Structural elucidation of a nickel boryl complex. A recyclable borylation Ni(II) reagent of bromobenzene

Article abstract of DOI:10.1039/b709832d

The first nickel complex bearing a boryl moiety, (PNP)Ni[B(catechol)] (PNP = N[2-P(CHMe₂)₂-4-methylphenyl]₂^-), has been prepared, structurally characterized, and analyzed by DFT; this rare species is shown to be a recyclable reagent for the borylation of bromobenzene, via an unusual cycle, by applying the ingredients catecholborane and NaBH ₄. The Royal Society of Chemistry.

Full text of DOI:10.1039/b709832d

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Structural elucidation of a nickel boryl complex. A recyclable
borylation Ni(II) reagent of bromobenzene{
Debashis Adhikari, John C. Huffman and Daniel J. Mindiola*
Received (in Berkeley, CA, USA) 27th June 2007, Accepted 27th August 2007
First published as an Advance Article on the web 11th September 2007
DOI: 10.1039/b709832d
When complex 111 is treated with catecholborane at room
The first nickel complex bearing a boryl moiety, (PNP)Ni[B-
(catechol)] (PNP 5 N[2-P(CHMe2)2-4-methylphenyl]22), has
been prepared, structurally characterized, and analyzed by
DFT; this rare species is shown to be a recyclable reagent for
the borylation of bromobenzene, via an unusual cycle, by
applying the ingredients catecholborane and NaBH4.
temperature in benzene, the boryl complex 2{ and H2 are cleanly
formed over a period of 12 h (Scheme 1).{ A similar strategy for
the synthesis of transition metal boryl complexes has been used by
Marder and co-workers.12 Compound 2 is a yellow colored solid
(mp 172 uC) that can be isolated in 81% yield after work-up of the
reaction mixture. Diamagnetic 2 evinces a broad 11B NMR
resonance at y47 ppm (JB–P coupling was not resolved even upon
cooling the solution to 245 uC), as well as a singlet at 51.2 ppm in
the 31P NMR spectrum, the latter being consistent with such a
species retaining C2v symmetry in solution. The former chemical
shift is comparable to other 3d transition metal boryl complexes
reported in the literature.13,14 To our surprise however, complex 2
is kinetically stable to heat (several days in C6D6 at 155 uC under
N2), and even air for brief periods of time (y30 min).
Group-transfer processes involving nickel reagents represent an
attractive paradigm given the low cost associated with this element
when compared to its heavier congeners. One particular catalytic
reaction that has found enormous promise has been the conversion
of aromatic and aliphatic hydrocarbons to the corresponding
boronic esters1 since these products can be used as feedstock for a
wide variety of cross-coupling reactions2 as well as substitution
chemistries at the C–B linkage.3 Under this premise, it has
been proposed that borylation reactions often proceed via reactive
M–BR2 intermediates (M 5 Ir, Rh, Pd and Pt).4–7 To our surprise,
borylation reactions involving Ni as a reagent have not been
documented. Arguably, the inability of Ni to emulate the
chemistry of its heavier congeners in processes such as borylations
might be attributed to lack of stable and isolable systems bearing
the prototypical Ni–BR2 linkage, which is in fact the case for this
element. The first report of a Ni–BR2 moiety was claimed by Noth
and Cundy, but this complex was ill-characterized.8 Recent studies
by Liang and co-workers have revealed that square-planar Ni(II)
Single crystal X-ray analysis of 2§ unequivocally exposes a
square-planar nickel complex bearing the terminal and planar
catecholboryl group (Ni–B, 1.9091(18) s, Fig. 1).{ This bond
length is within range of other structurally characterized boryl
˚
complexes for 3d metals such as Co (1.970(11) A), Fe (1.973(2) A),
˚
13
and Mn (2.108(6) A), but is significantly shorter than that
˚
15
˚
reported for the s-alkyl borane adduct of Ni(0) (2.172(6) A).
Although square-planar transition metal boryl complexes have
been documented, this geometry is rare when weighed against
most metal–boryl systems crystallographically characterized.14 In
2, the plane composing the O–B–O atoms bisects the imaginary
plane defined by the P–Ni–P atoms (y71.4u); therefore placing the
empty p(y) orbital of boron virtually along the (PNP)Ni xy-plane.
Intuitively, the orientation of the boryl fragment with respect to
the (PNP)Ni motif is not surprising since the filled d(xy) orbital on
Ni has the appropriate symmetry to backbond with the empty p(y)
orbital on boron.
2
complexes supported by the pincer ligand N[2-PR-phenyl]2
i
(R 5 Pr, Cy) can activate C–H bonds of benzene when the
system is promoted with a Lewis acid such as Al(CH3)3.9 Inspired
by their work, we speculated whether the closely related and robust
‘‘(PNP)Ni’’ surrogate, PNP 5 N[2-P(CHMe2)2-4-methylphe-
nyl]22,10could in fact accommodate a boryl group and then
transfer such a fragment to haloaromatics.
In order to address the geometry in 2 and to ascertain whether
the Ni–B linkage enjoyed MAL p-backbonding, we turned our
During the course of our studies involving the Ni(II) complex
(PNP)NiH (1),11 we discovered that such a precursor can be
converted to the square-planar nickel boryl compound
(PNP)Ni[B(catechol)] (2). Most notably, compound 2 can transfer
the B(catechol) moiety to bromobenzene, and the Ni(II) product
can be recycled back to 1 by applying NaBH4. Herein, we present
a new borylation cycle of bromobenzene by using the first Ni(II)
boryl reagent 2. This complex has been isolated, structurally
elucidated, and theoretically analyzed by DFT.
Department of Chemistry and the Molecular Structure Center, Indiana
University, Bloomington, IN 47405, USA.
E-mail: mindiola@indiana.edu; Tel: 001-812-855-2399
{ Electronic supplementary information (ESI) available: Experimental
preparation and reactivity (all compounds), crystallographic data, and
additional discussion. See DOI: 10.1039/b709832d
Scheme 1 Synthesis of complex 2 from 1 using catecholborane, and
subsequent borylation of bromobenzene. The preparation of 1 and 3 are
also illustrated.
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Fig. 1 Molecular structure of 2 omitting H-atoms, Et2O solvent, and
isopropyl methyls on phosphorus for the purpose of clarity. Thermal
ellipsoids are displayed at the 50% probability level. Selected metrical
parameters (distances in s, angles in u): Ni1–B31, 1.9091(18); Ni1–P2,
2.1447(4); Ni1–P18, 2.1408(4); Ni1–N10, 1.932(2); B31–O32, 1.419(2);
B31–O39, 1.423(2); P2–Ni1–P18, 174.42(7); N10–Ni1–P2, 87.45(4); N10–
Ni1–P18, 87.40(4); N10–Ni1–B31, 174.54(7); P2–Ni1–B31, 93.43(5); P18–
Ni1–B31, 91.89(5); Ni1–B31–O32, 129.2(2); Ni1–B31–O39, 122.8(2).
attention to high level DFT analysis.{ As anticipated for a square
planar d8 system, complex 2 has a filled d(z2) HOMO-1 orbital and
an unoccupied d(x2 2 y2) LUMO orbital augmented with PNP
aryl p* (Fig. 2). In addition, the HOMO orbital has ligand p*
character with considerable contribution from the PNP nitrogen
lone pair (Fig. 2). Upon further analysis of 2, we observe that
backbonding from Ni to B does not take place in this system since
the anticipated empty p(y) orbital on boron has been diffused
instead to the catechol p-framework thus extricating the Ni d(xy)
orbital from any bonding interaction (HOMO-3, Fig. 2).
Theoretical calculations by others have demonstrated that the
M–BR2 bond strength stems mainly from s donation of the boryl
ligand with minimal contribution from p-backbonding of the
transition metal to the vacant p orbital of boron when the R2
group in question bears a p-donor such as a catechol.16 As
expected for a hard and trans influencing ligand,17 the boryl–Ni s
bond is very low in energy combining a Ni d(x2 2 y2) and B p(x)
orbital (HOMO-8, Fig. S1).{ Thence, our orbital analysis and
natural bond order calculations (NBO 5 0.84){ suggest that
complex 2 does not contain multiple bond character between Ni
and B. Since NBO calculations do not account for the electrostatic
component between Ni and B, and the Ni–B linkage in 2 lacks p
overlap, our predicted Mayer bond order is less than one.
Hypothetically, this lack of orbital overlap between Ni and B
should render the catecholboryl group in 2 amenable to
substitution chemistry.
Fig. 2 Some of the most important molecular orbitals of 2 with isoden-
sity 5 0.05 au. The bottom scheme depicts a recyclable square-planar
Ni(II) reagent applied to borylation of bromobenzene, where the PNP2
cartoon represents the framework N[2-P(CHMe2)2-4-methylphenyl]2.
therefore prepared 3 independently, from H(PNP) and NiBr2, and
found that this system can in fact be converted smoothly to 1 when
subjected to NaBH4 (Scheme 1). Pure 1 from 3 and NaBH4 can be
isolated in 62% yield by recrystallization.{ To our disappointment
however, catalysis did not prevail when 10–20 mol% of 3 with
HB(catechol) and NaBH4 are mixed in neat bromobenzene for
18 h at 120 uC, since compound 1 reacts at a comparable rate with
bromobenzene to generate 3 and benzene. Despite this limitation,
we can perform the reaction stoichiometrically with 2 in neat
bromobenzene, to afford PhB(catechol) in 68% isolated yield.
Separation of PhB(catechol) from 3 was achieved using a
preparatory thin layer chromatography plate coated with silica.{
Most notably, compound 3 can also be separated from the mixture
in 88% and recycled back to 1 with NaBH4 (vide supra) thus
closing our cycle for the borylation of bromobenzene (Fig. 2).{
Isolation of the metal product and its recycling is an improvement
over reported cases in the preparation of phenylboronate esters
using Co,18 Rh, or Ir.19
Complex 2 does in fact transfer the boryl group, inasmuch as
treatment with bromobenzene at 120 uC over 18 h afforded
PhB(catechol) and (PNP)NiBr (3), quantitatively as inferred from
1H and 31P NMR spectroscopy (Scheme 1). The identity of the
former and latter products have been confirmed from independent
experiments as well as GC-MS and multinuclear NMR spectra
(1H, 11B, and 31P NMR spectra).{ Unfortunately, refluxing 2 in
chlorobenzene over 3 days at 120 uC resulted in only ,10%
borylation of the arene concurrent with formation of the known
precursor (PNP)NiCl (4). Our ability to generate the B–C coupled
product and 3 from bromobenzene insinuated that this process
could be made catalytic since precursors such as 4 have been
previously shown to convert to 1 by addition of NaBH4.11 We
Formation of 3 from 2 and bromobenzene as well as the
conversion of 2 from 1 with HB(catechol) might involve Ni(II)/
Ni(IV) redox couples, but we have not identified other inter-
mediates than the ones previously mentioned. Our proposed
4490 | Chem. Commun., 2007, 4489–4491
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data) and GoF 5 0.975. Inert atmosphere techniques were used to place a
yellow needle of approximate dimensions 0.25 6 0.25 6 0.15 mm onto the
tip of a 0.1 mm diameter glass fiber. A total of 55 063 reflections (212 ¡ h
¡ 12, 215 ¡ k ¡ 15, 220 ¡ l ¡ 20) were collected at T 5 132(2) K in
the h range of 2.05 to 27.56u, of which 8459 were independent, and 6455
were observed (Rint 5 0.0730); MoKa radiation (l 5 0.71073 s). A direct-
methods solution was calculated which provided most non-hydrogen atoms
from the E-map. Full-matrix least squares/difference Fourier cycles were
performed which located the remaining non-hydrogen atoms. All non-
hydrogen atoms were refined with anisotropic displacement parameters. All
hydrogen atoms were located in subsequent Fourier maps and included as
isotropic contributors in the final cycles of refinement. CCDC 652070 (2).
For crystallographic data in CIF or other electronic format, see DOI:
10.1039/b709832d
mechanism contrasts with Masuda’s palladium-catalyzed boryla-
tion of Ar–X (X 5 Br, I, OTf) to arylboronates in terms of
oxidation states as well as the order of intermediates formed
during the cycle. For example, the oxidative addition of an Ar–X
bond to Pd(0) differs from our proposed oxidative addition to a
Ni(II) center.20,21 Likewise, the formation of a Pd–B bond is
assumed to occur in the last step of the catalytic cycle, while in our
case a Ni–B bond must precede the oxidative addition step of the
Ar–X. Our recycling diagram depicted in Fig. 2 is interesting for
four reasons: (1) three key intermediates along the borylation
process can be readily isolated and characterized, which includes
the first Ni–boryl complex 2, (2) the process uses fairly stable Ni(II)
reagents such as 1–3, which can be prepared readily and recycled,
(3) our borylation cycle bypasses the use of the more common, but
expensive biscatecholdiborane reagent by applying HB(catechol)
as an alternate boryl source, and (4) our unique mechanism
scenario most likely involves a Ni(II)/Ni(IV) redox couple whereby
a Ni(II) boryl undergoes oxidative addition of Ar–Br.
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Based on these results, we have demonstrated that a Ni–boryl
complex can be prepared, characterized spectroscopically and
structurally in conjunction with theoretical analysis of its geometry
and bonding scheme. Such species is capable of transferring the
boryl fragment to bromobenzene to form PhB(catechol) and the
corresponding nickel(II) bromide. The new borylation cycle
presented here can be accomplished using relatively stable Ni(II)
reactants by applying cheaper reagents such as HB(catechol) and
NaBH4 as a means to borylate and recycle, respectively, the
(PNP)Ni(II) scaffold. Our proposed cycle differs from other group
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We thank Indiana University-Bloomington, the Dreyfus
Foundation, the Sloan Foundation, the US NSF (CHE-
0348941, PECASE award to DJM), and the Chemical Sciences,
Geosciences and Biosciences Division, Office of Basic Energy
Science, Office of Science, US Department of Energy (DE-FG02-
07ER15893) for financial support of this research. The authors
thank Dr. Hongjun Fan and Jayasree Srinivasan for insightful
discussions, and Professor Todd B. Marder, Professor Oleg V.
Ozerov, and the reviewers for invaluable suggestions to the
manuscript.
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Notes and references
{ Synthesis of (PNP)Ni[B(catechol)] (2): in a vial, (PNP)NiH (100 mg,
0.205 mmol) was dissolved in 5 mL of benzene and a solution of
catecholborane (25 mg, 0.205 mmol) in benzene was added dropwise. The
reaction mixture was stirred for 12 h and then dried in vacuo. The yellow-
brown materials were extracted with diethyl ether, filtered, and the filtrate
reduced in volume. The solution was then cooled to 235 uC to afford 2 as
yellow colored crystals (101 mg, 0.166 mmol, 81% yield). 1H NMR (25 uC,
399.8 MHz, C6D6): d 7.86 (d, 2H, C6H3), 7.24 (m, 2H, C6H4), 6.91 (br, 4H,
overlap of aromatic resonances), 6.84 (m, 2H, C6H3), 2.28–2.21 (br, 10H,
methyl and isopropyl methine resonances overlapped), 1.08 (dd, 12H,
CHMe2), 1.00 (dd, 12H, CHMe2). 13C NMR (25 uC, 100.6 MHz, C6D6): d
161.3 (aryl), 150.2 (aryl), 132.7 (aryl), 132.5 (aryl), 128.4 (aryl), 124.0 (aryl),
121.6 (aryl), 114.8 (aryl), 111.4 (aryl), 23.5 (CHMe2), 20.6 (MeAr), 18.5
(CHMe2), 17.6 (CHMe2). 31P NMR (25 uC, 121.5 MHz, C6D6) d 51.2. 11
B
NMR (25 uC, 160.6 MHz, C6D6) d 46.9. MS-CI, [M + H]+: calcd 605.2288,
found 605.2290. Mp 172 uC. UV-vis (hexane, 24 uC): 413 nm
(e 5 4042 M21 cm21).
¯
§ Crystal data for 2: C32H44BNNiO2P2?Et2O, Mr 5 680.26; triclinic; P1;
a 5 9.6521(7), b 5 12.1376(9), c 5 15.950(1) s, a = 94.662(2), b =
91.022(2), c = 100.029(2)u, V 5 1833.0(2) s3; Z 5 2; Dc 5 1.233 g cm23
R1 5 0.0473, wR2 5 0.0765 (I . 2s (I)); R1 5 0.0304, wR2 5 0.0842 (all
;
21 M. Melaimi, C. Thoumazet, L. Richard and P. L. Floch, J. Organomet.
Chem., 2004, 689, 2988–2994.
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