Crystal structures and 77Se NMR spectra of molybdenum(IV) areneselenolates having intramolecular NH...Se hydrogen bonds

Article abstract of DOI:10.1021/ic061140y

Salts of the monooxomolybdenum(IV,V) areneselenolates having intramolecular NH...Se hydrogen bonds, [Mo^IVO-(Se-2-RCONHC₆H ₄)₄]^2- (R = t-Bu, CH₃, CF ₃) and [Mo^VO(Se-2-t-BuCONHC₆H₄) ₄]^-, were synthesized and characterized by ¹H nuclear magnetic resonance (NMR), ⁷⁷Se NMR, electron spin resonance (ESR), UV-visible spectra, X-ray analysis, and electrochemical measurements. ⁷⁷Se-¹H correlated spectroscopy (COSY) indicated a significant correlation between amide ¹H and selenolate ⁷⁷Se atoms through an NH...Se hydrogen bond with ¹J(⁷⁷Se-¹H) = 5.4 Hz coupling. The hydrogen bonds contribute to the positive shift in the Mo(V)/Mo(IV) redox potential. In the crystal structure of (PPh₄)₂[Mo^IVO(Se-2- CH₃CONHC₆H₄)₄], an NH...O=Mo hydrogen bond was found. Ab inito calculations support the presence of intramolecular NH...O=Mo and NH...Se hydrogen bonds.

Full text of DOI:10.1021/ic061140y

Inorg. Chem. 2006, 45, 9374−9380
Crystal Structures and 77Se NMR Spectra of Molybdenum(IV)
Areneselenolates Having Intramolecular NH‚‚‚Se Hydrogen Bonds
Taka-aki Okamura,*,† Kaku Taniuchi,† Keonil Lee,† Hitoshi Yamamoto,† Norikazu Ueyama,*,† and
Akira Nakamura‡
Department of Macromolecular Science, Graduate School of Science, Osaka UniVersity,
Toyonaka, Osaka 560-0043, Japan, and OM Research, 7-2-1308, Minami Ogimachi,
Kita-ku, Osaka 530-0052, Japan
Received June 23, 2006
Salts of the monooxomolybdenum(IV,V) areneselenolates having intramolecular NH‚‚‚Se hydrogen bonds, [MoIVO-
-
(Se-2-RCONHC6H4)4]2 (R
)
t-Bu, CH3, CF3) and [MoVO(Se-2-t-BuCONHC6H4)4]-, were synthesized and
characterized by 1H nuclear magnetic resonance (NMR), 77Se NMR, electron spin resonance (ESR), UV
spectra, X-ray analysis, and electrochemical measurements. 77Se 1H correlated spectroscopy (COSY) indicated a
significant correlation between amide 1H and selenolate 77Se atoms through an NH‚‚‚Se hydrogen bond with
1J(77Se 1H)
5.4 Hz coupling. The hydrogen bonds contribute to the positive shift in the Mo(V)/Mo(IV) redox
potential. In the crystal structure of (PPh4)2[MoIVO(Se-2-CH3CONHC6H4)4], an NH‚‚‚
Mo hydrogen bond was
found. Ab inito calculations support the presence of intramolecular NH‚‚‚ Mo and NH‚‚‚Se hydrogen bonds.
−visible
−
−
)
O
d
O
d
Introduction
topes and one of them, 77Se, exhibits I ) 1/2 and enough
natural abundance (7.6%) to use for NMR measurements
without any enrichment.7
We have systematically investigated the function of
NH‚‚‚S hydrogen bonds in metalloproteins (e.g., molyb-
doenzyme,8,9 rubredoxin,10,11 ferredoxins,12 P-450,13,14 and
copper protein15) using a variety of model compounds. The
presence of NH‚‚‚S hydrogen bonds was established by IR
and 1H NMR spectroscopy. These methods also indicate the
Molybdenum enzymes, with the exception of nitrogenase,
are widely distributed in both eukaryotes and prokaryotes.
They are known to catalyze the reduction and oxidation of
many substrates, and their active sites contain molybdenum-
thiolate bonds.1 One of these enzymes, formate dehydroge-
nase H, has an additional molybdenum-selenolate (seleno-
cysteine) bond, which is essential for catalytic activity2,3 and
has been established by X-ray crystallography.4 The presence
of a hydrogen bond was proposed between selenocysteine
and the side chain of a histidine residue and has been believed
to play a role in proton-transfer reactions.
(5) Ralle, M.; Berry, S. M.; Nilges, M. J.; Gieselman, M. D.; van der
Donk, W. A.; Lu, Y.; Blackburn, N. J. J. Am. Chem. Soc. 2004, 126,
7244-7256.
(6) Johansson, L.; Gafvelin, G.; Arne´r, E. S. J. Biochim. Biophys. Acta
2005, 1726, 1-13.
Substituting a sulfur atom with selenium is a powerful
technique for determining metalloprotein coordination or
binding using X-ray absorption, ESR, NMR, and other
various spectroscopies.5,6 Selenium contains six stable iso-
(7) Emsley, J. The Elements; Oxford Press: Oxford, U.K.,1989.
(8) Ueyama, N.; Okamura, T.; Nakamura, A. J. Am. Chem. Soc. 1992,
114, 8129-8137.
(9) Baba, K.; Okamura, T.; Suzuki, C.; Yamamoto, H.; Yamamoto, T.;
Ohama, M.; Ueyama, N. Inorg. Chem. 2006, 45, 894-901.
(10) Okamura, T.; Takamizawa, S.; Ueyama, N.; Nakamura, A. Inorg.
Chem. 1998, 37, 18-28.
* To whom correspondence should be addressed. E-mail:
tokamura@chem.sci.osaka-u.ac.jp (T.O.), ueyama@chem.sci.osaka-u.ac.jp
(N.U.).
(11) Ueyama, N.; Okamura, T.; Nakamura, A. J. Chem. Soc., Chem.
Commun. 1992, 1019-1020.
(12) Ueyama, N.; Yamada, Y.; Okamura, T.; Kimura, S.; Nakamura, A.
Inorg. Chem. 1996, 35, 6473-6484.
† Osaka University.
‡ OM Research.
(1) Hille, R. Chem. ReV. 1996, 96, 2757-2816.
(2) Gladyshev, V. N.; Khangulov, S. V.; Axley, M. J.; Stadtman, T. C.
Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 7708-7711.
(3) Khangulov, S. V.; Gladyshev, V. N.; Dismukes, G. C.; Stadtman, T.
C. Biochemistry 1998, 37, 3518-3528.
(4) Boyington, J. C.; Gladyshev, V. N.; Khangulov, S. V.; Sun, P. D.
Science 1997, 275, 1305-1308.
(13) Ueyama, N.; Nishikawa, N.; Yamada, Y.; Okamura, T.; Oka, S.;
Sakurai, H.; Nakamura, A. Inorg. Chem. 1998, 37, 2415-2421.
(14) Ueyama, N.; Nishikawa, N.; Yamada, Y.; Okamura; T.; Nakamura,
A. J. Am. Chem. Soc. 1996, 118, 12826-12827.
(15) Okamura, T.; Ueyama, N.; Nakamura, A.; Ainscough, E. W.; Brodie,
A. M.; Waters, J. M. J. Chem. Soc., Chem. Commun. 1993, 1658-
1659.
9374 Inorganic Chemistry, Vol. 45, No. 23, 2006
10.1021/ic061140y CCC: $33.50
© 2006 American Chemical Society
Published on Web 10/12/2006

NH‚‚‚Se Hydrogen Bonds in Molybdenum Selenolates
Anal. Calcd for C48H40OPSe4Mo: C, 53.60; H, 3.75. Found: C,
53.17; H, 4.03.
strength of the NH bonds and the electron density or
negativity of the hydrogen atom. The strength of the
hydrogen bond was indirectly estimated from these data. We
have previously reported molybdenum arenethiolates having
intramolecular NH‚‚‚S hydrogen bonds, [MoIVO(S-2-
RCONHC6H4)4]2- and [MoVO(S-2-RCONHC6H4)4]- (R )
CH3, t-Bu, CF3), and have demonstrated the contribution of
the hydrogen bonds to the positive shift of the redox
potential.8 In this Study, molybdenum areneselenolates
having intramolecular NH‚‚‚Se hydrogen bonds were syn-
thesized, and the hydrogen bonds were detected directly by
77Se-1H nuclear spin-spin coupling. Although no example
of monooxomolybdenum(IV) areneselenolate has ever been
isolated, the NH‚‚‚Se hydrogen bond facilitated the isolation
of the complexes in the reduced state.
(PPh4)[MoVO(Se-2-t-BuCONHC6H4)4] (1b). The complex was
prepared by a ligand exchange method. A mixture of (Ph4P)[MoVO-
(SPh)4] (40 mg, 0.05 mmol) and bis(2-pivaloylaminophenyl)
diselenide (70 mg, 0.14 mmol) in 1,2-dimethoxyethane (DME, 2
mL) was stirred at room temperature overnight. The precipitate was
collected by filtration and was washed with diethyl ether. The blue
crude product was recrystallized from hot acetonitrile, and dark
blue microcrystals were obtained. Yield 20 mg (30%). Anal. Calcd
for C68H76N4O5PSe4Mo: C, 55.48; H, 5.20; N, 3.81. Found: C,
54.05; H, 5.16; N, 3.74. The disagreements of the elemental analyses
for 1b and 1a are probably caused by their hygroscopic character
and the instability of these Mo(V) compounds. The calculated values
for 1b‚(H2O)2: C, 54.15; H, 5.35; N, 3.72, agree with the found
ones.
(NEt4)[MoVO(Se-2-t-BuCONHC6H4)4] (1a). The complex was
synthesized by a similar method as described for the synthesis of
(Ph4P)[MoVO(Se-2-t-BuCONHC6H4)4] using (NEt4)[MoVO(SePh)4].
Dark blue microcrystals were obtained from hot acetonitrile. Anal.
Calcd for C52H76N5O5Se4Mo: C, 49.45; H, 6.07; N, 5.55. Calcd
for 1a‚(H2O)3: C, 47.42; H, 6.28; N,5.32. Found: C, 47.54; H,
5.89; N, 5.02.
Experimental Section
For the molybdenum compounds, all procedures including
synthesis, preparation of samples, and spectroscopic measurements
were performed under argon atmosphere by the Schlenk technique.
The isolated mercury complex is air-stable but the synthesis was
carried out in argon atmosphere. The syntheses of the ligands were
carried out in air. All solvents were dried and distilled under argon
before use. Bis(2-aminophenyl) diselenide,16 bis(2-acetylaminophe-
nyl) diselenide,16 [MoVO(SPh)4]- (NEt4+ and PPh4+ salts),17 (PPh4)2-
[MoIVO(S-4-ClC6H4)4],18 and (NEt4)[MoVO(SePh)4]19 were syn-
thesized by the reported methods.
(NEt4)2[MoIVO(Se-2-t-BuCONHC6H4)4] (2a). A mixture of
(NEt4)[MoVO(SePh)4] (220 mg, 0.25 mmol) and bis(2-pivaloy-
laminophenyl) diselenide (460 mg, 0.90 mmol) in DME (10 mL)
was stirred at room temperature overnight. The precipitate was
collected by filtration and was washed with DME and diethyl ether.
After drying under reduced pressure, tetraethylammonium boro-
hydride (37 mg, 0.26 mmol) was added, and the mixture was stirred
overnight in DME (10 mL) at room temperature. The precipitate
was collected by filtration and was washed with DME until the
washings were colorless. The crude powder was dissolved in 20
mL of acetonitrile and was filtered to remove insoluble materials.
The solution was concentrated under reduced pressure and the
residue was recrystallized from hot acetonitrile/diethyl ether to give
purple-red microcrystals. Yield 50 mg (14%). 1H NMR(acetonitrile-
d3) δ 9.08 (s, 4H), 8.11 (d, 4H), 7.56 (d, 4H), 6.98 (t, 4H), 6.68 (t,
4H), 3.08 (q, 16H), 1.21 (s, 36H), 1.15 (t, 24H). Anal. Calcd for
C60H96N6O5Se4Mo: C, 51.73; H, 6.95; N, 6.03. Found: C, 51.26;
H, 6.88; N, 6.00.
Bis(2-pivaloylaminophenyl) Diselenide. To a CH2Cl2 solution
(10 mL) of bis(2-aminophenyl) diselenide (1.0 g, 3.0 mmol)
containing pyridine (1 mL) was added slowly pivaloyl chloride (3.0
mL, 24 mmol) at room temperature. After stirring with refluxing
for 3 h, the solution was concentrated under reduced pressure. The
residue was dissolved in diethyl ether and was washed with water,
2% HCl aq, water, 4% NaHCO3 aq, and water, successively. The
organic layer was dried over sodium sulfate and was concentrated
under reduced pressure to give orange oil. The material was
recrystallized from hot n-hexane, and orange needles were obtained.
Yield, 1.2 g (83%). 1H NMR (acetonitrile-d3) δ 8.26 (s, 2H), 7.53
(d, 2H), 7.55 (d, 2H), 7.30 (t, 2H), 7.01 (t, 2H), 1.17 (s, 18H).
Anal. Calcd for C22H28N2O2Se2: C, 51.77; H, 5.53; N, 5.53.
Found: C, 51.53; H, 5.45; N, 5.38.
(NEt4)2[MoIVO(Se-2-CH3CONHC6H4)4] (3a). The complex was
synthesized by the same method as described for the synthesis of
(NEt4)2[MoIVO(Se-2-t-BuCONHC6H4)4] using bis(2-acetylami-
nophenyl) diselenide. The purple-red microcrystals were obtained
Bis(2-trifluoroacetylaminophenyl) Diselenide. To a tetrahy-
drofuran (THF) (10 mL) solution of bis(2-aminophenyl) diselenide
(0.5 g, 1.5 mmol) was added dropwise trifluoroacetic anhydride
(0.8 mL, 0.6 mmol) with cooling in an ice bath. The solution was
stirred overnight. After removal of solvents, water was added to
the residue. The precipitate was washed with water, 2% HCl aq,
water, 4% NaHCO3 aq, and water, successively. The crude product
was recrystallized from ethanol to give a yellowish white powder.
1
in a very low yield (ca. 10%). H NMR(acetonitrile-d3) δ 9.19 (s,
4H), 8.10 (d, 4H), 7.45 (d, 4H), 7.05 (t, 4H), 6.73 (t, 4H), 3.11 (q,
16H), 2.04 (s, 12H), 1.18 (t, 24H). Anal. Calcd for C48H72N6O5-
Se4Mo: C, 47.07; H, 5.92; N, 6.86. Calcd for 3a‚(H2O)1.5: C, 46.05;
H, 6.04; N, 6.71. Found: C, 45.98; H, 5.88; N, 6.81.
(PPh4)2[MoIVO(Se-2-CH3CONHC6H4)4] (3b). To a solution of
(NEt4)2[MoIVO(Se-2-CH3CONHC6H4)4] (9.1 mg, 7.9 × 10-6 mol)
in acetonitrile was added an acetonitrile solution (1 mL) of Ph4-
PBr (7.5 mg, 1.8 × 10-5 mol). Dark reddish-purple crystals were
deposited, which were washed with acetonitrile. The crystals were
recrystallized from hot acetonitrile to give reddish-purple plates.
This compound was characterized by X-ray analysis.
1
Yield 0.67 g (86%). H NMR(chloroform-d) δ 8.76 (s, 2H), 8.35
(d, 2H), 7.46 (m, 4H), 7.09 (t, 2H).
(PPh4)[MoVO(SePh)4]. The complex was synthesized by a
similar method reported for (NEt4)[MoVO(SePh)4].19 Dark-blue
powder was obtained and was found pure without recrystallization.
(PPh4)2[MoIVO(Se-2-CF3CONHC6H4)4] (4). A solution of
(PPh4)2[MoIVO(S-4-ClC6H4)4] (51 mg, 3.8 × 10-5 mol) and (Se-
o-CF3CONHC6H4)2 (48 mg, 9.0 × 10-5 mol) in acetonitrile (2 mL)
was stirred for 1 day. The solution was concentrated under reduced
pressure. Diethyl ether was added to the solution and the mixture
was refrigerated to afford red plates, which were washed with
(16) Keimatsu, S.; Satoda, I. Yakugaku Zasshi 1936, 56, 600-607.
(17) Boyd, I. W.; Dance, I. G.; Murray, K. S.; Wedd, A. G. Aust. J. Chem.
1978, 31, 279-284.
(18) Kondo, M.; Ueyama, N.; Fukuyama, K.; Nakamura, A. Bull. Chem.
Soc. Jpn 1993, 66, 1391-1396.
(19) Hanson, G. R.; Brunette, A. A.; McDonell, A. C.; Murray, K. S.; Wedd,
A. G. J. Am. Chem. Soc. 1981, 103, 1953-1959.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9375

Okamura et al.
diethyl ether and were dried in vacuo. Yield 24 mg (32%). 1H NMR
(acetonitrile-d3) δ 9.83 (s, 4H), 8.05 (d, 4H), 7.6-7.9 (m, 40H),
7.54 (d, 4H), 7.10 (t, 4H), 6.84 (t, 4H). Anal. Calcd for
C80H60N4O5F12P2Se4Mo: C, 51.69; H, 3.25; N, 3.01. Found: C,
51.47; H, 3.12; N, 4.50.
Table 1. Cryatallographic Data
3b
4
chemical formula
fw
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C80H72N4O5P2Se4Mo
1643.20
monoclinic
P21
14.728(15)
13.149(11)
19.335(15)
90
109.61(3)
90
3527(5)
2
C80H60N4O5F12P2Se4Mo
1859.09
triclinic
P1h
Hg(Se-2-t-BuCONHC6H4)2. To a methanol solution (10 mL)
of bis(2-pivaloylaminophenyl) diselenide (230 mg, 0.45 mmol) was
added sodium borohydride until the solution turned colorless at
room temperature. After the solution was stirred for 5 min, acetic
acid (4.0 mL) was added to hydrolyze the excess of sodium
borohydride. Mercury(II) dichloride (230 mg, 0.85 mmol) was
added to the solution which was stirred for 3 h. The clear solution
was concentrated under reduced pressure to give orange oil. After
addition of saturated NaCl aq (100 mL), the crude product was
extracted by diethyl ether (400 mL). The organic layer was separated
and washed successively with 2% HCl aq, water, 4% NaHCO3 aq,
water, and sat. NaCl aq. The solution was dried over anhydrous
sodium sulfate and was concentrated under reduced pressure. The
resulting orange solid was recrystallized from diethyl ether/n-hexane
to give air-stable colorless needles. Yield 105 mg (33%). 1H NMR
(Me2SO-d6); δ 8.94(s, 2H), 7.94(d, 2H), 7.66(d, 2H), 7.20(t, 2H),
6.90(t, 2H), 1.23(s, 18H); 13C NMR (Me2SO-d6); d 175.71, 139.86,
137.63, 127.60, 124.08, 121.67, 120.72, 27.35. Anal. Calcd for
C22H28N2O2Se2Hg: C, 37.17, H; 3.97; N, 3.94. Found: C, 37.34;
H, 4.04; N, 3.79.
12.624(9)
13.420(8)
13.886(9)
61.934(17)
66.77(2)
69.349(18)
1867(2)
1
1.654
2.254
8489
0.0429
Z
d
calc, g cm-3
1.547
2.351
11079
0.0475
abs coeff, mm-1
no. of ind reflns
R1a[I > 2σ(I)]
wR2b(all data)
0.0668
0.0867
a R1 ) ∑||Fo| - |Fc/∑|Fo|. b wR2 ) {∑[w(Fo
Fc )2]/∑[w(Fo )2]}1/2
.
2
-
2
2
Chart 1
Physical Measurements. Visible spectra in DMF solution were
recorded on a Jasco Ubest-30 spectrometer. 1H NMR spectra were
taken on a JEOL GSX-400 spectrophotometer operating at 399.65
MHz. 77Se NMR spectra were obtained at room temperature on a
76.10 MHz JEOL GSX 400 spectrometer using dimethyl selenide
in DMF-d7 as an external standard of the chemical shift. IR spectra
in solid state were recorded on a Jasco FT/IR-3 spectrophotometer.
Electrochemical measurements were carried out using a Yanaco
P-1100 instrument in DMF that contained 0.1 M tetra-n-butylam-
monium perchlorate as a supporting electrolyte. E1/2 value, deter-
mined as (Ep,a + Ep,c)/2, was referenced to the saturated calomel
electrode (SCE) electrode at room temperature. ESR spectra in
acetonitrile/DMF (4/1 v/v) were recorded on a JEOL RE-2X
spectrometer at room temperature and at -150 °C.
calculated using the program BORDER23 on the basis of the wave
function obtained in Gaussian calculations.
X-ray Analysis. Each single crystal of (PPh4)2[MoIVO(Se-2-
RCONHC6H4)4] (R ) CH3(3b), CF3(4)) was mounted in a loop
with Nujol, which was frozen immediately in a stream of cold
nitrogen at 100 K. Data collections were made on a Rigaku
RAXIS-RAPID Imaging Plate diffractometer with graphite mono-
chromated Mo KR radiation (0.71075 Å). The structures were
solved by direct method using SIR9220 and were expanded by
Fourier technique using SHELXL-97.21 Complex 4 has a crystal-
lographical center of symmetry. All hydrogen atoms were placed
at the calculated positions using the riding model. Non-hydrogen
atoms were refined anisotropically. The final Fourier map did not
show any significant features. The crystallographic data are listed
in Table 1.
Ab Initio Molecular Orbital Calculations. Single-point MO
calculations of the anion part of 3b, [MoIVO(Se-2-CH3-
CONHC6H4)4]2-, and a pair of the anion and the cation of 4,
[(PPh4)[MoIVO(Se-2-CF3CONHC6H4)4]]-, were performed using
Gaussian 0322 program package with 3-21G** basis set. The
coordinates of the crystal structure were used without any geo-
metrical optimization. The bond orders of hydrogen bonds were
Results and Discussion
Syntheses. To synthesize the monooxomolybdenum(V)
areneselenolate (Chart 1), (NEt4)[MoVO(Se-2-t-BuCON-
HC6H4)4] (1a), a similar method as described for (NEt4)-
[MoVO(S-2-t-BuCONHC6H4)4] was used (eq 1).8 Subsequent
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision B.05; Gaussian, Inc.: Wallingford, CT, 2003.
(23) Mayer, I.; Program BORDER, version 1.0; Chemical Research Center,
Hungarian Academy of Science: Budapest, Hungary, 2005.
(20) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giacovazzo,
C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435.
(21) Sheldrick, G. M. SHELXL-97-Program for the Refinement of Crystal
Structures; University of Goettingen: Goettingen, Germany, 1997.
9376 Inorganic Chemistry, Vol. 45, No. 23, 2006

NH‚‚‚Se Hydrogen Bonds in Molybdenum Selenolates
Figure 1. Molecular structure (anion part) of 3b.
Figure 2. Molecular structure (anion part) of 4.
reduction by NEt4BH4 produced the corresponding molyb-
denum(IV) complex (eq 2), where SeAr represents Se-2-
RCONHC6H4.
(4)° and 143.22(4)° for 3b and 151.61(4)° and 151.04(4)°
for 4. The differences between the two Se-Mo-Se angles
for each complex are 4.26°and 0.57°, respectively, which
indicate that each MoOSe4 core has a typical square-
pyramidal geometry. The corresponding thiolate complex 5
has a larger value (7.16°). A geometrical distortion toward
a trigonal bipyramidal structure has been known for the
molybdenum(V) thiolate (PPh4)[MoVO(S-2-CH3CONHC6H4)4]
(12.76°).8 The geometrical distortion is in agreement with
the shortening of the Mo-Q bond length, MoV-S (mean
2.397 Å) < MoIV-S (2.408 Å) < MoIV-Se (2.524 Å). The
mean O-Mo-Q-C (Q ) Se or S) torsion angles (-82.5°
for 3b, -85.1° for 4) are similar to that (81.4°) of 5. The
Mo-Se-C-C torsion angles (mean 68.6°) of 3b are also
similar to the Mo-S-C-C torsion angle (mean -69.8°) of
5. The large 4pπ orbital of the selenium atom enables the
formation of the NH‚‚‚Se hydrogen bond; however, the
direction of the amide NH is not so desirable. Interestingly,
one (N1H1) of the four amide NH groups in 3b is directed
to OdMo rather than to Se, where the torsion angle between
the amide plane and the benzene ring, C13-C12-N1-C17,
is -43°. This unexpected NH‚‚‚OdMo interaction is dis-
cussed in a subsequent section using molecular orbital (MO)
calculations. The four acylamino groups have been believed
to be located predominantly on the same side (all up) as the
ModO, as illustrated in Chart 1. When the bulky t-
BuCONH- group was used, a minor conformer with
alternatively directed (up-down-up-down) acylamino groups
(NEt4)[MoVO(SePh)4] + 2ArSeSeAr f
(NEt4)[MoVO(SeAr)4] + 2PhSeSePh (1)
NEt4BH4
(NEt4)[MoVO(SeAr)4]
8 (NEt4)2[MoIVO(SeAr)4] (2)
When (NEt4)[MoVO(SPh)4] was used as a starting material,
an incomplete ligand-exchange reaction was observed and
1a could not be isolated; however, (PPh4)[MoVO(SPh)4] gave
(PPh4)[MoVO(Se-2-t-BuCONHC6H4)4] (1b) because of the
poor solubility and good crystallinity characteristics of the
product. In the case of R ) CH3 or CF3, (NEt4)[MoVO(Se-
2-RCONHC6H4)4] could not be isolated and was found to
decompose easily during the purification process. (PPh4)2-
[MoIVO(Se-2-CF3CONHC6H4)4] (4) was synthesized suc-
cessfully from (PPh4)2[MoIVO(S-4-ClC6H4)4]18 and the di-
selenide of the ligand (eq 3).
(PPh4)2[MoIVO(S-4-ClC6H4)4] + 2ArSeSeAr f
(PPh4)2[MoIVO(SeAr)4] + 2(S-4-ClC6H4)2 (3)
Molecular Structure in the Crystal and Conformation
in Solution. Crystal structures of 3b and 4 are shown in
Figures 1 and 2, respectively. The selenolate complex 3b is
isomorphous and isostructual to the corresponding thiolate
complex (PPh4)2[MoIVO(S-2-CH3CONHC6H4)4] (5).8 The
MoOSe4 cores of both of these complexes exhibit a square
pyramidal structure. The mean Mo-Se bond length (2.524
Å) of 3b is longer than that of 5 (2.408 Å) by 0.116 Å
because of the difference of the atomic radii (ca. 0.1 Å),
which is similar to or slightly larger than the reported values
(0.107-0.112 Å) for [M(QPh)4]2- (M ) Zn(II), Cd(II),24 Fe-
(II)25,26). The two diagonal Se-Mo-Se angles are 147.48-
1
was detected in solution by H NMR. When the crystal
structure of 4 was examined, a new conformer (up-up-
down-down) was found. This conformation gave an open
+
ModO site to allow access of PPh4 cation to form
intermolecular ModO‚‚‚H-C hydrogen bond as shown in
Figure S1. The short O‚‚‚H contact (2.428 Å) shows a
(25) Coucouvanis, D.; Swenson, D.; Baenziger, N. C.; Murphy, C.; Holah,
D. G.; Sfaras, N.; Simopoulos, A.; Kostikas, A. J. Am. Chem. Soc.
1981, 103, 3350-3362.
(24) Ueyama, N.; Sugawara, T.; Sasaki, K.; Nakamura, A.; Yamashita, S.;
Wakatsuki, Y.; Yamazaki, H.; Yasuoka, N. Inorg. Chem. 1988, 27,
741-747.
(26) McConnachie, J. M.; Ibers, J. A. Inorg. Chem. 1991, 30, 1770-1773.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9377

Okamura et al.
Table 2. ESR Parameters for Mo(V) Complexesa
compounds giso g|
e
e
g
Aiso A|e
A
(PPh4)[MoVO(Se-2-t-BuCONHC6H4)4] 2.022 2.073 2.001 32.4 51.1 21.9
(1b)b
(NEt4)[MoVO(SePh)4]c
2.024 2.072 2.005 30.0 48.3 21.2
(NEt4)[MoVO(S-2-t-BuCONHC6H4)4]d 1.994 2.035 1.981 33.2 54.9 23.4
(NEt4)[MoVO(SPh)4]c
1.990 2.019 1.979 32.3 52.3 22.3
a Conditions: 1 mM in acetonitrile/DMF ()4:1 v/v) at 77 K and room
temperature. b At room temperature and -150 °C. c Reference 19. d Ref-
erence 8. e 10-4 cm-1
.
21.9 × 10-4 cm-1) at -150 °C is comparable to reported
values for [MoVO(SePh)4]- determined under similar condi-
tions.19 In the case of the thiolates, introduction of t-
BuCONH groups increased g| value by the distortion of
MoVOS4 core.8 Additionally, only one conformational isomer
was found at room temperature (giso ) 2.022, Aiso ) 32.4 ×
10-4 cm-1) without splitting, whereas the conformational
isomers of (NEt4)[MoVO(S-2-t-BuCONHC6H4)4] were de-
tected as splitting of peak in the previous study.8 In contrast
to the Mo-S bond, the Mo-Se bond is longer and its
presence results in the elimination of the geometrical
distortion. This finding is consistent with the crystal structure
of the corresponding Mo(IV) compounds, as described in
the previous section.
The one-electron reduction from MoV (d1) to MoIV (d2 low
spin) increased the band gap between the filled orbital of
the ligand and the vacant orbital of the metal ion, which
resulted in a high-energy shift of the LMCT band. (NEt4)2-
[MoIVO(Se-2-t-BuCONHC6H4)4] (2a) exhibited LMCT at
300 nm (25 000 M-1 cm-1) and a d-d transition at 590 nm
(300 M-1 cm-1) in DMF, as shown in Figure S3. The d-d
band of the corresponding Mo(V) complex was probably
masked by the intense LMCT band in this region. These
results are similar to those reported for the analogous thiolate
compounds.8
Figure 3. Absorption spectra of 1b (solid line), (PPh4)[MoVO(SePh)4]
(dashed line), (PPh4)[MoVO(S-2-t-BuCONHC6H4)4] (dotted-dashed line),
and (PPh4)[MoVO(SPh)4] (dotted line) in DMF at room temperature.
bonding character, where the bond order was calculated to
be 0.0320 by MO calculations.22,23
In solution, the most stable conformer is different from
1
that in the crystal. H NMR spectra of 4 in acetonitrile-d3
showed one set of signals for the coordinated ligand,
indicating that only one conformer was present in solution.
The pseudosymmetrical center of the asymmetric compound
4 and the intermolecular attractive ModO‚‚‚H-C interaction
are presumably favorable for packing. When the complex is
dissolved in acetonitrile, the formation of intramolecular
hydrogen bonds and the electrostatic interactions between
acylamino and ModO moieties8 dominate the confor-
mation, resulting in an all-up structure. If bulky t-BuCONH-
groups were introduced to the complex, a minor conformer
was detected, as shown in Figure S2. At the higher
temperature, the minor conformer increased. The content of
the minor conformer at 30 °C is 6% which is smaller than
that for the corresponding thiolate (20%).8 The calculated
thermodynamic parameters for this equilibrium, ∆H and ∆S,
are 54 kJ mol-1 and 150 J K-1 mol-1, respectively. The
longer Mo-Se bonds reduce the steric congestion of the
bulky t-Bu groups. The ESR spectrum of 1b in a mixture of
acetonitrile and DMF at room temperature did not show the
presence of the minor conformer as described in a subsequent
section.
Absorption Spectra. Monooxomolybdenum(V) arenethi-
olate and selenolate complexes exhibit characteristic ligand-
to-metal charge transfer (LMCT) bands in the visible
region.19 Figure 3 shows absorption spectra of 1b, (PPh4)-
[MoVO(SePh)4], (PPh4)[MoVO(S-2-t-BuCONHC6H4)4], and
(PPh4)[MoVO(SPh)4] in DMF. Substituting S with Se dis-
places the LMCT band to lower energy, as reported for
[MoVO(QPh)4]- (Q ) Se, S).19 In the case of (Ph4P)[MoVO-
(Q-2-t-BuCONHC6H4)4] (Q ) Se, S), a similar trend was
observed, except for splitting of the LMCT peak. In the
thiolate complex, splitting of an absorption band was
observed, which has been considered to be caused by
geometrical distortion from a square pyramidal to a trigonal
bipyramidal structure.8 The spectrum of 1b did not exhibit
such splitting, which suggests that the MoOSe4 core has a
symmetrical square pyramidal geometry and that the four
Mo-Se bonds are equivalent.
IR Spectra. The presence of NH‚‚‚Se hydrogen bonds
was established by IR spectra. 1b and 2a exhibited ν(NH)
at 3325 and 3290 cm-1, respectively, in the solid state (Figure
S4). Reduction of the Mo(V) complex to Mo(IV) resulted
in a low wavenumber shift of ν(NH) by 35 cm-1. In contrast,
the ν(CdO) bands were little affected by this reduction; 1b
and 2a exhibited ν(CdO) bands at 1673 and 1670 cm-1,
respectively, which are considered normal positions for free
ν(CdO) bands without intermolecular NH‚‚‚OdC hydrogen
bonds. The diselenide (Se-2-t-BuCONHC6H4)2 exhibited the
ν(NH) band at 3292 cm-1 and the ν(CdO) band at 1650
cm-1 in the solid state, suggesting the presence of intermo-
lecular NH‚‚‚OdC hydrogen bonds. Under similar condi-
tions, ν(NH) bands of (NEt4)[MoVO(S-2-t-BuCONHC6H4)4]
and (PPh4)2[MoIVO(S-2-CH3CONHC6H4)4] were found at
3330 cm-1 and at 3272 cm-1, respectively.8 The enhance-
ments of the NH‚‚‚Se and the NH‚‚‚S hydrogen bonds by
the reduction of the molybdenum center are explained by
the increase of the electron density on selenium and sulfur,
which is ascribed to the decrease of the electron donation to
the metal ion. Generally, a covalent metal-sulfur bond or a
high-valent metal ion results in weak NH‚‚‚S hydrogen
ESR Spectra. As shown in Table 2, the ESR spectrum of
1b (g| ) 2.073, g ) 2.001, A| ) 51.1 × 10-4 cm-1, A )
9378 Inorganic Chemistry, Vol. 45, No. 23, 2006

NH‚‚‚Se Hydrogen Bonds in Molybdenum Selenolates
Figure 4. 76.10 MHz 77Se NMR spectra of (a) 2a and (b) (Se-2-t-
BuCONHC6H4)2 in DMF-d7 at 30 °C. The asterisk denotes a signal of
impurity.
bonds, which is caused by the electron donation to electron-
deficient metal ion.8,27
Figure 5. 77Se-1H COSY spectrum of 2a in DMF-d7 at 30 °C. Two cross-
peaks were found, which were assigned to (a) Se and aromatic proton at
2-position through an aromatic ring (blue) and (b) Se and amide NH group
through an NH‚‚‚Se hydrogen bond (red).
Electrochemical Properties. Cyclic voltammograms of
1b and (PPh4)[MoVO(SePh)4] in DMF are shown in Figure
S5. Quasi-reversible Mo(IV)/Mo(V) redox couples were
found at -0.41 V versus SCE for 1b and at -0.79 V for the
benzeneselenolate.28 Analogous compounds, (PPh4)[MoVO-
(S-2-t-BuCONHC6H4)4] and (PPh4)[MoVO(SPh)4], showed
similar couples at -0.45 and -0.84 V, respectively, in the
same conditions. The NH‚‚‚Se hydrogen bond positively
shifted the redox potential by 0.38 V, which is almost the
same as the value (0.39 V) obtained for the effect of the
NH‚‚‚S hydrogen bond.8
The 77Se-1H correlated spectroscopy (COSY) clearly
indicates that the 77Se-1H coupling occurs through the
NH‚‚‚Se hydrogen bond and not via the other pathway.
Figure 5 shows the 77Se-1H COSY spectrum of 2a in DMF-
d7. Two cross-peaks, a and b, were found. The blue proton
in Figure 5 couples with Se via three covalent bonds
(H-C-C-Se), including the benzene ring, whereas the red
proton couples with Se through NH‚‚‚Se hydrogen bond. The
correlated peak b is obviously larger than a. This result
indicates clearly that the NH‚‚‚Se hydrogen bond is the
predominant pathway for the spin-spin coupling. In the case
of a covalent metal-selenolate bond, the hydrogen bond is
weaker. Actually, Hg(Se-2-t-BuCONHC6H4)2 exhibited a
weak NH‚‚‚Se hydrogen bond. The IR spectrum of this Hg
complex showed the ν(NH) band at 3341 cm-1 in the solid
state, which is higher than that of 2a by 51 cm-1. One-
dimensional 77Se NMR did not exhibit any splitting of the
signal, as shown in Figure S6a. However, such a weak
NH‚‚‚Se hydrogen bond could be detected by 77Se-1H
COSY spectra (Figure S6b). The distinct peak clearly
indicates the presence of NH‚‚‚Se hydrogen bonds.
Molecular Orbital (MO) Calculations. As mentioned
previously, unexpected NH‚‚‚OdMo contact was found in
the crystal structure of 3b. Ball-and-stick representations of
3b are shown in Figure 6. Three of four amide NH groups
were located to form intraligand NH‚‚‚Se hydrogen bonds.
However, one amide (N1-H) group is distinguishable from
the other NH groups. Evidently, this amide NH group is
directed to the terminal oxo ligand, suggesting the formation
of an NH‚‚‚OdMo hydrogen bond. In the molybdenum(V)
thiolate analogue, all amide NH groups were found to be a
good location for forming intramolecular NH‚‚‚S hydrogen
bonds rather than NH‚‚‚OdMo hydrogen bonds, and MO
calculations showed a weak repulsion between NH and
OdMo moieties.8 On the other hand, hydrogen bonds to
Detection of NH‚‚‚Se Hydrogen Bonds by 77Se NMR.
77Se is a stable and NMR-active isotope with I ) 1/2 and
exists 7.6% in natural abundance.7 The one-dimensional 77
-
Se NMR spectrum of 2a showed a doublet at 302.73 ppm
77
from Me2Se in DMF-d7. The splitting indicates that the
-
Se-1H spin-spin coupling constant 1J(77Se-1H) ) 5.4 Hz.
The J constant through the covalent Se-H bond in RSeH
1
has been reported to be 42-43 Hz.29 The 77Se-1H spin-
spin coupling constant through the NH‚‚‚SedC hydrogen
bond in selones has been reported as JSe-H ) ca. 12-13
Hz. 30 The relatively small J coupling constant of 2a indicates
weak spin-spin coupling through the hydrogen bond. On
the other hand, diselenide (Se-2-t-BuCONHC6H4)2 did not
exhibit any splitting of the peak at 425.16 ppm as shown in
Figure 4, suggesting that the NH‚‚‚Se hydrogen bond was
absent or too weak to be detected. The covalent Se-Se bond
did not cause enough negative charge on the selenium atom
to form a hydrogen bond. A similar situation was observed
in disulfide, which did not produce any NH‚‚‚S hydrogen
bonds despite a short N‚‚‚S contact.31
(27) Ueyama, N.; Taniuchi, K.; Okamura, T.; Nakamura, A.; Maeda, H.;
Emura, S. Inorg. Chem. 1996, 35, 1945-1951.
(28) Bradbury, J. R.; Masters, A. F.; McDonell, A. C.; Brunette, A. A.;
Bond, A. M.; Wedd, A. G. J. Am. Chem. Soc. 1981, 103, 1959-
1964.
(29) Schmidt, M.; Block, H. D. Chem. Ber. 1970, 103, 3348-3349.
(30) Wu, R.; Herna´ndez, C.; Odom, J. D.; Dunlap, R. B.; Silks, L. A. Chem.
Commun. 1996, 1125-1126.
(31) Ueyama, N.; Okamura, T.; Yamada, Y.; Nakamura, A. J. Org. Chem.
1995, 60, 4893-4899.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9379

Okamura et al.
Table 3. Intramolecular NH‚‚‚Se and NH‚‚‚OdMo Hydrogen Bonds in
the Crystal of 3b
calculated
N-H‚‚‚A
N-H
X-raya
bond orderb
A
N‚‚‚A
H‚‚‚A
H‚‚‚A
N1-H1
N2-H2
N3-H3
N4-H4
Se1
O5
Se2
O5
Se3
O5
Se4
O5
3.094(8)
3.28(1)
3.142(8)
3.822(9)
3.114(7)
3.35(1)
2.865
2.399
2.650
3.200
2.562
2.638
2.685
2.658
-0.0012
0.0314
0.0315
0.0003
0.0292
0.0132
0.0124
0.0175
3.120(8)
3.51(1)
a H atoms are at calculated positions. b Calculated by Gaussian0322 and
BORDER.23
position. To estimate the bond character or strength, bond
order was evaluated for the NH‚‚‚OdC and NH‚‚‚Se
hydrogen bonds, where the riding model of SHELXL-97 was
used to calculate the positions of the hydrogen atoms.21 The
bond orders as well as the interatomic distances in the crystal
structures are summarized in Table 3. Obviously, only N1
is directed to OdMo with short contact (2.399 Å), with and
without attractive NH‚‚‚Se interaction. The bond order was
a negative value (-0.0012) for N1H1‚‚‚Se1 and a large
positive value (0.0314) for N1H1‚‚‚O5)Mo. In comparison,
N2H2 formed predominantly NH‚‚‚Se hydrogen bonds. The
other NH groups formed both NH‚‚‚Se and NH‚‚‚OdMo
hydrogen bonds.
Figure 6. Ball-and-stick representation of the anion part of 3b, (a) top
and (b) side views, where methyl protons are omitted. An intramolecular
NH‚‚‚OdMo hydrogen bond is shown as a red dotted line.
Conclusions
The presence of NH‚‚‚Se hydrogen bonds in the mo-
nooxomolybdenum(IV) selenolate complex was established
by X-ray analysis and by IR and NMR measurements. The
77Se-1H COSY strongly supported the presence of NH‚‚‚Se
hydrogen bonds with covalent character. Replacing the sulfur
atom with selenium successfully produced isostructural
compounds and resulted in analogous spectral and electro-
chemical properties. Using the 77Se NMR technique pre-
sented us with indisputable evidence of the existence of
intramolecular NH‚‚‚Se and NH‚‚‚S hydrogen bonds. The
similarities between cysteine and selenocysteine in terms of
their structural and chemical properties suggest that the
NH‚‚‚Se hydrogen bond makes significant contributions and
possesses biological importance, as has been widely reported
for NH‚‚‚S hydrogen bonds in various enzymes.
Figure 7. Visual representation of HOMO for [MoIVO(Se-2-CH3-
CONHC6H4)4]2-
.
terminal OdMo have been pointed out in the active sites of
native molybdenum enzymes32,33 and the model com-
pounds.34,35
Ab initio MO calculations for 3b were performed to
confirm the presence of the NH‚‚‚Se and NH‚‚‚OdMo
hydrogen bonds on the basis of the crystal structure. Figure
7 shows HOMO of the anion part of 3b. The MO is located
mainly at the MoSe4 core with antibonding (and nonbonding)
pπ-dπ interaction, which is essentially the same as that of
the reported thiolate analogue.8 The MO contains dxy and
2
2
dx -y orbitals of molybdenum and px (or py) and pz orbitals
of selenium. The pπ orbital, which is directed to the proton
of the amide group, is perpendicular to the Mo-Se-C plane
and interacts with the dπ orbital of molybdenum. The
NH‚‚‚Se hydrogen bond stabilizes the Mo-Se bond by
decreasing the antibonding character of the Mo-Se bond.
Each amide NH group is directed toward the individual
Acknowledgment. This work was supported by the
Grant-in-Aid from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
Supporting Information Available: X-ray crystallographic data
for 3b and 4 in CIF format, geometrical parameters (Table S1),
X-ray structure and calculated bond order of 4 (Figure S1), 1H NMR
spectra (Figure S2) and absorption spectra (Figure S3) of 2a, IR
spectra of 1b and 2a (Figure S4), cyclic voltammograms of 1b
and the related compound (Figure S5), 77Se NMR and 77Se-1H
COSY spectra of the Hg complex (Figure S6). This material is
available free of charge via the Internet at http://pubs.acs.org.
(32) Schindelin, H.; Kisker, K.; Hilton, J.; Rajagopalan, K. V.; Rees, D.
C. Science 1996, 272, 1615-1621.
(33) Kisker, K.; Schindelin, H.; Pacheco, A.; Wehbi, W. A.; Garrett, R.
M.; Rajagopalan, K. V.; Enemark, J. H.; Rees, D. C. Cell 1997, 91,
973-983.
(34) Davies, E. S.; Beddoes, R. L.; Collison, D.; Dinsmore, A.; Docrat,
A.; Joule, J. A.; Wilson, C. R.; Garner, C. D. J. Chem. Soc., Dalton
Trans. 1997, 3985-3995.
(35) Cooney, J. J. A.; Carducci, M. D.; McElhaney, A. E.; Selby, H. D.;
Enemark, J. H. Inorg. Chem. 2002, 41, 7086-7093.
IC061140Y
9380 Inorganic Chemistry, Vol. 45, No. 23, 2006