Stabilization of high-valent terminal-oxo complexes: Interplay of d-orbital occupancy and coordination geometry

Full text of DOI:10.1021/ja953051i

J. Am. Chem. Soc. 1996, 118, 481-482
481
complex [Ru(O)PHAB] (2).10 As expected, the νMdO stretch
Stabilization of High-Valent Terminal-Oxo
Complexes: Interplay of d-Orbital Occupancy and
Coordination Geometry
Nathanael L. P. Fackler,† Songsheng Zhang,† and
Thomas V. O’Halloran*,†,‡
Departments of Chemistry and
Biochemistry, Molecular Biology and Cell Biology
Northwestern UniVersity, EVanston, Illinois 60208-3113
shifts to higher frequency upon oxidation (887 vs 935 cm-1 for
1 and 2, respectively).11 Differences in the IR spectra, however,
indicate that significant structural changes occur upon oxida-
tion: KBr pellets of 1 exhibit two amide νCdO stretches at 1670
and 1630 cm-1, but only a single amide νCdO stretch at 1704
cm-1 is evident for 2. X-ray diffraction analysis reveals that 2
is a square-pyramidal (SP) RuVI-monooxo species,10 but 1 is
a distorted trigonal-bipyramidal (TBP) RuV-oxo complex9
(Figure 1). Although a variety of dioxo-RuVI species are
known,12 2 represents the first structurally characterized example
of RuVI with a single terminal oxo group. The central Ru atom
sits 0.70 Å above the N2O2 basal plane, and the terminal oxo
occupies the apical position. The RuVIdO bond in 2 is shorter
than that in 1 and approaches that observed in a couple of
dioxo-RuVI complexes.13,14
Comparisons of the alkoxo- and amido-metal bond lengths
clearly indicate lower symmetry in 1 than in 2. The two Ru-N
bond lengths in 1 differ by 0.07 Å, and the two Ru-O bond
lengths differ by 0.1 Å, whereas the analogous parameters in 2
are indistinguishable from one another at the 3σ limit. The four
donor atoms of the PHAB ligand are significantly distorted from
planarity in 1 relative to 2, exhibiting mean deviations of the
N2O2 planes of 0.1830 and 0.03 Å, respectively. The trigonal
plane in 1, defined by Ru-N1-O1-O3 (see dotted line in
Figure 1) has a mean deviation of 0.05 Å, with Ru sitting 0.003
Å out of the plane and the axial N2-Ru-O2 subtending an
angle of 155.2(1)°. The terminal oxo occupies an equatorial
position, analogous to that reported for the only other structurally
characterized monooxo-RuV complex, Pr4N[RuV(O)(O2CO-
CEt2)2].15
ReceiVed September 5, 1995
To design highly selective catalysts for oxidation of organic
compounds, we are probing the electronic,1 thermodynamic,2,3
and geometric properties of the reactive species. Chelating
ligands that increase the stability of metal-oxo intermediates
can allow fine-tuning of chemo- and regioselectivity in the oxo-
and electron-transfer steps.4 Previous structural and mechanistic
studies of chelators containing amide donors5-7 underscore how
ligand basicity can stabilize high oxidation state metal centers;
however, here we focus on the importance of coordination
geometry. We report the unusual geometry of a novel RuV
oxidation catalyst that is formally analogous to a perferryl
species, a frequently postulated iron-oxo intermediate in
biological oxidations.8 By comparison with the related RuVI-
oxo complex, we show that the preferred coordination geometry
of these oxo-terminal complexes depends strongly on the formal
d-electron occupancy of the metal-oxo π*-orbitals. These
results have important implications for the design of ligands
that stabilize specific intermediates in catalytic reactions.
Ligands such as porphyrins and macrocyclic amides, which
impose a rigid planar array of four donor atoms, clearly stabilize
d0-d2 metal-oxo centers but may destabilize electron-rich
systems, such as the hypothetical d3 perferryl species, relative
to more flexible ligands that contain similar donor atoms. These
results also suggest a means by which subtle conformational
restraints imposed by oxo-transfer enzymes on the geometry
of metal cofactors can significantly tune the reactivity of the
metal-oxo intermediate.
While the barrier for interconversion of metal complexes
between SP and TBP geometries can be quite small,16 this is
not the case when metal-ligand multiple bonds are present.
Several differences in 1 and 2 indicate that the thermodynamic
preference for a particular five-coordinate geometry depends
on the occupancy of orbitals with significant d-character. First,
the distortion of the ligand in 1 is retained in solution and
Direct reaction of RuO4- with H4PHAB7 gives the paramag-
netic monooxoruthenium complex Pr4N[Ru(O)PHAB] (1).9
Subsequent oxidation by CeIV yields the diamagnetic monooxo
* Corresponding author: Department of Chemistry, Northwestern Uni-
versity, 2145 Sheridan Rd., Evanston, IL 60208-3113. Phone: (708) 491-
5060. Fax: (708) 491-7713.
† Department of Chemistry.
(10) (a) Anal. Calcd for [RuVI(O)PHAB], C34H24N2O5Ru: C, 63.65;
H, 3.77; N, 4.36. Found: C, 63.34; H, 3.77; N, 4.32 (Oneida). IR of a
KBr pellet: (νCdO) 1704 cm-1; (νMdO) 931 cm-1. UV-vis (CH2Cl2): λmax
(nm) (ꢀ, M-1 cm-1) 398 (3388), 458 (3436), 616 (1168). (b) Dark green
crystals of 2‚(CH3)2CO, grown from a concentrated acetone solution at -4
°C, were mounted on a glass fiber under oil (paratone N) to prevent solvent
loss. 2 crystallizes in the space group P1h, with a ) 9.259(1) Å, b ) 12.385-
(1) Å, c ) 14.143(3) Å, R ) 102.25(1)°, â ) 91.69(1)°, γ ) 103.58(1), V
) 1535.2(8), Fcalc ) 1.513 g cm-3, and Z ) 2. Data collection at -120 °C
from 2.8 e 2θ e 55° provided 5874 reflections with I > 3.00σ(I). The
structure was solved by Patterson methods (SHELXS) and refined in
TEXSAN 5.0 with 536 variables to a final R (Rw) value 0.024 (0.031).
(11) Isotopic labeling was achieved by reducing an acetonitrile solution
of 1 (100 mg, 0.12 mmol) with a substoichiometric aliquot of Ph3P under
a nitrogen atmosphere. After stirring overnight, 18O2-labeled dioxygen
(99%) was introduced. Precipitation with diethyl ether and crystallization
of the powder from acetonitrile provided partially labeled (∼34%) 1. IR
(KBr pellet): (νMdO) 885 cm-1; (νMdO*) 841 cm-1 (calcd νMdO*, 841 cm-1).
CeIV oxidation of 18O-labeled 1 in CH2Cl2, followed by crystallization of
the dark green oil from acetone, provided pure, partially labeled (∼13%)
2. IR (KBr pellet): (νMdO) 926 (941 sh) cm-1; (νMdO*) 887 cm-1 (calcd
‡ Department of Biochemistry, Molecular Biology and Cell Biology.
(1) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley-
Interscience: New York, 1988.
(2) Holm, R. H. Chem. ReV. 1987, 87, 1401-1449.
(3) Cook, G. K.; Mayer, J. M. J. Am. Chem. Soc. 1995, 117, 7139-
7156.
(4) Jacobsen, E. N.; Zhang, W.; Gu¨ler, M. L. J. Am. Chem. Soc. 1991,
113, 6703-6704.
(5) Margerum, D. W. Pure Appl. Chem. 1983, 55, 23-34.
(6) Collins, T. J. Acc. Chem. Res. 1994, 27, 279-285.
(7) MacDonnell, F. M.; Fackler, N. L. P.; O’Halloran, T. V. J. Am. Chem.
Soc. 1994, 116, 7431-7432.
(8) (a) Gunter, M. J.; Turner, P. Coord. Chem. ReV. 1991, 108, 115-
161. (b) Liu, K. E.; Wang, D.; Huynh, B. H.; Edmondson, D. E.; Salifoglou,
A.; Lippard, S. J. J. Am. Chem. Soc. 1994, 116, 7465-7466. (c) Lee, S.
K.; Fox, B. G.; Froland, W. A.; Lipscomb, J. D.; Mu¨nck, E. J. Am. Chem.
Soc. 1993, 115, 6450-6451. (d) Posner, G. H.; Cumming, J. N.;
Ploypradith, P.; Oh, C. H. J. Am. Chem. Soc. 1995, 117, 5885-5886.
(9) (a) Anal. Calcd for Pr4N[RuV(O)PHAB], C46H52N3O5Ru: C, 66.73;
H, 6.33; N, 5.07. Found: C, 66.18; H, 6.26; N, 5.02 (Oneida). IR of a
KBr pellet: (νCdO) 1670, 1630 cm-1; (νMdO) 887 cm-1. UV-vis (CH2-
Cl2): λmax (nm) (ꢀ, M-1 cm-1) 336 (3335), 420 (968), 538 (270). The
synthetic procedure is available as supporting information. (b) Black crystals
of 1 were grown by slow evaporation of an acetonitrile solution at room
temperature. 1 crystallizes in the space group P21/n, with a ) 11.981(2)
Å, b ) 11.245(2) Å, c ) 30.082(8) Å, â ) 98.90(2)°, V ) 4004(3), Fca3lc
) 1.373 g cm-3, and Z ) 4. Data collection at -120 °C from 2.8 e 2θ e
48° provided 4033 reflections with I > 3.00σ(I). The structure was solved
by Patterson methods (SHELXS) and refined in TEXSAN 5.0 with 654
variables to a final R (Rw) value 0.035 (0.036).
ν
MdO*, 876 (895 sh) cm-1).
(12) Che, C.-M.; Yam, V. W.-W. In AdVances in Inorganic Chemistry;
Sykes, A. G., Ed.; Academic Press, Inc.: San Diego, 1992; Vol. 39, pp
233-325.
(13) Perrier, S.; Kochi, J. K. Inorg. Chem. 1988, 27, 4165-4173.
(14) Dovletoglou, A.; Adeyemi, S. A.; Lynn, M. H.; Hodgson, D. J.;
Meyer, T. J. J. Am. Chem. Soc. 1990, 112, 8989-8990.
(15) Dengel, A. C.; Griffith, W. P.; O’Mahoney, C. A.; Williams, D. J.
J. Chem. Soc., Chem. Commun. 1989, 1720-1721.
(16) Holmes, R. R. Prog. Inorg. Chem. 1984, 32, 119.
0002-7863/96/1518-0481$12.00/0 © 1996 American Chemical Society

482 J. Am. Chem. Soc., Vol. 118, No. 2, 1996
Communications to the Editor
Chart 1
Figure 1. ORTEP representations of the anion 1 and the neutral
complex 2, showing 50% probability ellipsoids. Solvate molecules and
hydrogen atoms are omitted for clarity. Selected bond distances (Å)
and angles (deg) are as follows. (Left) 1: Ru-O1, 1.702(3); Ru-O2,
2.007(3); Ru-O3, 1.897(3); Ru-N1, 1.907(4); Ru-N2, 1.978(4); O2-
Ru-N2, 155.2(1), O3-Ru-O1, 117.3(1); O3-Ru-N1, 130.4(1); O1-
Ru-N1, 111.8(2). (Right) 2: Ru-O1, 1.661(1); Ru-O(2), 1.899(1);
Ru-O3, 1.895(1); Ru-N1, 1.929(2); Ru-N2, 1.923(2); O2-Ru-O3,
84.23(6); O2-Ru-N1, 82.17(6); O3-Ru-N2, 82.83(6); N1-Ru-N2,
80.52(7); O1-Ru-O3, 110.82(6); O1-Ru-O2, 113.19(7); O1-Ru-
N1, 109.99(7); O1-Ru-N2, 111.09(7); O2-Ru-N2, 135.63(6); O3-
Ru-N1, 139.12(6).
MdO center is orthogonal to the equatorial ligand plane and
antibonding with respect to the oxo. In the SP geometry, M-L
σ* contributions further destabilize the e set (dxz,dyz), which are
hybridized away from the basal ligands toward the apical
oxygen.22 By increasing one of the L-M-L angles to 180°,
as in the TBP geometry, the equatorial M-L σ* interactions
are effectively removed from the HOMO. This model is
consistent with the physical data: 2 is diamagnetic, providing
therefore is not a result of crystal packing forces. FT-IR of 1
in CH2Cl2 reveals both of the intense amide νCdO stretches
observed in the crystalline material, albeit shifted to higher
energy.17 Second, the PHAB ligand framework undergoes
an atypical distortion to maintain the TBP geometry of 1. All
other PHAB complexes characterized to date form SP com-
plexes, including 2, Ph4P[Mn(O)PHAB] (3), and the dimer
Li2[MnPHAB]2 (4).7
1
a sharp H-NMR,23 while 1 is paramagnetic [µeff (298 K) )
2.0 µB] and gives rise to an isotropic solid state EPR spectrum
[g (77 K) ) 1.98]. The dependence of the geometry of 1 on
the d-orbital occupation of M-O π*-orbitals leads us to suggest
that other d3 or d4 L4MdX complexes, where X is oxo, nitrido,
or sulfido and L4 is a macrocycle or porphyrin radical, may be
significantly more stable in TBP relative to SP or octahedral
geometries.
The stability of 1 and 2 is reflected in the M-O bond energy,
which can be estimated from reactivity comparisons.24 Both
complexes facilitate C-H bond activation and oxygen atom
transfer reactions, oxidizing benzyl alcohol to the aldehyde and
Ph3P to Ph3PdO. Notably, neither complex oxidizes styrene
to the oxide. The metal-oxo bond energies of 1 and 2 are
clearly less than the Ph3P-O bond energy of 135 kcal/mol.25
Furthermore, the air oxidation of Ph3P is catalyzed by 1,
emphasizing its unusual stability.26 Evaluations of the roles of
electronic structure and coordination geometry in these oxidation
mechanisms are underway.
The altered ligand conformation of 1 is also apparent in
pyramidalization or øN values:18 moderate distortion of one
amide (N2) is observed. The N1 amide and all of the amides
in the other SP PHAB complexes 2-4 maintain the planarity
characteristic of normal sp2 hybridization.18 The distortion in
N2 leads to an unfavorable reduction in resonance stabilization
from both the carbonyl and ligand phenyl backbone,19 evident
as a higher frequency νCdO stretch for amide N2 than for the
planar amide N1. A compensating Ru-N2 bonding interaction
cannot account for the energetic cost of this ligand distortion.
While the distortion does make N2 a stronger donor ligand,20 it
sits 0.07 Å further from the Ru center than amide N1, suggestive
of a weaker M-L bond. Thus, in spite of an unfavorable ligand
strain energy, the distorted geometry in 1 is closer to TBP than
SP.
Extended Hu¨ckel calculations21 suggest that in the absence
of ligand strain, a d3 metal-oxo center will generally prefer
TBP over SP geometry (Chart 1). This stabilization is reflected
in a lower energy for the occupied component of the dxz,dyz
pair. In both the SP and TBP geometries, the HOMO of a d3
Acknowledgment. We thank C. Stern, J. Ibers, J. Bollinger, M.
Mansuetto, O. Eisenstein, and F. MacDonnell for helpful discussions.
This work was supported by NSF (Presidential Young Investigator
Award CHE8657704), the Alfred P. Sloan Foundation, and a Camille
and Henry Dreyfus Teacher Scholar Award to T.V.O.
(17) Solution FT-IR spectra were obtained using a 0.050 mm KBr
solution cell with CH2Cl2 as solvent. Difference spectra (in CH2Cl2): (νCdO
)
Supporting Information Available: Synthetic procedures, atomic
positional parameters, intramolecular bond distances and angles,
complete ORTEP drawings, and infrared spectra for 1 and 2 (21 pages);
observed and calculated structure factors (68 pages). This material is
contained in many libraries on microfiche, immediately follows this
article in the microfilm version of the journal, can be ordered from the
ACS, and can be downloaded from the Internet; see any current
masthead page for ordering information and Internet access instructions.
1, 1713 and 1642 cm-1; 2, 1710 cm-1
.
(18) The pyramidalization term øN is defined as follows (Winkler, F.
K.; Dunitz, J. D. J. Mol. Biol. 1971, 59, 169-182): øN ) (ω2 - ω3 + π)
mod 2π, where ω2 ) O-C-N-M and ω3 ) O-C-N-C. øN maximizes
at 60° and can be understood as the point at which the central nitrogen
atom adopts a tetrahedral geometry. The following torsion angles and
Dunitz parameters are for amides numbered as N1-C1-O5 and N2-C2-
O4, respectively: 1, ω2 ) -170.8°, -173.6°, ω3 ) 8.5°, -22.7°, øN1
)
0.7°, and øN2 ) 29.1°; 2, ω2 ) 179.3°, -178.1°, ω3 ) 18.1°, -18.1°, øN1
) -18.8°, and øN2 ) 20.2°. The øN values observed for complexes 3 and
4 respectively are øN1 ) 5.4°, øN2 ) -7.9° and øN1 ) 10.8°, øN2 ) -17.8°,
øN3 ) -15.5°, øN4 ) 16.8°.
JA953051I
(22) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365-374.
(23) 1H-NMR was performed on a Varian 300 MHz FT-NMR with
benzene-d6 as solvent. Benzene backbone H, 6.7 and 8.3 ppm (q); gem
diphenyl H, 7.6 and 7.7 ppm (d), 7.0-7.1 ppm (m).
(24) Holm, R. H.; Donahue, J. P. Polyhedron 1993, 12, 571-589.
(25) Kirklin, D. R.; Domalski, E. S. J. Chem. Thermodyn. 1988, 20, 743-
745.
(26) Two CH2Cl2 solutions (25 mL) containing equal concentrations of
Ph3P, one containing 10-20 mg of 1, were stirred at room temperature
under a constant flow of air in flasks fit with water-cooled condensers.
31P-NMR (Ph4PCl as standard) showed an average turnover of 306 Ph3PdO/
Ru.
(19) Itai, A.; Toriumi, Y.; Saito, S.; Kagechika, H.; Shudo, K. J. Am.
Chem. Soc. 1992, 114, 10649-10650.
(20) Anson, F. C.; Collins, T. J.; Gipson, S. L.; Keech, J. T.; Kraft, T.
E.; Peake, G. T. J. Am. Chem. Soc. 1986, 108, 6593-6605.
(21) EH calculations were performed on idealized RuH4O models of 1
(C2V) and 2 (C4V) in CACAO v. 3.0. See: Mealli, C.; Proserpio, D. M. J.
Chem. Educ. 1990, 67, 399-402 and references therein. The trans basal
angles of the square pyramid were defined as 135°, the average of that
found in the X-ray structure of 2. M-L bond lengths were modeled after
those found for the TBP cis-dioxo complex (Ph3P)2N[O2RuCl3],13 the SP
model reflecting only the shorter of the two lengths.