Synthesis and structure of three unusual homoleptic aryloxides of nickel and palladium

Article abstract of DOI:10.1021/om010918e

Synthesis and structure of three unusual homoleptic aryloxides of nickel and palladium were investigated. Homoleptic group 10 aryloxides consisting of discrete molecules were isolated from the reaction of the sodium salts of bulky phenols with NiBr₂(dme) or PdCl₂(MeCN)₂. Results showed that nickel 2,6-diisopropylphenolate was a trimer.

Full text of DOI:10.1021/om010918e

1014
Organometallics 2002, 21, 1014-1016
Syn th esis a n d Str u ctu r e of Th r ee Un u su a l Hom olep tic
Ar yloxid es of Nick el a n d P a lla d iu m
J uan Ca´mpora* and Manuel L. Reyes
Departamento de Quı´mica Inorga´nica-Instituto de Investigaciones Quı´micas,
Universidad de Sevilla-Consejo Superior de Investigaciones Cientı´ficas,
c/ Americo Vespucio s/ n, Isla de la Cartuja, 41092 Sevilla, Spain
Kurt Mereiter
Institute of Mineralogy, Crystallography, and Structural Chemistry,
Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
Received October 22, 2001
Sch em e 1
Summary: Homoleptic group 10 aryloxides consisting of
discrete molecules have been isolated from the reaction
of the sodium salts of bulky phenols with NiBr2(dme) or
PdCl2(MeCN)2. The nickel 2,6-diisopropylphenolate is a
trimer, displaying σ-coordinated aryloxide ligands, while
the Ni and Pd derivatives of 2,6-di-tert-butylphenol are
monomers that adopt an unprecedented π-allylic, sand-
wich-like structure.
Alkoxide derivatives of the late transition elements
have aroused considerable interest, due to their involve-
ment in several catalytic processes, e.g. C-O bond
formation and cleavage, olefin and alkyne alkoxycar-
boxylation, and olefin polymerization.1 They also display
a high reactivity,2 which is sometimes reminiscent of
that of the related metal alkyls.3 Despite this interest,
late-transition-metal alkoxides have been much less
studied. Even though [M(OR)2]n complexes of Mn, Fe,
Co, and Zn have been characterized recently,4,5 when
OR is a bulky alkoxide or aryloxide group, corresponding
compounds of G10 elements have been hitherto unre-
ported. Herein we present the synthesis and structural
characterization of the first homoleptic aryloxide deriva-
tives of Ni and Pd.
The new compounds 1-3, derived from the bulky 2,6-
diisopropylphenolate (O-dipp) and 2,6-di-tert-butyl-
phenolate (O-dtbp), have been isolated following the
reactions summarized in Scheme 1.
Although our initial attempts to prepare the O-dipp
derivatives provided only ill-defined materials, we have
subsequently found that a rigorous elimination of the
volatiles under vacuum allows the isolation of the
extremely air-sensitive nickel compound 1,6 albeit in
very low yields (ca. 8%). Notwithstanding its para-
magnetism (µeff ) 2.08 B.M. per Ni atom at 298 K,
1
Evans method7), useful H and 13C NMR spectra can
be recorded, which reveal the unexpected complexity of
the molecules of 1. Except for the linear dependence of
the chemical shift with 1/T, there is no change in the
line shape of the NMR resonances in the temperature
interval from -80 to +80 °C. This is in accord with the
absence of equilibria involving other oligo- or monomeric
species and of intramolecular rearrangements. As Fig-
ure 1 shows,8 the molecules of 1 are trimeric in the solid
state and consist of a nearly linear arrangement of the
Ni atoms, separated by ca. 2.98 Å and bridged by
aryloxide groups.9 The coordination geometries of the
central and the terminal nickel atoms are remarkably
different, the former being in a distorted-square-planar
environment while the latter are bonded to one terminal
(Ni-O, 1.72 Å) and two bridging (Ni-O, 1.90 Å) aryl-
(1) (a) Yamamoto, A. Adv. Organomet. Chem. 1992, 34, 111. (b)
Mann, G.; Hartwig, J . F. J . Am. Chem. Soc. 1996, 118, 13109. (c)
Widenhoefer, R. A.; Zhong, H. A.; Buchwald, S. L. J . Am. Chem. Soc.
1997, 119, 6787. (d) Pe´rez, P. J .; Calabrese, J . C.; Bunel, E. E.
Organometallics 2001, 20, 337. (e) Drent, E.; Arnoldy, P.; Budzelaar,
P. H. M. J . Organomet. Chem. 1994, 475, 57. (f) Hampton, P. D.; Wu,
S.; Alam, T. M.; Claverie, J . P. Organometallics 1994, 13, 2066. (g)
Keim, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 235. (h) Younkin, T.;
Connor, E. F.; Henderson, J . I.; Friedrich, S. K.; Grubbs, R. H.;
Bansleben, D. A. Science 2000, 287, 460.
(2) (a) Bergman, R. G. Polyhedron 1995, 14, 3227. (b) Mehrotra, R.
C. Adv. Inorg. Chem. Radiochem. 1983, 26, 269. (c) Bryndza, H. E.;
Tam, W. Chem. Rev. 1988, 88, 1163. (d) Mehrotra, R. C. Coord. Chem.
Rev. 1981, 21, 113.
(3) (a) Ritter, J . C. M.; Bergman, R. G. J . Am. Chem. Soc. 1997,
119, 2580. (b) Dokter, D. W.; Fanwick, P. E.; Kubiak, C. P. J . Am.
Chem. Soc. 1996, 118, 4846. (c) Kapteijn, G. M.; Dervisi, A.; Verhoef,
M. J .; van den Broek, M. A. F. H.; Grove, D. M.; van Koten, G. J .
Organomet. Chem. 1996, 517, 123. (d) Kapteijn, G. M.; Verhoef, M. J .;
van den Broek, M. A. F. H.; Grove, D. M.; van Koten, G. J . Organomet.
Chem. 1995, 503, C26. (e) Torresan, I.; Michelin, R. A.; Marsella, A.;
Zanardo, F.; Pinna, F.; Strukul, G. Organometallics 1991, 10, 623. (f)
Green, L. M.; Meek, M. D. Organometallics 1989, 8, 659.
(4) (a) Murray, B. D.; Hope, H.; Power, P. P. J . Am. Chem. Soc. 1985,
107, 169. (b) Sigel, G. A.; Bartlett, R. A.; Decker, D.; Olmstead, M. M.;
Power, P. P. Inorg. Chem. 1987, 26, 1773. (c) Chen, H.; Power, P. P.;
Shoner, S. C. Inorg. Chem. 1991, 30, 2884. (d) Bartlett, R. A.; Ellison,
J . E.; Power, P. P.; Shoner, S. C. Inorg. Chem. 1991, 30, 2888.
(5) (a) Purdy, A. P.; George, C. F.; Brewer, G. A. Inorg. Chem. 1992,
31, 2633. (b) Geerts, R. L.; Huffman, J . C.; Caulton, K. G. Inorg. Chem.
1986, 25, 1803.
(6) 1H NMR (C6D6, 20 °C): δ -117.8 (s, ν1/2 ) 33 Hz, 2H, p-CH,
terminal), -24.4 (s, ν1/2 ) 27 Hz, 4H, CH), -6.0 (s, ν1/2 ) 25 Hz, 12H,
CH(CH3)2), -4.8 (s, ν1/2 ) 25 Hz, 12H, CH(CH3)2), 10.7 (br s, ν1/2 ) 46
Hz, 12H, CH(CH3)2), 16.8 (s, ν1/2 ) 25 Hz, 12H, CH(CH3)2), 17.8 (s, ν1/2
) 25 Hz, 12H, CH(CH3)2), 18.5 (br s, ν1/2 ) 39 Hz, 12H, CH(CH3)2),
38.6 (br s, ν1/2 ) 68 Hz, 4H, CH), 42.5 (br s, ν1/2 ) 78 Hz, 4H, CH), 45.8
(s, ν1/2 ) 25 Hz, 4H, CH), 53.9 (s, ν1/2 ) 23 Hz, 4H, CH), 54.2 (s, ν1/2
)
23 Hz, 4H, CH), 55.8 (br s, ν1/2 ) 87 Hz, 4H, CH). 13C{1H} NMR (C6D6,
20 °C): δ -319.3, -201.7, -69.4, -61.8, -6.4, 22.7, 34.2, 66.3, 88.0,
99.2, 179.0, 189.7, 195.7, 354.4, 356.2. IR (Nujol mull): 1584, 1556 st
ν(CdC). µeff(298 K) ) 6.25 B.M. Anal. Calcd for C72H102Ni3O6: C, 69.76;
H, 8.29. Found: C, 69.74; H, 8.22.
(7) Evans, D. F. J . Chem. Soc. 1959, 2003.
10.1021/om010918e CCC: $22.00 © 2002 American Chemical Society
Publication on Web 02/12/2002

Communications
Organometallics, Vol. 21, No. 6, 2002 1015
to moderate yields, whose elemental analysis and mass
spectra indicate the composition M(O-dtbp)2 (M ) Ni
(2),11 Pd (3)12). The synthesis of the Pd derivative 3 is
accompanied by the formation of the biphenol 4 and the
dibenzoquinone 5, which were isolated from the reaction
mixture. The presence of 4 and 5 suggests that the
oxidation of the phenoxide anion by Pd(II) competes
with the formation of 3, thus lowering the final yield of
the latter.13 Although the isolated yield of the Ni
compound 2 is also low, its synthesis is not accompanied
by the formation of 4 or 5. Instead, and as revealed by
the analysis of the mother liquors left after the crystal-
lization of 2, solvated species of composition Ni(O-
dtbp)2L2 (L2 ) dme (6a ), (THF)2 (6b))11 are formed,
which were isolated by fractional crystallization and
F igu r e 1. Molecular structure and numbering scheme of
1 (20% probability ellipsoids, hydrogen atoms and C6H14
solvent molecules omitted for clarity). Selected bond lengths
(Å) and angles (deg): Ni1-O1 ) 1.953(2), Ni1-O2 )
1.927(2), Ni1-O4 ) 1.956(2), Ni1-O5 ) 1.932(2), Ni2-
O1 ) 1.901(2), Ni2-O2 ) 1.918(2), Ni2-O3 ) 1.728(2),
Ni3-O4 ) 1.905(2), Ni3-O5 ) 1.899(2), Ni3-O6 ) 1.725(2),
Ni1-Ni3 ) 2.977(2), Ni1-Ni2 ) 2.990(2), O1-C1 )
1.381(3), O2-C13 ) 1.381(3), O3-C25 ) 1.325(3), O4-C37
) 1.386(3), O5-C49 ) 1.382(3), O6-C61 ) 1.322(3); O1-
Ni1-O2 ) 77.3(1), O1-Ni1-O4 ) 158.3(1), O1-Ni1-O5
) 107.9(1), O2-Ni1-O4 ) 108.7(1), O2-Ni1-O5 ) 150.3(1),
O4-Ni1-O5 ) 77.5(1), O1-Ni2-O2 ) 78.8(1), O1-Ni2-
O3 ) 144.2(1), O2-Ni2-O3 ) 136.9(1), O4-Ni3-O5 )
79.6(1), O4-Ni3-O6 ) 140.0(1), O5-Ni3-O6 ) 140.4(1),
Ni2-Ni1-Ni3 ) 174.42(2).
1
characterized by H NMR, IR, and elemental analysis.
Compounds 6 may be easily overlooked, due to their
high solubility in the crystallization solvent (petroleum
ether) and to their paramagnetic character. However,
they constitute the main product of the reaction and
their presence explains that isolated yields of 2 are as
low as 8% if no special caution is taken. In C6D6
solution, adducts 6 slowly lose the coordinated solvent,
generating 2. When we take advantage of this observa-
tion, the yield of 2 can be improved (up to 30%) by
extracting the reaction residue left after evaporation of
the THF solvent with toluene, followed by thorough
removal of the volatile components of the mixture in
vacuo.
Aryloxides 2 and 3 are diamagnetic and soluble in
hydrocarbon solvents. Their 1H and 13C{1H} NMR
spectra are very simple and show single tert-butyl and
meta and para aromatic signals, the last two displaying
large upfield shifts relative to those of the free HO-dtbp
(for instance, the aromatic p-H resonates at 3.75 ppm
in 2 and 4.74 ppm in 3). In the IR spectra prominent
oxide ligands in an unusual (for Ni(II)) trigonal-planar
coordination.10 The shorter Ni-O bonds to the terminal
O-dipp and the almost linear Ni-O-C bond angles
could suggest multiple-bond character. However, the
related complexes of Cr, Mn, Fe, and Co exhibit similar
structural features,4 which are therefore independent
of the metal electronic configuration. Hence, according
to Power4d it appears reasonable that the above struc-
tural properties are a consequence of a predominantly
ionic Ni2+‚‚‚‚(OAr)- bonding interaction. In fact, the
Ni‚‚‚O separations are close to the sum of the effective
ionic radii4d of O2- and Ni2+
.
The sodium salt of the bulkier phenol 2,6-di-tert-
butylphenol (NaO-dtbp) reacts with NiBr2(dme) or
PdCl2(MeCN)2 (Scheme 1), giving rise to very air sensi-
tive purple or deep green products, respectively, in low
(11) 2: 1H NMR (C6D6, 20 °C) δ 1.39 (s, 36H, CMe3), 3.75 (t, 2H,
3
3J HH ) 3.4 Hz, p-CH arom), 6.52 (d, 4H, J HH ) 6.4 Hz, m-CH arom);
13C{1H} NMR (C6D6, 20 °C) δ 29.6 (CMe3), 35.6 (CMe3), 61.2 (p-CH
arom), 117.6 (m-CH arom), 128.0 (Cq-But), 176.8 (CdO); IR (Nujol
mull) 1612, 1582, st. ν(CdO); MS (EI) m/z 469 (M+). Anal. Calcd for
(8) Diffraction experiments were performed on a Bruker SMART
CCD diffractometer using graphite-monochromated Mo KR radiation
(λ ) 0.710 73 Å). Crystal data for 1: triclinic, space group P1h (No. 2),
a ) 14.303(8) Å, b ) 16.003(9) Å, c ) 19.644(11) Å, R ) 86.30(1)°, â )
71.70(1)°, γ ) 74.89(1)°, V ) 4121(4) Å3, Z ) 2, C84H130Ni3O6 (includes
two C6H14 solvent molecules that are present in the crystal lattice), T
) 208(2) K, data/parameters 14 438/822, R1 ) 0.064, wR2 ) 0.126
(all data). Crystal data for 2: orthorhombic, P212121 (No. 19), a )
9.795(5) Å, b ) 13.866(7) Å, c ) 19.531(11) Å, V ) 2653(2) Å3, Z ) 4,
C28H42NiO2, T ) 223(2) K, data/parameters 3906/196; R1 ) 0.060, wR2
) 0.097 (all data). Crystal data for 3: monoclinic, space group C2/c
(No. 15), a ) 20.617(4) Å, b ) 7.219(2) Å, c ) 18.048(5) Å, â ) 91.75(1)°,
V ) 2684.9(12) Å3, Z ) 4, C28H42O2Pd, T ) 213(2) K, data/parameters
3813/149, R1 ) 0.020, wR2 ) 0.0482 (all data).
(9) Linear trinuclear complexes of nickel can exhibit interesting
magnetic properties. See: (a) Ginsberg, A. P.; Martin, R. L.; Sherwood,
R. C. Inorg. Chem. 1968, 7, 932. (b) Beissel, T.; Birkelbach, F.; Bill,
E.; Glaser, T.; Kesting, F.; Krebs, C.; Weyhermu¨ller, T.; Wieghardt,
K.; Butzlaff, C.; Trautwein, A. X. J . Am. Chem. Soc. 1996, 118, 12376.
(c) Higgs, T. C.; Spartalian, K.; O’Connor, C. J .; Matzanke, B. F.;
Carrano, C. J . Inorg. Chem. 1998, 37, 2263.
C
28H42NiO2: C, 71.66; H, 9.02. Found: C, 71.38; H, 9.00. 6a : 1H NMR
(C6D6, 20 °C) δ -42.8 (br s, ν1/2 ) 535 Hz, 2H, p-CH arom), 1.0 (s, ν1/2
) 39 Hz, 5H, CH2 (dme)), 8.4 (s, ν1/2 ) 34 Hz, 6H, CH3 (dme)), 18.2 (br
s, ν1/2 ) 329 Hz, 36H, CMe3), 44.3 (s, ν1/2 ) 29 Hz, 4H, m-CH arom);
IR (Nujol mull) 1579 st ν(CdC). Anal. Calcd for C32H52NiO4: C, 68.70;
H, 9.37. Found: C, 68.84; H, 9.14. 6b: 1H NMR (C6D6, 20 °C) δ -47.4
(s, ν1/2 ) 24 Hz, 2H, p-CH arom), 1.6 (s, ν1/2 ) 34 Hz, 8H, CH2 (THF)),
6.8 (s, ν1/2 ) 31 Hz, 8H, CH2 (THF)), 21.5 (br s, ν1/2 ) 49 Hz, 36H,
CMe3), 44.8 (s, ν1/2 ) 14 Hz, 4H, m-CH arom); IR (Nujol mull) 1579 st
ν(CdC). Anal. Calcd for C36H58NiO4: C, 70.48; H, 9.53. Found: C,
70.22; H, 9.36.
(12) 3: 1H NMR (C6D6, 20 °C) δ 1.37 (s, 36H, CMe3), 4.74 (t, 2H,
3
3J HH ) 6.3 Hz, p-CH arom), 6.25 (d, 4H, J HH ) 6.3 Hz, m-CH arom);
13C{1H} NMR (C6D6, 20 °C) δ 30.1 (CMe3), 35.1 (CMe3), 70.6 (p-CH
arom), 119.3 (m-CH arom), 132.7 (Cq-But), 180.7 (CdO); IR (Nujol
mull) 1616, 1588, 1571 st ν(CdO); MS (EI) m/z 516 (M+). Anal. Calcd
for C28H42PdO2: C, 65.04; H, 8.19. Found: C, 65.24; H, 8.27.
(13) Compounds 4 and 5 can also be produced by chemical or
electrochemical oxidation of HO-dtbp. See: (a) Fujiyama, H.; Kohara,
I.; Iwai, K.; Nishiyama, S.; Tsuruya, S.; Masai, M. J . Mol. Catal. 1999,
188, 417. (b) Torii, S.; Dhimane, A. L.; Araki, Y.; Inokuchi, T.
Tetrahedron Lett. 1989, 30, 2105. (c) Omura, K.; Tetrahedron Lett.
2000, 41, 685.
(10) Hope, H.; Olmstead, M. M.; Murray, B. D.; Power, P. P. J . Am.
Chem. Soc. 1985, 107, 712.

1016 Organometallics, Vol. 21, No. 6, 2002
Communications
Sch em e 2
absorptions in the region 1620-1570 cm-1, attributable
to the ν(CdO) vibration, can be found. Both features
resemble closely those recently reported for π-aryloxide
complexes of Ni and Pd14 and suggest that 2 and 3
possess a sandwich-type structure. This conclusion is
supported by the X-ray studies summarized in Figure
2.8 In the two compounds, which are not isostructural
with respect to the crystal lattice, each O-dtbp ligand
coordinates to the metal in a η3 fashion, a rare feature
among the π-aryloxo complexes. These compounds
constitute unique examples of transition-metal homo-
leptic π-aryloxides,15 an observation that appears the
more striking when one considers that the iron deriva-
tive [Fe(O-2,4,6-t-Bu3C6H2)2]2 exhibits a σ-aryloxide
coordination.4c The bond distances within the rings of
the η3-coordinated aryloxide ligands suggest that their
aromaticity is largely lost. Thus, the C1-O, C1-C2, and
C3-C4 bond lengths (mean values 1.238, 1.487, and
1.432 Å, respectively; low-temperature values) are
comparable to those of the CdO, C-C, and CdC bonds
in organic enones (1.215, 1.44, and 1.36 Å, respec-
tively),16 while the C4-C5 and C5-C6 distances are
close to 1.41 Å, a value characteristic of bis(π-allyl)
complexes of Ni and Pd.17 The loss of aromaticity is also
evident in the appreciably puckered rings, where the
allylic and enone moieties are contained in planes that
form angles of 16.7° in 1 and 18.0° in 2. Their solution
NMR spectra indicate higher molecular symmetry than
is found in the solid state. A plausible explanation is
fast exchange of the metal center between the two
possible η3-allylic sites averaging the t-Bu and m-H
resonances (Scheme 2). The symmetrical spectrum is
maintained down to -100 °C, a noticeable line broaden-
ing being observable only at the lowest temperatures
studied, indicating a very low energy barrier for the
exchange. It is suspected that the same type of process
might also be facile in π-quinone complexes of palladium
structurally related to 2,18 but in contrast some η3-
hexadienyl palladium systems only undergo this isomer-
ization process above the ambient temperature.19
In summary, we have shown that group 10 homoleptic
aryloxides that consist of discrete molecules can be
prepared by simple ligand exchange reactions, provided
that the aryloxide group contains bulky substituents.
While the nickel derivative of O-dipp is a trimer
displaying a structure related to that of other 3d
transition-metal homoleptic alkoxides and aryloxides,
F igu r e 2. Molecular structures and numbering schemes
of 2 and 3 (20% probability ellipsoids, hydrogen atoms
omitted for clarity). Selected bond lengths (Å): 2, Ni-C4
) 2.052(2), Ni-C5 ) 2.003(2), Ni-C6 ) 2.139(2), Ni-C18
) 2.052(2), Ni-C19 ) 2.004(2), Ni-C20 ) 2.140(2), O1-
C1 ) 1.242(3), C1-C2 ) 1.485(3), C2-C3 ) 1.372(3), C3-
C4 ) 1.424(3), C4-C5 ) 1.399(3), C5-C6 ) 1.422(3), C6-
C1 ) 1.481(3), O2-C15 ) 1.242(3), C15-C16 ) 1.485(3),
C16-C17 ) 1.372(3), C17-C18 ) 1.423(3), C18-C19 )
1.399(3), C19-C20 ) 1.421(3), C20-C15 ) 1.480(3); 3, Pd-
C5 ) 2.163(1), Pd-C4 ) 2.224(1), Pd-C6 ) 2.276(1), O-C1
) 1.234(1), C1-C2 ) 1.489(1), C2-C3 ) 1.361(2), C3-C4
) 1.441(2), C4-C5 ) 1.405(2), C5-C6 ) 1.414(2), C6-C1
) 1.499(1).
the Ni and Pd derivatives of the more hindered O-dtbp
ligand adopt an unprecedented π-allylic structure. The
high stability of Ni and Pd π-allyl complexes undoubt-
edly favors the latter configuration, but it remains an
open question whether stable σ-aryloxides of the heavier
group 10 elements can be obtained. Although we have
been hitherto unable to isolate and characterize a well-
defined [Pd(O-dipp)2]n compound, the synthesis of new
homoleptic alkoxides and aryloxides of Ni, Pd, and Pt
is being actively pursued in our laboratory.
Ack n ow led gm en t. Financial support from the Di-
reccio´n General de Ensen˜anza Superior e Investigacio´n
Cient´ıfica y Te´cnica (Project 1FD97-0919) and Repsol-
YPF. M.L.R. thanks Repsol-YPF for a research fellow-
ship.
(14) Ca´mpora, J .; Reyes, M. L.; Hackl, T.; Monge, A.; Ruiz, C.
Organometallics 2000, 19, 2950.
(15) A trinuclear zinc complex displaying a central bis(π-aryloxide)
unit has been reported: Uhlenbrock, S.; Wegner, R.; Krebs, B. J . Chem.
Soc., Dalton Trans. 1996, 3731.
(16) Gordon, A. J .; Ford, R. A. The Chemist’s Companion; Wiley:
New York, 1972; p 108.
(17) (a) Goddard, R.; Krueger, C.; Mark, F.; Stansfield, R.; Zhang,
X. Organometallics 1985, 4, 285. (b) Gozum, J . E.; Pollina, D. M.;
J ensen, J . A.; Girolami, G. S. J . Am. Chem. Soc. 1988, 110, 2688.
(18) (a) Fox, G. A.; Pierpont, C. G. J . Chem. Soc., Chem. Commun.
1988, 806. (b) Fox, G. A.; Pierpont, C. G. Inorg. Chem. 1992, 31, 3718.
(19) Maasarani, F.; Pfeffer, M.; Le Borgne, G. J . Chem. Soc., Chem.
Commun. 1986, 488.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, thermal parameters, and bond lengths and angles
for 1-3. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM010918E