Optically active 4- and 5-coordinate transition metal complexes of bifurcated dipeptide Schiff bases

Article abstract of DOI:10.1021/ic025996o

Symmetrical and unsymmetrical benzophenone Schiff bases of bifurcated dipeptides [e.g., Ar₂C=N-CHR¹CONH～HNCO-CHR²-N= CAr₂] have been synthesized using Boc methodology. These ligands may be regarded as chiral porphyrin mimics because of the α-carbons of the amino acids. The Schiff bases function as effective ligands for transition metals, particularly the late transition metals Ni(II), Cu(II), and Zn. Upon metal insertion, there is loss of the amide protons, resulting in N₄ chelating ligands that retain the amino acid based chirality as well as newly generated metal-centered chirality, which for the Ni(II) complexes have been shown by X-ray analysis to be A (left-handed helix) if the amino acids are S. For Ni(II) and Cu(II), metal insertion results in highly colored complexes and is easily followed by UV-vis spectrophotometry. Several Ni(II) complexes were also characterized by ¹H NMR. Co(II) and Mn(II) complexes were characterized by CW EPR. Two Cu(II) complexes, 7f-Cu^II and 7k-Cu^II, were characterized by EPR (ENDOR and ESEEM), which clearly showed the pentacoordinate nature of 7k-Curr^II.

Full text of DOI:10.1021/ic025996o

Inorg. Chem. 2003, 42, 566−574
Optically Active 4- and 5-Coordinate Transition Metal Complexes of
Bifurcated Dipeptide Schiff Bases
Robin Polt,* Brian D. Kelly, Brian D. Dangel, Udaya Bhaskar Tadikonda, Regina E. Ross,
Arnold M. Raitsimring, and Andrei V. Astashkin
Department of Chemistry, The UniVersity of Arizona, Tucson, Arizona 85721
Received September 3, 2002
1
Symmetrical and unsymmetrical benzophenone Schiff bases of bifurcated dipeptides [e.g., Ar2CdNsCHR s
2
CONH∼HNCOsCHR sNdCAr2] have been synthesized using Boc methodology. These ligands may be regarded
as chiral porphyrin mimics because of the R-carbons of the amino acids. The Schiff bases function as effective
ligands for transition metals, particularly the late transition metals Ni(II), Cu(II), and Zn. Upon metal insertion, there
is loss of the amide protons, resulting in N chelating ligands that retain the amino acid based chirality as well as
4
newly generated metal-centered chirality, which for the Ni(II) complexes have been shown by X-ray analysis to be
Λ (left-handed helix) if the amino acids are S. For Ni(II) and Cu(II), metal insertion results in highly colored complexes
and is easily followed by UV−vis spectrophotometry. Several Ni(II) complexes were also characterized by 1H NMR.
II
II
Co(II) and Mn(II) complexes were characterized by CW EPR. Two Cu(II) complexes, 7f‚Cu and 7k‚Cu , were
II
characterized by EPR (ENDOR and ESEEM), which clearly showed the pentacoordinate nature of 7k‚Cu .
Introduction
Metalloenzymes are capable of many remarkable regio-
and stereoselective transformations in vivo, to which syn-
thetic organic chemistry can presently only aspire.1 These
aspirational goals are extremely beneficial, serving originally
to stimulate synthetic studies to produce enzyme “mimics”
that help unravel the principles of metalloenzyme function,2
and have been applied directly to problems in enantioselec-
tive catalysis more recently.3 Complexes of Ni(II), Cu(II),
Co(II), and Pd(II) and picolinamide based ligands (e.g., 1,
Figure 1) were characterized some time ago,4 and there is
growing interest in nonplanar salen complexes.5 As a con-
Figure 1. Metal-centered chirality. Achiral Nomoyama complexes 1. The
metal-centered chirality of 2 is enhanced by the steric interactions of the
overlapping aryl groups.
sequence of the modular character, peptides are of particular
interest as ligands for catalyst libraries.6 Recently, we have
begun to explore the catalytic activity of enantiomerically
pure transition metal complexes 2,7 which could be regarded
as porphyrin mimics,8 or alternatively as bifurcated peptide
complexes.9
* To whom correspondence should be addressed. E-mail: polt@
u.arizona.edu.
(1) (a) Ermler, U.; Grabarse, W.; Shima, S.; Goubeaud, M.; Thauer, R.
K. Curr. Opin. Struct. Biol. 1998, 8, 749-758. (b) Gray, H. B.;
Malmstrom, B. G.; Williams, R. J. P. JBIC, J. Biol. Inorg. Chem.
2000, 5, 551-559.
(2) (a) Pinto, A. L.; Hellinga, H. W.; Caradonna, J. P. Proc. Natl. Acad.
Sci. U.S.A. 1997, 94, 5562-5567. (b) Watt, R. K.; Ludden, P. W.
Cell. Mol. Life Sci. 1999, 56, 604-625. (c) Hage, R.; Kerschner, J.
Trends Inorg. Chem. 1998, 5, 145-159. (d) Lippard, S. J.; Berg, J.
M. Curr. Opin. Chem. Biol. 2000, 4, 137-139. (e) Henderson, R. A.
J. Chem. Soc., Dalton Trans. 1995, 503-11.
Experimental Section
General Procedure for Preparation of C2-Symmetric Bifur-
cated Dipeptides (4a, 4c, 4e-g). Phenylenediamine (1 equiv) was
(3) (a) Pierre, J.-L. Chem. Soc. ReV. 2000, 29, 251-257. (b) Fenton, D.
E. Chem. Soc. ReV. 1999, 28, 159-168. (c) Berkessel, A. Sel. React.
Met.-Act. Mol., Proc. Symp., 3rd 1998, 25-33. (d) Costas, M.; Chen,
K.; Que, L. Coord. Chem. ReV. 2000, 200-202, 517-544. (e) Robert,
A.; Meunier, B. Biomimetic Oxid. Catal. Transition Met. Complexes
2000, 543-562.
(5) Yamada, S. Coord. Chem. ReV. 1999, 190-192, 537-555.
(6) Severin, K.; Bergs, R.; Beck, W. Angew. Chem., Int. Ed. 1998, 37,
1635-1654.
(7) (a) Dangel, B.; Clarke, M.; Haley, J.; Sames, D.; Polt, R. J. Am. Chem.
Soc. 1997, 119, 10865-6. (b) Dangel, B. D.; Polt, R. Org. Lett. 2000,
2, 3003-6.
(4) Nonoyama, M.; Yamasaki, K. Inorg. Chim. Acta 1973, 7, 373-7.
566 Inorganic Chemistry, Vol. 42, No. 2, 2003
10.1021/ic025996o CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/28/2002

Complexes of Bifurcated Dipeptide Schiff Bases
dissolved in DMF (dimethylformamide) (200 mL). DIEA (3 equiv)
was added in a single portion and the reaction mixture cooled to 0
°C. Boc-amino acid (2.2 equiv) and BOP (2.5 equiv) were added
in one portion. The solution was allowed to warm to room
temperature for 24 h, after which complete conversion of starting
material was observed by TLC. The reaction was diluted with
EtOAc and washed with 1% NaCl (3 × 200 mL), 1 M HCl (2 ×
200 mL), 1% NaCl (1 × 200 mL), saturated NaHCO3 (2 × 200
mL), and saturated NaCl (1 × 200 mL). The organic layer was
dried over MgSO4, filtered, and reduced to a foam in vacuo.
0.7 mL, 5.02 mmol). NiBr2 (900 mg, 4.12 mmol) was added, and
the slurry was heated to reflux for 30 h. A green precipitate formed
upon heating (believed to be nickel oxide). The progress of the
reaction was monitored by TLC (70% toluene/30% ethyl acetate).
The green solid was filtered and discarded. The mother liquor was
stripped to a solid by rotary evaporation. The brown residue was
dissolved in methylene chloride (40 mL) and extracted with a 10%
NaHCO3 solution (2 × 40 mL). The organic layer was dried over
anhydrous CaCl2, filtered, and stripped to a solid. Purification by
flash chromatography (70% toluene/30% ethyl acetate) and sub-
sequent vapor crystallization (ethyl acetate/hexane) gave 209 mg
of 7f‚NiII as maroon crystals in an overall 65% yield. Mp > 200
°C. [R]D ) (-)486 (2.87 × 10-3 g/100 mL in MeOH). IR (KBr
General Procedure for One-Pot Preparation of Non-C2-
Symmetric Bifurcated Dipeptides (4b, 4d, 4h-k). Phenylenedi-
amine (1 equiv) was dissolved in DMF (200 mL). DIEA (1.5 equiv)
was added in a single portion and the reaction mixture cooled to 0
°C. The first boc-amino acid (0.97 equiv) and BOP (1.25 equiv)
were added in one portion. The reaction was held at 0 °C for 45
min, by which time TLC showed complete mono-acylation. The
second boc-amino acid (1.1 equiv) was added in one portion
followed by addition of BOP (1.25 equiv) and DIEA (1.5 equiv).
The solution was allowed to warm to room temperature for 24 h,
after which complete conversion of starting material was observed
by TLC. The reaction was diluted with EtOAc and washed with
1% NaCl (3 × 200 mL), 1 M HCl (2 × 200 mL), 1% NaCl (1 ×
200 mL), saturated NaHCO3 (2 × 200 mL), and saturated NaCl (1
× 200 mL). The organic layer was dried over MgSO4, filtered,
and reduced to a foam in vacuo.
pellet): 3056, 1642, 1570, 1483, 1448, 1373, 1029, 749, 705 cm-1
.
1H NMR (CDCl3) δ (ppm): 6.3-8.5 (m, 34 H), 3.8-4.0 (m, 4 H),
2.4-2.5 (dd, JAX ) 12 Hz, JBX ) 3.2 Hz, 2 H). 13C NMR (CDCl3)
δ (ppm): 182, 174, 119-142, 78, 44. UV-vis λmax [DCM
(dichloromethane)]: 234 nm (ꢀ ) 33670 M-1 cm-1, A ) 2.002),
254 nm (ꢀ ) 31954 M-1 cm-1, A ) 1.90), 370 nm (ꢀ ) 4901 M-1
cm-1, A ) 0.29141). FAB MS ) 787.2603 g/mol.
2(S)-(Benzhydrylidene-amino)-N-{2-[2(S)-(benzhydrylidene-
amino)-3-phenyl-propionylamino]-phenyl}-3-phenyl-propiona-
mide-copper(II) Complex (7f‚CuII). In a 100 mL glass round-
bottom flask, 6f (100 mg, 0.137 mmol) was dissolved in DMF (5
mL) and DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) (72.3 mg, 0.457
mmol). CuCl2 was added to the solution. The resulting brown
solution was stirred at room temperature for 10 min. The progress
of the reaction was monitored by TLC (70% toluene/30% ethyl
acetate). The DMF was azeotropically removed with toluene. The
resulting residue was dissolved in DCM (10 mL) and extracted
with 10% NaHCO3 (2 × 30 mL). A second extraction with saturated
NaHCO3 (125 mL), followed by a brine wash, was performed. The
organic layer was dried over K2CO3, filtered, and stripped to a solid.
Purification by flash chromatography and subsequent vapor crystal-
lization from ethyl acetate and hexanes gave 72.1 mg of 6f as brown
crystals in an overall 60% yield. Mp > 200 °C. [R]D ) (-)1920
(1.87 × 10-3 g/100 mL in CHCl3). IR (KBr pellet): 3056, 2925,
1623, 1570, 1474, 1460, 1380, 1291, 758, 700 cm-1. UV-vis λmax
(DCM): 222 nm (ꢀ ) 41 834 M-1 cm-1, A ) 1.0153), 244 nm (ꢀ
General Procedure for Boc Deprotection (5a-m). Compound
4(a-m) was dissolved in 50 mL of anhydrous MeOH and chilled
to 0 °C. In a separate flask, 50 mL of MeOH was chilled to 0 °C.
AcCl (4 equiv) was added slowly. The resultant methanolic HCl
solution was added to the chilled reaction mixture dropwise over
30 min. The reaction mixture was allowed to warm to room
temperature for 24 h. After all the starting material had been
consumed, as judged by TLC, MeOH was removed by rotary
evaporation. The residual HCl was removed by dissolving the
residue in MeOH and subsequent rotary evaporation (5 × 200 mL).
This material was placed in vacuo until a brittle foam was obtained,
and then triturated in Et2O until a fine suspension was achieved.
The HCl salt was collected by filtration and was dried overnight
under vacuum and over P2O5.
) 47 005 M-1 cm-1, A ) 1.1408), 272 nm (ꢀ ) 32 528 M-1 cm-1
,
A ) 0.78946), 340 nm (ꢀ ) 3496 M-1 cm-1, A ) 0.08485). FAB
MS ) 791.2463 g/mol.
General Procedure for Schiff Base Formation (6a-m).
Compound 5(a-m) was suspended in 60 mL of CH2Cl2. Ben-
zophenone imine (2.05 equiv) was added in a single portion. The
resultant slurry was stirred at room temperature for 12 h. Upon
completion, the reaction was diluted with 100 mL of CH2Cl2 and
washed with saturated NaHCO3 (3 × 100 mL). The organic layer
was taken, dried over K2CO3, filtered, and concentrated in
vacuo.
2(S)-(Benzhydrylidene-amino)-N-{2-[2(S)-(benzhydrylidene-
amino)-3-phenyl-propionylamino]-phenyl}-3-phenyl-propiona-
mide-zinc(II) Complex (7f‚ZnII). In a flame-dried 50 mL glass
round-bottom flask, 6f (219 mg, 0.3 mmol) was dissolved in 3 mL
of freshly distilled THF. To the clear homogeneous solution, a 0.5
M Et2Zn solution in THF (0.64 mL, 0.32 mmol) was added in a
single portion. The resultant reaction mixture was heated to reflux
for 1 h. The Zn complex is relatively stable, and its formation can
be monitored by TLC (80% hexanes/20% ethyl acetate). Two UV
active spots are observed corresponding to the complex (Rf ) 0.2)
and 6f (Rf ) 0.75) which is generated from the hydrolysis of the
complex during the TLC development.
2(S)-(Benzhydrylidene-amino)-N-{2-[2(S)-(benzhydrylidene-
amino)-3-phenyl-propionylamino]-phenyl}-3-phenyl-propiona-
mide-nickel(II) Complex (7f‚NiII). In a 100 mL glass round-bottom
flask equipped with a reflux condenser, 6f (300 mg, 0.411 mmol)
was dissolved in methanol (10 mL) and triethylamine (508 mg,
Determination of Kinetics of Nickel(II) Insertion. A 100 mL
three neck round-bottom flask was equipped with a reflux con-
denser, glass stopper, and septum. NiBr2 (1.25 × 10-5 mol) and 6f
(1.25 × 10-5 mol) were dissolved in 60 mL of MeOH and heated
to reflux under an argon atmosphere. The initial UV-vis spectrum
was measured, followed by addition of NEt3 (3.12 × 10-6 mol) at
time zero. The UV-vis spectra were measured using a fiber optic
dip probe attached to a SI440 spectrophotometer (Spectral Instru-
ments, Inc., Tucson, Arizona). Full spectra (200-900 nm) were
(8) (a) Nanthakumar, A.; Miura, J.; Diltz, S.; Lee, C.-K.; Aguirre, G.;
Ortega, R.; Ziller, J. W.; Walsh, P. J. Inorg. Chem. 1999, 38, 3010-
13. (b) Hsiao, Y.; Hegedus, L. S. J. Org. Chem. 1997, 62, 3586-91.
(c) Schaus, W. E.; Larrow, J. F.; Jacobsen, E. N. J. Org. Chem. 1997,
62, 4197-4199. (d) Kobayashi, N.; Kabayashi, Y.; Osa, T. J. Am.
Chem. Soc. 1993, 115, 10994-5. (e) Busch, D. H. Acc. Chem. Res.
1978, 11, 392-400.
(9) (a) Bair, M. L.; Larsen, E. M. J. Am. Chem. Soc. 1971, 93, 1140-
1148. (b) Gergely, A.; Farkas, E. J. Chem. Soc., Dalton Trans. 1977,
381-386. (c) Raghunath, P.; Agarwal; Perrin, D. D. J. Chem. Soc.,
Dalton Trans. 1977, 53-57.
Inorganic Chemistry, Vol. 42, No. 2, 2003 567

Polt et al.
Table 1. Summary of Crystallographic Details for 7b‚NiII, p-MeO-7b‚NiII, 7f‚NiII, p-MeO-7f‚NiII, rac-7g‚NiII, CH2Cl2-7g‚NiII, 7l‚NiII, 11
7b‚NiII
p-MeO-7b‚NiII
7f‚NiII
p-MeO-7f‚NiII
rac-7g‚NiII
CH2Cl2-7g‚NiII
7l‚NiII
11
unique reflns
(total)
8500 (23421)
7878 (42284)
15136 (15136) 10513 (39718) 7526 (37222)
10774 (12178) 17022 (22997) 7515 (28537)
θ
max (deg)
29.32
25.77
24.97
28.63
28.26
27.23 28.338 24.455
empirical
formula
Mr
C43H34N4NiO2 C47H42N4NiO6 C50H40N4NiO2 C54H48N4NiO6 C38H32N4NiO4 C38H32N4NiO4 C42H36N4NiO6 C41H33N3O
697.45
orthorhombic
P212121
9.3318(7)
15.6129(12)
23.3240(18)
90
90
90
3398.2(4)
1.363
4
902.48
monoclinic
P21/c
13.4825(13)
11.5511(11)
26.762(3)
90
99.012(2)
90
4116.4(7)
1.456
787.572
orthorhombic
P212121
17.259(4)
14.290(3)
34.939(7)
90
90
90
8617(3)
1.282
4
907.67
orthorhombic
P212121
12.4041(8)
13.5171(9)
26.1586(17)
90
90
90
4385.9(5)
1.375
4
709.85
monoclinic
P21/c
21.888(2)
14.0327(14)
10.7457(11)
90
101.070
90
3239.1(6)
1.456
667.39
triclinic
P1
751.47
triclinic
P1
583.70
orthorhombic
P212121
9.7799(6)
17.6968(10)
18.0760(10)
90
90
90
3128.5(3)
1.239
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
Vc (Å3)
Dc (Mg/m3)
Z
8.6240(10)
10.1739(12)
18.951(2)
94.644(2)
99.792(2)
91.997(2)
1631.2(3)
1.359
8.629(2)
9.665(3)
23.136(6)
79.747(9)
88.743(9)
89.707(7)
1898.3(9)
1.463
4
4
2
2
4
µ (Mo KR)
0.616
0.659
0.499
0.501
0.731
0.642
0.708
0.075
(mm-1
)
R1a [I g 2σ(I) 0.0457
data]
0.0914
0.068
0.0366
0.0562
0.0339
0.0486
0.0432
wR2
GOF
0.0654
0.808
0.2146
0.943
0.933
0.994
0.0711
0.920
0.1135
0.926
0.0658
0.955
0.0933
0.953
0.0728
0.881
larg diff peak, +0.741,
+0.607,
-0.673
+0.336,
-0.214
+0.289,
-0.304
+0.653,
-0.560
+0.513,
-0.228
+0.708,
-0.771
+0.193,
-0.155
hole (e Å3)
-0.383
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(Fo - Fc )]/∑[w(Fo )2]}1/2
.
measured at regular intervals (every 15 min for the first hour and
every 30 min after that) by inserting the probe through the septum.
The insertion of NiII was monitored by observing the increase in
absorbance of the band due to 7f‚NiII at 370 nm.
Table 2. Yields for Bifurcated t-Boc Protected Dipeptides
entry
amino acids
R1(PG1), R2(PG2)
yield (4)
a
b
c
d
e
f
g
h
i
Gly, Gly
Gly, Phe
Ala, Ala
Ala, Phe
Val, Val
Phe, Phe
Ser, Ser
H, H
H, CH2Ph
CH3, CH3
CH3, CH2Ph
CH(CH3)2, CH(CH3)2
CH2Ph, CH2Ph
CH2OBn, CH2OBn
CH2Ph, CH2OBn
CH2Ph, CH2STrt
CH2Ph, CH2ImDNP
CH2Ph, CH2ImBn
92%
99%
97%
85%
90%
98%
98%
93%
88%
93%
73%
X-ray Structure Determination of 7b‚NiII, p-MeO-7b‚NiII, 7f‚
NiII, p-MeO-7f‚NiII, rac-7g‚NiII, CH2Cl2-7g‚NiII, 7l‚NiII, 11.
Crystal, data collection, and refinement parameters are given in
Table 1. Suitable crystals were mounted in a random orientation
on a thin glass fiber. X-ray diffraction data were collected at 170-
(2) K on a Bruker SMART 1000 CCD detector with Mo KR
radiation (λ ) 0.71073 Å). The frames were integrated using the
Bruker SAINT10 package’s narrow frame algorithm. Empirical
absorption and decay corrections were applied using the program
SADABS (Sheldrick 1997). The structure solutions were achieved
utilizing direct methods followed by Fourier synthesis using
SHELXS in the Bruker SHELXTL11 software package. Refinements
were performed using SHELXL and illustrations made using XP.
A complete list of bond distances and angles is available for all
compounds in CIF format as Supporting Information.
Phe, Ser
Phe, Cys
Phe, His
Phe, N-Bn(His)
j
k
t-Boc-protected dipeptide 4. This sequence can be performed
with or without isolation of the mono-acylated intermediates
3. The yields for these reactions were generally excellent,
with the exception of S-trityl-protected t-Boc-cysteine (44%
for R1 ) Phe, R2 ) S-trityl-Cys), probably because of the
extreme bulk exhibited by this side chain. Yields were
increased when the bulkier of the two residues was coupled
first. Cleavage of the Boc groups from 4 was accomplished
with methanolic HCl, which was generated in situ from AcCl
and MeOH. The resulting salts 5‚2HCl were conveniently
stored as white or off-white crystalline solids.
Condensation of the dipeptide salts 5‚2HCl with com-
mercially available (redistilled) Ph2CdNH or the corre-
sponding para-methoxy-substituted ketimine, (p-MeOPh)2Cd
NH, was accomplished under a variety of conditions. For
aliphatic side chains, the solvents CH2Cl2, ClCH2CH2Cl, or
CH3CN generally worked well without catalysis (Tables 2
and 3). For the bifurcated dipeptides bearing more polar side
chain residues, cosolvent mixtures of CH3CN/DMF or the
solvent NMP (N-methylpyrrolidinone) was used. Alterna-
tively, free bases 5 could be condensed in the presence of a
catalytic amount of a protic acid (e.g., camphorsulfonic acid,
HBr), or in the presence of BF3‚Et2O. In most cases, the
Ligand Synthesis
The Schiff base ligands are easily accessible via the t-Boc-
protected intermediates (Table 2, Scheme 1). The t-Boc
amino acids (R1) can be reacted with ortho-phenylenediamine
to provide the mono-acylated intermediates, 3. Mono-acyla-
tion is favored because of large differences in the acylation
rates for the diamine versus acylation rates for the mono-
acylated product, which possesses an unfavorable hydrogen
bonding pattern and proceeds slowly at 0 °C. Subsequent
acylation with a second t-Boc amino acid (R2) proceeded at
acceptable rates at RT (room temperature) to provide the
(10) SAINT, V5; Program for reduction of data collected on Bruker AXS
CCD area detector systems; Bruker Analytical X-ray Systems:
Madison, WI, 1997.
(11) SHELXTL, V5; Program suite for solution and refinement of crystal
structures; Bruker Analytical X-ray Systems: Madison, WI, 1997.
568 Inorganic Chemistry, Vol. 42, No. 2, 2003

Complexes of Bifurcated Dipeptide Schiff Bases
Scheme 1. Synthesis of Bifurcated Dipeptide Ligands (Chiral Porphyrin Mimics)a
a Reagents: i, BOP, iPr2NEt, DMF, 0 °C, 30 min; ii, t-Boc-NH-CHR2COOH, BOP, iPr2NEt, DMF, RT, 5-8 h; iii, AcCl, MeOH, 0 °C, inverse addition;
iv, for side chain modifications, see Experimental Section; v, Ph2CdNH, CH2Cl2, RT.
Table 3. Yields for Schiff Base Ligands and Intermediates
yield yielda
(5)
[R]D (6)
(6) c ≈ 1,CHCl3
entry amino acids
R1
R2
a
b
c
d
e
f
g
h
i
Gly, Gly
Gly, Phe
Ala, Ala
Ala, Phe
Val, Val
Phe, Phe
Ser, Ser
Phe, Ser
Phe, Cys
Phe, His
H
H
H
99% 49%
CH2Ph
CH3
CH2Ph
92% 23% (-)-104°
98% 45% (+)-50°
98% 73% (-)-22°b
CH3
CH3
CH(CH3)2 CH(CH3)2 98% 69% (-)-20°
CH2Ph
CH2OH
CH2Ph
CH2Ph
CH2Ph
CH2Ph
CH2OH
CH2OH
CH2SH
CH2Im
99% 72% (-)-159°
99% 98% (+)-11°
99% 92% (-)-90°
93% 58% (-)-38°
88% 79% (-)-109°
j
k
Phe, N-Bn-His CH2Ph
CH2ImBn 80% 79% (-)-80°
98% (+)-41°
lc Ser, Ser
CH2OAc CH2OAc
a Unoptimized yields for 6a-k. b Measured in CH3CN. c Via acylation
of 6g.
Figure 2. Other diamine analogues of 6f.
THF) in the presence of NEt3 to produce maroon crystals of
a square planar complex. Zinc insertion was accomplished
by heating the ligand in the presence of Et2Zn in either THF
or toluene under argon.
Scheme 2. Metal Insertiona
Purification of the Cu(II) and Ni(II) complexes was easily
accomplished with silica gel chromatography. The Zn
complexes could be subjected to TLC with some streaking
but were too polar for column chromatography. The metals
Co(II) (CoCl2) and Mn(II) (Mn(OAc)2) formed complexes
with 6k, and were characterized by CW EPR, but did not
crystallize. All of the metals produced visible M+ ions of
the insertion product using FAB MS. The insertion reaction
was tolerant of a wide variety of amino acid side chains:
R1, R2 ) -H, -CH3, -CH2Ph, -CH(CH3)2, -CH2OH,
-CH2SH, -CH2OBn, -CH2SC(Ph)3, -CH2-imidazole, and
-CH2-imidazole-τN-Bn. Attempts to isolate the metal
insertion products with NiBr2 and the closely related ligands
8, 9, 10, and 11 failed (Figure 2).
The kinetics of Ni(II) insertion, although observable at RT
by spectrophotometric methods (Figure 3), is complex. For
NiBr2 at 60 °C in MeOH using pseudo-first-order condi-
tions,12 the initial rate of insertion was ∼10-4 mol/min with
a large excess 6f, which slowed to ∼10-6 mol/min within
2-3 h of reaction. The rate of insertion is retarded by excess
Ni2+ ions and is enhanced by the addition of a weak base
(NEt3, iPr2NEt, DBU). Related work by Margerum and others
with linear peptides indicates that the initially formed O,N
amide Ni(II) complexes rearrange slowly to the more stable
N,N amide complexes.13 Interaction of the amide carbonyl
with the imine-bound transition metals strongly acidifies the
amide (δ-π back-bonding), but the factors which control
a Reagents: i, NiBr2, Et3N, THF, 12 h, reflux; ii, CuCl2, DBU, DMF, 1
min, RT; iii, Et2Zn, THF, 1 h, reflux.
resultant Schiff bases ligands 6 were isolated as glassy solids
after column chromatography.
Metal Insertion
Insertion of Cu(II) proceeded readily at RT with CuCl2 in
the presence of DBU in DMF or CH3CN to provide stable,
highly colored, EPR-active complexes (Scheme 2). Insertion
of Ni(II) as NiBr2 required higher temperatures (refluxing
(12) Childers, R. F.; Wentworth, R. A. D. Inorg. Chem. 1969, 8, 2218-
20.
Inorganic Chemistry, Vol. 42, No. 2, 2003 569

Polt et al.
A large downfield shift was observed for the equatorial R
hydrogen of 7a‚NiII (Figure 6, Table 5), which is probably
due to the deshielding cone of an aromatic ring of NdCPh2,
and not due to magnetic anisotropy introduced by the Ni(II)
nucleus. A downfield shift is also observed for one of the â
hydrogens of 7f‚NiII and for p-MeO-7f‚NiII. This effect was
not observed for the less hindered 7g‚NiII and 7l‚NiII side
chains.
To gain some insight into the metal binding nature of the
ligands, several Ni(II) complexes were subjected to single-
crystal X-ray crystallography. In one case, a metal-free ligand
(11) was subjected to crystallography for comparison (Figure
4). Interestingly, the three nitrogens appear to be somewhat
pre-organized for metal binding. Because the Ph2CdN
groups can hydrogen bond to the amide N-H, this causes
all three nitrogens to become coplanar. It is noteworthy that
the amino acid side chain is projected over the binding pocket
for the metal.
Complex 7b‚NiII provided maroon crystals from EtOAc/
hexanes which were subjected to single-crystal X-ray analysis
(Figure 5). It is noteworthy that the axial nature of the
phenylalanine side chain observed in the ligand 11 is retained
in this case, similar to what was observed for 7f‚NiII and
p-MeO-7f‚NiII, which have two phenylalanine side chains.7a
With the exception of the phenylalanine side chain in 7b‚
NiII, this asymmetric complex still retains approximate C2
symmetry with the distal Ph2CdN projecting above the plane
on the same side as the side chain.
Figure 3. Ni(II) insertion at RT (370 nm).
Table 4. Isolated Metal Complexes
ligand
metal
colora
yield
complex
6a
6k
6b
6f
6g
6l
6k
6f
6k
6f
NiBr2
CoCl2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
CuCl2
CuCl2
Et2Zn
Me2Zn
maroon
redc
maroon
maroon
textb
71%
79%
86%
83%
60%
96%
94%
92%
95%
7a‚NiII
7k‚CoII
7b‚NiII
7f‚NiII
7g‚NiII
7l‚NiII
7k‚NiII
7f‚CuII
7k‚CuII
7f‚Zn
maroon
yellow
violet
green
nonec
>90%
>90%
6f
nonec
7f‚Zn
a Solid state. b Crystals from CH2Cl2 were maroon. Crystals from EtOAc/
hexanes were dichroic. c Oil.
the subsequent O f N rearrangement are controlled by a
number of effects that are complex and difficult to predict
(e.g., steric demand of the ligand, additional ligation sites,
oxophilicity of the metal). Because the O f N rearrangement
must occur twice for each metal insertion, it is not likely
that it will be possible to sort out kinetics for the discrete
steps required for complete insertion of the metal.
Insertion of Ni(II), Cu(II), and Zn into 6f to provide the
corresponding complexes 7 was straightforward (Table 4).
The Ni(II) and Cu(II) complexes were easily purified by
column chromatography on SiO2. All of the complexes
provided mass spectra with visible M+ ions. The Ni(II) and
In all Ni(II) complexes studied by X-ray thus far, the
complexes were approximately square planar, with average
Ni-N bond lengths of 1.84 Å for the amide nitrogens, and
1.93 Å for the imine nitrogens (standard deviation <2%).
The presence or absence of the p-methoxy group on the
Schiff base moiety did not have a noticeable effect on bond
lengths or other features in the ORTEP representations
(Figures 7 and 8). Because several complexes crystallized
with more than one unique molecule in the unit cell, there
are more data here than the number of crystalline complexes
would imply. The crystal packing of these complexes is
interesting in its own right and will be reported elsewhere.
All the Ni(II) complexes studied showed square planar
geometry with an average tetrahedral distortion of 0.07 Å
(0.011-0.137 Å, standard deviation ) 0.040 Å, Table 6),
as measured by how far N(4) is out of the plane defined by
the first 3 nitrogens. This is a clear expression of the metal
centered chirality. The overlapping Ar2CdN groups define
a screw axis running through the metal, which has a
predictable sense of helicity, or the ligand may even
predetermine the sense of helicity.14 That is, if the amino
acids are S, then the metal-centered helicity is left-handed,
or Λ. Indeed, this is apparent even in the less hindered
tridentate ligand 11, which does not even contain a metal.
1
Zn complexes were characterized by H NMR, and several
Ni(II) complexes were subjected to single-crystal X-ray
analysis. The Cu(II) complexes provided good EPR spectra,
with visible hyperfine couplings to the Cu, and superhyper-
fine couplings to the nitrogen ligands.
Structure of the Complexes
1
The H NMR spectra of the ligands (Table 5, Figure 6)
showed downfield shifts upon insertion of Ni(II). In the cases
where R1 ) R2, perfect C2 symmetry was observed, as
expected for a square planar complex. C2 symmetry was not
observed for the corresponding Zn complexes, and it is
postulated that the Zn complexes are best described as square
pyramidal.
(14) (a) Mamula, O.; Von Zelewsky, A.; Bark, T.; Stoeckli-Evans, H.;
Neels, A.; Bernardinelli, G. Chem. Eur. J. 2000, 6, 3575-85. (b)
Muniz, K.; Bolm, C. Chem. Eur. J. 2000, 6, 2309-16. (c) Va´zquez,
M.; Bermejo, M. R.; Sanmart´ın, J.; Garc´ıa-Deibe, A. M.; Lodeiro,
C.; Mah´ıa, J. J. Chem. Soc., Dalton Trans. 2002, 870-877.
(13) (a) McDonald, M. R.; Fredericks, F. C.; Margerum, D. W. Inorg.
Chem. 1997, 36, 3119-3124. (b) Kirvan, G. E.; Margerum, D. W.
Inorg. Chem. 1985, 24, 3245-53. (c) Sigel, H.; Martin, R. B. Chem.
ReV. 1982, 82, 385-426.
570 Inorganic Chemistry, Vol. 42, No. 2, 2003

Complexes of Bifurcated Dipeptide Schiff Bases
Table 5. Effect of Ni(II) Insertion on Ligand Chemical Shifts and Coupling Constants
cmpd
residues
H(â) (δ)
H(â′) (δ)
H(R) (δ)
J
âR (Hz)
J
â′R (Hz)
Jââ′ (Hz)
6a
Gly, Gly
Gly, Gly
4.09
4.30 (ax)
3.56 (eq)
4.29
4.05
3.91
3.62
4.23
3.97
4.24
7a‚NiII
16.9
(JRR′
12.9
)
6d
Phe
Ala
Phe
Ala
3.14
1.33
3.76
1.69
3.12
3.89
3.09
3.88
3.35
2.65
9.7
6.8
9.1
6.8
9.2
9.3
9.3
6.9
a
3.0
4.9
7d‚NiII
13.2
6f
Phe, Phe
Phe, Phe
Phe, Phe
Phe, Phe
Ser, Ser
Ser, Ser
Ser(OAc)
Ser(OAc)
Ser(OAc)
Ser(OAc)
Phe
3.29
2.46
3.26
3.3
4.8
3.1
6.1
a
4.5
6.4
13.1
13.2
12.9
13.7
a
11.2
13.1
7f‚NiII
6fp-MeO
7fp-MeO‚NiII
6ga
2.43
4.16
∼4.2-4.1
∼3.8-3.7
∼3.9
7g‚NiII
6l
4.08
3.72
3.60
4.7
6.8
4.44
4.29
4.26
7l‚NiII
6j
4.50
4.32
3.87
5.2
5.4
11.4
3.09
2.66
3.08
3.15
3.37
2.93
3.23
3.13
4.32
4.11
4.20
4.27
9.5
5.2
9.3
7.3
3.1
5.1
3.3
4.7
13.0
14.8
13.0
14.5
His
Phe
BnHis
6k
a Oxazolidine-â-hydroxyimine tautomerism causes severe overlap in the case of 6g. J values not determined.
the ligand environment was different for these two com-
plexes. Continuous wave (CW, X-band) ESR spectra were
recorded of both Cu(II) complexes in a frozen solution of
CH2Cl2 at 77 K (Figure 10). Both showed the hyperfine
coupling and g-values expected for Cu(II) in similar pseudopla-
nar N4 compexes17 (g ) 2.01, 1.97, g| ) 2.22, 2.19; Azz ∼
400 and 500 MHz for 7k‚CuII and 7f‚CuII, respectively),
but whereas the superhyperfine coupling to the nitrogen
ligands was quite distinct for 7f‚CuII, the superhyperfine
coupling was obscure for the pentacoordinate case 7k‚CuII.
Obviously, the loss of resolution reflects the fact that
hyperfine/quadrupole parameters of the four strongly coupled
14N are less equivalent in 7k‚CuII than those of 7f‚CuII. To
qualitatively understand this difference, we performed pulsed
ENDOR measurements of 14N for both complexes. The sets
of ENDOR spectra acquired in various positions of the EPR
spectra are shown in the Supporting Information.
Figure 4. ORTEP representation of ligand 11.
The ENDOR spectra of 7f‚CuII and 7k‚CuII were acquired
at a single selective low field position (g|, mI ) -3/2) (Figure
11). In both cases, ENDOR spectra are spread between 10
and 25 MHz and show 5-4 distinctive peaks. In this
particular field position, we can neglect the disorder due to
molecular orientation and evaluate the hyperfine/quadrupole
parameters (Table 7).18
Figure 5. ORTEP representation of 7b‚NiII, front view.
(15) Kawamoto, T.; Hammes, B. S.; Haggerty, B.; Yap, G. P. A.;
Rheingold, A. L.; Borovik, A. S. J. Am. Chem. Soc. 1996, 118,
285-6.
This same type of interaction has been observed in Cu(II)
complexes by Borovik.15
The source of this chirality may be attributed to the
extreme A1,3 strain about the Ar2CdNsCH moiety, which
has been observed in the context of the observed weak acidity
of the R proton.16
Unfortunately, none of the Cu(II) complexes crystallized
in a form that was suitable for X-ray analysis. Because the
d9 Cu(II) complexes complicate NMR analysis, an electron
spin resonance (ESR) study of 7f‚CuII and 7k‚CuII (Figure
9) was performed. Because the colors of these two complexes
were quite different (violet for the tetracoordinate 7f‚CuII,
and green for 7k‚CuII), it was obvious from the start that
(16) O’Donnell, M. J.; Bennett, W. D.; Bruder, W. A.; Jacobsen, W. N.;
Knuth, K.; LeClef, B.; Polt, R. L.; Bordwell, F. G.; Mrozack, S. R.;
Cripe, T. A. J. Am. Chem. Soc. 1988, 110, 8520-5.
(17) (a) AdVanced EPR, Applications in Biology and Biochemistry; Hoff,
A. J., Ed.; Elsevier: Amsterdam, 1989. (b) See examples in: Pilbrow,
J. R. Transition Ion Electron Paramagnetic Resonance; Clarendon
Press: Oxford, 1990; Chapters 5 and 11. (c) Brown, T. G.; Hoffman,
B. M. Mol. Phys. 1980, 39, 1073. (d) Goldfarb, D.; Fauth, J.-M.; Tor,
Y.; Shanzer, A. J. Am. Chem. Soc. 1991, 113, 1941-8. (e) Van Dam,
P. J.; Reijerse, E. J.; Van Der Meer, M. J.; Guajardo, R.; Mascharak,
P. K.; De Boer, E. Appl. Magn. Reson. 1996, 10, 71-86. (f) Bender,
C. J.; Peisach, J. J. Chem. Soc., Faraday Trans. 1998, 94, 375-386.
(g) Slutter, C. E.; Gromov, I.; Epel, B.; Pecht, I.; Richards, J. H.;
Goldfarb, D. J. Am. Chem. Soc. 2001, 123, 5325-5336.
(18) Details may be found in the Supporting Information.
Inorganic Chemistry, Vol. 42, No. 2, 2003 571

Polt et al.
Figure 6. ABX spectra provided by H(R), H(â), and H(â′) for ligands and complexes.
the quadrupole interactions were determined in the g-frame,
and the relation between the g-frame and the molecular frame
requires work that is beyond the scope of this paper. On the
basis of the observed values of the quadrupole interactions,
we can only make some qualitative conclusions about the
structure of 7f‚CuII and 7k‚CuII. Assuming that all nitrogens
have a similar quadrupole tensor, that the z-axis of the
quadrupole tensor is directed along the Cu-N bond, and that
the asymmetry parameter η is ∼0.5, one can estimate that
the angle between the xy-plane of the g-frame and the Cu-N
bond of the imine nitrogens in 7k‚CuII is 30-50°. If we
then assume that the value of the Q0 element of the
quadrupole tensor is about 4 MHz,22 we can determine that
the amide nitrogens are closer to the xy-plane of g-frame,
making this angle 0-20°. All these data show that both
complexes are not planar.
Figure 7. ORTEP representation of 7l‚NiII, top view.
Because of the low magnetic momentum of 14N, pulsed
ENDOR is not appropriate for the investigation of weakly
coupled 14N ligands. Therefore, to examine the presence of
the fifth (axial or apical?) coordinated nitrogen in 7k‚CuII,
we applied electron spin-echo envelope modulation,
which is suitable for the investigation of the weakly coupled
Cu-14N. As shown, the ESEEM measurements were per-
formed in various positions of the EPR spectrum. For the
sake of comparison, similar measurements were performed
for 7f‚CuII, where the fifth coordinated 14N was not present.
The ESEEM measurements of 7k‚CuII clearly showed the
modulation related to 14N, and those of 7f‚CuII did not. The
FT ESEEM spectra for the mostly nonselective field position
for these two compounds are shown in Figure 12.
While the spectrum of 7f‚CuII does not show any 14N
related lines, the FT spectrum of 7k‚CuII shows 4 lines,
typical for weakly coupled 14Ns under so-called “cancellation
conditions”. From line positions, one can immediately
evaluate a hyperfine interaction constant of 1.5-2.0 MHz
and parameters of quadrupole interactions: nuclear quadru-
pole coupling constant of 0.75 MHz and an asymmetry
parameter of 0.3-0.4. These values are quite typical for 14N.
Therefore, ESEEM data unambiguously show that 7k‚CuII
is a pentacoordinate complex.
Figure 8. ORTEP representation of 7l‚NiII, side view.
In both complexes, there are two groups of two 14N with
approximate parameters, A ∼ 28-30 MHz and A ∼ 39-41
MHz, which shows that structures of 7f‚CuII and 7k‚CuII
are, in general, similar. In accordance with Iwaizumi et al.,19
the hyperfine interaction of 38-48 MHz correspond to sp2
nitrogens, while those of 33-27 MHz correspond to sp3
nitrogens ligated to Cu(II). Both types of nitrogens are easily
seen in complex structure.
In principle, more structural information can be obtained
from the evaluated parameters of quadrupole interaction
because the axes of the quadrupole tensor and the direction
of the metal-N bonds are interrelated.20
The magnitude of the principal elements of the quadrupole
tensor Q0 and the asymmetry parameter η of the metal bound
ligand and corresponding free ligand can, however, be
substantially different. For instance, the numerous studies
of planar metal porphyrin complexes21 have shown that the
main axis of the quadrupole tensor can be either parallel or
perpendicular to the Cu-N bond, albeit remaining in a
porphyrin plane. In addition, in this work, the parameters of
Redox Behavior of 7f‚CuII and 7f‚NiII
Atom transfer reactions seemed to be an obvious applica-
tion of the Schiff base ligand system. Thus, we wished to
examine the ability of this ligand to support higher oxidation
(21) Van Doorslaer, S.; Bachmann, R.; Schweiger, A. J. Phys. Chem. A
1999, 103, 5446-5455 and references therein.
(22) Electron Spin-Echo EnVelope (ESEEM) Spectroscopy; Dikanov, S.
A., Tsvetkov, Yu. D., Eds.; CRC Press: Boca Raton, FL, 1992.
(19) Iwaizumi, M.; Kudo, T.; Kita, S. Inorg. Chem. 1986, 25, 1546-50.
(20) Jiang, F.; Karlin, K. D.; Peisach, J. Inorg. Chem. 1993, 32, 2576-82.
572 Inorganic Chemistry, Vol. 42, No. 2, 2003

Complexes of Bifurcated Dipeptide Schiff Bases
Table 6. Summary of Ni(II) Ligation from X-ray Data
complex
R1
R2
R3
N1-Ni (Å)
N2-Ni (Å)
N3-Ni (Å)
N4-Ni (Å)
distortiona (Å)
7b‚NiII
H
H
CH2Ph
CH2Ph
CH2Ph
CH2OH
CH2OH
CH2OAc
CH2OAc
CH2Ph
CH2Ph
CH2Ph
CH2Ph
CH2Ph
CH2OH
CH2OH
CH2OAc
CH2OAc
H
OCH3
H
H
OCH3
H
H
H
H
1.922(2)
1.915(7)
1.944(4)
1.921(5)
1.9479 (17)
1.943(3)
1.957(2)
1.963(3)
1.968(3)
1.906(2)
1.909(7)
1.942(4)
1.963(5)
1.9329 (18)
1.922(3)
1.903(3)
1.925(3)
1.921(3)
1.833(2)
1.829(7)
1.843(5)
1.843(5)
1.8455 (18)
1.834(3)
1.837(3)
1.829(3)
1.845(3)
1.833(2)
1.821(7)
1.830(4)
1.849(5)
1.8442 (17)
1.840(3)
1.846(2)
1.852(3)
1.846(3)
0.100
0.128
0.026
0.137
0.066
0.011
0.066
0.071
0.068
p-MeO-7b‚NiII
b7f‚NiII
c7f‚NiII
p-MeO-7f‚NiII
rac-7g•NiII
CH2Cl2 7g•NiII
7l‚NiII
c7l•NiII
a Out-of-plane distortion of one N4 relative to N1-N3. b C2-symmetric complex. c Non-C2-symmetric complex. Complexes 7f‚NiII and 7l‚NiII crystallize
with two molecules per unit cell, which differ principally by a rotation about ø1 of one of the side chains.7a
Table 7. Hyperfine and Quadrupole Parametersa
7f‚CuII
A1
A2
A3
A4
41 MHz
P1
39.2 MHz
P2
31.2 MHz
P3
30.6 MHz
P4
1.06 MHz
1.06 MHz
0 MHz
0 MHz
7k‚CuII
A1
A2
A3
A4
40.5 MHz
P1
38.5 MHz
P2
29 MHz
P3
28 MHz
P4
Figure 9. Cu(II) complexes for ESR study.
1.2 MHz
1.2 MHz
0.5 MHz
0.82 MHz
14N
a Hyperfine and quadrupole parameters of 14N (14NAzz and
P )
zz
evaluated from ENDOR spectra at g|, mI ) -3/2. The details are shown in
Supporting Information.
Figure 10. First derivative CW spectrum of 7f‚CuII.First derivative X-band
CW EPR spectra of 7k‚CuII and 7f‚CuII. Arrows show field positions
(Gauss) where pulsed ENDOR measurements were obtained (Figure 11).
Dashed lines show field positions for ESEEM measurements (Figure 12).
Figure 12. ESEEM spectra for 7k‚CuII and 7f‚CuII. Cosine FT primary
ESEEM of 7f‚CuII and 7k‚CuII acquired in the most nonselective field
positions (Figure 10). Experimental conditions: mw frequency ) 9.421;
mw pulse durations ) 2 × 10 ns; τ0 ) 200 ns; T ) 13 K.
Amide ligands are known to stabilize higher oxidation
states (e.g., Ni(III) and Cu(III)).23 Cyclic voltammetry of 7f‚
CuII showed a highly reversible II/III couple similar to what
was observed by Ruiz24 and others25 with tetracoordinate
copper complexes derived from substituted o-phenylenedi-
amines (Table 8). The corresponding Ni(II)/(III) couple for
7f‚NiII was also reversible, with E/V ) +0.52 V. Although
Figure 11. Pulsed ENDOR spectra of Cu(II) complexes. Davies ENDOR
spectra of 7k‚CuII and 7f‚CuII acquired in most selective field positions,
shown in Figure 10 by arrows. Microwave frequency ) 9.421 MHz;
microwave pulse durations ) 30 ns,15 ns, 30 ns; RF pulse duration ) 5
µs; nominal RF power ∼ 1 kW; τ ) 450 ns; T ) 13 K; pulse repetition
rate ) 200 Hz.
(23) Bour, J. J.; Birker, J. P. M. W. L.; Steggerda, J. J. Inorg. Chem. 1971,
10, 1202-5.
(24) Ruiz, R.; Surville-Barland, C.; Aukauloo, A.; Anxolabehere-Mallart,
E.; Journaux, Y.; Cano, J.; Munoz, M. C. J. Chem. Soc., Dalton Trans.
1997, 745-751.
states of transition metals. The reversible nature of the II/III
redox of copper and nickel suggested that these complexes
may be of value in catalysis.
Inorganic Chemistry, Vol. 42, No. 2, 2003 573

Polt et al.
Table 8. E/V Values for Cu(II)/(III) in CH3CN
irreversibility of the Ni III/IV oxidation states. The trans-
epoxides were the major product in every case, regardless
of the starting olefin geometry.27
Conclusions
The stability of the complexes and the variety of metals
accepted by the Schiff base ligands, coupled with the
simplicity of their synthesis and the widespread availability
of enantiomerically pure amino acids of either R or S
configuration, suggests that these ligands may be considered
“privileged”. Future work in this laboratory will revolve
around the use of these ligands in solution and in their resin-
bound forms to develop new enantioselective catalysts for
oxidations and carbon-carbon bond formation.
Acknowledgment. We thank the National Science Foun-
dation (Grants CHE-9201112, CHE-9526909 and CHE-
9729350) for partial support of this work and for the
provision of the X-ray diffraction apparatus (Grant CHE-
9610374). R.E.R. is a DuPont-Arizona Research Experience
undergraduate fellowship recipient.
Supporting Information Available: X-ray crystallographic files
in CIF format (for 7b‚NiII, p-MeO-7b‚NiII, 7f‚NiII, p-MeO-7f‚NiII,
rac-7g‚NiII, CH2Cl2-7g‚NiII, 7l‚NiII, 11). ESR details for 7f‚CuII,
7k‚CuII, 7k‚CoII, and 7k‚MnII. This material is available free of
charge via the Internet at http://pubs.acs.org.
a Reference 23. b This work. c Reference 22.
IC025996O
Ni(IV)dO species have been suggested as putative inter-
mediates in nickel catalyzed oxygen atom transfer reactions,26
results from epoxidation studies support Ni(III)-O• as a more
likely intermediate, especially in light of the observed
(27) The oxidation of simple cis- and trans-olefins was examined using
complex 7f‚iII and NaOCl (commercial bleach) as an oxidant.
[Meunier, B.; Guilmet, E.; De Carvahlo, M. E.; Poilblanc, R. J. Am.
Chem. Soc. 1984, 106, 6668-9.] No attempt was made to optimize
these reactions. While the ee’s were not large (∼4%), turnover numbers
of 12-14 with trans-â-methylstyrene and 7f‚NiII indicate that the
ligand system can function under the strongly oxidizing conditions of
catalysis. In several cases, the unchanged catalyst could be recovered
after the reaction. Other oxidants were also surveyed. PhIdO was also
effective as an oxidant, but NaIO4 and H2O2 were not. [Kinneary, J.
F.; Wagler, T. R.; Burrows, C. J. Tetrahedron Lett. 1988, 29, 877-
80.]
(25) Anson, F. C.; Collins, T. J.; Richmond, T. G.; Santarsiero, B. D.; Toth,
J. E.; Treco, B. G. R. T. J. Am. Chem. Soc. 1977, 99, 4527.
(26) (a) Yoon, H.; Wagler, T. R.; O’Connor, K. J.; Burrows, C. J. J. Am.
Chem. Soc. 1990, 112, 4568-70. (b) Fernandez, I.; Pedro, J. R.;
Rosello, A. L.; Ruiz, R.; Ottenwaelder, X.; Journaux, Y. Tetrahedron
Lett. 1998, 39, 2869-2872.
574 Inorganic Chemistry, Vol. 42, No. 2, 2003