Copper(I) Complexes of a Heavily Fluorinated β-Diketiminate Ligand: Synthesis, Electronic Properties, and Intramolecular Aerobic Hydroxylation

Article abstract of DOI:10.1021/ic035056j

The aza-Wittig reaction between Ar^f-N=PPh₃ [Ar ^f = 3,5-(CF3)2C6H3] and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione affords a new, highly fluorinated β-diketimine, 1. Metalation by mesitylcopper-(I) in benzene gives rise to the

Full text of DOI:10.1021/ic035056j

Inorg. Chem. 2003, 42, 7354−7356
Copper(I) Complexes of a Heavily Fluorinated â-Diketiminate Ligand:
Synthesis, Electronic Properties, and Intramolecular Aerobic
Hydroxylation
David S. Laitar, Casey J. N. Mathison, William M. Davis, and Joseph P. Sadighi*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge Massachusetts 02139
Received September 7, 2003
f
f
The aza-Wittig reaction between ArsNdPPh3 [Ar ) 3,5-
reactivity of copper(I) â-diketiminate complexes with di-
oxygen has been studied in detail.4
(CF ) C H ] and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione affords a
3 2
6 3
We have now prepared a heavily fluorinated â-diketimi-
nate ligand,5,6 1, by a convenient aza-Wittig reaction,7 and
synthesized several copper(I) complexes from it. This ligand
is quite electron-poor, as judged by an infrared study of
several related (â-diketiminate)copper(I) carbonyl complexes.
Despite its electron-poor metal center, a copper(I) â-diketim-
inate complex reacts readily with dioxygen, with clean ortho-
hydroxylation of a ligand N-aryl group.8 The ligand backbone
remains unaffected.
new, highly fluorinated â-diketimine, 1. Metalation by mesitylcopper-
(I) in benzene gives rise to the Cu(I) â-diketiminate as its η2-
benzene adduct, 2a. Copper(I) carbonyl complexes of 1, and of
three less-fluorinated analogues, have been generated in situ and
compared by IR spectroscopy; the two backbone CF groups exert
3
a stronger electronic influence than the four N-aryl CF groups.
3
Dinuclear adduct 2b reacts readily with O , leading to ortho-
2
f
hydroxylation of a ligand N-Ar group.
Ligand 1 is synthesized in good yield by the aza-Wittig
reaction of 2 equiv of [3,5-bis (trifluoromethyl)-phenylimino]-
triphenylphosphorane with 1,1,1,5,5,5-hexafluoro-2,4-pen-
tanedione in toluene at 90 °C. We have been unable to
prepare this ligand by traditional routes.9 Reaction of 1 with
The aerobic oxidation of organic substrates represents an
ongoing goal in synthetic catalysis.1 Copper(I) complexes,
which can react with dioxygen to form strong oxidants,2,3
some of them capable of C-H bond hydroxylation,3 are
interesting candidates for catalyst precursors. Recently, the
(4) (a) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998-1999. (b)
Spencer, D. J. E.; Aboelella, N. W.; Reynolds, A. M.; Holland, P. L.;
Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 2108-2109. (c) Spencer,
D. J. E.; Reynolds, A. M.; Holland, P. L.; Jazdzewski, B. A.; Duboc-
Toia, C.; Le Pape, L.; Yokota, S.; Tachi, Y.; Itoh, S.; Tolman, W. B.
Inorg. Chem. 2002, 41, 6307-6321. (d) Aboelella, N. W.; Lewis, E.
A.; Reynolds, A. M.; Brennessel, W. W.; Cramer, C. J.; Tolman, W.
B. J. Am. Chem. Soc. 2002, 124, 10660-10661.
(5) â-Diketimines formally derived from 1,1,1-trifluoro-2,4-pentanedione
have been synthesized: (a) Fustero, S.; de la Torre, M. G.; Pina, B.;
Fuentes, A. S. J. Org. Chem. 1999, 64, 5551-5556. Such ligands
have been used to advantage in catalysis: (b) Allen, S. D.; Moore, D.
R.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2002, 124,
14284-14285.
(6) (a) The synthesis of N,N′-bis(2-mercaptophenyl) â-diketimines by
condensation of the aniline with 1,1,1,5,5,5-hexafluoro-2,4-pentanedi-
one has been reported, but no specific details are given: Sharma, R.
K.; Singh, Y.; Rai, A. K. Main Group Met. Chem. 2000, 23, 777-
780. (b) While this work was in progress, the N,N′-bis(2,6-diisopro-
pylphenyl) analogue of 1, and its lithium salt, were reported: Carey,
D. T.; Cope-Eatough, E. K.; Vilaplana-Mafe´, E.; Mair, F. S.; Pritchard,
R. G.; Warren, J. E.; Woods, R. J. Dalton Trans. 2003, 1083-1093.
(7) (a) For a review, see: Molina, P.; Vilaplana, M. J. Synthesis 1994,
1197-1218. (b) Preparation of R-diimines by this reaction: Zhong,
H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002, 124,
1378-1399.
(8) The synthesis and autoxidation of 2b have been presented in oral
form: Sadighi, J. P.; Laitar, D. S. Abstracts of Papers, 224th National
Meeting of the American Chemical Society, Boston, MA, Aug. 18-
22, 2002; American Chemical Society: Washington, DC, 2003; INOR-
318.
* To whom correspondence should be addressed. E-mail: jsadighi@
mit.edu.
(1) See for example: (a) Muldoon, J.; Brown, S. N. Org. Lett. 2002, 4,
1043-1045. (b) Nishiyama, Y.; Nakagawa, Y.; Mizuno, N. Angew.
Chem., Int. Ed. 2001, 40, 3639-3641. (c) Do¨bler, C.; Mehltretter,
G.; Beller, M. Angew. Chem., Int. Ed. 1999, 38, 3026-3028. (d) Stahl,
S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37,
2180-2192.
(2) (a) Kitajima, N.; Moro-oka, Y. Chem. ReV. 1994, 94, 737-757. (b)
Karlin, K. D.; Kaderli, S.; Zuberbu¨hler, A. D Acc. Chem. Res. 1997,
30, 139-147. (c) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.;
Kaderli, S.; Young, V. G., Jr.; Que, L., Jr.; Zuberbu¨hler, A. D.; Tolman,
W. B. Science 1996, 271, 1397-1400. (d) Straub, B. F.; Rominger,
F.; Hofmann, P. Chem. Commun. 2000, 1611-1612. (e) Taki, M.;
Itoh, S.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6203-6204.
(3) Selected references: (a) Pidcock, E.; Obias, H. V.; Zhang, C. X.;
Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 7841-
7847. (b) Holland, P. L.; Rodgers, K. R.; Tolman, W. B. Angew.
Chem., Int. Ed. 1999, 38, 1139-1142. (c) Itoh, S.; Taki, M.; Nakao,
H.; Holland, P. L.; Tolman, W. B.; Que, L., Jr. Angew. Chem., Int.
Ed. 2000, 39, 398-400. (d) Taki, M.; Teramae, S.; Nagatomo, S.;
Tachi, Y.; Kitagawa, T.; Itoh, S.; Fukuzumi, S. J. Am. Chem. Soc.
2002, 124, 6367-6377. (e) Mirica, L. M.; Vance, M.; Jackson Rudd,
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Am. Chem. Soc. 2002, 124, 9332-9333. (f) Zhang, C. X.; Liang, H.-
C.; Kim, E-i.; Shearer, J.; Helton, M. E.; Kim, E.; Kaderli, S.; Incarvito,
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7354 Inorganic Chemistry, Vol. 42, No. 23, 2003
10.1021/ic035056j CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/14/2003

COMMUNICATION
Table 1. νCO for Inter-Related (â-Diketiminate)Cu(CO) Complexes
We have found analogues without the backbone CF3 groups
to be unstable and have been unable to isolate them.
To assess the electron-withdrawing nature of ligand 1, less-
fluorinated analogues 4-6 were synthesized, and the νCO of
the copper carbonyl complexes derived from them were
measured (Table 1).14,15 The all-methyl analogue of 1 gives
rise to the lowest carbonyl stretching frequency, resulting
from the most electron-rich metal center, as expected. Two
CF3 groups at the ligand backbone exert a larger electron-
withdrawing effect than four at the meta positions of the
N-aryl groups: νCO increases by 24 versus 17 cm-1.
Replacement of all methyl groups with CF3 groups results
in an increase in νCO of 37 cm-1, a substantial effect given
the modest metal-to-ligand back-bonding from Cu(I).16
Despite the electron-poor nature of ligand 1, the copper-
(I) complex 2b reacts readily with dioxygen. Exposure of a
hexane solution of 2b to dry air leads to the rapid formation
of a brown precipitate; the oxidation proceeds in near-
quantitative yield after 30 min at room temperature. The 1H
and 19F NMR of the product indicate the formation of an
asymmetric, paramagnetic species.
Figure 1. Representation of 2a, shown as 50% elipsoids. For clarity, all
hydrogen atoms were omitted, and only one orientation of each of the aryl
CF3 groups, which were modeled with six fluorines at half occupancy, is
shown. Selected bond distances (Å) and angles (deg): Cu(1)-C(22)
2.117(4), Cu(1)-C(23) 2.102(4), C(22)-C(23) 1.385(6), N(1)-Cu(1)-N(1)
99.62(12), C(22)-Cu(1)-C(23) 38.35(17), Cu(1)-C(22)-C(27) 108.1(3),
Cu(1)-C(23)-C(24) 106.1(3).
Scheme 1. Synthesis and Autoxidation of Cu(I) Complexes of 1
Single crystals of the oxidation product, 3, were grown
by slow diffusion of hexane into a saturated solution of the
mesitylcopper(I) in benzene solution produces benzene
adduct 2a in high yield. The coordinated benzene is labile,
and upon concentration in vacuo, 2a is converted to the
dinuclear benzene adduct 2b (Scheme 1).
1
product complex in C6F6. The H and 19F NMR, IR, and
UV-vis spectra obtained from the crystals match those of
the bulk material. Analysis of a single crystal by X-ray
diffraction reveals a dinuclear structure for 3, in which one
of the ligand N-aryl groups has been hydroxylated to give a
bridging phenolate (Figure 2).17 The hydroxyl hydrogen atom
was detected on the Fourier difference map in the X-ray
study, and its presence is further corroborated by the stretch
at 3671 cm-1 in the IR spectrum of 3, consistent with other
known µ-hydroxocopper complexes.4a The structure displays
considerable tetragonal distortion at the copper centers; the
dihedral angle between the two (â-diketiminate)copper planes
is 53.68°.
Crystals suitable for X-ray crystallography were grown
by slow cooling of a hot hexane/benzene solution of 2a
(Figure 1). The X-ray structure10 shows 2a to be a monomeric
η2-benzene adduct. The benzene ring remains essentially
planar, and the C(22)-C(23) bond length (1.385(6) Å) is
equivalent within error to that of free benzene (1.39 Å),11
indicating little π-back-bonding from copper.12 Benzene
complexes of copper(I) with poorly donating anions have
been structurally characterized;13 to our knowledge, this is
the first such adduct supported by a â-diketiminate ligand.
The UV-vis spectra of starting material 2b and oxidation
product 3, measured in dichloromethane solution, are shown
(9) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.;
Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514-1516.
(10) Crystal data for C27H13N2F18Cu: monoclinic, space group P21/n, a )
8.5814(14) Å, b ) 24.486(4) Å, c ) 14.198(2) Å, â ) 100.135(3)°,
V ) 2936.8(8) Å3, Z ) 4, Fcalcd ) 1.744 g/cm3, F(000) ) 1520. A
total of 16229 reflections were collected in the θ range 2.21-25.00°
of which 5738 were unique (Rint ) 0.0415). No absorption correction
was applied. The least squares refinement converged normally with
residuals of R1 (based on F) ) 0.0583, wR2 (based on F2) ) 0.1215,
and GOF ) 1.315 based on I > 2σ(I).
(11) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.
G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1-S19.
(12) Similarly, the CdC bond length in ethylene is unchanged by
coordination to a [(phen)Cu]+ fragment: Munakata, M.; Kitagawa,
S.; Kosome, S.; Asahara, A. Inorg. Chem. 1986, 25, 2622-2627.
(13) (a) Turner, R. W.; Amma, E. L. J. Am. Chem. Soc. 1963, 85, 4046-
4047. (b) Dines, M. B.; Bird, P. H. J. Chem. Soc., Chem. Commun.
1973, 1, 12. (c) Silverthorn, W. E. AdV. Organomet. Chem. 1975, 13,
47-137.
(14) Full experimental details on the synthesis of ligands and the generation
of the Cu(I) carbonyl complexes are provided in the Supporting
Information.
(15) For a recent example of metal carbonyl IR stretching frequencies as
a measure of ligand electronics, see ref 7b.
(16) Similar differences in stretching frequency have been reported between
Cu(CO) complexes of fluorinated and nonfluorinated tris(pyrazolylbo-
rate) ligands: Rasika Dias, H. V.; Lu, H.-L. Inorg. Chem. 1995, 34,
5380-5382.
(17) Crystal data for C48H14N4O2F42Cu2: monoclinic, space group: P21/
n, a ) 14.102(7) Å, b ) 21.704 Å, c ) 18.354(8) Å, â ) 99.907(7)°,
V ) 5534(4) Å3, Z ) 4, Fcalcd ) 1.925 g/cm3, F(000) ) 3128. A total
of 22 451 reflections were collected in the θ range 2.21-23.30° of
which 7963 were unique (Rint ) 0.0674). No absorption correction
was applied. The least squares refinement converged normally with
residuals of R (based on F) ) 0.0444, wR2 (based on F2) ) 0.1153,
and GOF) 1.024 based on I > 2σ(I).
Inorganic Chemistry, Vol. 42, No. 23, 2003 7355

COMMUNICATION
The magnetic susceptibility of 3 been measured for a
powder sample using a SQUID magnetometer. The effective
magnetic moment µeff, at ambient temperature and a field
strength of 0.5 T, is 1.95 µB; the magnetic behavior is
consistent with weak antiferromagnetic coupling between the
two copper(II) centers.18 For comparison, the square planar
bis[(â-diketiminate) copper(II) µ-hydroxide] reported by Dai
and Warren has a smaller effective magnetic moment, 1.39
µB.4a
Intramolecular aerobic hydroxylation of an aryl group is
well-precedented;2b,3a,b in this case, the aryl group is oxidized
cleanly and rapidly despite the presence of two electronically
deactivating CF3 groups. Reaction of other (â-diketiminate)-
copper(I) species with O2 generally gives rise to dimeric
Cu(III) µ-oxo complexes, or to monomeric Cu(III) η2-peroxo
complexes, depending on the ligand.4 Aerobic oxidation of
the â-diketiminate ligand backbone, which has been reported
for Cu(II) and Zn(II) complexes,19 was not observed in the
oxidation of 2.
In conclusion, an aza-Wittig reaction, using 1,1,1,5,5,5-
hexafluoro-2,4-pentanedione, affords the new, heavily flu-
orinated â-diketimine ligand 1 in good yield. This ligand is
quite electron-poor, as reflected by the infrared stretching
frequencies for four inter-related copper(I) carbonyl com-
plexes. Nonetheless, a benzene adduct of its copper(I)
complex reacts rapidly with dry air under ambient conditions,
generating an oxidizing intermediate sufficiently powerful
to hydroxylate a ligand-based 3,5-bis(trifluoromethyl)phenyl
group, while leaving the diketiminate backbone unchanged.
The chemistry of related complexes, protected against N-aryl
group oxidation, is currently under investigation.
Figure 2. Representation of 3, shown as 50% elipsoids. For clarity, the
following were omitted: the fluorine atoms of all aryl CF3 groups, all
hydrogen atoms except for the hydroxyl proton, and a cocrystallized C6F6
molecule. Selected interatomic distances (Å) and angles (deg): Cu(1)-
Cu(2) 2.9345(12), Cu(1)-O(1) 1.935(3), Cu(2)-O(1) 2.028(3), Cu(1)-
O(2) 1.908(3), Cu(2)-O(2) 1.911(3), O(1)-C(7) 1.325(4), N(1)-Cu(1)-
N(2) 97.05(12), N(3)-Cu(2)-N(4) 97.31(13), O(1)-Cu(1)-O(2) 78.92(11),
O(1)-Cu(2)-O(2) 76.57(10), Cu(1)-O(1)-Cu(2) 95.51(11), Cu(1)-O(2)-
Cu(2) 100.41(12).
Acknowledgment. We gratefully acknowledge Prof.
Stephen J. Lippard for donation of an IR spectrometer, and
Prof. Daniel G. Nocera for the use of his UV-vis spectro-
photometer. Dr. Daniel Grohol kindly assisted us in obtaining
magnetic susceptibility data. We thank these colleagues and
Prof. Christopher C. Cummins for helpful discussions. We
also thank the MIT Department of Chemistry for support of
this research through a startup grant, and the MIT School of
Science for a generous grant from the James H. Ferry, Jr.
Fund. We gratefully acknowledge a Lester Wolfe predoctoral
fellowship for D.S.L. We also acknowledge NSF Awards
CHE-9808061 and DBI-9729592 for supporting our NMR
facilities.
Figure 3. UV-vis spectra of 2b (s) and 3 (- - -) in CH2Cl2.
in Figure 3. The starting material spectrum shows a single
absorption at 379 nm (ꢀ ) 26 900 M-1 cm-1). The spectrum
of dinuclear Cu(II,II) complex 3 displays an absorption
maximum at 376 nm (ꢀ ) 22 300 M-1 cm-1), with new
bands at 320 nm (ꢀ ) 13 300 M-1 cm-1) and at 430 nm
(ꢀ ) 14 700 M-1 cm-1). This pattern is reminiscent of that
observed upon oxygenation of other copper(I) â-diketimi-
nates,4c even though the oxidation state obtained in this case
is different.
Supporting Information Available: Experimental details (PDF)
and X-ray crystallographic data (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
IC035056J
(18) See Supporting Information for plots of ø and of µeff versus T (Figure
S1).
(19) Yokota, S.; Tachi, Y.; Itoh, S. Inorg. Chem. 2002, 41, 1342-1344.
7356 Inorganic Chemistry, Vol. 42, No. 23, 2003