Heteroleptic nickel complexes of a bulky bis(carbene)borate ligand

Article abstract of DOI:10.1016/j.poly.2018.09.026

Bis(carbene)borate ligand transfer to the nickel nitrosyl synthon Ni(NO)(PPh₃)₂Br provides the new nickel nitrosyl complex Ph₂B(^tBuIm)₂Ni(NO)(PPh₃). The solid state structure reveals a trigonal pyramidal nickel ion with a very long bond to the apical PPh₃ ligand. The complex reversibly dissociates PPh₃ in solution to afford three-coordinate Ph₂B(^tBuIm)₂Ni(NO), with NMR data providing evidence for PPh₃ binding at low temperatures. Ligand transfer to Ni(PMe₃)₂Cl₂ provides the square planar complex, Ph₂B(^tBuIm)₂Ni(PMe₃)Cl, which shows no evidence for rearranging to the form a homoleptic complex with two bis(carbene)borate ligands. This complex is a suitable synthon for the [Ph₂B(^tBuIm)₂Ni]⁺ fragment, as demonstrated by the synthesis of the π-allyl complex Ph₂B(^tBuIm)₂Ni(η³-C₇H₇). The π-allyl complex reacts with O₂ to provide benzaldehyde and an unstable complex that is tentatively identified as the nickel(II) hydroxide [Ph₂B(^tBuIm)₂Ni(μ-OH)]₂.

Full text of DOI:10.1016/j.poly.2018.09.026

Accepted Manuscript
Heteroleptic Nickel Complexes of a Bulky Bis(carbene)borate Ligand
Jorge L. Martinez, Wei-Tsung Lee, Maren Pink, Chun-Hsing Chen, Jeremy M.
Smith
PII:
S0277-5387(18)30578-3
https://doi.org/10.1016/j.poly.2018.09.026
POLY 13425
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
3 August 2018
12 September 2018
14 September 2018
Please cite this article as: J.L. Martinez, W-T. Lee, M. Pink, C-H. Chen, J.M. Smith, Heteroleptic Nickel Complexes
of a Bulky Bis(carbene)borate Ligand, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.09.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Heteroleptic Nickel Complexes of a Bulky Bis(carbene)borate
Ligand
Jorge L. Martinez, Wei-Tsung Lee,† Maren Pink, Chun-Hsing Chen, Jeremy M. Smith*
Department of Chemistry
Indiana University
800 E. Kirkwood Ave.
Bloomington IN 47405, USA
smith962@indiana.edu
† Current Address: Department of Chemistry and Biochemistry, Loyola University Chicago, Chicago, IL
60660, USA
Abstract
Bis(carbene)borate ligand transfer to the nickel nitrosyl synthon Ni(NO)(PPh3)2Br provides the new nickel
nitrosyl complex Ph2B(tBuIm)2Ni(NO)(PPh3). The solid state structure reveals a trigonal pyramidal nickel
ion with a very long bond to the apical PPh3 ligand. The complex reversibly dissociates PPh3 in solution to
afford three-coordinate Ph2B(tBuIm)2Ni(NO), with NMR data providing evidence for PPh3 binding at low
temperatures. Ligand transfer to Ni(PMe3)2Cl2 provides the square planar complex,
Ph2B(tBuIm)2Ni(PMe3)Cl, which shows no evidence for rearranging to the form a homoleptic complex with
two bis(carbene)borate ligands. This complex is a suitable synthon for the [Ph2B(tBuIm)2Ni]+ fragment, as
demonstrated by the synthesis of the -allyl complex Ph2B(tBuIm)2Ni(3-C7H7). The -allyl complex reacts
with O2 to provide benzaldehyde and an unstable complex that is tentatively identified as the nickel(II)
hydroxide [Ph2B(tBuIm)2Ni(-OH)]2.
Keywords
Nickel complexes; nitrosyl ligands, NHC complexes, bidentate ligands; dioxygen reactivity
Introduction
Among his many scientific contributions, Bill Jones has elegantly demonstrated the utility of strongly
donating bulky ligands in nickel chemistry. For example, the Ni(dippe) fragment (dippe = iPr2PCH2CH2PiPr2)
is notable for its involvement in organonitrile C-CN bond cleavage 1 as well as thiophene and
benzothiophene desulfurization.2 Moreover, using this same fragment, Bill’s group foreshadowed a new
era in metal-ligand multiple bonds3 by providing some of the first evidence for a terminal metal-ligand
multiple bond involving a late 3d transition metal, in the form of a transient nickel sulfide.4
The strong -donor nature and synthetic flexibility of N-heterocyclic carbenes (NHC) have driven the
development of these ligands well beyond their initial application as phosphine surrogates. Although the
utility of nickel complexes with monodentate NHCs is relatively well established,5 nickel complexes with
, , ,
bidentate bis(carbene) donor ligands are still relatively rare.6 7 8 9 Despite this limited body of work, the
application of bidentate bis(carbene) nickel complexes in metal-ligand multiple bond chemistry8e and C-C

bond cleavage8h hints at the potential functionality of the nickel bis(carbene) fragment. One impediment
towards realizing this potential is the highly favorable formation of undesired square planar Ni(II)
complexes containing two bis(carbene) ligands, even with quite bulky bis(carbene) ligands.9
We have had a longstanding interest in the chemistry of 3d metals based on borate ligands containing
multiple NHC donors,10 including bis(carbene)borates. While the combination of negative charge and
strong -donors may be expected to favor higher oxidation states, these ligands have also shown affinity
for low valent metals, for example in providing the first access to square planar Fe(0).11 In the case of
nickel, we previously reported the synthesis of the square planar homoleptic Ni(II) complex of the
dihydrobis(tert-butylimidazol-2-ylidene)borate ligand, [H2B(tBuIm)2]2Ni, however we were unable to
prepare heteroleptic Ni(II) complexes containing a single bis(carbene)borate ligand.9 The formation of
four-coordinate homoleptic complexes of this ligand is similarly facile for other 3d metals, providing
complexes such as [H2B(tBuIm)2]2Fe12 and [H2B(tBuIm)2]2Co.11b
We have subsequently found that simple modification of this bis(carbene)borate ligand, namely to
diphenylbis(tert-butylimidazol-2-yl)borate, 13 allows for the isolation of stable low-coordinate iron
complexes that show no evidence for rearrangement to the undesired homoleptic analogues (Figure 1).14
We hypothesized that this modified ligand would similarly provide access to low coordinate Ni(II)
complexes in a strongly donating environment. In this paper, we report on the synthesis and reactivity of
heteroleptic nickel(II) bis(carbene)borate complexes using this modified ligand. These complexes also
provide additional evidence for the strong donor ability of the bis(carbene)borate ligand.
Figure 1. Bis(carbene)borate ligands to stabilize homoleptic and heteroleptic iron(II) complexes.
Experimental
General Considerations. All manipulations involving air- and moisture sensitive compounds and their
preparations were performed under a nitrogen atmosphere by standard Schlenk techniques or in an M.
Braun Labmaster glovebox. Glassware was dried at 130 °C overnight before cooling under dynamic
vacuum in an antechamber. Diethyl ether (Et2O), n-pentane, tetrahydrofuran (THF), and toluene were
purified by the Glass Contour solvent purification system. Before use, an aliquot of each solvent was
tested with 1 drop of solution of sodium benzophenone ketyl in THF. Celite was dried under vacuum at
130 °C overnight. The reagents Ph2(tBuImH)2(OTf)13 and Ni(NO)Br(PPh3)215 were prepared according to
literature procedures. All reagents were purchased from commercial vendors and used as received.
Benzene-d6 (C6D6, Cambridge Isotope Laboratories) was degassed by three consecutive freeze-pump-thaw
cycles on a Schlenk line and placed over molecular sieves for at least 12 h prior to use. THF-d8 (Sigma-
Aldrich) was dried over molecular sieves overnight prior to use. 1H NMR data were recorded on a Varian
Unity 400 spectrometer (400 MHz) at 25 °C. VT IR data were recorded using a Mettler Toledo ReactIR 15.

Synthesis of Ph2B(tBuIm)2Ni(NO)(PPh3)
A solution of LDA (39 mg, 0.36 mmol) in toluene (2 mL) was added to a slurry of Ph2B(tBuIm)2OTf (100 mg,
0.18 mmol) in toluene (3 mL). After stirring at ambient temperature for 2 h, Ni(PPh3)2(NO)Br was added
as a solid. The resulting dark green solution was stirred for 3 h before the volatiles were removed under
reduced pressure. The residue was extracted with pentane, filtered through Celite and dried in vacuo to
yield a purple solid (86 mg, 96 %). Crystals suitable for X-ray diffraction were grown from concentrated
pentane at -35 °C. The NMR spectral data is consistent with an equilibrium involving loss of PPh3 from
Ph2B(tBuIm)2Ni(NO)(PPh3), see the discussion for details. 1H NMR (400 MHz, C6D6, ): 7.49 (d, JHH = 6.8, 4H,
B(C6H5)2), 7.36 (m, 6H, P(C6H5)3), 7.34 (d, JHH = 7.6, 4H, B(C6H5)2), 7.24 (t, JHH = 7.2, 2H, B(C6H5)2), 7.03 (m,
9H, P(C6H5)3), 6.79 (d, JHH = 1.6, 2H, ImH), 6.76 (d, JHH = 1.6, 2H, ImH), 1.75 (s, 18H, C(CH3)3). 13C{1H} (C6D6 ,
500 MHz): 184.47, 137.95, 137.87, 134.48, 134.27, 134.12, 128.91, 128.83, 128.78, 127.57, 126.40,
125.28, 116.97, 57.09, 31.68. 31P{1H} (400 MHz, C6D6, δ): -4 (1P, PPh3). IR (cm-1): NO = 1767 cm-1 (THF); NO
= 1722 cm-1 (KBr). HRMS (APCI/Q-TOF) m/z [M+H]+ Anal. Calcd for C44H48BN5NiOP 762.3038; Found
762.3032.
Synthesis of Ph2B(tBuIm)2NiCl(PMe3)
In a scintillation vial, a 1 M solution of PMe3 in THF (5.98 mL) was added to solid NiCl2 (0.258 g, 1.92 mmol),
affording a dark purple solution. The vial was taken outside the glovebox and heated at 35 °C for 3 h before
returning the mixture to the glovebox. Volatiles were removed under pressure to yield a pink solid (crude
NiCl2(PMe3)2).16 In a separate scintillation vial, a solution of LDA (0.39 g, 3.64 mmol) in Et2O (5 mL) was
added dropwise to a slurry of Ph2B(tBuImH)2 (1.01 g, 1.79 mmol) in Et2O (10 mL), affording a clear solution
after 15 min. The carbene solution was added to solid in situ prepared NiCl2(PMe3)2 at −78 °C. The mixture
was slowly warmed to room temperature and the reaction was stirred for 14 h. Volatiles were removed
under reduced pressure and the residue stirred in toluene (15 mL) for 1 h before filtering through Celite.
The solvent was removed under vacuum to yield a yellow solid (0.74 g, 71 % yield based on Ph2B(tBuImH)2).
Crystals suitable for X-ray diffraction were grown by layering n-pentane onto a THF solution at -35 °C. 1H
NMR (400 MHz, C6D6, ): 7.86 (s, 2H, B(C6H5)2), 7.45 (m, 2H, B(C6H5)2), 7.31 (m, 2H, B(C6H5)2), 7.24 (s, 4H,
B(C6H5)2), 6.91 (s, 1H, ImH), 6.71 (s, 1H, ImH), 6.45 (s, 1H, ImH), 6.25 (s, 1H, ImH), 1.87 (s, 9H, C(CH3)3), 1.57
(s, 9H, C(CH3)3), 0.42 (d, JPH = 9.0 Hz , 9H, P(CH3)3). 13C{1H} (C6D6): 168.09, 166.92, 164.95, 164.63, 136.68,
134.24, 127.40, 127.32, 126.70, 126.22, 126.14, 117.73, 117.53, 57.18, 56.66, 32.06, 31.28, 12.97, 12.72.
31P{1H} (400 MHz, C6D6, δ): -18 (1P, P(CH3)3).
Synthesis of Ph2B(tBuIm)2Ni(3-C7H7):
A THF solution of benzyl magnesium chloride (1 M, 1.1 mL) was added to Ph2B(tBuIm)2NiCl(PMe3) (320
mg, 0.55 mmol) in THF (4 mL) and stirred for 14 h. The THF was removed under reduced pressure and the
residue was redissolved in toluene (5 mL). Anhydrous 1,4-dioxane (203 mg, 2.3 mmol) was added
dropwise to the toluene solution to precipitate the magnesium salts. The mixture was vigorously stirred
for 1 h before filtering through Celite. Volatiles were removed under reduced pressure to yield an orange
solid. Crystals suitable for X-ray diffraction were grown at room temperature from a saturated Et2O
solution in two crops (176 mg 57 % yield). 1H NMR (400 MHz, C6D6, δ): 7.75 (s, 2H, B/CH2(C6H5)), 7.63 (m,
2H, B/CH2(C6H5)), 7.43 (b, 1H, B/CH2(C6H5)), 7.37-7.29 (m, 5H, B/CH2(C6H5)), 7.20 (s, 1H, B/CH2(C6H5)), 7.18
(s, 1H, B/CH2(C6H5)), 7.16 (C6D6 overlap, B/CH2(C6H5)), 7.09 (d, JHH = 1.7 Hz, 1H, ImH), 7.0-6.87 (m, 4H,
1ImH+3 B/CH2(C6H5)), 6.56 (d, JHH = 1.6 Hz, 1H, ImH), 6.44 (d, JHH = 1.5 Hz, 1H, ImH), 4.38 (d, JHH = 6.7 Hz,

1H, CH2(C6H5)), 2.04 (d, JHH = 3.6 Hz, 1H, CH2(C6H5)), 1.47 (s, 9H, C(CH3)3), 0.94 (s, 9H, C(CH3)3). 13C{1H} (C6D6,
500 MHz): 178.01, 172.65, 137.07, 136.60, 134.01, 131.06, 127.46, 127.33, 127.19, 127.01, 126.70,
126.20, 125.91, 117.81, 116.89, 116.84, 92.45, 56.09, 54.88, 33.02, 31.74, 30.69. HRMS (APCI/Q-TOF) m/z
[M+H]+ Anal. Calcd for C33H40BN4Ni 561.2700; Found 561.2710.
Reaction of Ph2B(tBuIm)2Ni(3-C7H7) with O2:
A solution of Ph2B(tBuIm)2Ni(3-C7H7) (8.0 mg) in C6D6 (1 mL) was added to a J. Young NMR tube. The tube
was degassed with three freeze-pump-thaw cycles and 1 atm of O2 was added to the frozen solution. The
proposed nickel hydroxide complex can be observed within the first 30 min of reaction before the onset
of significant decomposition. Decomposition is observed even in the absence of excess O2. Benzaldehyde
was isolated by vacuum transfer.
Results and Discussion
Synthesis and Characterization of Ph2B(tBuIm)2Ni(NO)(PPh3)
Reaction of the bis(carbene)borate ligand transfer agent Ph2B(tBuIm)2Li(OEt2) with Ni(NO)Br(PPh3)2
provides the {NiNO}10 complex17 Ph2B(tBuIm)2Ni(NO)(PPh3) as a purple solid in high yield after workup
(Scheme 1). This complex has been structurally and spectroscopically characterized. The solid state
structure of the complex, as revealed by single crystal X-ray diffraction, reveals a four-coordinate Ni center
bound by the bis(carbene)borate, nitrosyl and PPh3 ligands (Figure 2). The nickel ion has a trigonal
pyramidal geometry and is 0.508 Å out of the plane defined by the bis(carbene)borate and nitrosyl donors.
A notable feature of the structure is the long Ni-P distance (2.3099(5) Å), which is almost 0.1 Å longer than
in the related tris(thioimidazolyl)borate and tris(selenoimidazolyl)borate complexes BmMeNi(PPh3)(NO)18
and BseMeNi(PPh3)(NO).19 In fact, this distance is also significantly longer than the average Ni-P distance in
CSD for all four-coordinate nickel PPh3 complexes (2.21(5) Å). This long distance suggests a weak
interaction between nickel and phosphine ligand,20 which has been evaluated in solution (see below).
Scheme 1
It is also worth noting that the Ni-N-O angle in Ph2B(tBuIm)2Ni(NO)(PPh3) is close to linear at 173.2(2) °,
whereas the related complexes BmMeNi(PPh3)(NO)14 and BseMeNi(PPh3)(NO)15 both feature bent nitrosyl
ligands (ca. 150 °). While the latter complexes are nominally five-coordinate due to a weak interaction
between the ligand B-H group and nickel, this is not consequential for nitrosyl ligand bending. Rather,
bending stabilizes these low symmetry molecules, firstly by reducing overlap in the M-N * orbital, and
secondly by allowing this orbital to mix with an NO * orbital.15,21 The fact that similar bending is not
observed in Ph2B(tBuIm)2Ni(NO)(PPh3) suggests the complex may be better described as a three-
coordinate complex where the weak Ni-PPh3 interaction has only a minor perturbation on the electronic
structure. All structurally characterized three-coordinate nickel nitrosyl complexes reported to date
,21,22,23,24
feature linear Ni-N-O linkages.22

Figure 2. X-ray crystal structure of Ph2B(tBuIm)2Ni(NO)(PPh3), thermal ellipsoids shown at 50 % probability,
hydrogen atoms omitted for clarity. Black, blue, red, pink, green and orange ellipsoids correspond to C, H,
O, B, Ni and P atoms, respectively. Selected bond lengths (Å) and angles (°): Ni-P 2.3099(5), Ni-C 1.981(2),
1.990(2), Ni-N 1.659(2), N-O 1.177(2); Ni-N-O 173.2(2).
.
Table 1. IR data (NO) for selected neutral three- and four-coordinate {NiNO}10 complexes
Three-coordinate complexes
Complex
Four-coordinate complexes
Complex
NO
NO (cm-1)
ref

Ph2B(tBuIm)2Ni(NO)(PPh3)
1767, 1730 (THF)
this work
Ph2B(tBuIm)2Ni(NO)(PPh3) 1722 (KBr)
1785 (ATR)b
1731, 1692 (KBr)
23
24
1784 (C6H6)
1728 (KBr)
a - Interaction of B-H and nickel is weak.
b - ATR = Attenuated Total Reflectance.

1772 (NaCl)
1766 (NaCl)
22
1718 (C6H6)
1688 (KBr)
26
In the solid state, the NO ligand of Ph2B(tBuIm)2Ni(NO)(PPh3) is characterized by a strong band in the IR
spectrum (NO = 1722 cm-1). The position of this band compares favorably with that observed for
(BmtBu)Ni(PPh3)(NO) in the solid state (Table 1), although the latter complex was not characterized by
single crystal X-ray diffraction. It is notable that NO for both of these complexes is at a significantly higher
frequency than for (BmMe)Ni(PPh3)(NO), where the nitrosyl ligand is bent.15 Intriguingly, two bands are
observed in the solid state spectrum of (BseMe)Ni(PPh3)NO, one at a frequency similar to
Ph2B(tBuIm)2Ni(NO)(PPh3) and one similar to (BmMe)Ni(PPh3)(NO). These similarities suggest the presence
of both linear and bent nitrosyl ligands in the solid state structure of (BseMe)Ni(PPh3)NO, as was
hypothesized by the authors.16
While four-coordinate in the solid state, spectroscopic studies reveal the loss of PPh3 in solution to provide
three-coordinate Ph2B(tBuIm)2Ni(NO). Thus, the solution IR spectrum shows a large change in the nitrosyl
stretching frequency (NO = 1767 cm-1) from that observed in the solid state (NO = 1722 cm-1). It is notable
that this frequency is similar to that observed for other three-coordinate nickel nitrosyl complexes (Table
1). The addition of excess PPh3 (up to 500 mM) to a 5 mM solution of Ph2B(tBuIm)2Ni(NO) complex has no
effect on the intensity and position of this band, and moreover does not lead to the appearance of new
NO bands, leading us to estimate an upper bound for PPh3 binding, Keq < 10-4 M-1.27 Although the room
temperature IR spectrum does not show evidence for the formation of Ph2B(tBuIm)2Ni(NO)(PPh3) in
solution, even in the presence of excess of PPh3, decreasing the temperature leads to the growth of a new
band at 1730 cm-1, consistent with the formation of Ph2B(tBuIm)2Ni(NO)(PPh3) at low temperature. This
1
conclusion is supported by VT H NMR spectroscopy, where coalescence of the resonances associated
with PPh3 is observed at -60 °C in C7D8. These spectroscopic changes are concomitant with the blue
solution changing to the purple color observed for Ph2B(tBuIm)2Ni(NO)(PPh3) in the solid state.28
Synthesis and Reactivity of Square Planar Complexes
The square planar nickel(II) complex Ph2B(tBuIm)2NiCl(PMe3) is formed in good yield from the reaction of
Ph2B(tBuIm)2Li(OEt2) and the nickel(II) complex NiCl2(PMe3)2 (Scheme 2). The low temperature and nickel
source are both critical to successful metalation, with unidentified side products observed at higher
temperatures, and other nickel reagents, including NiX2 (X = Cl, Br, I), NiCl2(dme), NiCl2(PCy3)2 and
NiBr2(PPh3)2 providing intractable materials.
Scheme 2.

Single crystal X-ray diffraction reveals the expected square planar geometry, in which nickel has the same
coordination sphere as the cationic bis(carbene)methane complex [H2C(tBuIm)2NiCl(PMe3)]+.7 Indeed, the
two complexes have comparable bond lengths to the nickel ion, despite the charge differences. Apart
from the bent C-Ni-P angle (163.20(9) °), which is likely due to steric interactions between PMe3 and tert-
butyl group of the bis(carbene)borate ligand, and is similarly observed for [H2C(tBuIm)2NiCl(PMe3)]+, the
structure of Ph2B(tBuIm)2NiCl(PMe3) is unremarkable. This structure is maintained in solution, as
determined by 1H and 31P{1H} NMR spectroscopy.
Figure 3. X-ray crystal structures of (a) Ph2B(tBuIm)2NiCl(PMe3), selected bond lengths (Å): Ni-Cl 2.2205(9);
Ni-P 2.219(1); Ni-C 1.921(3); 1.862(3); and (b) Ph2B(tBuIm)2Ni(3-C7H7), selected bond lengths (Å) and
angles (°): Ni-C (carbene) 1.916(2); 1.876(2). Ni-C (benzyl) 1.974(2), 2.100(2), 2.228(2) Thermal ellipsoids
shown at 50 % probability, hydrogen atoms omitted for clarity. Black, blue, red, light green, pink, green
and orange ellipsoids correspond to C, H, O, Cl, B, Ni and P atoms, respectively.
While the structure of Ph2B(tBuIm)2NiCl(PMe3) is in itself unremarkable, it is worth noting that this
complex shows no tendency to rearrange to a homoleptic analogue containing two bis(carbene)borates.
For example, no change in the 1H NMR spectrum is observed when the complex is heated at 110 °C. This
stability should be contrasted with observations for the related dihydrobis(carbene)borate and
dihydrobis(carbene)methane ligands, where there is a strong driving force to forming the homoleptic Ni(II)
complexes.9 Thus, Ph2B(tBuIm)2NiCl(PMe3) is expected to serve as a suitable platform for the further
development of nickel chemistry with the bis(carbene)borate behaving as a spectator ligand.
In an initial demonstration of this potential synthetic utility, we treated Ph2B(tBuIm)2NiCl(PMe3) with
PhCH2MgBr to afford orange Ph2B(tBuIm)2Ni(3-C7H7) in moderate isolated yield (Scheme 2). This complex
has also been structurally characterized (Figure 3). The structural metrics regarding the benzyl ligand are
similar to those of other structurally characterized nickel(II) 3-benzyl complexes,29 however the angle

between plane of the allyl ligand and that defined by nickel and bis(carbene)borate donors (46.2 °) is
relatively small. We attribute this small angle to the steric influence of the tert-butyl groups of the
bis(carbene)borate ligand, which prevents the 3-benzyl ligand from binding with a more perpendicular
orientation.
The solid state structure of Ph2B(tBuIm)2Ni(3-C7H7) is maintained in solution, as determined by 1H NMR
1
spectroscopy. A distinctive feature of the H NMR spectrum are the resonances attributed to the two
benzylic protons, which are observed as doublets at  4.38 and 2.04 ppm. The chemical inequivalence of
these two protons, along with the chemical inequivalence of the resonances assigned to the tert-butyl
groups ( 1.47 and 0.94 ppm), suggests that there are no fluxional processes involving the 3-benzyl
ligand.
The benzyl complex Ph2B(tBuIm)2Ni(3-C7H7) is sensitive to O2, reacting cleanly to yield a new diamagnetic
nickel complex. While the thermal instability of this complex has hindered efforts at crystallographic
characterization, we propose that this product is a Ni(II) hydroxide on the basis of 1H NMR spectroscopy,
1
likely the dimer [Ph2B(tBuIm)2Ni(-OH)]2. Most notably, the H NMR spectrum of this complex reveals a
single resonance for the tert-butyl groups of the bis(carbene)borate ligand at  1.52 ppm, consistent with
a two-fold symmetric structure. In addition, a new upfield shifted resonance integrating for a single proton
is observed at  -6.62 ppm, which we tentatively assign to the hydroxide ligand, based on its similarity to
[IPrNi(Cl)(-OH)]2. 30 In addition to the nickel complex, the reaction also yields an equivalent of
1
benzaldehyde, which was characterized by H NMR spectroscopy following vacuum transfer of the
reaction volatiles.
This transformation is similar to observations for (NHC)Ni(3-allyl)Cl complexes, where O2 oxidizes the -
allyl ligands to ,-unsaturated carbonyls.24 This oxidation reaction has been proposed to follow a
mechanism involving initial formation of a Ni(III) peroxo intermediate that abstracts a hydrogen atom
from the allyl ligand to yield a hydroperoxo ligand. Homolysis of the O-O bond leads to intramolecular
hydroxylation of the allyl ligand, which is followed by hydrogen atom transfer to provide the carbonyl and
nickel hydroxide products. The reaction of Ph2B(tBuIm)2Ni(3-C7H7) with O2 likely follows a similar
mechanism.
Summary and Conclusions
Similarly to our observations for iron, introducing phenyl groups for the boron hydrides of a bulky
bis(carbene)borate ligand allows access to heteroleptic Ni(II) complexes. Specifically, the conformation of
the borate ligand orients the phenyl groups such that they prevent another bis(carbene)borate ligand
from binding to the metal, as we have previously observed for iron.14 The square planar complex
Ph2B(tBuIm)2Ni(PMe)3Cl is an entry point to Ni(II) chemistry in which the bis(carbene)borate ligand is
expected to behave as a spectator ligand. Ligand substitution provides the corresponding allyl complex
Ph2B(tBuIm)2Ni(3-C7H7) that reacts with O2 to provide benzaldehyde and a putative Ni(II) hydroxide
complex. The stability of the Ni bis(carbene)borate fragment in all of these transformations worth noting.
The bulky bis(carbene)borate ligand also provides access to a three-coordinate nickel nitrosyl complex
through the reversible loss of PPh3 from Ph2B(tBuIm)2Ni(NO)(PPh3). While PPh3 binding is weak at room
temperature, low temperature spectroscopic data suggest that the four coordinate complex observed in
the solid state is also formed in solution. While this ligand binding and uncertainty regarding the linearity

of the nitrosyl ligand in reference complexes are a complicating factor, the frequency of NO in the IR
spectrum is consistent with previous reports13 that bis(carbene)borates are a very strongly donating
bidentate ligands.
Appendix A. Supplementary data
CCDC 1859951-1859953 contains the supplementary crystallographic data for Ph2B(tBuIm)2Ni(NO)(PPh3)
Ph2B(tBuIm)2NiCl(PMe3) and Ph2B(tBuIm)2Ni(3-C7H7). These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
Funding from IU, the NSF (CHE-1112299, CHE-1566258) and DOE-BES (DE-FG02-08ER15996) is gratefully
acknowledged. We thank Carly Hanson (Loyola University Chicago) for experimental assistance.
Supporting Information
Additional spectroscopic data, X-ray crystallographic data.
References
TOC picture
A bulky diphenylbis(carbene)borate ligand provides access to heteroleptic nickel complexes
Highlights
Unlike its dihydrobis(carbene)borate analogue, which unavoidably provide homoleptic Ni(II) complexes,
a bulky diphenylbis(carbene)borate ligand stabilizes heteroleptic Ni(II) complexes.
A four-coordinate {NiNO}10 complex provides access to the three-coordinate species via phosphine loss.
Reaction of a -benzyl complex with O2 leads to the formation of benzaldehyde.

A bulky diphenylbis(carbene)borate ligand provides access to heteroleptic nickel complexes
1 (a) Garcia, J.J.; Jones, W.D. Organometallics 2000, 19, 5544-5545; (b) Garcia, J.J.; Brunkan, N.M; Jones, W.D. J. Am.
Chem. Soc. 2002, 124, 9547-9555; (c) Brunkan, N.M.; Bretensky, D.M.; Jones, W.D. J. Am. Chem. Soc. 2004, 126,
6327-3641; (d) Garcia, J.J.; Arevalo, A.; Brunkan, N.M.; Jones, W.D. Organometallics 2004, 23, 3997-4002; (e) Atesin,
T.A.; Li, T.; Lachaize, S.; Brennessel, W.W.; Garcia, J.J.; Jones, W.D. J. Am. Chem. Soc. 2007, 129, 7562-7569; (f) Atesin,
T.A.; Li, T.; Lachaize, S.; Garcia, J.J.; Jones, W.D. Organometallics 2008, 15, 3811-3817; (g) Swartz, B.D.; Reinartz,
N.M.; Brennessel, W.W.; Garcia, J.J.; Jones, W.D. J. Am. Chem. Soc. 2008, 130, 8548-8554; (h) Li, T.; Garcia, J.J.
Brennessel, W.W.; Jones, W.D. Organometallics 2010, 29, 2430-2445; (i) Swartz, B.D.; Brennessel, W.W.; Jones, W.D.
Synlett 2018, 29, 747-753.
2 (a) Vicic, D.A.; Jones, W.D. J. Am. Chem. Soc. 1997, 119, 10855-10856; (b) Vicic, D.A.; Jones, W.D. Organometallics
1998, 17, 3411-3413; (c) Vicic, D.A.; Jones, W.D. J. Am. Chem. Soc. 1999, 121, 7606-7617; (d) Torres-Nieto, J.;
Brennessel, W.W.; Jones, W.D.; Garcia, J.J. J. Am. Chem. Soc. 2009, 131, 4120-4126;(e) Grochowski, M.R.; Li, T.;
Brennessel, W.W.; Jones, W.D. J. Am. Chem. Soc. 2010, 132, 12412-12421.

3 Selected reviews: (a) Saouma, C.T.; Peters, J.C. Coord. Chem. Rev. 2011, 255, 920-937; (b) Ray, K.; Heims, F.; Pfaff,
F.F. Eur. J. Inorg. Chem. 2013, 3784-3807.
4 Vicic, D.A.; Jones, W.D. J. Am. Chem. Soc. 1999, 121, 4070-4071.
5 Prakasham, A.P.; Ghosh, P. Inorg. Chim. Acta 2015, 431, 61-100.
6 Charra, V.; de Frémont, P.; Braunstein, P. Coord. Chem. Rev. 2017, 341, 53-176.
7
Douthwaite, R.E.; Haüssinger, D.; Green, M.L.H.; Silcock, P.J.; Gomes, P.T.; Martins, A.M.; Danopoulos, A.A.
Organometallics 1999, 18, 4584-4590.
8 (a) Douthwaite, R.E.; Green, M.L.H.; Silcock, P.J.; Gomes, P.T. Organometallics 2001, 20, 2611-2615; (b) Berding, J.;
Lutz, M.; Spek, A.L.; Bouwman, E. Organometallics 2009, 28, 1845-1854; (c) Huynh, H.V.; Jothibasu, R. Eur. J. Inorg.
Chem. 2009, 1926-1931; (d) Paulose, T.A.P.; Wu, S.-C.; Olson, J.A.; Chau, T.; Theaker, N.; Hassler, M.; Quail, J.W.;
Foley, S.R. Dalton Trans. 2012, 41, 251-260; (e) Harrold, N.D.; Hillhouse, G.L. Chem. Sci. 2013, 4, 4011-4015; (f) Guo,
J.; Lv, L.; Wang, X.; Cao, C.; Pang, G.; Shi, Y.; Inorg. Chem. Commun. 2013, 31, 74-78; (g) Huffer, A.; Jeffrey, B.; Waller,
B.J.; Danopoulos, A.A. C.R. Chemie 2013, 16, 557-565; (h) Brendel, M.; Braun, C.; Rominger, F.; Hofmann, P. Angew.
Chem. Int. Ed. 2014, 53, 8741-8745; (i) Huynh, H.V.; Koh, C.H.M.; Nguyen, V.H. Dalton Trans. 2017, 46, 11318-11326;
(j) Nirmala, M.; Saranya, G.; Viswanathamurthi, P.; Bertani, R.; Sgarbossa, P.; Malecki, J.G. J. Organomet. Chem. 2017,
831, 1-10.
9 (a) Nieto, I.; Bontchev, R.P.; Smith, J.M. Eur. J. Inorg. Chem. 2008, 2476-2480; (b) Santini, C.; Marinelli, M.; Pellei,
M. Eur. J. Inorg. Chem. 2016, 2312-2331.
10 Smith, J.M. Comments Inorg. Chem. 2008, 28, 189-233.
11 Hickey, A.K.; Lutz, S.A.; Chen, C.-H.; Smith, J.M. Chem. Commun. 2017, 53, 1245-1248.
12 (a) Lee, W.-T.; Jeon, I.-R.; Xu, S.; Dickie, D.A.; Smith, J.M. Organometallics, 2014, 33, 5654-5659; (b) Liu, Y.-Z.; Wang,
J.; Zhao, Y.; Chen, L.; Chen, X.-T.; Xue, Z.-L. Dalton Trans. 2015, 44, 908-911.
13 (a) Shishkov, I.V.; Rominger, F.; Hofmann, P. Organometallics 2009, 28, 3532-3536; (b) Xiong, Y.; Szilvási, T.; Yao,
S.; Tan, G.; Driess, M. J. Am. Chem. Soc. 2014, 136, 11300-11303.
14 Hickey, A.K.; Lee, W.-T.; Chen, C.-H.; Pink, M.; Smith, J.M. Organometallics 2016, 35, 3069-3073.
15 Feltham, R.D. Inorg. Chem. 1964, 3, 116-119.
16 Dahl, O. Acta Chem. Scand. 1969, 23, 2342-2354.
17 Enemark, J.H.; Feltham, R.D. Coord. Chem. Rev. 1974, 13, 339-406.
18 Maffett, L.S.; Gunter, K.L.; Kreisel, K.A.; Yap, G.P.A.; Rabinovich, D. Polyhedron 2007, 26, 4758-4764.
19 Landry, V.K.; Parkin, G. Polyhedron 2007, 26, 4751-4757.
20 An important caveat to such an analysis: Kaupp, M.; Danovich, D.; Shaik, S. Coord. Chem. Rev. 2017, 344, 355-362.
21 Mingos, D.M.P. Inorg. Chem. 1973, 12, 1209-1211.
22 (a) Chong, K.S.; Rettig, S.J.; Storr, A.; Trotter, J. Can. J. Chem. 1979, 57, 3090-3098; (b) Iluc, V.M.; Miller, A.J.M.;
Hillhouse, G.L. Chem. Commun. 2005, 5091-5093; (c) Wright, A.M.; Wu, G.; Hayton, T.W. J. Am. Chem. Soc. 2012,
134, 9930-9933; (d) Wright, A.M.; Zaman, H.T.; Wu, G. Hayton, T.W. Inorg. Chem. 2013, 52, 3207-3216; (e)
Tskhovrebov, A.G.; Solari, E.; Scopelliti, R.; Severin, K. Inorg. Chem. 2013, 52, 11688-11690.
23 Puiu, S.C.; Warren, T.H. Organometallics 2003, 22, 3974-3976.
24 Hartmann, N.J.; Wu, G.; Hayton, T.W. J. Am. Chem. Soc. 2016, 138, 12352-12355.
25 Kriege-Simondsen, J.; Elbaze, G.; Dartiguenave, M.; Feltham, R.D.; Dartiguenave, Y. Inorg. Chem. 1982, 21, 230-
236.
26 Varonka, M.S.; Warren, T.H. Organometallics 2010, 29, 717-720.
27 This is based on the estimate that 5% conversion would provide an observable band for Ph2B(tBuIm)2Ni(NO)(PPh3)
in the IR spectrum.
28 While adding one equivalent of B(C6F5)3 to a solution of Ph2B(tBuIm)2Ni(NO)(PPh3) in benzene does lead to the
formation of Ph2B(tBuIm)2Ni(NO), as detected by 1H NMR and IR spectroscopy, a number of other products are also
observed.
29 (a) Anderson, T.J.; Vicic, D.A. Organometallics 2004, 23, 623- 625; (b) Wasilke, J.-C.; Komon, Z.J.A.; Bu, X.; Bazan,
G.C. Organometallics 2004, 23, 4174-7177; (c) Sujith, S.; Noh, E.K.; Lee, B.Y.; Han, J.W. J. Organomet. Chem. 2008,
693, 2171-2176; (d) Matsubara, R.; Gutierrez, A.C.; Jamison, T.F. J. Am. Chem. Soc. 2011, 133, 19020-19023.
30 (a) Dible, B.R.; Sigman, M.S. J. Am. Chem. Soc. 2003, 125, 872-873; (b) Dible, B.R.; Sigman, M.S. Inorg. Chem. 2006,
45, 8430-8441.