Coordination of a Diphosphine-Ketone Ligand to Ni(0), Ni(I), and Ni(II): Reduction-Induced Coordination

Article abstract of DOI:10.1021/acs.organomet.5b00264

The coordination chemistry of the diphosphine-ketone ligand 2,2′-bis(diphenylphosphino)benzophenone (^Phdpbp) with nickel is reported. The ketone moiety does not bind to Ni(II) in the complex (^Phdpbp)NiCl₂, whereas reduction to Ni(I) or Ni(0) induces η²(C,O) coordination of the ketone to form the pseudotetrahedral complexes (^Phdpbp)NiCl and (^Phdpbp)Ni(PPh₃). DFT calculations indicate that the metal-ketone bond is dominated by π back-donation; hence, ^Phdpbp functions as a hemilabile acceptor ligand in this series of complexes. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00264

Communication
pubs.acs.org/Organometallics
Coordination of a Diphosphine−Ketone Ligand to Ni(0), Ni(I), and
Ni(II): Reduction-Induced Coordination
Bartholomeus W. H. Saes,† Dide G. A. Verhoeven,† Martin Lutz,‡ Robertus J. M. Klein Gebbink,†
and Marc-Etienne Moret*,†
†Organic Chemistry & Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG
Utrecht, The Netherlands
‡Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Padualaan 8,
3584 CH Utrecht, The Netherlands
S
* Supporting Information
ABSTRACT: The coordination chemistry of the diphosphine−
ketone ligand 2,2′-bis(diphenylphosphino)benzophenone
(Phdpbp) with nickel is reported. The ketone moiety does not
bind to Ni(II) in the complex (Phdpbp)NiCl2, whereas reduction
to Ni(I) or Ni(0) induces η2(C,O) coordination of the ketone to
form the pseudotetrahedral complexes (Phdpbp)NiCl and
(Phdpbp)Ni(PPh3). DFT calculations indicate that the metal−
ketone bond is dominated by π back-donation; hence, Phdpbp
functions as a hemilabile acceptor ligand in this series of complexes.
Scheme 1. Synthesis of the Phdpbp Ligand
he development of greener and less expensive chemical
Tprocesses has motivated a widespread investigation of
complexes of first-row transition metals as potential homoge-
neous catalysts to replace or, better, improve on traditional
systems that are widely based on noble metals.1 Progress in this
area has been intimately related to the development of tailored
ligands: cooperative ligands2 that actively participate in
chemical reactionse.g. by accepting and releasing electrons,3
hydrogenation catalysts but to our knowledge has not been
protons,4 or hydride equivalents5and hemilabile ligands6 that
used in first-row transition-metal chemistry.
Here we report on the coordination chemistry of the Phdpbp
ligand to nickel in three different oxidation states, showing that
it can act as a wide bite angle bidentate ligand for Ni(II) and
facilitate reaction steps by adapting their coordination mode to
the electronic structure of the metal center along the reaction
coordinate. In particular, multidentate ligands containing a
adopts a pincer-like geometry in Ni(I) and Ni(0) complexes, in
Lewis acidic moiety tethered to one or more chelating arms
which the ketone moiety is coordinated in an η2 fashion
have recently been subjected to intense scrutiny7 and were
dominated by π back-donation.
found to facilitate several catalytic reactions such as hydro-
In an improvement on Ding’s original five-step synthesis,12
genation5 (bifunctional H2 activation) and N2 reduction8
ligand Phdpbp (1) was synthesized in 84% yield from o-
bromo(diphenylphosphino)benzene15 by lithiation with n-BuLi
(hemilabile behavior).
Ketones are known to form relatively weak coordination
followed by reaction with 0.5 equiv of N,N-dimethylchlor-
bonds in two distinct modes: η1(O) with electrophilic metals9
oformamide (Scheme 1). Reaction of 1 with (dme)NiCl2 (dme
and η2(C,O) with electron-rich metals.10 Because of the
= 1,2-dimethoxyethane) in dichloromethane afforded 83% of
1
electronegativity of oxygen, the latter binding mode, described
by the Dewar−Chatt−Duncanson model, is often dominated
by π back-bonding: for example, the formation of complexes of
the type (Et3P)2Ni(η2-benzophenone) from (Et3P)4Ni and
substituted benzophenones is strongly accelerated by electron-
withdrawing substituents.10a Thus, we reasoned that a ketone
moiety tethered to chelating arms would be of interest as a
hemilabile, pincer-type ligand featuring a strong π acceptor in
the central position.11 The ligand 2,2′-bis(diphenylphosphino)-
benzophenone12 (Phdpbp (1); Scheme 1) has been applied as a
coligand for chirality transfer in rhodium13 and ruthenium14
the 1:1 complex 2 as brown crystals (Scheme 2). Broad H
NMR resonances ranging from −6.6 to 20.7 ppm and a
solution effective magnetic moment (μeff) of 2.8 μB indicate a
paramagnetic S = 1 state consistent with a high-spin Ni(II)
center. Complex 2 displays an intense IR band at 1634 cm−1,
only slightly shifted from the CO band in the free ligand 1
(1661 cm−1), suggesting at most a weak interaction of the C
O moiety with the metal.
Received: March 27, 2015
© XXXX American Chemical Society
A
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Scheme 2. Coordination of the Phdpbp Ligand to Ni(0),
Ni(I), and Ni(II)
isolated from the reaction mixture (see the Supporting
Information). Its crystal structure18 (Figure 1) reveals a
mononuclear, four-coordinate complex in which the ketone
unit is coordinated to the nickel in an η2 fashion (Ni−C =
2.006(2) Å, Ni−O = 1.9740(15) Å). Significant π back-
donation into the CO fragment is evidenced by an elongated
CO bond (1.310(2) Å vs 1.213(3) Å in 112) and the
pyramidalization of the C71 atom, which displays a sum of
valence angles of 354.1(3)°. The overall coordination geometry
is best described as slightly distorted tetrahedron, in line with
known tris(phosphine)NiI−X (X = I, Br, Cl) complexes.19 To
our knowledge, 3 is the first example of a structurally
characterized η2(C,O)-ketone complex of Ni(I). The η1(O)
binding bode is known in the Ni(I) complex [(Nacnac)Ni(O
CPh2)] (Nacnac = HC[CMeNC6H3(i-Pr)2]2),20 which displays
a much shorter C−O distance (1.239(7) Å), indicating a lower
extent of π back-donation than in 3. The preference for the
η2(C,O) mode in 3 can be attributed to (i) a more electron rich
Ni(I) center and (ii) geometrical constraints imposed by the
rigid o-phenylene linkers.
Samples of 3even after multiple recrystallizations
1
contain a diamagnetic component evidenced by aromatic H
NMR resonances between 6.5 and 8 ppm (next to the expected
broad, paramagetically shifted spectrum) and a single 31P
resonance at 30.6 ppm, which we tentatively attribute to a Ni−
Ni bonded species.21,22 The IR spectrum of 3 displays no
absorption corresponding to an unbound ketone but exhibits
two absorptions at 1331 and 1340 cm−1 that do not appear in
the spectrum of the free ligand and are assigned to the bound
CO moiety (see the Supporting Information). More insight
into the electronic structure of compound 3 was obtained by
EPR spectroscopy and DFT calculations.23 The room-temper-
ature EPR spectrum of 3 (Figure 2) displays a broad doublet
Crystallization of 2 from CH2Cl2 resulted in two crystal
forms, which were both analyzed by X-ray crystal structure
determinations. The results for the solvent-free 2 are discussed
here (Figure 1). For the results of 2·CH2Cl2, see the
Supporting Information. The Ni−C (3.4031(12) Å) and Ni−
O (3.1012(10) Å) distances are large enough to exclude
coordination of the carbonyl moiety but are nevertheless
shorter than the sum of van der Waals radii16 (Ni−C = 4.17 Å;
Ni−O = 3.90 Å), which is likely imposed by the rigidity of the
o-phenylene linkers but may result in a weak interaction.
Hence, the coordination geometry is best described as distorted
tetrahedral with a P−Ni−P bite angle of 112.996(13)° and a
large Cl−Ni−Cl angle (133.302(14)°). Similar distortions have
been observed in NiCl2 complexes of other diphosphine ligands
with large bite angles and ascribed to lone-pair repulsion
between Cl atoms.17a
Figure 2. (left) Experimental (black) and simulated (red) room-
temperature X-band EPR spectra of 3. Simulation parameters: g =
2.177, Aiso(31P) = 380 Hz. (right) Spin density of 3 calculated at the
B3LYP/6-31G(d,p) level.
The nickel(I) complex 3 was accessed via comproportiona-
tion of (dme)NiCl2 and Ni(cod)2 in the presence of ligand 1
(Scheme 2). Complex 3 was also observed by NMR following
direct reduction of 2 with sodium naphthalide but could not be
Figure 1. Molecular structures of compounds 2−4 in the crystal (50% probability level), determined by single-crystal X-ray structure determination.
For clarity, only the ipso C atoms of phosphorus-bound phenyl groups are represented. Only one of the two independent molecules of 3 is
represented, and an Et2O molecule in the solid-state structure of 4 is omitted.
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(Aiso(31P) = 380 MHz) centered at g = 2.177, consistent with a
metal-centered radical with superhyperfine coupling to one of
the 31P nuclei. A spectrum recorded at 100 K in frozen toluene
(Figure S25 in the Supporting Information) displays a complex
pattern that can be adequately simulated as a rhombic signal
(gx= 2.325, gy = 2.175, gz = 2.026) with anisotropic hyperfine
coupling to two inequivalent 31P nuclei (A1(31P) =
[360,300,500] MHz, A2(31P) = [210,100,95] MHz). The spin
density obtained from DFT at the B3LYP/6-31G(d,p) level
(Figure 2) suggests that the inequivalency of the two 31P nuclei
both at 100 K and at room temperaturewhere only one
hyperfine coupling is resolvedcan be ascribed to delocaliza-
tion of the unpaired electron on a single P atom: NBO24,25
analysis of the spin density ascribes a natural spin density
(NSD) of 1.026 to Ni and of respectively 0.094 and 0.004 to
the two phosphorus atoms, supporting the predominant
metalloradical character of 3. Negative NSDs on the ketone
C (−0.12) and O (−0.04) atoms suggest that a charge-transfer
configuration (Ni(II) + ketyl radical anion) might contribute to
the overall electronic structure of 3.
= 1.97−1.99 Å; Ni−O = 1.84−1.87 Å),10 which might be due
to more steric congestion.
More insight into the bonding of the carbonyl fragment to Ni
was obtained from NBO24,25 analysis performed on ligand 1
and complexes 2−4 (Table 1). Coordination of the Phdpbp
Table 1. Wiberg Bond Indices (WBI) and Natural Charges
(q) from Densities Calculated at the B3LYP/6-31+G(d,p)
Level
1
2
3
4
WBI(C−O)
WBI(Ni−O)
WBI(Ni−C)
q(C)
1.75
1.67
1.30
1.23
<0.01
<0.01
0.56
0.33
0.30
0.46
0.50
0.58
0.18
0.12
q(O)
−0.54
0.04
−0.55
0.01
−0.69
−0.51
−0.73
−0.61
q(C) + q(O)
ligands through its P atoms in 2 induces a slight decrease of the
C−O Wiberg bond index (WBI), consistent with the observed
shift of the corresponding IR band from 1661 to 1634 cm−1;
however, the orbital interaction between the Ni center and the
CO moiety is minimal, with Ni−C and Ni−O WBIs below
0.01. In contrast, the C−O WBI decreases upon binding from
1.75 in the free ligand 1 to 1.30 in 3 and 1.23 in 4. Interestingly,
the Ni−O WBIs in 3 (0.33) and 4 (0.30) are lower than the
corresponding Ni−C WBIs (0.46 in 3, 0.50 in 4), suggesting
that back-donation into the π*(C−O) orbital (which has a
larger coefficient on C) contributes more to the bonding than
donation from the π(C−O) orbital. The primarily acceptor
character of the CO moiety is additionally corroborated by a
decrease of the total charge of the C−O fragment by 0.55 and
0.65 e upon coordination to Ni(I) in 3 and to Ni(0) in 4,
respectively. In accordance, NBO analysis performed on the
Ni(0) complex 4 characterizes the (Ni−C−O) triangle as
engaging in three-center−four-electron bonding (Figures S29
and S30 in the Supporting Information).
In summary, we have studied the coordination chemistry of
the o-phenylene-bridged diphosphine−ketone (Phdpbp) ligand
1 with nickel, showing that the central ketone moiety in 1 can
act as a hemilabile moiety. In the Ni(II) complex 2, the Phdpbp
ligand acts as a flexible, wide bite angle diphosphine ligand17
and the ketone moiety is not bound to the metal. Moving to
more reduced Ni(I) and Ni(0) complexes induces coordination
of the ketone so that the Phdpbp ligand acts as a tridentate,
pincer-like ligand. Consequences of this acceptor-hemilabile
behavior in small-molecule activation and catalysis are currently
under investigation in our laboratories.
To study its coordination to Ni(0) centers, ligand 1 was
treated with 1 equiv of Ni(cod)2 to afford a brown solid that is
formulated as the ligand/metal 3/2 complex 5 (Scheme 2) on
the basis of NMR data. The 31P NMR spectrum of 5 at room
temperature consists of three broad peaks that sharpen upon
heating to 100 °C, becoming three mutually coupled doublets
of doublets at 44.4, 21.3, and 5.6 ppm (Figure S20 in the
Supporting Information). The 1H NMR spectrum of 5 exhibits
a complex set of aromatic signals ranging from 8.8 to 6.2 ppm
and no additional signal that would suggest the presence of
other organic ligands. The simplest structure that is consistent
with this data is a symmetrical dinuclear complex in which a
central Phdpbp ligand bridges two Ni(0) centers chelated by an
additional Phdpbp ligand each (Scheme 2). This structural
model is further corroborated by bands at 1679 and 1310 cm−1
in the IR spectrum of 5 corresponding to the unbound and
bound CO units, respectively.
When a solution of 5 in toluene-d7 was treated with excess
PPh3 in an NMR tube, 2 equiv of the mononuclear complex 4
formed with concomitant release of 1 equiv of the free ligand 1
(Scheme 2). Complex 4 could also be synthesized as a brown
solid from the reaction of 1, Ni(cod)2, and PPh3 or by direct
reduction of 2 with 2.2 equiv of sodium naphthalide in the
presence of PPh3. Its 31P NMR spectrum consists of a triplet at
38.2 ppm (2JP,P = 25 Hz) and a doublet at 17.9 ppm in a 1/2
integral ratio, indicating that the two 31P nuclei from the
1
Phdpbp ligand are equivalent on the H NMR time scale, in
contrast with compound 5. This can be explained by the less
sterically congested structure of 4, which allows for rapid
exchange of the P atoms (see the Supporting Information for
an extended discussion). The η2-coordinated ketone is
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, figures, and CIF and XYZ files giving experimental
and computational details, characterization data for all
compounds, crystallographic data, and DFT-optimized coor-
dinates. The Supporting Information is available free of charge
on the ACS Publications website at DOI: 10.1021/
acs.organomet.5b00264.
characterized by a doublet (2JP,C = 14 Hz) of triplets (2JP,C
=
9 Hz) at 120.4 ppm in the 13C NMR spectrum and an IR
absorption at 1309 cm−1.
The X-ray crystal structure of 4 (Figure 1) reveals a
pseudotetrahedral environment of the metal similar to that
found in 3, albeit with a significantly larger P−Ni−P angle
(120.65(2)° vs 107.57(2)°). The CO bond (1.330(3) Å) is
somewhat more elongated than that in 3, consistent with
stronger π back-donation from Ni(0). The Ni−C (2.001(2) Å)
and Ni−O (2.0091(14) Å) distances are longer than those in
three-coordinate (R3P)2Ni(benzophenone) complexes (Ni−C
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.-E.M.: m.moret@uu.nl.
Notes
■
The authors declare no competing financial interest.
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Dapporto, P.; Sriyunyongwat, V.; Albright, T. A. Acta Crystallogr., Sect
C: Cryst. Struct. Commun. 1983, 39, 995.
(20) Bai, G.; Wei, P.; Stephan, D. W. Organometallics 2005, 24, 5901.
ACKNOWLEDGMENTS
■
The X-ray diffractometer was financed by The Netherlands
Organization for Scientific Research (NWO). We acknowledge
funding from the European Union Seventh Framework
Programme (FP7/2007-2013) under grant agreement PIIF-
GA-2012-327306 (IIF-Marie Curie grant awarded to M.-E.M.),
the Dutch National Research School Combination Catalysis
(NRSC-C), and the Sectorplan Natuur- en Scheikunde
(Tenure-track grant at Utrecht University). The DFT work
was carried out on the Dutch national e-infrastructure with the
support of the SURF Foundation. We thank L. Witteman for
assistance with NBO calculations.
(21) (a) Velian, A.; Lin, S.; Miller, A. J. M.; Day, M. W.; Agapie, T. J.
Am. Chem. Soc. 2010, 132, 6296. (b) Gruger, N.; Wadepohl, H.; Gade,
̈
L. H. Dalton Trans. 2012, 41, 14028.
(22) The fraction of diamagnetic material appears not to depend on
the concentration of the sample as expected for a simple monomer−
dimer equilibrium, suggesting a more complex process (see the
Supporting Information).
(23) All calculations were performed with: Frisch, M. J. et al.
Gaussian 09, Revision D.01; Gaussian, Inc., Wallingford, CT, 2013. The
full reference is given in the Supporting Information (ref S8).
(24) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural
Bond Orbital Donor-Acceptor Perspective; Cambridge University Press:
New York, 2005.
(25) NBO analyses performed with: Glendening, E. D.; Badenhoop,
J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.;
Landis, C. R.; Weinhold, F. NBO 6.0; Theoretical Chemistry Institute,
University of Wisconsin, Madison, WI, 2013; http://nbo6.chem.wisc.
edu/.
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