Synthesis and electronic spectra of fluorinated aryloxide and alkoxide [NiX4]2- anions

Article abstract of DOI:10.1021/ic9003593

Five new homoleptlc [NIX₄]^2- compounds have been prepared with the fluorinated ligands OC₆F₅ (OAr ^F), OC₆H₃(CF₃)₂ (OAr'), and OC₄F₉ (0

Full text of DOI:10.1021/ic9003593

Inorg. Chem. 2009, 48, 4274-4276
Synthesis and Electronic Spectra of Fluorinated Aryloxide and Alkoxide
[NiX4]2- Anions
BingNa Zheng,† Maria O. Miranda,‡ Antonio G. DiPasquale,§ James A. Golen,§ Arnold L. Rheingold,§
and Linda H. Doerrer*,‡
Chemistry Department, Boston UniVersity, 590 Commonwealth AVenue, Boston, Massachusetts
02215, Chemistry Department, Barnard College, 3009 Broadway, New York, New York 10027,
and UniVersity of California, San Diego, 9500 Gilman DriVe, La Jolla, California 92093
Received February 22, 2009
Five new homoleptic [NiX4]2- compounds have been prepared with
the fluorinated ligands OC6F5 (OArF), OC6H3(CF3)2 (OAr′), and
OC4F9 (ORF) and characterized with X-ray crystallography, magnetic
susceptibility, and elemental analysis. Electronic spectral studies
show that these ligands engender a ligand-field environment similar
to that of fluoride and thus act electronically like fluoride, but with
none of the drawbacks of F- as a transition-metal ligand.
Scheme 1. Synthesis of Fluorinated Homoleptic Nickel Aryloxide
Complexes
Homoleptic and mononuclear aryloxide complexes of the
form [M(OAr)n]m- or analogous alkoxides such as [M(OR)n]m-
have proven challenging to synthesize because of the propensity
of these ligands to bridge multiple metal centers. Previous work
demonstrated that extensive fluorination of aryloxide rings
permits the isolation of homoleptic phenolate anions with CoII
and CuII.1 Under analogous conditions with nonfluorinated
phenolate, only low yields of dimeric structures are obtained.1
Herein we report the extension of this chemistry to NiII with
two fluorinated aryloxides and one fluorinated alkoxide that has
resulted in tetrahedral nickel anions surrounded by heavily
fluorinated oxygen-donor ligands. UV-vis and near-IR spec-
troscopic analysis has been carried out to describe these newer
ligands in the context of the traditional spectrochemical series.
The homoleptic anions with fluorinated aryloxides of the
form [Co(OAr)4]2- and [Cu(OAr)4]2- were readily obtained
by metathesis reactions of metal halides MX2 with potassium
or thallium salts of the appropriate aryloxide anion.1 Such
reactions were unsuccessful in the case of nickel. With NiCl2
or NiCl2(DME), incomplete substitution was observed with
either potassium or thallium aryloxide salts, and with NiI2,
reduction to Ni0 occurred.
An alternative synthesis via the fluorinated phenols HOArF
and HOAr′ succeeded. Alcoholysis of [Cp2Ni] with 2 equiv
of phenol and 2 equiv of the apposite aryloxide in tetrahy-
drofuran at reflux for several days was necessary. As shown
in Scheme 1, potassium aryloxides and crown compounds
were used to synthesize {K(18C6)}2[Ni(OArF)4] (1), {K(benzo-
18C6)}2[Ni(OArF)4] (2), {K(18C6)}2[Ni(OAr′)4] (3), and
thallium aryloxide, TlOArF, was used for the preparation of
[Tl2Ni(OArF)4] (4). The formation of Ni-OAr linkages via
alcoholysis is rare but not unprecedented. Previously, [Cp2Ni]
had been reacted with phenols to make phosphine phenolate
complexes,2,3 a bridging phosphide with pendant phenol,4
and an N-heterocyclic functionalized salen derivative.5 An
earlier report was made of green material isolated from the
reaction of TlOArF and NiCl2 in CH3CN characterized only
as having the stoichiometry {Ni(OArF)2}.6
Notably, the metathesis reaction of NiI2 and 4 equiv of KORF
works well for the alkoxide case {K(18C6)}K[Ni(ORF)4] (5),
as depicted in Scheme 2. This result suggests that the sensitivity
*
To whom correspondence should be addressed. E-mail:
doerrer@bu.edu.
(2) Heinicke, J.; Koehler, M.; Peulecke, N.; Keim, W.; Jones, P. G. Z.
Anorg. Allg. Chem. 2004, 630, 1181–1190.
(3) Heinicke, J.; Peulecke, N.; Karaghiosoff, K.; Mayer, P. Inorg. Chem.
2005, 44, 2137–2139.
†
Barnard College.
Boston University.
University of California, San Diego.
‡
§
(1) Buzzeo, M. C.; Iqbal, A. H.; Long, C. M.; Millar, D.; Patel, S.; Pellow,
M. A.; Saddoughi, S. A.; Smenton, A. L.; Turner, J. F. C.; Wadhawan,
J. D.; Compton, R. G.; Golen, J. A.; Rheingold, A. L.; Doerrer, L. H.
Inorg. Chem. 2004, 43, 7709–7725.
(4) Heinicke, J.; Jux, U. Inorg. Chem. Commun. 1999, 2, 55–56.
(5) Li, W.; Sun, H.; Chen, M.; Wang, Z.; Hu, D.; Shen, Q.; Zhang, Y.
Organometallics 2005, 24, 5925–5928.
(6) Nyholm, R. S.; Hollebone, B. R. J. Chem. Soc. A 1971, 332–337.
4274 Inorganic Chemistry, Vol. 48, No. 10, 2009
10.1021/ic9003593 CCC: $40.75  2009 American Chemical Society
Published on Web 03/30/2009

COMMUNICATION
Scheme 2. Synthesis of Homoleptic Nickel Alkoxide Complex 5
Figure 1. ORTEP of 2 (left) with no F atoms and (right) the anion of 2
alone with 50% thermal ellipsoids.
of NiI2 to reduction, in the presence of aryloxide but not
alkoxide ligands, may be due to the relative ease of oxidation
of aryloxide anions versus alkoxide ones.
Repeated attempts to make 5 with 2 equiv of 18C6 were
unsuccessful. Purple crystalline material that analyzed as 5
was obtained with some additional colorless material, prob-
ably 18C6. We hypothesize that the 3-fold coordination of
the uncrowned K+ by the three CF3 groups, vide infra, is
more favorable than coordination by the crown ether.
All of the structures of the divalent nickelate complexes 1-5
are approximately tetrahedral at NiII with {NiO4} coordination,
except in 4, where the coordinated Tl atoms create greater
distortion. X-ray crystallographic studies were carried out
according to the parameters collected in Table S1 of the
Supporting Information. Only the connectivity of 1 was obtained
from a low-quality data set, but the stoichiometry was con-
firmed.7 Within the limits of the data set, the structure of 1 is
very similar to that of 2. The precise interatomic metrical
parameters for 2-5 from the diffraction experiments are
summarized in Table S2 of the Supporting Information. The
X-ray crystal structure of {K(18C6)OArF} (6) has also been
determined for comparison purposes, and an ORTEP is
presented in Figure S1 of the Supporting Information.
The structure of compound 2 is tetrahedral at Ni with a
dihedral angle between the O1-Ni-O2 and O3-Ni-O4
planes of 85.6°. On the left side of Figure 1, the cations and
anion are shown, revealing that two O atoms are bridged by
an encapsulated {K(18C6)}+ cation on each side of the
complex. The average K-OAr distance is 2.815(13) Å, which
is longer than the K-OAr bond distance of 2.6123(11) Å in
6. The weak interactions between K and the aryloxide O
atoms cause the subtended O-Ni-O angles to be smaller
[average ) 89.0(5)°] than those without bridging K centers
[average ) 120(8)°]. The Ni-O distances exhibit a short
(average ) 1.934 Å) and long (average ) 1.948 Å) distance
in each pair bridged by a single K and are slightly longer
than other Ni-O bond lengths in {NiO4} environments in
the CSD,8 vide infra, because of the less basic O atoms. The
OArF ligand has previously been bound as a terminal ligand
to Ni in [(Ph3P)2Ni(OArF)2] [Ni-O ) 1.852(2) Å]9 and as
a bridging ligand between two Ni centers in [(ArF)2Ni(µ2-
OArF)2Ni(ArF)2],10and[{CpCo{P(dO)(OMe)2}3}2Ni2(OH2)2(µ2-
OArF)2].11
Figure 2. ORTEP of the anion of 5 (left) with F-chelated K+ and (right)
alone with 50% thermal ellipsoids.
only been measured previously in [(Ph3P)2Ni(OAr′)2] [Ni-O
) 1.857(2) Å].9
The unit cell of 4 has two independent [Tl2Ni(OArF)4]
molecules, only one of which is shown in Figure S3 of the
Supporting Information, with a [Ni(OArF)4]2- core similar to
that of 1 and 2, distorted somewhat by the bridging Tl atoms.
The stoichiometry of 5 includes one {K(18C6)}+ cation
and one crown-free K per Ni center. The crown-free
coordination of K3 creates a pseudo-3-fold axis along the
K3 · · · Ni1 vector, as shown in Figure 2. The K3-Ni1-O4
angle is 165.44(9)°, while the other three K3-Ni1-Ox
angles average 56.5(4)°. In the crystal structure, there are
three different K+ environments, as shown in Figure S4 of
the Supporting Information. One {K(18C6)}+ unit, contain-
ing K2, bridges two [Ni(ORF)4]2- anions via two bridging
fluoride atoms F31. Another {K(18C6)}+ unit containing K1
is not closely bound to any other moiety in the crystal
lattice. The two remaining K+ ions, K3, are coordinated
by three O atoms, O1- O3, from the alkoxide ligands
and six F atoms from adjacent perfluoro-tert-butyl groups,
shown on the right side of Figure S4 of the Supporting
Information. There are no previous reports of compounds
containing the NiORF unit.
There are only a few other examples of structurally
characterized {NiO4} coordination in the literature, and all
contain NiII that is part of one or more chelate rings, with
an average Ni-O bond distance in the CSD of 1.87(7) Å.
The majority of the examples are derivatives of nickel
Compound 3, depicted in Figure S2 of the Supporting
Information, is quite similar structurally to 2 with a slightly
smaller dihedral angle of 82.4°. The Ni-OAr′ linkage has
(9) Kim, M.; Zakharov, L. N.; Rheingold, A. L.; Doerrer, L. H. Polyhedron
2005, 24, 1803–1812.
(10) Sanchez, G.; Ruiz, F.; Serrano, J. L.; Garcia, G.; Lopez, G.; Casabo,
J.; Molins, E.; Miravitlles, C. An. Quim. Int. Ed. 1997, 93, 49–54.
(11) Ma, D.-Q.; Hikichi, S.; Akita, M.; Moro-oka, Y. Dalton Trans. 2000,
1123–1134.
j
(7) P1, a ) 13.383(6) Å, b ) 13.508(6) Å, c ) 18.540(8) Å, R )
82.676(6)°, ꢀ ) 81.787(6)°, γ ) 73.522(6)°.
(8) Allen, F. H. Acta Crystallogr. 2002, B58, 380–388.
Inorganic Chemistry, Vol. 48, No. 10, 2009 4275

COMMUNICATION
bis(acetylacetonate),12-19 some bis(o-catecholate) com-
pounds,20,21 a few bimetallic species with bridging alkox-
ides,22-25 two salicylaldehyde complexes,26,27 two bridging
aryloxides,28,29 and one polyoxometallate30 complex.
Previously studied (pseudo)halogen [NiX4]2- complexes
with D2d symmetry have magnetic moments in the range
3.5-4.0 µB. The compounds 1-5 are paramagnetic, with
an average µeff of 3.51 µB for the K2NiX4 species, as
determined by Evans’ method at room temperature.31,32
These values are closest to that for [NiI4]2-, 3.49 µB, and, in
combination with UV-vis data, suggest that the fluorinated
aryloxide and alkoxide ligands are among the stronger
ligands in the “weak-field” manifold. Figgis has termed these
ligands “medium field”33 in which there is more spin-orbit
coupling, and more free-ion-like behavior, consistent with
the Racah parameters obtained experimentally, vide infra.
The UV-vis-near-IR spectra of compounds 2-5 as well
as that of [NiCl4]2- are shown in Figure 3, and their
absorption energies are compared with previously published
data according to the method of Lever34,35 to obtain values
for the Racah parameter B and the ligand-field parameter
Dq. All four spectra of the nickel complexes with fluorinated
ligands show significantly blue-shifted spectra, compared to
[NiCl4]2-, indicating a stronger ligand field. The strongly
electron-withdrawing and reduced π-basic character of the
ligands results in a large ligand-field splitting, as described
by Dq. The large Racah parameter B for 5 indicates strong
interelectronic repulsion and primarily σ-donor character
Figure 3. UV-vis-near-IR spectra of [NiX4]2- species.
Table 1. Electronic Spectral Data and Ligand-Field Parameters
E(ν3)
E(ν2)
B
Dq
[NiX4]2-
(cm-1
)
(cm-1
)
(cm-1
)
(cm-1
)
ref
[Ni(NCO)4]2-
[NiCl4]2-
16 200
14 760
13 320
16 660
16 820
16 000
19 300
9460
7470
6995
511
405
379
877
867
800
1096
311
206
201
502
540
560
427
34, 36
34, 37
34, 37
this work
this work
this work
this work
[NiBr4]2-
[Ni(OArF)4]2-, 2
[Ni(OAr′)4]2-, 3
[Ni(OArF)4]2-, 4
[Ni(ORF)4]2-, 5
9290
10,000
10400
7840
from the ligand. On the basis of these data, we write the
following abbreviated spectrochemical series:
Br < Cl < NCO , OAr′ ∼ OArF < ORF
These data demonstrate that the fluorinated aryloxide and
alkoxide ligands are as strong in ligand-field terms as
fluoride. Unlike fluoride, however, these ligands are much
less prone to bridging, as complexes 1-3 and 5 attest. Thus,
these ligands serve as an electronic equivalent of fluoride
and are therefore expected to stabilize oxidation states for
Ni higher than Ni2+. A cyclic voltammogram (CV) of 5 with
Epa of -0.2 V vs Ag/Ag+ is shown in Figure S5 of the
Supporting Information.
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In summary, several new homoleptic tetrahedral nickelate
complexes [Ni(OAr)4]2- and [Ni(OR)4]2- with extensively
fluorinated aryloxide and alkoxide ligands have been prepared.
Ligand-field studies show that these ligands are best described
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Acknowledgment. We thank Boston University, NSF
CAREER (Grant CHE-0134817), and ACS-PRF AC-48022
for financial support.
Supporting Information Available: Full experimental details,
tabulated X-ray crystallographic information (Tables S1 and S2)
for compounds 2-6, ORTEPs for compounds 3-6 (Figures
S1-S4), a CV of 5 (Figure S5), and time-of-flight electrospray
ionization mass spectrometry analysis of 4 (Figure S6). This material
is available free of charge via the Internet at http://pubs.acs.org.
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4276 Inorganic Chemistry, Vol. 48, No. 10, 2009