Monooxomolybdenum(IV) complex with extremely bulky dithiolate ligands - Acceleration of O-atom transfer by distorted square pyramidal conformation

Article abstract of DOI:10.1246/cl.2005.44

Q₂[Mo^IVO{1,2-S₂-3,6-(Ph ₃CCONH)₂C₆H₂}₂] (Q = NEt₄ (1), PPh₄ (2)) was synthesized and characterized as a model for Mo-enzymes. The crystal structu

Full text of DOI:10.1246/cl.2005.44

4
4
Chemistry Letters Vol.34, No.1 (2005)
Monooxomolybdenum(IV) Complex with Extremely Bulky Dithiolate Ligands
Acceleration of O-atom Transfer by Distorted Square Pyramidal Conformation
—
ꢀ
Koji Baba, Taka-aki Okamura, Hitoshi Yamamoto, Tetsuo Yamamoto, Mitsuo Ohama, and Norikazu Ueyama
Department of Macromolecular Science, Graduate School of Science, Osaka University, Osaka, 560-0043
(Received September 6, 2004; CL-041047)
IV
Q2[Mo O{1,2-S2-3,6-(Ph3CCONH)2C6H2}2] (Q = NEt4
1), PPh4 (2)) was synthesized and characterized as a model
for Mo-enzymes. The crystal structure of 2 shows the slightly
(
IV
distorted square pyramidal core, Mo OS4. The O-atom transfer
reaction from Me3NO to Me3N in the presence of 1 proceeds at
IV
3
0 to 48 times higher rates than in the presence of [Mo O{1,2-
2
ꢁ
S2-(C6H4)}2] (3).
Molybdenum enzymes and related tungsten enzymes, which
are known to undergo O-atom transfer, are of great interest be-
cause they have a unique core structure and can act upon diverse
substrates. Their active sites characteristically have one or two
pterin dithiolene ligands, which cannot be found in other metal-
loenzymes, and one or two oxo ligands. In Figure 1, the active
1
Figure 2. Structure of the anion part of 2
perspective. (b) Space-filling view (top). (c) Space-filling view
side).
.
3
CH CN. (a) ORTEP
(
2
site of arsenite oxidase from Alcaligenes faecalis is illustrated.
Arsenite oxidase can be assigned to the dimethylsulfoxide
ꢀ
the amide groups and the short N–S distances (2.92–2.97 A),
NHꢃꢃꢃS hydrogen bonding is expected. Large differences
between 2 and 3 are not observed for other bond lengths and
angles.
2 shows a single broad NH band at 3291 cm in the solid
state, the low-frequency-shifted band indicates the presence of
IV
(DMSO) reductase family. The Mo OS4 core in the reduced
state has a distorted square pyramidal structure; the C1–C2–
ꢂ
C3–C4 dihedral angle is about 14 . Many model complexes with
IV
ꢁ1
a Mo OS4 core have been synthesized but with an approximate-
3
ly regular square pyramidal sturucture. In this study, we synthe-
sized novel monooxomolybdenum complexes (1, 2) using an ex-
tremely bulky dithiolate ligand, [1,2-S2-3,6-(Ph3CCONH)2-
9
NHꢃꢃꢃS hydrogen bonding, as expected from the crystal struc-
ture of 2.
ꢁ1
The shifts of the Mo=O in 1 from that of 3 are þ24 cm for
2ꢁ
C6H2] (Figure 1), to clarify the relationship between function
and distorted structure.
ꢁ1
the resonance Raman band and þ23 cm for the IR band,
ꢁ1
whereas the Mo–S Raman band shifts þ10 cm from that of
3
. The LMCT band (348 nm) of 1 in DMF solution is red-shifted
1
0
IV
V
by 20 nm from 3 (328 nm). The Mo /Mo redox potential of
in DMF solution is positively shifted by 0.41 V from 3
ꢁ0:38 V vs SCE). As compared with L = 1,2-S2-C2(CH3)2, en-
hancements of Mo=O, red-shifts, and positive shifts are ob-
served for [Mo OL2] (L = 1,2-S2-C2(COOCH3)2, 1,2-S2-
C2(CF3)2, 1,2-S2-C2(CN)2) with electron-withdrawing substitu-
1
(
IV
2ꢁ
1
1
ents. Because the inductive effect of the triphenylacetoamido
1
2
group is thought to be weak, the delocalization of charge on
sulfur atoms by NHꢃꢃꢃS hydrogen bonding should be the main
reason of these shifts.
Figure 1. (a) Distorted square pyramidal structure of active site
in arsenite oxidase. (b) Synthetic bulky dithiolate ligand.
Figure 3 shows the spectroscopic time course of the O-atom-
transfer reaction between 1 and Me3NO, one of the substrates for
Synthesis of 1 or 2 involved a ligand-exchange reaction of
V
V
4
(
3
NEt4)[Mo O(SPh)4] or (PPh4)[Mo O(SPh)4] with {1,2-S2-
5
,6-(Ph3CCONH)2C6H2}2, accompanied with reduction by
6
NEt4BH4.
In Figure 2, the crystal structure of the anion part of
.
7
2
8CH3CN is shown. The space-filling representation clearly
shows that the bulky triphenyl groups form a unique conforma-
tion like as two engaged gears to avoid their steric hindrance.
ꢂ
The C11–C12–C31–C32 dihedral angle of 2 is 5.6 , which is
ꢂ
ꢂ
slightly larger than that of 3 (5.0 and 2.7 for two structures
8
in a unit cell). The amide groups are not coplanar with the ben-
zene rings and the C(amide)–N–C(benzene ring)–C(benzene
Figure 3. Visible spectral change during O-atom-transfer reac-
ꢂ
ꢂ
ring) dihedral angles are 5–25 . Considering the orientation of
tion in DMF at 27 C with ½1ꢄ ¼ 1 mM and ½Me3NOꢄ ¼ 2 mM.
0
0
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.1 (2005)
45
4
5
J. R. Bradbury, M. F. Mackay, and A. G. Wedd, Aust. J. Chem., 31,
2
423 (1978).
Ligand synthesis is as follows. (n-Bu4N)2{1,2-(S2O3)2-3,6-(NH2)2-
C6H2} was derived from a cation-exchange reaction of K2{1,2-
(
S2O3)2-3,6-(NH2)2C6H2}17 and n-Bu4NBr using a liquid–liquid ex-
traction. To a chloroform solution (30 mL) of (n-Bu4N)2{1,2-(S2O3)2-
3
6
,6-(NH2)2C6H2} (1.0 g, 1.2 mmol) including Et3N (0.84 mL,
.0 mmol) was slowly added Ph3CCOCl (0.90 g, 2.9 mmol), which
is synthesized from Ph3COOH and thionyl chloride. The mixture
was stirred overnight at reflux. The solution was then cooled to room
temperature and washed with water (100 mL) and dried over Na2SO4.
The solution was concentrated in volume to 5 mL, and then diethyl
ether (30 mL) was added. The resulting white powder of (n-Bu4N)2-
{
1,2-(S2O3)2-3,6-(Ph3CCONH)2C6H2} was collected and washed
with diethyl ether. (n-Bu4N)2{1,2-(S2O3)2-3,6-(Ph3CCONH)2C6H2}
620 mg, 0.46 mmol) and i-PrSNa (380 mg, 3.9 mmol) were suspend-
Figure 4. Proposed mechanism of O-atom-transfer reaction be-
tMw ee 3e Nn O d. ithiolene complexes, 1 (———) or 3 (ꢃ ꢃ ꢃ ꢃ ꢃ ꢃ), and
(
ed in methanol (20 mL), and the reaction mixture was heated at reflux
for 12 h under Ar. Ethanol was evaporated under reduced pressure.
The residue was dissolved in dichloromethane (100 mL). The solution
was washed with brine, 2% HCl, brine, 4% NaHCO3, and brine; dried
over Na2SO4; and concentrated in vacuo. The residual solid was
dissolved in methanol (30 mL), and the solution was heated under
reflux overnight. The obtained yellow precipitate of {1,2-S2-3,6-
(Ph CCONH) C H } was collected with filtration and washed with
the DMSO reductase family.13 Trimetylamine N-oxide reductase
is crystallographically characterized only in the oxidized form;
two pterin dithiolene, two oxo, and Oꢀ of Ser147 are revealed
13
to be the ligands. 1 reacts quickly with Me3NO to give
VI
2ꢁ
[
Mo O2{1,2-S2-3,6-(Ph3CCONH)2C6H2}2] and Me3N. The
3
2
6
2 2
initial rate constant (kobs) is analyzed by pseudo-first-order kinet-
ics using the LMCT band (531 nm) for the Mo O2 complex.
The isosbestic point is observed at 350 nm. The kobs
methanol and diethyl ether. Yield, 130 mg (20%); Anal. Calcd for
C92H68N O S : C, 77.72; H, 4.82; N, 3.94. Found: C, 76.86; H,
VI
4
4 4
4
1
.76; N, 4.02%. IR (KBr): ꢁNH 3361, 3345, 3316, ꢁC=O 1704,
1
ꢁ
685 cm
.
ꢁ
1
(
0.026 s ) for 1 (1 mM) is 48 times larger than that of 3 in
1
6
1 was synthesized as follows: An acetonitrile (5 mL) solution of {1,2-
S2-3,6-(Ph3CCONH)2C6H2}2 (100 mg, 0.29 mmol) and NEt4BH4
(45 mg, 0.31 mmol) was added with stirring to a solution of (NEt4)-
0
the presence of 2 mM Me3NO. The kobs for 1 increases to
ꢁ
1
0
.17 s at 10 mM Me3NO and is 30 times larger than 3.
IV
[MoVO(SPh) ] (100 mg, 0.15 mmol) in a mixed solvent of acetonitrile
Previously, we reported that the Mo complex with inter-
molecular NHꢃꢃꢃS hydrogen bonding accelerates Me3NO reduc-
4
(
10 mL) and water (1 mL), resulting in the formation of a yellow pow-
der that was collected with filtration. The obtained yellow powder of 1
was washed with acetonitrile and diethyl ether and dried in vacuo.
Yield, 230 mg (87%); Anal. Calcd for C108H108N6O5MoS4: C,
72.30; H, 6.07; N, 4.68. Found: C, 69.26; H, 6.14; N, 5.06%. Absorp-
1
4
tion by about 6 times. However, it is not explainable that the
large acceleration exhibited by 1 is caused only by the presence
IV
of NHꢃꢃꢃS hydrogen bonding. In the case of [Mo O(1,2-1,2-S2-
2ꢁ
ꢁ1
ꢁ1
tion spectrum (DMF): ꢂmax ("M cm ) 348 (12000), 406 (sh)
3
-Ph3SiC6H3)2] with bulky ligands, the trans–cis rearrange-
(
980), 457 (530) nm. IR (KBr): ꢁNH 3332, 3315, 3280, ꢁC=O 1680,
Mo=O 928 cm . Raman: ꢁMo=O 926, ꢁMo{S 366 cm
ment is the rate-determining step in the proposed reaction mech-
ꢁ
1
ꢁ1
1
ꢁ
. H NMR
1
0
anism of 3 (Figure 4). However, 1 did not show such a rate-de-
termining step in spite of much bulkier ligands. Therefore, a new
reaction mechanism for 1 should be proposed in Figure 4. In so-
lution, the free rotation of Ph–C and C–C(=O) bonds in Ph3C–
C(=O)–NH and the increase of the C(amide)–N–C(benzene
ring)–C(benzene ring) dihedral angles by thermal vibration cre-
ate temporarily more distortion than in the solid state. But, in the
cis form, such steric repulsion is decreased. Thus, the extremely
large oxidation rate for 1 is attributed to the release from the
distortion. The initial step of the reaction is considered to be
Me3NO attack to a vacant space cis to the oxo ligand, formed
by the distortion. Such cis-attack is discussed for the active site
in Mo-, W-enzymes.
Recently a distorted square pyramidal structure has also
been reported in the active center of dithionite-reduced DMSO
reductase from Rhodobacter capsulatus.16 The Mo center in
the reduced form is hexacoordinated with two dithiolenes, an
(
7
DMF-d7, anion): ꢃ 9.34 (s, 4), 7.97 (s, 4), 7.49 (d, 24), 7.37 (t, 24),
.26 (t, 12). Using a PPh4 cation, 2 was synthesized by a similar
method described for 1. Calcd for C116H108N4O5P2MoS4: C, 76.00;
H, 4.92; N, 2.53. Found: C, 73.67; H, 5.08; N, 2.45%.
.
7
Crystal data for 2 8CH3CN. C156H132N12O5P2S4Mo, Mr ¼ 2540:86,
monoclinic, P21=n, a ¼ 26:071ð6Þ, b ¼ 14:944ð2Þ, c ¼ 35:217ð8Þ Aꢀ ,
ꢄ ¼ 93:81ð2Þ , Z ¼ 4, V ¼ 13691ð5Þ Aꢀ , Dcalcd ¼ 1:233 gcm
ꢂ
3
ꢁ3
,
ꢁ1
ꢅ ¼ 0:240 mm , 18479 reflections measured, 17861 independent
int
(R ¼ 0:1034), R1 ¼ 0:0672 (I > 2ꢆðIÞ), wR2 ¼ 0:2347 (all data).
8
9
1
S. Boyde, S. R. Ellis, C. D. Garner, and W. Clegg, J. Chem. Soc.,
Chem. Commun., 1986, 1541.
A disulfide, (S-2-Ph3CCONHC6H4)2, is utilized as a standard
ꢁ1
reference. It has a free ꢁNH at 3341 cm in CH2Cl2 solution.
0 H. Oku, N. Ueyama, M. Kondo, and A. Nakamura, Inorg. Chem., 33,
209 (1994).
11 a) D. Coucouvanis, A. Hadjikyriacou, A. Toupadakis, S. M. Koo, O.
Ileperuma, M. Draganjac, and A. Salifoglou, Inorg. Chem., 30, 754
(1991). b) K. Wang, J. M. McConnachie, and E. I. Stiefel, Inorg.
Chem., 38, 4334 (1999). c) S. Sarkar and S. K. Das, Proc.-Indian
Acad. Sci., Chem. Sci., 104, 437 (1992). d) B. S. Lim, J. P. Donahue,
and R. H. Holm, Inorg. Chem., 39, 263 (2000).
12 T. Okamura, S. Takamizawa, N. Ueyama, and A. Nakamura, Inorg.
Chem., 37, 18 (1998).
13 M. Czjzek, J. P. Dos Santos, J. Pommier, G. Giordano, V. Mejean, and
R. Haser, J. Mol. Biol., 284, 435 (1998).
1
5
oxo ligand, and Oꢀ of Ser147. The dihedral angle of four carbon
atoms in two dithiolenes is about 31 . The large distortion from
ꢂ
the square pyramidal conformation in the active site of DMSO
IV
reductase will accelerate the O-atom transfer from Mo O to
VI
Mo O2.
1
4 H. Oku, N. Ueyama, and A. Nakamura, Inorg. Chem., 36, 1504
1997).
(
References and Notes
15 F. Schneider, J. L o¨ we, R. Huber, H. Schindelin, C. Kisker, and
J. Kn a¨ blein, J. Mol. Biol., 263, 53 (1996).
16 A. S. McAlpine, A. G. McEwan, A. L. Shaw, and S. Bailey, J. Biol.
Inorg. Chem., 2, 690 (1997).
17 A. G. Green and A. G. Perkin, J. Chem. Soc., 83, 1201 (1903).
1
2
3
R. Hille, Met. Ions Biol. Syst., 39, 187 (2002).
P. J. Ellis, T. Conrads, R. Hille, and P. Kuhn, Structure, 9, 125 (2001).
J. H. Enemark, J. J. Cooney, J. J. Wang, and R. H. Holm, Chem. Rev.,
1
04, 1175 (2004) and the references therein.
Published on the web (Advance View) December 4, 2004; DOI 10.1246/cl.2005.44