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Novel Binuclear Cobalt Dioxygen ComplexЧA Step on the Path to Dioxygen Activation.

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171 Tui-nwcr numbers were determined for a catalyst: TBHP:substrate ratio of
I .i0.700 In C‘H,Ch’ undcr Ar at room temperature for a reaction time of 5
111111
1x1 Prepnred from [DJrBuMgCI using the synthetic method outlined
i n . C.
Walling. S. A . Buckler. .I An?. C%rii?. Soc. 1955, 77. 6032 -6038.
“4 As\ignmcnt of proton rcsoiianccs in 3: d = 32 (py a-H). 19 (py /I-H), and 6
(py 7 - H ) .
[IOl J T <;ro\c>. .I. Chiw k11i<c1985. 62, 928-9il.
11 I \ :I) I<. J Gu<IpLrdo,S. E. Hudson. S J Brown. P. K Mascharak. .
I
Am. Chmm.
.So(. 1993. 115. 7971 ~ 7 9 7 7b)
, A. Sauer-Masarwa, N Herron, C . M . Fendrick.
L). 13. B U S C ~//iorX.
.
C h n . 1993, 32- 1086. 1089.
1121 For curnp:ii-ison. the Raman spectrum ofthe Fe-00-cumyl intermediate shows
features at 454. 554. 574. 696, 728. and 792 cm-’.
1131 .I) G Tian. I A . Berry, J. P. Klinman. Biochenii.c/,:1~
1994. 33. 226-234: h) J. S.
Valcntiiic. Mi. Warn, K.Y, H o in Tiir AC,/iw/io!?of Dio\~~cn
riiirl HomoXrneous
( b r r i l i ~ r i c O~ \ i ( / r / l i o r i (Eds.: D. H. R. Barton. A. E. Martell. D. T. Sawyer).
Plrnuin. Ken York. 1993. pp. 1x3- 198; c) W. Nam. R. Y. Ho. J. S. Valentine.
.I A u i . C’hwi. Sot.. 1991. 113. 7052 7054: d ) J. W. Sam, X.-J. Tang, J. Peisach.
;hid 1994. 116. 5250 -5256: e ) 0. M. Reinaud. K . H. Theopold. J. A m . Chrm
.Sm. 1994. (16. 6979 -6980; i’) S. Mahapatra. J. A. Halfen. E. C. Wilkinson. L.
Quc. .lr.. W B Tolinan. i h i d 1994. 116. 9785-9786.
ence between the Tp“ and the Tp’ complexes. Upon progressive
cooling of a solution of the paramagnetic [Tp”Co(OJ] complex
in CD,CI,, the five isotropically shifted ’ H N M R resonances
were gradually replaced by a set of peaks in the range 6 = 0- 10.
indicating the formation of a diamagnetic compound. These
spectroscopic changes were reversible. and they were accompanied by changes in the color of the solution. At 270 K the solution appeared brown, similar to the color of [Tp’CojO,)]; cooling to 220K produced a dark green solution. Similar color
changes were observed in toluene, THF, and acetonitrile. Higher
concentrations of [Tp”Co(O,)] increased the fraction of the diamagnetic complex. Based on these observations we propose that
a monomer-dimer equilibrium (Scheme 1 ) causes the spectroscopic changes.
Novel Binuclear Cobalt Dioxygen ComplexA Step on the Path to Dioxygen Activation**
Olivia M . Reinaud, Glenn P. A. Yap,
A r n o l d L. Rheingold, and Klaus H. Theopold*
The “activation” of dioxygen by complexation to transition
metals is an important theme of contemporary chemical research.”] The synthesis and characterization of novel dioxygen
complexes (with O,, O ; , or 0:- ligands) is thus part of the
search for catalytic oxidation reactions that use air as an environmentally benign oxidant.”] Such complexes may also serve
a s models for the active sites of biomolecules that facilitate
transport and utilization of dioxygen in living organisms.
Herein we describe the formation of a cobalt dioxygen complex
containing an unprecedented structural motif. The cobalt is coordinated by sterically congested tris(pyrazoly1)borate moieties,l-31a class of ligands that have shown a propensity for
stabilizing unusual binding modes in dioxygen complexes.[41
As previously described, exposure of solid [Tp”Co(CO)]
(Tp” = hydrotris(3-isopropyl-5-methylpyrazolyl)borate)to an
excess of 0, gas yielded the dioxygen complex [Tp”Co(O,)] .Is1
The spectroscopic characterization of this paramagnetic compound left little doubt that it is a close analogue of the structurally characterized complex [Tp’Co(O,)] (Tp’ = hydrotris(31rri-butyl-5-1nethylpyrazolyl)borate),[~~~
which was the first
example of a dioxygen complex with a symmetrically side-on
bound superoxido ligand. However, in marked contrast to the
very stable Tp‘ complex, the sterically less encumbered
[Tp”Co(O,)] decomposed in solution at room temperature, producing [Tp”Co(p-OJCoTp”] as a transient intermediate, and
ultimately leading to hydrogen atom abstraction from the ligand. Below -10°C [Tp”Co(02)]was stable for hours, but its
variable-temperature N M R spectra revealed yet another differ[*] Prof. Dr. K . H. Theopold. Dr. G. P. A. Yap. Prof. Dr. A. L Rheingold
Departmen! of Chemistry and Biochemistry
C‘cntcr for < ‘ii-ilytic Science and Technology
Univci-sit)
Delaware
N e w i r k . [IF. 19716 (USA)
klef.ix I n 1 code (302)831-6335
e-mail: thcopoldfir strauss.udel.edu
+
[**I
Dr 0. M. Rcinaud
L:iboi-;i1oirc dc Recherchcs Organiques del‘ESPCI associe au CNRS
10 riic Vaiiquclin. F-75231 Paris Cedex 05 (France)
Thi\ rnearch was supported by the U. S. Department of Energy (ER14273)
; i d by thc (’NRS and NATO.
Scheme 1. Equilibrium between monomeric and dimeric formi of [Tp”Co(O,)].
Cooling a concentrated solution of [Tp”Cu(O,)] in acetonitrile to - 30 “C yielded green crystals of [(Tp”Co(O,)),
2CH,CN] suitable for a low-temperature X-ray structure determination; apparently the dimer is much less soluble in this solvent and crystallizes out selectively. The molecular structure
featuring crystallographic inversion symmetry is depicted in
Figure 1 .I6] Each cobalt atom is bound to three nitrogen atoms
of its Tp” ligand and to two oxygen atoms; the coordination
environment about the metal closely approximates an idealized
square pyramid. There are two dioxygen molecules. both linking
Fig. 1. The molecular structure of[(Tp”Co(O,)),.2CH,CN]; thesolvent molecules
are omitted for clarity. Selected interatomic distances [A] and angles [ 1: Co-O(l)
1.836(3). C o - 0 ( 2 ) 1.839(4), Co-N(I1) 2.056(4). Co-N(21) 2.107(4). Co-N(31)
2.070(4). O(I)-O(ZA) 1.354(5). Co-CoA 3.50; O(l)-Co-0(2) 88.8(2). Co-O(l)O(2A) 110.4(3),Co-O(2)-O(lA) 110.9(3), O(l)-Co-N(I 1 ) 171.3(2).O(l)-Co-N(II)
95.5(2). O(I)-Co-N(31)
91.6(2), N(ll)-Co-N(21) 89.8(2), N(1 I)-Co-N(31) 86.3(2),
N(21)-Co-0(31) 90 9(2).
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the cobalt atoms in a tram p-qt:q' fashion; the resulting sixmembered Co,O, ring adopts a chair conformation. To our
knowledge, this structural motif is unprecedented in the coordination chemistry of 0,. The 0-0 distance of 1.354(5) 8, is
intermediate between the ranges usually associated with
p-superoxido (1.24-1.32 A) and p-peroxido hgands (1.381.53 8,).[']However, the Co-N bond lengths (average Co-N =
2.08 8,) are characteristic of a tris(pyrdzoly1)borate moiety
bound to Co" centers. Besides, five-coordinate Co"' complexes
are rather rare in general,[*] and we have yet to find an example
of a stable Co"' complex containing one of these sterically demanding Tp ligands. Thus we suggest that the structural evidence is most consistent with the designation of [{Tp"Co(O,)),]
as a Co" superoxido complex.
Since thermochemical parameters for the interaction of O2
with transition metals are important, we measured the temperature dependence of the monomer-dimer equilibrium described
above. The relative concentrations of [Tp"Co(OJ] and
[(Tp"Co(O,)),] were measured by 'H NMR spectroscopy over
the temperature range 220-260 K. The temperature dependence
of the equilibrium constant in the form of a van't Hoff plot is
depicted in Figure 2. The thermochemical parameters
distance of 3.50 8, rules out any direct interaction; instead we
suggest that the diainagnetism of [{Tp"Co(O,)),] may result
from strong antiferromagnetic coupling between low-spin Co"
ions (S = 1/2) and the adjacent superoxido ligands ( S = 1/2).
Interestingly, dimeric [{Tp"Co(O,)},] is thermally less stable
than monomeric [Tp"Co(O,)]. Whereas the monomer is perfectly stable in the solid state at ambient temperature, green crystals
of the dimer decomposed rapidly at room temperature, yielding
a gas (O,?) and a pink solid, very similar in nature to the decomposition products of [Tp"Co(p-O,)CoTp"] . These observations
suggest that [ (Tp"Co(0,)) ,] may also be an intermediate in the
solution reaction of [Tp"Co(O,)] . Its formation, followed by
loss of one ofits two dioxygen moieties, constitutes a reasonable
mechanistic pathway for the formation of [Tp"Co(p-O,)CoTp"],
(much preferable to preequilibrium dissociation of 0, from
monomeric [Tp"Co(O,)]) followed by trapping of the coordinatively unsaturated [Tp"Co] by [Tp"Co(OJ] . The first proposal
would also explain the surprising difference in reactivity between [Tp"Co(O,)] and its more hindered analogue [Tp'Co(O,)];
a dimeric form of the latter is obviously much less accessible.
Indeed, this reaction pathway may be crucial to our success in
constructing a catalyst for the activation of O,, which depends
on simultaneous binding of one Oz molecule to two metals
atoms in the presence of excess 0,.
E.xperimentul Procedure
-
[Tp"Co(OJ]: An annlytically pure sample of [Tp"Co(CO)](500 mg, 1.07 mmol) was
ground into a fine powder in a dry box. The powder was transferred into a glass
ainpoulc. which was then evacuated for 2 h to remove any trace of solvent. Upon
exposure to 0.8 iltm of dry 0,. the color of the solid changed rapidly from green to
brown (within 1 min): the exposure was continued for 2 11. To ensure quantitative
reaction, two more cycles o f evacuation and exposurc to O2 were carried out. The
solid thus obtained in quantitative yield was pure [Tp"Co(02)].
. ..
3 . 6 ~ 1 0 . ~3 . 8 ~ 1 0 . ~4 ~ 1 0 . ~4 . 2 ~ 1 0 . ~
4 . 4 ~ 1 0 4~ . 6 ~ 1 0 ~
T-' /K-' ---+
-3
~
i
Fig. 2. Temperature dependence o f t h e equilibrium constant K,, (see Scheiue I ) . o:
[Co] = 0 . 0 3 3 ~ :a : [Co]= 0 . 4 0 ~in CD,CI, a t 25 C. Results of the van't Hoff
analysis: A// = -14.5(5) kcalmol-', A S = - 60(3) cal inol-' K - I .
[jTp"Co(O,)~],]: Single crystals of [/Tp"Co(O,)),.2CH,CN] were obtained by
cooling a SoluLion of[Tp"Co(O,)] (50 mg) in MeCN (1 m L ) to -30 C for 2 d. The
mother liquor was then pipetted out of the vial and cold oil introduced in order to
coat the green crystals. and to protect them from cont;ict with air during collection
of the X-ray diffraction data. A crystal of dimensions 0.36 x 0.38 x 0.3X mm was
mounted on a Siemens P4 diffractometer at 222 K, and 491 7 rcflections were coIIected, ofwhich 3033 were judged observed ( F < 4.0cr(F)). The structure was solved by
direct rncthods. using Siemens SHELXTL PLUS s o f h a r e .
German vcrsion:
derived therefrom are A H = - 14.5(5) kcal mol
and A S =
- 60(3) cal mol-' K - I . As theconversion ofthemonomer into
the dimer involves the opening of two strained COO, rings and
the formation of one six-membered ring, the reaction enthalpy
provides a measure of the strain energy of roughly 7 kcalmol- I
per three-membered ring.
We also monitored the magnetic susceptibility of
[Tp"Co(O,)]/[ (Tp"Co(02))2] solutions at various temperatures
by the Evans
At 270K, where monomeric
[Tp"Co(O,)] is the only species detectable by NMR. the effective
magnetic moment perfwas 3.3(1) p". This value is the same as
that measured for solid [Tp"Co(02)] at room temperature,[']
and it is consistent with two unpaired electrons ( S = 1 ) resulting
from strong antiferromagnetic coupling of a Co" ion ( S = 312)
to the superoxido ligand ( S = 1/2). When the solution was
cooled. the magnetic moment decreased rapidly, concomitant
with the appearance ofthe NMR signals of the dimer. Although
we have not been able to extend these measurements to low
enough temperatures to actually convert all of the cobalt complex into the dimeric form, extrapolation of the magnetic
data indicates diamagnetic character for [{Tp"Co(O,)) ,I. The
narrow line widths, absence of isotropic shifts, and lack of significant temperature-dependence in the 'H NMR spectrum of
the dimer are also consistent with this notion.["] The Co-Co
~
Received: April 27. 1995 [Z79351E]
2171 -2173
A n g i w . Chcm. 1995, 107.
-
Keywords: cobalt compounds dioxygen complexes tris(pyrazolyl)bora tes
[ I ] a ) R. A. Sheldon. J. K . Kochi, Mctrtl-Catuli~re~l
0.ridution.s nf Orjiutiic. Cunipoutid\. Academic. New York, 1981 : b) O.qgeii Coniphses und O.YJ.~~IW
A~rirutron h i . f i u m i t i n n ,Mi,lul.s (Eds.: A E. Martell, D. T. Sawyer). Plenum, New
York. 1988: c ) Dto.Y):zrti Ai l i w l i o n and Hotiiogcneou.\ Curulj~ricO.~tdution.
.Ylnd. Sw/.S<,i.Cu/o/. 1991. 06.
[2] Mc~lulDin.vJ:qoi C o n i p l e ~ i 'A~ .P c r s p w t i w (Ciieni. K c v . 1994, Y4. 597)
[3] S. Trotimenko, C h i i . Reir 1993. Y3, 943.
[4] ii) M. J. Baldwin. D E. Root. J. E. Pate. K. Fujisdwa, N . Kitajima. E I.
Solomon, J ,4111. Chem Sm. 1992. f i4. 10421 : b) J. W, Egan, Jr.. B. S. Hliggert y , A. L. Rheingold, S. C. Sendlinger. K . H . Theopold. ihid. 1990. ff2.2445: c)
K . FiiJtsawa. M. Tanaka. Y. Moro-oka, N. Kitajima. ;/d.
1994. 116. 12079.
[ 5 ] 0 . M . Rcinaud. K. H. Theopold. J Am. Chetn. S o c . 1994. 116. 6979.
[6j [Tp"Co(Oi)l2.ZCH,CN: monoclinic P2,:n. II = 13.256(6). h = 13.868(4). c =
16.463(6)A. /{ = 96.35(3) I'= 3005(2) A'. 1 = 2 . R = 0.0563. R, = 0.06XO.
Further details of the crystal structure investigation may be obtained from the
Fachinformationszentruni Karlsruhc, D-76344 Eggenstein-Leopoldshafen
(Germany). on quoting the depository number CSD-59 113.
[7] R. R Conry. K. D. Karlin in EiiI;ycInpedfu 01 /no,xonic. Cheniislri.. L
'
X 3 (Ed.
R. B. King). Wiley. New York. 1994. p. 1036.
[XI :I) L G Marzili. M. F. Summers. N. Bresci;ini-Pahor. E, Zangrando. I.-P.
Charland. L. Randaccio. J. 4177. Chon. Soi. 1985. 107, 6880; b) S. Bruckner. M.
Calligaris. G. Nardin. L. Randaccio. Inorg. ('him A u r t 1969. 3. 308.
[9] a) D. F. Evans, J. Climi. SOC.1959. 2003: b) D. Ostfeld. I. A. Cohen. J Chtwi.
Ed. 1972, 4Y. X29: c ) S . K. Sur, J. .Mugnct. R P ~1989.
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82. 169.
[lo] I . Bertini. C. Luchinat. .VMR of Purumugnclic Moliwdrr In Bfol~)~~iculS~.srern.\.
Benjamin,Cummings, Menlo Park. CA, 1986. p. 19ff.
.
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