a 1” pad of silica. Removal of the volatiles in vacuo gave the product
as a red/orange solid. Overall yield: 0.197 g, 89%. X-ray quality crystals were grown by layering
1
a concentrated toluene solution with MeCN. H NMR (500 MHz, C6D6) δ 8.01 (d, J = 7.2 Hz,
2H), 7.62 (apparent dd, J = 5.9, 3.4 Hz, 4H), 7.48 (t, J = 7.2 Hz, 2H), 7.36 – 7.30 (m, 2H), 7.27
(t, J = 7.5 Hz, 2H), 7.11 (apparent dd, J = 6, 3.4 Hz, 4H), 2.04 (dp, J = 14.2, 7.3 Hz, 4H), 0.74 (d,
J = 6.9 Hz, 6H), 0.72 (d, J = 6.9 Hz, 6H), 0.61 (d, J = 7.1 Hz, 6H), 0.58 (d, J = 7.1 Hz, 6H). 31P
(202 MHz, C6D6) δ 45.6 (s). 11B{1H} (160 MHz) δ 27.8 (bs). 13C{1H} NMR (126 MHz, C6D6) δ
159.3, 143.1-142.9 (m), 135.0, 132.0 – 131.4 (m), 130.6, 130.1, 129.9, 126.9, 125.4, 27.7
(apparent dt, J = 18.3, 8.5 Hz), 20.0, 18.9. UV-vis (THF): λmax (nm) (εmax (M−1cm−1)) 318 (sh,
2.5 x 104), 372 (1.7 x 104), 451 (7.9 x 103), 577 (sh, 2.8 x 103). Anal. Calcd. for C36H44B2NiP2 (1
x C4H10O): C, 69.31 H, 7.85. Found: C, 69.98 H, 8.62.
8

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Acknowledgements
This research was supported by the National Science Foundation (CHE-1752876) and the
American Chemical Society Petroleum Research Fund (57314-DNI3). NMR spectra were
collected on instruments funded by an NSF MRI grant (CHE-1626673) and an Army Research
Office instrumentation grant (W911NF-16-1-0523). We thank Dr. Fook Tham for assistance
with X-ray crystallographic studies.
Appendix A. Supplementary data
Crystallographic data for 3 and 4 have been uploaded to the Cambridge Structural Database
under accession numbers CCDC 1885406 and CCDC 1885407.
References
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