Synthesis of oxorhenium(V) complexes with diamido amine ancillary ligands and their role in oxygen atom transfer catalysis

Article abstract of DOI:10.1021/ic901434u

The detailed syntheses of complexes of the form [Re(0)(X)(RNCH ₂CH₂)₂N(Me)] (X=Me, Cl, I, R=mesityl, C ₆F₅), 1-3, incorporating diamidoamine ancillary ligands are described. X-ray crystal structures f

Full text of DOI:10.1021/ic901434u

1
1058 Inorg. Chem. 2009, 48, 11058–11066
DOI: 10.1021/ic901434u
Synthesis of Oxorhenium(V) Complexes with Diamido Amine Ancillary Ligands
and Their Role in Oxygen Atom Transfer Catalysis
Yuee Feng, Joel Aponte, Paul J. Houseworth, Paul D. Boyle, and Elon A. Ison*
Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh,
North Carolina 27695-8204
Received July 22, 2009
The detailed syntheses of complexes of the form [Re(O)(X)(RNCH CH ) N(Me)] (X=Me, Cl, I, R=mesityl, C F ),
2
2 2
6 5
1
-3, incorporating diamidoamine ancillary ligands are described. X-ray crystal structures for the complexes
[Re(O)(Me)((C F )NCH CH ) N(Me)], 1a, [Re(O)(I)((C F )N CH CH ) N(Me)], 3a, and [Re(O)(I)((Mes)NCH ) N-
Me)], 3b, are reported. The geometry about the metal center in 1a is best described as a severely distorted square
6
5
2
2 2
6 5
2
2 2
2 2
(
pyramid with the oxo ligand in the apical position. In contrast, the geometry about the metal center in 3a is best
described as a severely distorted trigonal bipyramid, with the iodo ligand occupying the apical position and the diamido
nitrogens and the oxo ligand occupying the equatorial plane. The catalytic activities of these complexes for oxygen
atom transfer, OAT, from pyridine-N-oxides, PyO, to PPh were also examined. The reactions exhibited a clear
3
dependence on the diamido ligand substituent and the X ligand (Me, I, Cl) attached to the metal, with the combined
effect that electron-withdrawing substituents on the diamido ligand and poor σ donors directly attached to the metal
center increases the rate of catalytic activity. The kinetics of OAT from pyridine-N-oxides to Re were also investigated.
The reactions displayed clean first order kinetics in Re and saturation kinetics for the dependence on PyO. Changing
the PyO substrate had no effect on the saturation value, k , suggesting that the OAT reaction in these five-coordinate
sat
complexes appears to be governed by isomerization of the starting complex. Attempts to isolate a postulated Re(VII)
intermediate were not successful because of hydrolytic degradation. The product of hydrolytic degradation
-
-
[
((C F )N(H)CH CH )) NH(Me)][X], (X=ReO , or I ), 4 can be isolated, and its X-ray crystal structure is reported.
6 5 2 2 2 4
Although the Re(VII) intermediate could not be isolated, its activity in OAT reactions was probed by competition
experiments with PPh and four para-substituted triarylphosphines (p-X-Ph) P (X = OMe, Me, Cl, CF ). These
3
3
3
experiments led to a Hammett that yielded a reaction constant of F=-0.30 ( 0.01. This data suggests a positive
charge buildup on phosphorus for the OAT reaction and is consistent with the nucleophilic attack of phosphorus on an
electrophillic metal oxo.
0
are [Re(O)(hoz) (CH CN)][B(C F ) ] (hoz=2,(-2 -hydrox-
2 3 6 5 4
Introduction
1-5
yphenyl)-2-oxazoline), methyltrioxo rhenium, MeRe(O)3,
In recent years oxorhenium complexes have been shown to
be effective catalysts for oxygen atom transfer, OAT, reac-
tions. Among the complexes that are active for this reaction
6
-8
(
MTO),
and MeRe(O)(thiolate)L (L = PAr , pyridine,
3
9-20
tetramethylthiourea, R S).
These complexes have been
2
(
(
9) Wang, Y.; Espenson, J. H. Org. Lett. 2000, 2(22), 3525–3526.
*
To whom correspondence should be addressed. E-mail: eaison@ncsu.edu.
Phone: (919) 513-4376. Fax: (919) 515-8909.
1) Corbin, R. A.; Ison, E. A.; Abu-Omar, M. M. Dalton Trans. 2009, 15,
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1
911–1916.
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(
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850–2855.
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(
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(
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(
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(
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(
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(
pubs.acs.org/IC
Published on Web 11/04/2009
r 2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11059
3
1-35
Chart 1
polymerization and metathesis catalysts.
We were
attracted to them because of the ease of synthesis, and the
fact that the electronics and sterics at the amido nitrogens
could be easily varied. Oxorhenium complexes incorpor-
ating these ligands were synthesized according to the
methods outlined in Scheme 1.
Methyl rhenium complexes 1a-b were prepared by
treating a dichloromethane solution of MeRe(O)3,
MTO, with 1 equiv of PPh and an equivalent of the
appropriate ligand, and allowing the solution to stand at
room temperature overnight. This procedure, led to black
crystals of 1, which in the case of 1a were suitable for
single-crystal X-ray diffraction studies. Complex 1 is
stable in air.
2
1,22
utilized in a wide variety of reactions, including epoxidations,
23-26
3
aldehyde olefinations,
cies,
oxidations of sulfur containing spe-
andeventhereductionofperchlorates. The utility of
2
,27,28
3,29
rhenium complexes for these reactions lies in the fact that unlike
catalysts that incorporate the isoelectronic group 6 metals (Mo,
and W), that are prevalent in biological oxotransferases, the
kinetics of OAT with rhenium is facile and occurs at non-forcing
conditions.
The known OAT rhenium catalysts incorporate terminal
oxo and ancillary ligands such as thiolates, and oxazolines.
While these complexes are effective, a major limitation is that
the ancillary ligands do not allow for tuning the sterics and
electronics around the metal center so that systematic im-
provements to the catalytic activity can be made. To address
this issue, we have synthesized a series of oxorhenium com-
plexes of the form [Re(O)(X)(RNCH CH ) N(Me)] (X =
30
The chlororhenium complex Re(O)Cl(MesNCH2-
CH ) NMe (2b) was obtained as a yellow solid from
2 2
mer-Re(O)Cl (PPh ) , MeN((MesN(H)(CH ) ) , and 2
3
3 2
2 2 2
equiv of 2,6-lutidine, in refluxing ethanol for 16 h. The
synthesis of complex 2a was previously reported by
Schrock and co-workers from the reaction of [Re(O)Cl ]-
4
3
3
[
have found, however, that 2a, an air-stable green solid,
NBu ] and the ligand MeN(C F NH(CH ) ) . We
4 6 5 2 2 2
can be more conveniently prepared by stirring a suspen-
sion of mer-Re(O)Cl (PPh ) and MeN(C F N(H)-
2
2 2
Me, Cl, I; R = mesityl, C F ; Chart 1), that incorporate
3
3 2
6 5
6
5
(CH ) ) at room temperature for 5 days.
2 2 2
diamidoamine ancillary ligands.
Finally the green iodorhenium complexes 3were obtained
by stirring a suspension of purple Re(O) I(PPh ) , and
The design of these complexes allows for a systematic
investigation of the catalytic activity as the electronics and
sterics of the X substituent on the metal and the R substituent
on the diamido ligand are varied. In this article we examine
the kinetics of OAT for six oxorhenium complexes incorpor-
ating diamidoamine ancillary ligands. We describe in detail
the syntheses of these complexes, examine the structural
consequences of varying the electronics at the metal center,
investigate in detail the mechanism for OAT from pyridine-
N-oxides, and compare the catalytic activities of these com-
plexes for OAT from pyridine-N-oxides to PPh3.
2
3 2
1
1
equiv of the appropriate ligand, in dichloromethane for
2 h. These air-stable complexes were isolated as powders
after filtration to remove any unreacted starting material,
concentration of the filtrate, and precipitation with hexanes.
1 13
All complexes were fully characterized by H, C or
19
F
NMR and elemental analysis. Complexes 1a, 3a, and 3b
were also characterized by X-ray crystallography.
Crystallographic Studies. As mentioned above, crystals
of 1a suitable for X-ray crystallography were obtained by
allowing the reaction mixture to stand at room tempera-
ture for 12 h. Crystals of 3a and 3b were obtained by slow
diffusion of pentane into a concentrated CH Cl solution
Results and Discussion
Syntheses of Complexes. Diamidoamines have
been utilized as ligands for early transition metal olefin
2
2
at room temperature. The crystal structure of 2a was
3
3
previously reported. Interesting structural differences
are apparent when complex 1a is compared to the halide
containing complexes 2a, 3a, and 3b.
(
21) Zhou, M. D.; Zhao, J.; Li, J.; Yue, S.; Bao, C. N.; Mink, J.; Zang,
S. L.; Kuehn, F. E. Chem.;Eur. J. 2007, 13(1), 158–166.
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Rosch, N. Angew. Chem., Int. Ed. 1998, 37(16), 2211–2214.
(
The geometry about the metal center in 1a is best
described as a severely distorted square pyramid with
the oxo ligand in the apical position (Figure 1). The
(
(
23) Kuhn, F. E.; Santos, A. M. Mini-Rev. Org. Chem. 2004, 1(1), 55–64.
24) Pedro, F. M.; Hirner, S.; Kuhn, F. E. Tetrahedron Lett. 2005, 46(45),
7
777–7779.
25) Santos, A. M.; Romao, C. C.; Kuhn, F. E. J. Am. Chem. Soc. 2003,
25(9), 2414–2415.
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C. C.; Kuhn, F. E. Chem.;Eur. J. 2004, 10(24), 6313–6321.
27) Schultz, B. E.; Hille, R.; Holm, R. H. J. Am. Chem. Soc. 1995, 117(2),
27–828.
28) Arterburn, J. B.; Perry, M. C.; Nelson, S. L.; Dible, B. R.; Holguin,
M. S. J. Am. Chem. Soc. 1997, 119(39), 9309–9310.
29) Abu-Omar, M. M.; Mcpherson, L. D.; Arias, J.; Bereau, V. M.
Angew. Chem., Int. Ed. 2000, 39(23), 4310.
˚
Re-O bond length (1.685(4) A) is consistent with the
(
36-39
assignment of a triple bond.
˚
The Re atom lies
0.7184(2) A above the plane made by the atoms N1, N2,
N2a, and C1.
1
(
(
In contrast, the geometry about the metal center in 3a is
best described as a severely distorted trigonal bipyramid
(Figure 2), with the iodo ligand occupying the apical
position and the atoms, N2, N3 and O1, occupying the
equatorial plane. The angles for the atoms in the equatorial
8
(
(
(
(
30) Enemark, J. H.; Cooney, J. J. A. Chem. Rev. 2004, 104(2), 1175–1200.
31) Cochran, F. V.; Schrock, R. R. Organometallics 2001, 20(11), 2127–
2
129.
32) Cochran, F. V.; Hock, A. S.; Schrock, R. R. Organometallics 2004, 23
4), 665–678.
33) Cochran, F. V.; Bonitatebus, P. J.; Schrock, R. R. Organometallics
000, 19(13), 2414–2416.
34) Lopez, L. P. H.; Schrock, R. R.; Bonitatebus, P. J. Inorg. Chim. Acta
006, 359(15), 4730–4740.
35) Guerin, F.; Mcconville, D. H.; Vittal, J. J.; Yap, G. A. P. Organo-
metallics 1998, 17(23), 5172–5177.
(
(36) Herrmann, W. A.; Roesky, P. W.; Wang, M.; Scherer, W. Organo-
metallics 1994, 13(11), 4531–4535.
(37) Herrmann, W. A.; Ding, H.; Kuhn, F. E.; Scherer, W. Organome-
tallics 1998, 17(13), 2751–2757.
(38) Herrmann, W. A.; Correia, J. D. G.; Rauch, M. U.; Artus, G. R. J.;
Kuhn, F. E. J. Mol. Catal. A: Chem. 1997, 118(1), 33–45.
(39) Romao, C. C.; Kuhn, F. E.; Herrmann, W. A. Chem. Rev. 1997, 97
(8), 3197–3246.
(
(
2
2
(
(

1
1060 Inorganic Chemistry, Vol. 48, No. 23, 2009
Feng et al.
Scheme 1
plane, O1-Re1-N2, 120.04(12); O1-Re1-N3,
compared with 1b, [Re(O)(Me)((Mes)NCH CH ) N(Me)].
2
With 1 mol % catalyst, PPh is completely converted to
3
OPPh by 3a in less than 10 min. In contrast, less than
3
2 2
1
19.63(12); N2-Re1-N3, 118.44(11), are all consistent
with a trigonal bipyramidal geometry. In comparison, for
complex 1a, these angles are O1-Re1-N2, 113.08(9);
O1-Re1-N2a, 113.08(9); N2-Re1-N2a, 133.43(17).
The I1-Re1-N1 angle in 3a is 163.23(7)ꢀ which is
distorted by 17ꢀ from the idealized angle in a trigonal
bipyramid. However, for complex 1a this distortion is
significantly more severe (C1-Re1-N1, 143.36(18)). The
crystal structure for 3b (Supporting Information), which
contains mesityl substituents on the diamido ligand is
very similar to 3a, and the geometry about the metal
center can again be described as a severely distorted
trigonal bipyramid. Thus the preference for a square
pyramidal geometry versus a trigonal bipyramidal geo-
metry seems to arise from the electronics of the X ligand
directly attached to the metal center and not from the
diamido substituents. The preference for a trigonal bipyr-
amidal geometry in 3a and 3b is due to the increased
stability in a trigonal bipyramidal crystal field that occurs
when an electronegative atom (Cl, I) is placed in the apical
position. The methyl complex, 1a, encounters a larger
crystal field splitting when the high-field ligand, methyl, is
in the basal plane of a square pyramid as opposed to in the
10% of PPh is converted to OPPh after 6 h by 1b under
3
3
the same reaction conditions. Thus the combined effect of
electron withdrawing substituents on the diamido ligand
and poor σ donors directly attached to the metal center
appears to dramatically increase the rate of catalytic
activity. This is exemplified further when complex 3a,
[Re(O)(I)((C F )NCH CH ) N(Me)], is compared with
6
5
2
2 2
3b, [Re(O)(I)((Mes)NCH CH ) N(Me)] and complex 1a,
2
2 2
[Re(O)(Me)((C F )NCH CH ) N(Me)] is compared with
6
5
2
2 2
1b, [Re(O)(Me)((Mes)NCH CH ) N(Me)]. Complex 3a
2 2
2
converts 100% of PPh to OPPh in 10 min while this
3
3
conversion is achieved with 3b in 250 min. Similarly, 10%
of PPh is converted to OPPh within 2 min by 1a, while
complex 1b requires 350 min to convert the same amount
3
3
of PPh . Thus the general trend from the catalytic data is
3
3a ≈ 2a > 1a >3b >2b > 1b. From the data, changing
the substituent on the diamido ligand appears to have a
more dramatic effect on the rate of catalysis.
A drawback of this catalytic system especially for the
halide complexes, 2-3, is that the complexes are suscep-
tible to hydrolytic degradation. This is seen in Figure 3
when the halo complexes 2a and 3a are compared. While
both complexes appear initially to perform at approxi-
mately the same rate, after 10 min 3a reaches its maximum
conversion (≈ 75%). In fact, the maximum conversion for
both 3a and 2a varies depending on the amount of water
4
0
apical position in the TBP structure.
Catalytic OAT. To investigate the effect of substituents
on catalytic activity in OAT reactions we examined the
reactivity of catalyst 1-3 for the catalytic OAT from
pyridine-N-oxides to PPh according to eq 1.
3
present in the solvent. An ESI-MS of the reaction mixture
, and
catalyst
-
PPh3 þ PyO
s
f
OPPh3 þ Py
ð1Þ
Reactions were performed at room temperature in
after a catalytic run revealed the presence of ReO
protonated ligand, ((C F )N(H)CH CH ) NH(Me)).
4
6
5
2
2 2
Mechanism of OAT. The generally accepted mechan-
ism for transition metal catalyzed OAT usually involves
the formation of a high-valent transition metal oxo
intermediate which subsequently reacts with an oxygen
CD Cl (1 mL) with 1 mol % catalysts (0.0054 mmol),
2
PPh (0.54 mmol), and PyO (0.65 mmol). The disappear-
3
ance of PPh and the appearance of OPPh was mon-
3
itored by P NMR. As shown in Figure 3, there is a clear
dependence in OAT reactions on substituents on the
diamido ligand and the X ligand (Me, I, Cl) attached
to the metal center. This dramatic effect is seen when
2
3
3
1
3
,11-13,19,41
atom acceptor.
As depicted in Figure 4 the
catalytic cycle can be divided into two halves; one-half
involves oxidation of the metal center by an oxygen
atom donor, while the other half involves oxidation of
the substrate by a high-valent metal oxo intermediate.
complexes 3a, [Re(O)(I)((C F )NCH CH ) N(Me)], are
2 2
6
5
2
(
40) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions
(41) Arias, J.; Newlands, C. R.; Abu-Omar, M. M. Inorg. Chem. 2001, 40
(9), 2185–2192.
in Chemistry; John Wiley and Sons: New York, 1999.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11061
Figure 3. Catalytic OAT with catalysts 1-3 for the reaction of PPh
with PyO according to the equation 1. Reactions were performed with 1
mol % catalyst (0.0054 mmol) based on PPh (0.54 mmol) and 1.2 equiv.
of PyO (0.65 mmol) in CD Cl (1 mL) at room temperature. Reactants
and products were monitored by P NMR spectroscopy. Conversion %
refers to the relative ratio of OPPh :PPh
3
3
2
2
Figure 1. X-ray single crystal structure of Re(O)(Me)((C
6 5 2
F )NCH -
31
CH
2
)
2
N(Me)], 1a. Thermal ellipsoids are 50%. Selected bond distances
A) and angles (deg). Re1-O1, 1.685(4); Re-N2, 1.972(3); Re-C1,
.122(5); Re-N1, 2.162(4); O1-Re1-N2, 113.08(9); N2-Re1-N2a,
33.43(17); O1-Re1-N1, 112.43(17); N2-Re1-N1, 78.70(9), C1-Re1-N1,
43.36(18); O1-Re1-C1, 104.20(18).
3
3
.
˚
(
2
1
1
Figure 4. Simplified mechanism for OAT with oxorhenium complexes.
Figure 2. X-ray single crystal structure of Re(O)(I)((C
6 5
F )-
NCH CH N(Me)], 3a. Thermal ellipsoids are 50%. Selected bond
2
2 2
)
˚
distances (A) and angles (deg). Re1-O1, 1.691(2); Re1-N2, 1.950(3);
Re1-I1, 2.6954(3); Re1-N1, 2.127(3); O1-Re1-N2, 120.04(12);
O1-Re1-N3, 119.63(12); N2-Re1-N3, 118.44(11); N2-Re1-N1,
7
9.39(11); N3-Re1-N1, 79.84(11); O1-Re1-I1, 99.66(8); N1-Re1-I1,
163.23(7).
The observation that the catalytic reaction is more facile for
catalysts with electron-withdrawing substituents on the
diamido ligand, and poor σ donor X ligands suggests that
for this system the substrate oxidation/metal reduction step
is turnover limiting.
To investigate the kinetics of OAT in these complexes
and to examine the effects that substituents have on both
halves of the catalytic cycle, we investigated in detail the
mechanism for both the OAT from pyridine-N-oxide and
its derivatives, and the OAT from the putative Re(VII)
intermediate to PPh3.
Figure 5. Typical absorbance versus time plot for the reaction of 2a with
PyO. Reactions were performed at 298 K in CH CN at 368 nm. Condi-
tions were [Re] = 0.3 mM, [PyO] =0.05 M. The kobs valueswere obtained
by non-linearfitting ofAbs368 versus timeto the equation: Abs = Abs
Abs - Abs ) exp(-kobst).
3
t
¥
þ
(
0
¥
Scheme 2
OAT from Pyridine-N-oxides to Re(V). The reactions of
a (Scheme 2) with pyridine-N-oxide were investigated under
2
pseudo-first-order conditions using UV-vis spectroscopy.
The progress of the reaction was monitored at 368 nm,
where the rhenium(V) oxo-chloro complex, 2a, absorbs

1
1062 Inorganic Chemistry, Vol. 48, No. 23, 2009
Feng et al.
but the product does not (see Supporting Information).
The disappearance of 2a monitored at 368 nm displayed
clean first order kinetics in Re (Figure 5). A plot of kobs
versus [PyO] revealed saturation kinetics for the depen-
dence on PyO (Figure 6).
Saturation kinetics in PyO is consistent with several
kinetically indistinguishable mechanisms that are depicted in
Scheme 3. From the crystal structure for 2a and 3a the
metal-halide bonds are particularly long. This suggests
that these bonds may be weak and thus a plausible
mechanism (Scheme 3A) could involve dissociation of
the halide ligand to produce a cationic 14 electron inter-
mediate which quickly reacts with PyO to produce the
Re(VII) dioxo species.
Two alternative mechanisms (Scheme 3B and Scheme 3C)
involve isomerization of the starting complex prior to
oxidation by PyO. In the mechanism depicted in
Scheme 3B, the amine ligand dissociates from the metal
in a prior equilibrium step to form a 14-electron inter-
mediate, which subsequently reacts with PyO to form the
products. In Scheme 3C, the complex rearranges to create
an open coordination site cis to the Re-O bond. As
described earlier, the solid state structures described for
1a, 2a, 3a, and 3b are characterized by different isomers
depending on the ligands attached to the metal. Thus,
geometrical isomerization prior to OAT to Re by PyO is a
reasonable hypothesis.
The last mechanism in Scheme 3 (Scheme 3D) is associa-
tive, involving attack of the PyO ligand cis to Re-O bond to
form, in a prior equilibrium step, an adduct of PyO where
this ligand is cis to the oxo and the Cl ligand is trans. This is
then followed by OAT to form Re(VII).
To investigate these mechanisms further, we compared
the kinetic behavior of five different para-substituted
pyridine-N-oxides, 4-X-PyO, (X =Me, H, phenyl, CN,
OMe). Changing the PyO substrate would have an effect
on the saturation value, k , in Scheme 3D, as the saturation
sat
value is governed by k . However, changing the substrate
8
would not affect the saturation values k , k , andk according
1
3
5
to Schemes 3A-C. As shown in Table 1 changing the para
substituent on the PyO does not affect the saturation value.
Thus the data precludes mechanism Scheme 3D.
Figure 6. PyO dependence for the reaction of PyO with 2a in CH
98 K. Conditions were [Re] = 0.3 mM, [PyO] = 0.015 M-0.2 M. Data
was fit with non-linear least-squares fitting to an equation describing
saturation in [PyO]: kobs = k [PyO]/(k þ [PyO]).
3
CN at
In addition, the temperature dependence of the rate
constant ksat was also investigated from 288 to 308 K.
2
‡
0
00
From this analysis the entropy of activation, ΔS , was
Scheme 3

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11063
Table 1. Comparison of Saturation Rate Constants for the Reaction of 2a with
para-Substituted Pyridine-N-oxides for the Reaction
Scheme 4
a
-1
substrate
ksat (s
)
PyO
0.030(1)
0.030(1)
0.021(2)
0.039(8)
0.034(6)
CN-PyO
Ph-PyO
Me-PyO
OMe-PyO
a
Saturation rate constants obtained by non-linear fits to the equa-
0
tion: rate=-d[Re]/dt={ksat[Re][PyO]}/{K þ {pyO]}.
Figure 8. X-ray single crystal structure [(C
Me)][ReO ], 4. Thermal ellipsoids are 50%. The structure exhibits a
compositional disorder in ReO
6 5 2 2 2
F )(NHCH CH ) HN-
(
4
-
-
-
4
and I . The I ion is not shown.
based on mechanism C that is consistent with the experi-
0
mental data is depicted in eq 2, where ksat=k and K =
5
k
5/k6.
-
rate ¼ -
d½Reꢀ
dt
¼
ksat½Reꢀ½PyOꢀ
ð2Þ
0
K þ ½PyOꢀ
ksat ¼ k5
Figure 7. Comparison of saturation rate constants, ksat, between 2a
and 3a.
0
K ¼ k-5=k6
found to be negative (-16.7 eu) while the enthalpy of
‡
activation, ΔH was positive (14.4 kcal/mol.K). These
activation parameters are not consistent with the disso-
OAT from Re(VII) to PPh . As mentioned above the
3
traditional mechanism for OAT involves initial oxidation
of the metal to Re(VII) by an oxygen atom donor,
followed by the transfer of an oxo from this high-valent
4
2
ciative pathway depicted in Scheme 3A. Further, as
shown in Figure 7, the rate constant at saturation (ksat)
for 2a is about 8 times larger than for 3a, this is again not
3
,11-13,19,27,41,43
Re intermediate to an oxygen atom acceptor.
-
consistent with Scheme 3A as one would expect I , the
better leaving group, to dissociate more readily than Cl
Despite several attempts the postulated Re(VII) inter-
mediate could not be isolated in these complexes as the
hydrolytic degradation of this intermediate quickly
ensued. As an illustration, when 3a is treated with
4-cyanopyridine-N-oxide, a color change from green to
red quickly takes place. However, isolation of the red
-
if dissociation of the halide ligand were the operational
mechanism.
Thus the mechanisms depicted in Schemes 3B and
Scheme 3C are the only viable mechanisms. While we
prefer mechanism C, primarily because it avoids a high
energy 14-electron intermediate, this mechanism cannot
be ruled out with the available data. In any event, the
saturation kinetics of the PyO substrate in the OAT
reaction in these five-coordinate complexes appears to
be governed by isomerization of the starting complex.
This is, in contrast to the six-coordinate complexes
product and crystallization by slow diffusion of Et O into
2
a concentrated acetonitrile solution of this complex at room
temperature results in the formation of [(C F )(N(H)-
CH CH ) NH(Me)][X], (X=ReO , or I ), 4 (Scheme 4).
6
5
-
-
2
2 2
4
The X-ray structure (Figure 8) confirms the assignment
of 4, although the structure exhibits a compositional
-
-
1
19
disorder in ReO4 and I . H and F NMR data along
with elemental analysis are also consistent with the as-
signment for this molecule. Similarly, the treatment of
complex 2a and 3b with pyridine-N-oxide results in an
initial color change from green to red, and then over
[
Omar and co-workers, where saturation is governed by
Re(O)(hoz) (CH CN)][B(C F ) ], reported by Abu-
2 3 6 5 4
3
the oxidation from Re(V) to Re(VII). A rate equation
(
42) Inorganic and Organometallic Reaction Mechanisms; Atwood, J. D,
Ed.; VCH: New York, 1997; p 13.
(43) Abu-Omar, M. M. Comments Inorg. Chem. 2003, 24(1), 15–37.

1
1064 Inorganic Chemistry, Vol. 48, No. 23, 2009
Feng et al.
Scheme 5
3
metal as observed by Abu-Omar and co-workers. The data
presented suggests that the catalytic OAT reactions presented
proceed via the generation of an electrophillic high-valent Re
oxo species. The catalytic efficiency of these complexes is
thwarted, however, by the high reactivity of this species and
its susceptibility to hydrolytic degradation.
Experimental Section
Reagents and Instrumentation. Ligands (MesNHCH
2
CH
CH ) NMe and complexes Re-
2 2
) -
4
6
NMe and (C
(
33
6
F
5
NHCH
2 2 2
3
CH NMe and Re(O)
3
47
O)Cl(C NCH
6
F
5
2
2
)
2
2
I(PPh
3
)
2
were pre-
pared as previously reported. Other reagents were purchased
from commercial sources and used as received. Solvents were
degassed and purified with a solvent purification system
1
13
31
19
(
spectra were recorded on a Varian Mercury 400 MHz or a
MBraun Inc.) prior to use. H and C, P and F NMR
1
13
Varian Mercury 300 MHz spectrometer. All H and C NMR
spectra were referenced against tetramethylsilane using reso-
nances due to the residual protons in the deuterated solvents or
the C resonances of the deuterated solvents. P NMR spectra
were obtained on a Varian spectrometer operating at 162 MHz
Figure 9. Hammett Plot obtained from competition experiments with
3 3
para-substituted triaryl phosphines (p-X-Ph) P (X = OMe, Me, Cl, CF )
asdepictedinScheme4. The ratio ofproductswas obtainedbyintegration
of the P NMR spectrum for each experiment.
3
1
13
31
1
longer periods of time to yellow. The H NMR spectrum
of these solutions reveals the presence of free ligand.
Although the Re(VII) intermediate could not be iso-
lated, its activity in OAT reactions was probed by com-
19
3 4
and referenced against external 85% H PO . F NMR spectra
were obtained on a Varian spectrometer operating at 377 MHz
and referenced against the external standard C F (163 ppm).
6
6
Unless otherwise noted, NMR spectra were acquired at room
temperature. Elemental analyses were performed by Atlantic
Microlabs, Inc. UV-vis spectra were recorded on a Cary 100
Bio spectrophotometer equipped with a thermostatted 6 ꢁ 6
multicell peltier. Mass spectrometry was performed by the NC
State Mass Spectrometry Facility. X-ray crystallography was
performed at the X-ray Structural Facility of North Carolina
State University. Data fitting was done using KaleidaGraph 4.0
software.
petition experiments with PPh and four para-substituted
3
triarylphosphines (p-X-Ph) P (X=OMe, Me, Cl, CF ) as
3
3
depicted in Scheme 5. Reactions were performed in
CD CN at room temperature with the ratio of Re/PyO/
3
PPh :(p-X-Ph) P = 1:20:100:100. The ratio of OPPh /
3
3
3
3
1
(
p-X-Ph) PO was determined by integration of the
3
P
NMR spectrum for each experiment. From the relative
ratios of the products a Hammett plot (Figure 9) that
yielded a reaction constant of -0.30 ( 0.01 was obtained.
This data suggests a positive charge buildup on phos-
phorus for the OAT reaction and is consistent with the
nucleophilic attack of phosphorus on an electrophillic
Re(O)Me(C
(262.3 mg, 0.802 mmol), (C F NHCH CH ) NMe (326.7 mg,
6 5 2 2 2
F NCH CH ) NMe (1a). Triphenylphosphine
6
5
2
2 2
0.802 mmol), and methyltrioxorhenium (200 mg, 0.802 mmol)
were added sequentially to a small reaction vial with about 2 mL
of dichloromethane. The product was allowed to crystallize for
1
1
3,44,45
16 h to yield purple crystals (138 mg, 27.6%). H NMR
metal oxo.
(
acetone-d
NCH CH
.83 (m, 2H, NCH
(acetone-d , δ): -151.15 (m, 4F, phenyl-F), -164.573 (m, 2F,
6
, δ): 4.47 (m, 2H, NCH
N), 3.51(s, 3H, N-CH ), 3.32 (m, 2H, NCH
CH N), 2.21 (s, 3H, Re-CH
2
CH
2
N), 3.77(m, 2H,
CH N),
). F NMR
2
2
3
2
19
2
Conclusion
2
2
2
3
We have demonstrated that oxorhenium complexes of the
form [Re(O)(X)(RNCH CH ) N(Me)] (X=Me, Cl, I, R=
6
phenyl-para-F), -167.07 (m, 4F, phenyl-F). Elemental Analysis:
Theory (C: 32.53; N: 6.32; H: 2.12), Found (C: 32.63; N: 6.35; H:
2
2 2
mesityl, C F ), incorporating diamidoamine ancillary
6
5
2
.07).
Re(O)Me(MesNCH
420.9 mg, 1.6 mmol), (MesNHCH CH ) NMe (567.5 mg,
ligands, can be effectively tuned for OAT reactions by vary-
ing the electronics of the amido substituents and the nature of
the X ligand attached directly to the metal. Catalytic OAT is
favored by electron-withdrawing substituents on the amido
nitrogens and poor σ donors attached directly to the metal.
The rate of OAT in these complexes is governed by isomer-
ization of the metal complex rather than oxidation of the
2
2 2
CH ) NMe (1b). Triphenylphosphine
(
2
2 2
1.6 mmol), and methyltrioxorhenium (400 mg, 1.6 mmol) were
added sequentially to a small reaction vial with about 2 mL of
dichloromethane. The product was allowed to crystallize for
1
1
6 h to yield gray crystals (384 mg, 42.1%). H NMR (CD Cl ,
2
2
(46) Hultzsch, K. C.; Hampel, F.; Wagner, T. Organometallics 2004, 23
(
44) Pestovsky, O.; Bakac, A. J. Am. Chem. Soc. 2003, 125(48), 14714–
4715.
45) Seymore, S. B.; Brown, S. N. Inorg. Chem. 2000, 39(2), 325–332.
(11), 2601–2612.
1
(47) Kennedy-Smith, J. J.; Nolin, K. A.; Gunterman, H. P.; Toste, F. D.
J. Am. Chem. Soc. 2003, 125(14), 4056–4057.
(

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11065
Table 2. Selected Crystallographic Data and Collection Parameters for Re(O)Me(C
6
F
5
NCH
2
CH
2
)
2
NMe (1a), Re(O)I(C
6
F
5
NCH
2
CH
2
)
2
NMe (3a), and Re(O)I-
-
-
(
MesNCH
2
CH
2
)
2
NMe (3b), and [(C
6
F
5
)(N(H)CH
complex 1a
ORe
2
CH
2
)
2
NH(Me)][X], (X = ReO
4
, or I ), 4
complex 3a
complex 3b
11IN ORe
complex 4
empirical formula
formula wt
crystal system
space group
C
18
F
10
H
14
N
3
C
680.64
23
H
33IN
3
ORe
C
17
F
10
776.38
H
3
17 14 10 0.0784 3
C H F I N O3.6864Re0.9216
664.51
orthorhombic
Pnma
13.2914(4)
22.0434(5)
6.7339(2)
1972.95(9)
4
690.90
monoclinic
Cc
22.5189(5)
7.4115(2)
12.6984(3)
2028.73(9)
4
orthorhombic
Pnma
triclinic
P1
˚
a, A
13.0515(5)
21.5677(9)
8.4982(3)
2392.17(16)
4
6.8210(3)
8.1645(4)
19.3225(10)
1020.13(9)
4
˚
b, A
˚
c, A
3
V (A )
˚
Z
-3
Dcalcd, g cm
2.237
1.929
2.528
2.262
crystal size (mm)
R1, wR2 {I > 2σ(I)}
GOF
0.25 ꢁ 0.23 ꢁ 0.06
0.0298, 0.0538
0.950
0.30 ꢁ 0.12 ꢁ 0.08
0.0443, 0.1263
1.099
0.12 ꢁ 0.08 ꢁ 0.06
0.0269, 0.0595
1.022
0.17 ꢁ 0.05 ꢁ 0.02
0.0311, 0.0625
1.085
δ): 6.86 (s, 4H, Mes-meta-H), 4.37 (ddd, 2H, J=4.4 Hz, 11.7 Hz,
bottom flask with 40 mL of benzene. The mixture was allowed to
reflux for 12 h. After cooling to room temperature, the solution
was filtered, and the solvent was removed under reduced
pressure. The resulting solid was dissolved in methylene chlor-
ide, and hexanes was added to precipitate the product. The
2
CH
NCH
NCH
3.4 Hz, NCH CH N), 3.56 (dd, 2H, J=5.9 Hz, 12.8 Hz, NCH -
2
2
2
2
N), 3.40 (s, 3H, N-CH
3
), 3.10 (dd, 2H, J=3.7 Hz, 11.0 Hz,
2
CH
CH
2
N), 2.69 (ddd, 2H, J = 6.2 Hz, 11.7 Hz, 22.7 Hz,
N), 2.39 (s, 3H, Re-CH ), 2.28 (s, 6H, Mes-CH ),
2
2
3
3
1
3
2
.25 (s, 6H, Mes-CH ), 1.84 (s, 6H, Mes-CH ). C NMR
, δ): 136.39, 134.45, 133.89, 129.14, 71.98, 69.86,
product was filtered and washed with ether to collect 150 mg of
3
3
1
yellow-greenish solid (yield 40%). H NMR (CD
(
CD
Cl
2 2
2
Cl
2
, δ) 6.88
(s, 2H, Mes-meta-H); 6.85 (s, 2H, Mes-meta-H); 4.03 (m, 2H,
-1
49.36, 21.03, 18.90, 10.91. IR (KBr pellet) 958 cm (νReꢂO).
Elemental Analysis: Theory(C: 50.68; N: 7.39; H: 6.38), Found-
NCH CH N); 3.25 (s, 3H, N-CH ); 3.09 (m, overlap, 6H,
2
2
3
(
C: 50.14; N: 7.35; H: 6.44). UV/vis (CH Cl solution): λ
(ε)=
NCH CH N); 2.34 (s, 6H, Mes-CH ); 2.27 (s, 6H, Mes-CH );
2
2
max
2
2
3
3
1
). C NMR (CD
3
3
27 (1064).
Re(O)Cl(C
1.97(s, 6H, Mes-CH , δ): 135.18, 134.98,
134.85, 129.03, 72.89, 66.08, 21.01, 20.11, 19.59. IR (KBr pellet)
933 cm (νReꢂO). m/z (ESI) 680.1264 ([MþH] . C23
N OI requires 680.1271).
3
3
Cl
2 2
6
F
5
NCH
2
CH
NHCH
2
)
2
NMe (2a). 2,6-Lutidine (2.0 mL,
-
1
þ
17.23 mmol), (C
and Re(O)Cl (PPh ) (1.0 g, 1.2 mmol) were added to a 100 mL
6
F
5
2
CH NMe (424.3 mg, 1.2 mmol),
2
)
2
H
34Re-
3
3 2
-
-
round-bottom flask with 60 mL of ethanol and stirred for 5 days.
The resulting green solid was filtered and washed with excess
diethyl ether to yield 638 mg product (77.6%). This complex
was prepared by different route previously by Schrock and
[(C F )N(H)CH CH ) NH(Me)][X], (X=ReO , or I ) 4. A
6
5
2
2 2
4
100 mL round-bottom flask was charged with Re(O)I-
(C NCH CH NMe (3a) (323 mg, 0.416 mmol), 4-cyano-
pyridine-N-oxide (55 mg, 0.458 mmol), and CH CN (40 mL),
6
F
5
2
2 2
)
3
1
co-workers, (Organometallics 2000, 19, 2414-2614). H NMR
and a homogeneous green solution was formed. The mixture
was stirred at room temperature overnight, and color changes
from green to yellow to brown and dark red were observed
within 1 h. The solution was concentrated to 10 mL, and diethyl
ether (30 mL) was added to form a red precipitate. A 220 mg red
(
CD
CH
vis (CH Cl solution): λ
max
2
Cl
N); 3.41 (s, 3H, N-CH
(ε)=330 (866).
2
, δ) 4.18 (m, 2H, NCH
2
CH
2
N); 3.48 (m, 2H, NCH
2
-
2
3
); 3.27 (m, 4H, NCH
CH N); UV/
2 2
2
2
Re(O)Cl(MesNCH CH ) NMe (2b). 2,6-Lutidine (2.0 mL,
2
2 2
1
7.23 mmol), (MesNHCH
2
CH
and Re(O)Cl (PPh (1.0 g, 1.2 mmol) were added to a 100 mL
2
)
2
NMe (424.3 mg, 1.2 mmol),
solid was collected through filtration and washed with diethyl
1
3
)
2
ether (76% yield). H NMR (CD
3
CN, δ) 7.26 (br, 1H, NH-
)N(H)); 3.66 (m, 4H, N(H)CH ); 3.45
CH ); 3.33 (m, 2H, (C )N(H)CH
CN, δ): -160.14
3
round-bottom flask with 60 mL of ethanol and refluxed for 16 h.
The resulting yellow solid was filtered and washed with excess
diethyl ether to yield 638 mg product (90.2%). H NMR
(Me)); 4.66 (m, 2H, (C
(m, 2H, (C )N(H)CH
CH ); 2.97 (d, 3H, NH(CH
(m, 4F), -166.85 (m, 4F), -173.22 (m, 2F).
6
F
5
2
F
6 5
2
2
6
F
5
2
-
1
19
2
3 3
); F NMR (CD
(
2 2
CD Cl , δ) 6.87 (s, 2H, Mes-meta-H), 6.85 (s, 2H, Mes-meta-
H), 3.95 (m, 2H, NCH CH N), 3.32 (s, 3H, N-CH ), 3.19
Kinetics. Equal volumes of solutions of 2a in acetonitrile (0.6 mM)
and PyO (0.02-0.4 M) were mixed in a UV-vis cell at 298 K to give
reaction solutions with half the loaded concentrations (30 μM in Re
and 0.01-2 M in PyO. The decrease in absorbance of 2a was
monitored at 368 nm under these pseudo-first order conditions. The
2
2
3
(
m, 6H, NCH CH N), 2.30 (s, 6H, Mes-CH ), 2.25 (s, 6H, Mes-
2 2 3
1
3
3 3 2 2
CH ), 1.99 (s, 6H, Mes-CH ). C NMR (CD Cl , δ): 135.11,
1
34.77, 129.30, 129.08, 71.87, 66.619, 57.60, 19.50, 19.35. UV/vis
(
(
CH Cl solution): λ (ε) = 340 (1460). m/z (ESI) 588.1923
2
2
þ
max
[MþH] . C23
H
34ReN
3
OCl requires 588.1915).
k
obs values were obtained by non-linear fitting of Abs368 versus time to
the equation: Abs =Abs - Abs ) exp(-kobst).Plotsofkobs
þ (Abs
versus [PyO] showed saturation kinetics. Fits of the data to the
appropriate rate law, eq 2, yielded a first order rate constant (k
Re(O)I(C F NCH CH ) NMe (3a). Iododioxobis(triphenyl-
2
t
¥
0
¥
6
5
2 2
phosphine)rhenium (314 mg, 0.35 mmol) and (C
CH NMe (251 mg, 0.558 mml) were added to a 10 mL round-
bottom flask with 2 mL of CH Cl . The dark purple mixture was
6 5 2
F NHCH -
2
)
2
)
5
0
and an apparent equilibrium constant (K ). Details for the rate law
derivation are in the Supporting Information.
2
2
allowed to stir for a day at room temperature and a blue-
greenish product was precipitated. This product was filtered to
yield a blue-greenish solid, which was washed with 10 mL of
General Procedure for X-ray Determination. The sample was
mounted on a nylon loop with a small amount of NVH immer-
sion oil. The frame integration was performed using SAINT.
1
48
hexanes to give 80 mg product (29%). H NMR (acetone-d , δ)
6
4
(
(
.22 (m, 2H, NCH
s, 3H, N-CH ); 3.20 (m, 4H, NCH
acetone-d , δ): -149.05 (m, 2F), -149.62 (m, 2F), -167.02
2
CH
2
N); 3.39 (m, 2H, NCH
2
CH19
2
N); 3.30
The resulting raw data was scaled and absorption corrected
using a multiscan averaging of symmetry equivalent data using
3
2
CH N); F NMR
2
4
9
SADABS.
The structure was solved by direct methods using the XS
program (Table 2). Most non-hydrogen atoms were obtained
6
(
3
1
m, 2F), -167.72 (m, 4F). UV/vis (CH Cl solution): λ
(ε)=
2
2
max
5
0
52 (3154). Elemental Analysis: Theory (C: 26.30; N: 5.41; H:
.43), Found (C: 26.49; N:5.37; H: 1.50).
2 2 2
Re(O)I(MesNCH CH ) NMe (3b). Iododioxobis(tripheny-
(
48) SAINT; Bruker-Nonius: Madison, WI, 2006.
lphosphine)rhenium (521 mg, 0.554 mmol) and (MesNHCH2-
CH ) NMe (196 mg, 0.554 mml) were added to a 100 mL round-
(49) SADABS; Bruker-Nonius : Madison, WI, 2004.
(50) SHELXTL, XS; Bruker-AXS: Madison, WI, 2008.
2
2

1
1066 Inorganic Chemistry, Vol. 48, No. 23, 2009
Feng et al.
from the initial solution. The remaining atom positions were
recovered from a subsequent difference Fourier map. The
hydrogen atoms were introduced at idealized positions and were
allowed to ride on the parent atom. The structural model was fit
unit cell dimensions were determined from a symmetry con-
strained fit of 9881 reflections with 5.16ꢀ < 2θ < 59.7ꢀ. The data
collection strategy was a number of ω and j scans which
collected data up to 66.22ꢀ (2θ).
2
to the data using full matrix least-squares based on F . The
calculated structure factors included corrections for anomalous
dispersion from the usual tabulation. The structure was refined
The molecule resides on a crystallographic mirror which im-
posed a positional disorder in N-methyl group (atom C12)
which further induced a positional disorder between atoms C11
5
1
0
using the XL program from SHELXTL, graphic plots were
produced using the NRCVAX crystallographic program suite.
Additional information and other relevant literature references
and C11 .
X-ray Structure Determination for [(C F )(N(H)CH -
5
6
2
-
-
CH ) NH(Me)][X], (X=ReO , or I ), 4. All X-ray measure-
2
2
4
0
can be found in the reference section of the Facility s Web page
(
ments were made on a Bruker-Nonius Kappa Axis X8 Apex2
diffractometer at a temperature of 110 K. The unit cell dimen-
sions were determined from a symmetry constrained fit of 9980
reflections with 5.82ꢀ < 2θ < 64.32ꢀ. The data collection
strategy was a number of ω and j scans which collected data
up to 70.54ꢀ (2θ).
http://www.xray.ncsu.edu).
6 5 2
X-ray Structure Determination for Re(O)Me(C F NCH -
CH
Bruker-Nonius X8 Apex2 diffractometer at a temperature of
10 K. The unit cell dimensions were determined from a
symmetry constrained fit of 9825 reflections with 5.96ꢀ <
θ < 64.28ꢀ. The data collection strategy was a number of ω
and j scans which collected data up to 65.86ꢀ (2θ).
The final difference map showed a large peak (5.31e /A) 2.07
2 2
) NMe (1a). All X-ray measurements were made on a
1
The structure exhibits a compositional disorder between
-
-
2
ReO
4
and I . The occupancies of the perrhenate anion and
the iodide anion were refined and normalized to an occupancy of
1.0. The occupancy of the perrhenate was refined to be
0.9216(14). The rhenium atom position and the iodine atom
-
˚
˚
˚
A from RE1 and 1.58 A from C1. However, attempts to refine
this peak as a bona fide atomic position (as either C or O) led to
unreasonable displacement parameters.
˚
position were offset by 1.12 A.
X-ray Structure Determination for Re(O)I(C
6
F
5
NCH
2
-
CH ) NMe (3a). All X-ray measurements were made on a
Acknowledgment. Acknowledgement is made to North
Carolina State University, ACS-PRF Type-G (PRF# 47820-
G3), and the ORAU Ralph E. Powe Junior Faculty Enhance-
ment Award for financial support. The authors thank the
Department of Chemistry of North Carolina State University
and the State of North Carolina for funding the purchase of the
Apex2 diffractometer.
2
2
Bruker-Nonius X8 Apex2 diffractometer at a temperature of
10 K. The unit cell dimensions were determined from a
symmetry constrained fit of 9922 reflections with 5.26ꢀ < 2θ
1
<
63.62ꢀ. The data collection strategy was a number of ω and j
scans which collected data up to 64.74ꢀ (2θ).
2 2 2
X-ray Structure Determination for Re(O)I(MesNCH CH ) -
NMe (3b). All X-ray measurements were made on a Bruker-
Nonius X8 Apex2 diffractometer at a temperature of 173 K. The
Supporting Information Available: X-ray data for 3b and CIF
files for 1a, 3a, 3b, and 4. Rate law derivation for Scheme 3C
and selected UV-vis spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
(
51) SHELXTL, XL; Bruker-AXS: Madison, WI, 2008.