An arene-stabilized cobalt(I) Aryl: Reactions with CO and NO

Article abstract of DOI:10.1021/ic8015782

The half-sandwich cobalt(I) complex (η⁶-C₇H ₈)CoAr*-3,5-ⁱPr₂ (Ar*-3,5- ⁱPr₂ = -C₆H-2,6-(C₆H ₂-2,4,6-ⁱPr₃)₂-3,5-

Full text of DOI:10.1021/ic8015782

Inorg. Chem. 2008, 47, 10205-10207
An Arene-Stabilized Cobalt(I) Aryl: Reactions with CO and NO
Hao Lei,† Bobby D. Ellis,† Chengbao Ni,† Fernande Grandjean,‡ Gary J. Long,§ and Philip P. Power*,†
Department of Chemistry, UniVersity of California, DaVis, California 95616, Department of
Physics, UniVersity of Liège, B-4000 Sart-Tilman, Belgium, and Department of Chemistry,
Missouri UniVersity of Science and Technology, Rolla, Missouri 65409
Received August 25, 2008
The half-sandwich cobalt(I) complex (η -C7H8)CoAr*-3,5-iPr2 (Ar*-
3,5-iPr2 ) -C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2) was synthesized
by reduction of [3,5-iPr2Ar*Co(µ-Cl)]2 in toluene. It reacts with CO
or NO to afford the unusual complexes [3,5-iPr2Ar*C(O)Co(CO)]
or [3,5-iPr2Ar*N(NO)OCo(NO)2].
(η6-C6H6)CoMe.6 A change in the spin state from a triplet
6
to a singlet was also predicted when the centroid-Co-Me
geometry changes from a linear (180°) to a bent (135°)
configuration. Indeed, a low-spin state is observed for the
Ar′CoCoAr′ [Ar′ ) -C6H3-2,6-(C6H3-2,6-iPr2)2] dimer, which
has a bent Co configuration.1c However, the high-spin triplet
configuration has not been experimentally verified for an
essentially linear (η6-arene)Co(η1-aryl) system. We now
Investigations of the sterically crowded first-row transition-
metal moiety MAr*-3,5-iPr2 (Ar*-3,5-iPr2 ) -C6H-2,6-
(C6H2-2,4,6-iPr3)2-3,5-iPr2; M ) Cr1, Mn2, Fe2) have shown
that stable η6 complexes with aromatic rings such as benzene
or toluene are not formed in the case of chromium.1,3 In
contrast, for manganese and iron, the stable inverted sand-
wich [(µ-η6:η6-C7H8){MnAr*-3,5-iPr2}2] and half-sandwich
species [(η6-C6H6)FeAr*-3,5-iPr2] can be readily isolated.2
The instability of the chromium(I) arene complex4 is
noteworthy because the related ꢀ-diketiminate/arene complex
[(µ-η6:η6-C7H8)Cr{(C6H3-2,6-iPr2)NC(Me)}2CH] is isolable
under ambient conditions.5 Calculations for (η6-C6H6)MMe
model species (M ) Cr, Fe, Co) supported a preference for
weak η2 rather than η6 chromium(I)-ring interactions,
whereas strong η6-C6H6-Co interactions were predicted for
(6) La Macchia, G.; Gagliardi, L.; Power, P. P.; Brynda, M. J. Am. Chem.
Soc. 2008, 130, 5105.
(7) 3,5-iPr2Ar*Co(η6-C7H8) (2): A brown solution of [3,5-iPr2Ar*Co(µ-
Cl)]2 (1) (4.69 g, 7.11 mmol), prepared analogously to [Li(OEt2)-
24
Ar′CoI2]2 from 3,5-iPr2Ar*Li3 and CoCl2, in ca. 20 mL of toluene,
was added to a suspension of KC8 (1.02 g, 7.54 mmol) at 0 °C in ca.
20 mL of toluene. After stirring for ca. 24 h, the solvent was removed
under reduced pressure and the dark residue was extracted with ca.
100 mL of hexane. The solution was filtered, and the green filtrate
was concentrated to ca. 20 mL, which afforded X-ray-quality bright-
green crystals of 2·n-hexane after storage at -18 °C for 1 day. Yield:
4.10 g (80%). Mp: 175 °C (black oil). Elem anal. Calcd for C49H69Co:
C, 82.08; H, 9.70. Found: C, 82.56; H, 10.1. 1H NMR (300.08 MHz,
C6D6, 27 °C): δ 16.18 (s), 15.67 (s), 14.45 (br d), 8.70 (s), 3.22 (d),
2.53 (d), 2.12 (s), 1.26 (m), 0.91 (s), 0.30 (s), -0.83 (t), -59.16 (s),
-62.31 (s). UV/vis [hexanes; λmax, nm (ε, L·mol-1 ·cm-1)]: 242
(13 700), 260 (8600), 274 (9700), 316 (5700), 404 (1800). 3,5-
iPr2Ar*C(O)Co(CO) (3): A green solution of 2 (0.52 g, 0.73 mmol)
in ca. 50 mL of diethyl ether was treated with dry CO gas (1 atm) at
room temperature for 2 h. The solution became red within ca. 10 min.
After further stirring for ca. 24 h, the solvent was removed under
reduced pressure, and the residue was extracted with ca. 15 mL of
hexane. Storage at -18 °C for several days gave X-ray-quality red
crystals of 3. Yield: 0.16 g (32%). Mp: 221 °C. Calcd for C44H61CoO2:
C, 77.61; H, 9.03. Found: C, 78.15; H, 8.80. 1H NMR (300.08 MHz,
C6D6, 23 °C): δ 7.34 (s, 1H), 7.07 (m, 4H), 2.96 (sept, 1H), 2.83
(sept, 1H), 2.73 (sept, 1H), 2.56 (sept, 2H), 2.44 (sept, 2H), 1.86 (sept,
1H), 1.34 (d, 6H), 1.29 (d, 6H), 1.24 (d, 6H), 1.20 (d, 6H), 1.14 (d,
6H), 1.03 (d, 6H), 0.98 (d, 6H), 0.86 (d, 6H). IR (Nujol, cm-1) ν
1965 (s). UV/vis [hexanes; λmax, nm (ε, L·mol-1 ·cm-1)]: 222 (73 600),
294 (22 500). 3,5-iPr2Ar*N(NO)OCo(NO)2 (4): A green solution of 2
(0.50 g, 0.70 mmol) in ca. 50 mL of diethyl ether was treated with
dry NO gas (1 atm) at room temperature for 2 h. The solution turned
brown immediately. After stirring for ca. 24 h, the solvent was removed
under reduced pressure, and the residue was extracted with ca. 50
mL of hexane. The solution was filtered. and the dark-red filtrate was
concentrated to ca. 10 mL, which afforded X-ray-quality reddish-brown
crystals of 4 after storage at -18 °C for several days. Yield: 0.10 g
(19%). Mp: 163 °C. Calcd for C42H61CoN4O4: C, 67.78; H, 8.26; N,
7.52. Found: C, 68.31; H, 8.24; N, 7.12. 1H NMR (300.08 MHz, C6D6,
23 °C): δ 7.67 (s, 1H), 7.13-7.24 (m, 4H), 2.83 (m, 8H), 1.50 (d,
12H), 1.17-1.38 (m, 36H). IR (Nujol, cm-1) ν 1690 (s). UV/vis
[hexanes; λmax, nm (ε, L·mol-1 ·cm-1)]: 224 (82 600), 272 (34 800),
364 (85 100).
* To whom correspondence should be addressed. E-mail: pppower@
ucdavis.edu.
†
University of California.
University of Liège.
‡
§
Missouri University of Science and Technology.
(1) (a) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long,
G. J.; Power, P. P. Science 2005, 310, 844. (b) Wolf, R.; Ni, C.;
Nguyen, T.; Brynda, M.; Long, G. J.; Sutton, A. D.; Fischer, R. C.;
Fettinger, J. C.; Hellman, M.; Pu, L.; Power, P. P. Inorg. Chem. 2007,
46, 11277. (c) Nguyen, T.; Merrill, W. A.; Ni, C.; Lei, H.; Long, G. J.;
Power, P. P. Angew. Chem., Int. Ed. 2008, in press.
(2) Ni, C.; Ellis, B. D.; Fettinger, J. C.; Long, G. J.; Power, P. P. Chem.
Commun. 2008, 1014.
(3) Wolf, R.; Brynda, M.; Ni, C.; Long, G. J.; Power, P. P. J. Am. Chem.
Soc. 2007, 129, 6076.
(4) (a) Bauschlicher, C. W.; Partridge, H.; Langhoff, S. R. J. Phys. Chem.
1992, 96, 3273. (b) Lin, C.-Y.; Chen, Q.; Chen, H.; Freiser, B. S. J.
Phys. Chem. A 1997, 101, 6023. (c) Yang, C.-N.; Klippenstein, S. J.
J. Phys. Chem. A 1999, 103, 1094. (d) Pandey, R.; Rao, B. K.; Jena,
P.; Blanco, M. A. J. Am. Chem. Soc. 2001, 123, 3799. (e) Jaeger,
T. D.; van Heijnsbergen, D.; Klippenstein, S. J.; von Helden, G.;
Meijer, G.; Duncan, M. A. J. Am. Chem. Soc. 2004, 126, 10981.
(5) Tsai, Y.-C.; Wang, P.-Y.; Chen, S.-A.; Chen, J.-M. J. Am. Chem. Soc.
2007, 129, 8066.
10.1021/ic8015782 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 22, 2008 10205
Published on Web 10/15/2008

COMMUNICATION
Figure 1. Solid-state molecular structure of 2 (H atoms and solvent
molecules are not shown; thermal ellipsoids are shown at 30% probability).
Selected bond distances (Å) and angles (deg): Co(1)-C(1) 2.021(2),
Co(1)-centroid 1.659(1); C(1)-Co(1)-centroid 167.6(2).
report the synthesis and characterization of 3,5-iPr2Ar*Co(η6-
C7H8). We also describe its reactions with CO and NO to
give the new complexes 3,5-iPr2Ar*C(O)Co(CO) and
3,5-iPr2Ar*N(NO)OCo(NO)2.
Figure 2. Plot of the effective magnetic moment and inverse molar
magnetic susceptibility of 2 (inset) versus temperature. Both solid lines
correspond to the best fit obtained by using the method given in ref 12.
The magnetic properties have been fit with S1 ) S2 ) 1, g ) 2.08(1), J )
-1.8(1) cm-1, D ) 35(3) cm-1, and NR ) 0.00059(5) emu ·mol-1
.
Reduction of [3,5-iPr2Ar*Co(µ-Cl)]2 (1) with KC8 in
toluene afforded bright-green crystals of 3,5-iPr2Ar*Co(η6-
C7H8) (2) in 80% yield.7 The X-ray structure of 2 (Figure
1) confirmed the η1 and η6 bonding patterns for the 3,5-
iPr2Ar* and toluene ligands, respectively, as well as a +1
oxidation state at cobalt.8 Monovalent cobalt complexes have
received increased attention because of their ability to activate
small molecules and their effectiveness at binding oxo or
nitrene functionalities in group transfer reactions.9 The
structure of 2 is related to that of [(HC{C(Me)NC6H3-2,6-
Me2}2)Co(η6-C7H8)], which has a Co-centroid distance of
1.747(2) Å.9a The Co(1)-centroid distance in 2 is 1.659(1)
Å, which is almost 0.1 Å shorter, possibly as a result of the
lower coordination number at the cobalt center. However, it
is longer than the corresponding distance (1.602 Å) in the
cobalt(I) complex Co(PMe3)2(BPh4),10 where cobalt interacts
with a phenyl ring in an η6 fashion. The C(1)-Co(1)-centroid
angle in 2 is 167.6°, and the Co(1)-C(1) σ-bond length is
2.021(2) Å, which is close to 1.984(2) Å observed in the
square-planar bis(imido)pyridindylcobalt(I) complex [C5H3-
N{2,6-C(Me)N(C6H3-2,6-iPr2)}2CoCH2SiMe3]11 and is es-
sentially the same as the 2.008(3)-2.019(3) Å range found
in Ar′CoCoAr′.1c The H NMR spectrum of 2 exhibits
1
paramagnetically shifted resonances between 20 and -70
ppm, suggesting a high-spin d8 (S ) 1) electronic configu-
ration for the cobalt(I) center, which was further confirmed
by the study of its magnetic properties. The variation of the
inverse molar magnetic susceptibility with respect to tem-
perature is shown in Figure 2.12 The plot is linear above ca.
150 K, yielding a Curie constant of 1.42 emu · K· mol-1, a
Weiss temperature of -42 K, and µeff of 3.37 µB. Below
150 K, the inverse molar susceptibility deviates only slightly
from linearity possibly as a result of zero-field splitting and
very weak long-range coupling. The data are thus consistent
with the presence of two unpaired electrons and a high-spin
configuration at the metal center.
The reactivity of complex 2 toward CO and NO was also
studied. The reaction of 2 with CO proceeded smoothly and
within minutes produced the new acyl/carbonyl complex
[3,5-iPr2Ar*C(O)Co(CO)] (3) as red crystals in 32% yield.7
An X-ray crystal structure of 3 (Figure 3) revealed that the
cobalt center is coordinated to a terminal carbonyl group as
well as an acyl group formed by insertion of a CO molecule
into the Co-C σ bond.8 The facile insertion of CO into the
metal-ligand bond may be contrasted with the behavior of
related cobalt(I) and iron(I) ꢀ-diketiminate complexes, where
insertion is not observed for the bond to the stabilizing bulky
ligand.9,13 There is also a strong interaction between cobalt
and the flanking aryl ring. The Co-centroid distance
(8) Crystallographic data for 2·n-hexane, 3, and 4 at 90 K with Mo KR
(λ ) 0.710 73 Å) radiation. 2·n-hexane: a ) 30.400(2) Å, b )
10.9345(8) Å, c ) 14.7054(11) Å, V ) 4888.3(6) Å3, M ) 803.14,
orthorhombic, space group Pna21, Z ) 4, R1 ) 0.0438 [I > 2σ(I)
data], wR2 ) 0.1242 for all data. 3: a ) 13.867(2) Å, b ) 14.236(2)
Å, c ) 21.655(4) Å, R ) 75.604(3)°, ꢀ ) 85.861(3)°, γ ) 84.494(3)°,
3
j
V ) 4116.6(12) Å , M ) 680.86, triclinic, space group P1, Z ) 4, R1
) 0.0632 [I > 2σ(I) data], wR2 ) 0.1773 for all data. 4: a ) 9.533(3)
Å, b ) 25.390(8) Å, c ) 20.019(7) Å, ꢀ ) 97.844(5)°, V ) 4800(3)
Å3, M ) 744.88, monoclinic, space group P21/n, Z ) 4, R1 ) 0.1841
[I > 2σ(I) data], wR2 ) 0.5236 for all data.
(9) (a) Dai, X.; Kapoor, P.; Warren, T. H. J. Am. Chem. Soc. 2004, 126,
4798. (b) Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.;
Lachicotte, R. J. J. Am. Chem. Soc. 2002, 124, 14416. (c) Jenkins,
D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 11238.
(d) Hu, X.; Castro-Rodriguez, I.; Meyer, K. J. Am. Chem. Soc. 2004,
126, 13464. (e) Hu, X.; Meyer, K. J. Am. Chem. Soc. 2004, 126, 16322.
(f) Ingleson, M.; Fan, H.; Pink, M.; Tomaszewski, J.; Caulton, K. G.
J. Am. Chem. Soc. 2006, 128, 1804. (g) Ingleson, M. J.; Pink, M.;
Caulton, K. G. J. Am. Chem. Soc. 2006, 128, 4248. (h) Ingelson, M. J.;
Pink, M.; Fan, H.; Caulton, K. G. J. Am. Chem. Soc. 2008, 130, 4262.
(i) Ingleson, M. J.; Pink, M.; Fan, H.; Caulton, K. G. Inorg. Chem.
2007, 46, 10321. (j) Shay, D. T.; Yap, G. P. A.; Zakharov, L. N.;
Rheingold, A. L.; Theopold, K. H. Angew. Chem., Int. Ed. 2005, 44,
1508. (k) Thyagarajan, S.; Shay, D. T.; Incarvito, C. D.; Rheingold,
A. L.; Theopold, K. H. J. Am. Chem. Soc. 2003, 125, 4440. (l) Reinaud,
O. M.; Theopold, K. H. J. Am. Chem. Soc. 1994, 116, 6979. (m)
Detrich, J. L.; Konecny, R.; Vetter, W. M.; Doren, D.; Rheingold,
A. L.; Theopold, K. H. J. Am. Chem. Soc. 1996, 118, 1703.
(11) Kooistra, T. M.; Knijnenburg, Q.; Smits, J. M. M.; Horton, A. D.;
Budzelaar, P. H. M.; Gal, A. W. Angew. Chem., Int. Ed. 2001, 40,
4719.
(12) Ginsberg, A. P.; Martin, R. L.; Brookes, R. W.; Sherwood, R. C. Inorg.
Chem. 1972, 11, 2884.
(13) Smith, J. M.; Sadique, A. R.; Cundari, T. R.; Rodgers, K. R.; Lukat-
Rodgers, G.; Lachicotte, R. J.; Flaschenriem, C. J.; Vela, J.; Holland,
P. L. J. Am. Chem. Soc. 2006, 128, 756.
(10) Carlos Ananias de Carvalho, L.; Dartiguenave, M.; Dartiguenave, Y.;
Beauchamp, A. L. J. Am. Chem. Soc. 1984, 106, 6848.
10206 Inorganic Chemistry, Vol. 47, No. 22, 2008

COMMUNICATION
Figure 3. Solid-state molecular structure of 3 (one of two independent
molecules shown; H atoms and solvent molecules are not shown; thermal
ellipsoids are shown at 30% probability). Selected bond distances (Å) and
angles (deg): Co(1)-C(1) 1.739(5), Co(1)-C(2) 1.926(5), Co(1)-centroid
1.599(2), C(1)-O(1) 1.162(5), C(2)-O(2) 1.227(5); Co(1)-C(1)-O(1)
176.6(4), Co(1)-C(2)-O(2) 126.8(4).
Figure 4. Solid-state molecular structure of 4 (H atoms and solvent
molecules are not shown; thermal ellipsoids are shown at 30% probability).
Selected bond distances (Å) and angles (deg): Co(1)-N(3) 1.643(9),
Co(1)-N(4) 1.618(11), Co(1)-O(1) 1.920(8), Co(1)-O(2) 1.944(8),
N(1)-O(1) 1.334(10), N(1)-N(2) 1.274(12), N(2)-O(2) 1.299(11),
N(3)-O(3) 1.168(12), N(4)-O(4) 1.189(14); O(1)-Co(1)-O(2) 79.9(3),
N(3)-Co(1)-N(4) 111.0(5), Co(1)-N(3)-O(3) 164.7(11), Co(1)-N(4)-O(4)
159.8(11).
[1.599(2) Å] is much shorter than that in 2 [1.659(1) Å] and
is close to that in Co(PMe3)2(BPh4) (1.602 Å) mentioned
above.10 The average Co(1)-C(1) and C(1)-O(1) distances
are 1.739(5) and 1.162(5) Å, respectively. These are com-
parable to those in other cobalt(I) carbonyl compounds, such
as [{PhB(CH2PPh2)3}Co(CO)2]9c [Co-C ) 1.729(5) and
1.744(5) Å; C-O ) 1.170(5) and 1.140(5) Å] and
[(TIMENxyl)Co(CO)]Cl9d [TIMEN ) tris[2-(3-arylimidazol-
2-ylidene)ethyl]amine; Co-C ) 1.8463(19) Å, C-O )
1.101(3) Å]. The relatively short metal-carbon and long
C-O bond lengths in 3 imply a strong back-bonding between
the cobalt center and the terminal carbonyl.14 The IR
spectrum of 3 featured a CO stretching band at 1965 cm-1,
indicating considerable Co-CO back-bonding. No resonance
broadening or shifting was observed in the 1H NMR spectrum
of 3, as predicted for an 18-valence electron diamagnetic
species. The possible mechanism of formation of 3 may
involve the initial carbonylation of 2 with the elimination
of toluene. A migratory insertion of one of the carbonyls
into the Co-C(aryl) σ bond, facilitated by a strong Co-η6
flanking ring interaction, may then occur.
to form (η5-C5H5)2Zr(CH2Ph)(η2-O2N2CH2Ph),17 (η5-C5H5)-
W(NO)(CH2SiMe3)(η2-O2N2CH2SiMe3),18 and WMe4(η2-
O2N2Me)2,19 respectively. In contrast, similar double insertion
into late-transition-metal-carbon σ bonds is rare.20 Both
terminal NO ligands in 4 feature wide Co-N-O angles
[Co(1)-N(3)-O(3) 164.7(11)°, Co(1)-N(4)-O(4) 159.8(11)°]
such that an 18-electron configuration is attained. The IR
spectrum of 4 shows N-O bands at 1660 and 1720 cm-1
consistent with its behavior as a three-electron ligand.21 No
cobalt-arene interaction was apparent. The structure of the
3,5-iPr2Ar*N(NO)O ligand in 4 resembles that of Cupferron
[PhN(NO)O]NH4, a common analytical reagent,22 and the
related derivatives of NONOates, [R2NN(NO)O]-, which are
smooth nonenzymatic releasers of nitric oxide in physiologi-
cal media.23
In conclusion, the unusual arene-stabilized cobalt(I) aryl
complex 2 has been prepared and characterized. It readily
undergoes addition/migratory insertion reactions with CO or
NO. Further exploration on the reactivity of 2 with other
small molecules and atom-transfer reagents is underway.
Exposure of 2 to a large excess of NO (1 atm) resulted in
reddish-brown crystals of [3,5-iPr2Ar*N(NO)OCo(NO)2] (4)
in 19% yield.7 Unfortunately, despite numerous attempts,
high-quality crystals could not be grown. Nonetheless, an
X-ray data set of sufficient quality to establish its structure
was obtained,8 and this is shown in Figure 4. The complex
features a four-coordinate cobalt center with distorted
tetrahedral geometry. The [3,5-iPr2Ar*N(NO)O]- anion,
formed by insertion of NO into the Co-C σ bond15 with
concomitant NO coupling and N-N bond formation, binds
cobalt as a bidentate ligand in a η2-O,O fashion. Such double
NO insertion behavior is known for early-transition-metal-
alkyl or -aryl complexes16 such as (η5-C5H5)2Zr(CH2Ph)2, (η5-
C5H5)W(NO)(CH2SiMe3)2, and WMe6, which react with NO
Acknowledgment. We are grateful to the National Science
Foundation (Grant CHE-0641020) for financial support.
B.D.E. thanks the NSERC of Canada for a postdoctoral
fellowship.
Supporting Information Available: 1H NMR spectra for 2-4
and X-ray crystallographic data (in CIF format) for 2-4. This ma-
terial is available free of charge via the Internet at http://pubs.acs.org.
IC8015782
(18) Legzdins, P.; Rettig, S. J.; Sanchez, L. Organometallics 1988, 7, 2394.
(19) Fletcher, S. R.; Skapski, A. C. J. Organomet. Chem. 1973, 59, 299.
(20) Puiu, S. C.; Warren, T. H. Organometallics 2003, 22, 3974.
(21) (a) Ford, P. C.; Lorkovic, I. M. Chem. ReV. 2002, 102, 993. (b) Hayton,
T. W.; Legzdins, P.; Sharp, W. B. Chem. ReV. 2002, 102, 935.
(22) (a) Popov, A. I.; Wendlandt, W. W. Anal. Chem. 1954, 26, 883. (b)
Wendlandt, W. W. Anal. Chem. 1955, 27, 1277. (c) Elving, P. J.;
Olson, E. C. Anal. Chem. 1955, 27, 1817.
(23) (a) Hrabie, J. A.; Keefer, L. K. Chem. ReV. 2002, 102, 1135. (b)
Wasser, I. M.; de Vries, S.; Moe¨nne-Loccoz, P.; Schro¨der, I.; Karlin,
K. D. Chem. ReV. 2002, 102, 1201. (c) Yi, G.-B.; Khan, M. A.; Richter-
Addo, G. B. Inorg. Chem. 1995, 34, 5703. (d) Schneider, J. L.; Young,
V. G., Jr.; Tolman, W. B. Inorg. Chem. 1996, 35, 5410.
(14) Hocking, R. K.; Hambley, T. W. Organometallics 2007, 26, 2815.
(15) (a) Weiner, W. P.; White, M. A.; Bergman, R. G. J. Am. Chem. Soc.
1981, 103, 3612. (b) Weiner, W. P.; Bergman, R. G. J. Am. Chem.
Soc. 1983, 105, 3922. (c) Niu, S.; Hall, M. B. J. Am. Chem. Soc.
1997, 119, 3077. (d) Niu, S.; Hall, M. B. J. Phys. Chem. A 1997,
101, 1360.
(16) McCleverty, J. A. Chem. ReV. 1979, 79, 53.
(17) Fochi, G.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Chem. Soc.,
Dalton Trans. 1986, 445.
(24) Sutton, A. D.; Nguyen, T.; Fettinger, J. C.; Olmstead, M. M.; Long,
G. J.; Power, P. P. Inorg. Chem. 2007, 46, 4809.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10207