Synthesis and structure of lipophilic dioxo-molybdenum (VI) bis(hydroxamato) complexes

Article abstract of DOI:10.1016/j.poly.2010.07.014

New methods for the synthesis of complexes of molybdenyl with hydrophobic alkyl-substituted hydroxamic acids have been developed. A number of novel coordination compounds of the general formula MoO₂L₂ (where HL is decano-, N-methyl-decano-, N-methyl-hexano-, N-methyl-1-adamantano- and N-tert-butyl-hexanohydroxamic acids) have been synthesized. All compounds obtained have been characterized by IR, ¹H NMR spectroscopy and X-ray crystallography. The effect of the electron donating and steric properties of ligand substituents on the structure of complexes is discussed.

Full text of DOI:10.1016/j.poly.2010.07.014

Polyhedron 29 (2010) 2900–2906
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and structure of lipophilic dioxo-molybdenum (VI)
bis(hydroxamato) complexes
*
Valeriy I. Dzyuba, Lyudmila I. Koval , Vladimir V. Bon, Vasyl I. Pekhnyo
V.I. Vernadskii Institute of General and Inorganic Chemistry, Ukrainian National Academy of Sciences, Prospect Palladina 32-34, 03680 Kyiv 142, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history:
New methods for the synthesis of complexes of molybdenyl with hydrophobic alkyl-substituted hydroxa-
mic acids have been developed. A number of novel coordination compounds of the general formula
MoO2L2 (where HL is decano-, N-methyl-decano-, N-methyl-hexano-, N-methyl-1-adamantano- and N-
tert-butyl-hexanohydroxamic acids) have been synthesized. All compounds obtained have been charac-
terized by IR, 1H NMR spectroscopy and X-ray crystallography. The effect of the electron donating and
steric properties of ligand substituents on the structure of complexes is discussed.
Received 14 April 2010
Accepted 14 July 2010
Available online 13 August 2010
Keywords:
Lipophilic molybdenyl complexes
Hydroxamic acids
Ó 2010 Elsevier Ltd. All rights reserved.
Aliphatic substituents
X-ray structure
1. Introduction
The present article describes methods, developed by us, for the
synthesis of complexes of molybdenyl with hydroxamic acids
The richness of the coordination chemistry of hydroxamic acids
stimulates extensive scientific researches in different areas of tech-
nology. As an example, it has been found recently that oxo- and
oxoperoxo-molybdenum (VI) hydroxamato complexes possess un-
ique properties (selectivity and high desired-product yield) of cat-
alysts for the oxidation of olefins to epoxides [1]. However, the
overwhelming majority of publications dealing with metal and
metalloid hydroxamato complexes relate to chemical biology and
medicinal chemistry, which is reflected in an excellent review arti-
cle by Codd [2].
containing aliphatic substituents both at the nitrogen atom and
at the carbonyl carbons. The following novel coordination
compounds have been synthesized and investigated: bis(decanohy-
droxamato)-dioxo-molybdenum, (VI) MoO2(DH)2 (1); bis(N-methyl-
decanohydroxamato)-dioxo-molybdenum (VI), MoO2(N-MDH)2
(2); bis- (N-methyl-hexanohydroxamato)-dioxo-molybdenum (VI),
MoO2(N-MHH)2 (3); bis(N-tert-butyl-hexanohydroxamato)-dioxo-
molybdenum (VI), MoO2(N-tBuHH)2 (4); bis(N-methyl-1-adamant-
anohydroxamato)-dioxo-molybdenum (VI), MoO2(N-MAdH)2 (5)
(Scheme 1).
It is widely accepted that lipophilic compounds have a better
cellular permeability than hydrophilic ones. A number of com-
plexes of vanadyl and vanadate with hydroxamic acids as suitable
2. Experimental
hydrophobic ligands showed
a considerable glucose-lowering
2.1. Materials
activity in tests on experimental diabetic mice [3]. For a number
of potentially anti-diabetic and at the same time hydrophobic
complexes of the general formula MoO2L2, where HL is 2,4-pen-
tanedione, L-cysteine ethyl ester, N,N-diethyldithiocarbamic acid,
a ligand-exchange mechanism of assimilation of [MoO4]2ꢀ anion
by biological media, including erythrocytes, has been established
[4]. The presence of aliphatic substituents in hydroxamic acids
gives high lipophilicity to them as proligands and to their coordi-
2,4-Pentanedione, tert-butylamine, triethylamine, benzoyl per-
oxide, benzene, toluene, methanol of Merck; 1-adamantanecarbonyl
chloride, hexanoyl chloride, decanoyl chloride, N-methyl-hydroxyl-
amine hydrochloride, hydroxylamine hydrochloride, ethyl decan-
oate, molybdenum trioxide of Aldrich; dimethylacetamide of
Lab-Scan were used as original substances and solvents without
additional purification. Dimethylformamide (DMF) of Merck was
purified as was described by us earlier [7]. All chemicals used were
of reagent grade. Silica gel TLC plates of Merck were used for qual-
itative analysis. Bis(2,4-pentanedionato)-dioxo-molybdenum (VI),
MoO2(acac)2, has been synthesized by a known procedure [8].
Molybdenum dioxodichloride, MoO2Cl2, was prepared by sub-
stantially modifying a published method [9]. The stock, which
nation compounds [5].
A number of N-methyl substituted
hydroxamic acids have been patented as active substances of
drugs [6].
* Corresponding author. Tel.: +380 44 424 3461; fax: +380 44 424 3070.
E-mail address: l_koval@ionc.kiev.ua (L.I. Koval).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.07.014

V.I. Dzyuba et al. / Polyhedron 29 (2010) 2900–2906
2901
R2
O
CH3
CH3
1 - R1
= H;
O
O
R2 =
4 - R1
R2
=
O
N
CH2(CH2)7CH3
CH2(CH2)7CH3
CH2(CH2)3CH3
CH2(CH2)3CH3
C
N
=
;
Mo
1
R2
CH3
2 -
=
R
CH3;
CH3;
O
=
R2
=
O
3 - R1
=
R2
R1
=
R2
C
R1 =
CH3;
5 -
R1
Scheme 1.
was prepared by grinding 29.0 g (0.201 mol) of molybdenum triox-
ide and 11.8 g (0.201 mol) of sodium chloride in a porcelain mor-
tar, was placed in a tubular quartz reactor with a gas inlet on the
one side and a device for receiving the sublimed product on the
other side. This device consists of a coaxial two-necked round-bot-
tom flask, a glass rod with a Teflon scraper and a rod guide
equipped with gas outlet. On gradual heating (ꢁ5 °C/min) with a
tube furnace in an argon stream (ꢁ10 l/h) above 400 °C, molybde-
num dioxodichloride sublimate appears on the cold part of the
reactor, which is removed with the scraper into the receiving flask.
After the reaction mixture has reached a temperature of ꢁ600 °C,
sublimate condensation continues for another ca. 1 h. Without
interrupting argon supply, the receiving flask with yellowish-white
crystalline product is detached sequentially from the rod guide and
tubular reactor and is then quickly sealed with stoppers; the com-
pound obtained is transferred in a dry box to a storage bottle.
MoO2Cl2 yield: 5.9 g (ꢁ30% in terms of MoO3).
1H NMR (300 MHz, CDCl3, 25 °C, TMS), d, ppm: 8.52 (br.s, 1H,
OH), 3.35 (s, 3H, N-CH3), 2.32 (m, 2H, b-CH2), 1.65 (m, 2H,
3
c
-CH2), 1.23 (m, 12H, (CH2)6), 0.88 (t, JH–H = 6.9 Hz, 3H, CH3).
N-methyl-1-adamantanohydroxamic acid (N-MAdHA) was syn-
thesized by the somewhat changed above procedure, namely, 1-
adamantanocarbonyl chloride (0.1 mol, 19.9 g) was added drop-
wise to the reaction mixture as a solution in dry ether; the product
obtained was purified by sublimation (bath temperature 65 °C,
12 Pa). Yield: 9.4 g, 45%., m.p. 111–112 °C (lit. [13] 112–113 °C).
Anal. Calc. for C12H19NO2 (MW 209.29): C, 68.87; H, 9.15; N, 6.69.
Found: C, 68.67; H, 9.25; N, 6.73%. IR (KBr, cmꢀ1): 3152, 2915,
2850, 1594, 1587, 1514, 1455, 1419, 1393, 1360, 1321, 1263,
1209, 1182, 1107, 1071, 986, 957, 911, 798, 727, 694, 526, 465.
1H NMR (300 MHz, CDCl3, 25 °C, TMS); d, ppm: 8.54 (br.s, 1H,
OH), 3.46 (s, 3H, N-CH3), 2.05 (m, 3H,
c-CH of Ad), 2.01 (m, 6H,
b-CH2 of Ad), 1.73 (m, 6H, d-CH2 of Ad).
2.3. Synthesis of the complexes
2.3.1. Method 1
2.2. Synthesis of the proligands
To
a solution of proligand (30 mmol) and triethylamine
Decanohydroxamic acid (DHA) has been synthesized by the
procedure presented in [10], m.p. 88–89 °C (lit. 88 °C [11]). IR
(KBr, cmꢀ1): 3260, 3060, 2945, 2920, 2840, 2760, 1665, 1623,
1565, 1467, 1420, 1113, 1080, 1055, 1050, 1045, 1012, 985, 968,
740, 721, 650, 559, 495, 480, 445. 1H NMR (300 MHz, DMSO-d6,
25 °C, TMS), d, ppm: 10.31 (s, 1H, NH), 8.64 (s, 1H, OH), 1.92 (m,
(30 mmol, 3.03 g) in dry DMF (100 ml) was added dropwise a solu-
tion of molybdenum (VI) dioxydichloride (15 mmol, 2.98 g) in dry
DMF with vigorous stirring in argon atmosphere. After the termi-
nation of addition, the lemon yellow colour reaction mixture was
stirred for another 3 h. The starting substances were fed and the
reaction was carried out in complete isolation from atmospheric
moisture. After the termination of stirring, the (C2H5)3NꢂHCl pre-
cipitate formed was filtered off and washed on a filter with three
3 ml portions of DMF; the solvent was evaporated under 25 hPa
vacuum. The solid yellow precipitate was dried under 12 Pa vac-
uum at 50 °C to remove the traces of the solvent; pure complexes
were isolated by crystallization.
2H, b-CH2), 1.47 (m, 2H,
c-CH2), 1.24 (m, 12H, (CH2)6), 0.86 (t,
3JH–H = 6.9 Hz, 3H, CH3).
N-methyl-hexanohydroxamic acid (N-MHHA) and N-methyl-
decanohydroxamic acid (N-MDHA) have been synthesized by a
procedure analogous to that for N-methyl-acetohydroxamic acid
[12].
N-MHHA: To a mixture of sodium carbonate (0.1 mol, 10.6 g)
and N-methyl-hydroxylamine hydrochloride (0.1 mol, 8.35 g) in
160 ml of dry methanol was added dropwise hexanoyl chloride
(0.1 mol, 13.5 g) with stirring at a temperature of not over 5 °C
(cooling ice bath). After the termination of addition, the reaction
mixture was stirred for another 0.5 h and then filtered; the solvent
was removed on a rotary evaporator. The residue was filtered again
and distilled in vacuo, b.p. 70–72 °C, 12 Pa (lit. [6] 87–90 °C;
0.1 mm Hg). Yield: 7.5 g, 52%. Anal. Calc. for C7H15NO2 (MW
145.21): C, 57.90; H, 10.41; N, 9.65. Found: C, 58.11; H, 10.32; N,
9.34%. IR (film, cmꢀ1): 3191, 2960, 2941, 2871, 1630, 1615, 1510,
1470, 1445, 1397, 1206, 1100, 943, 900, 843, 721, 615, 503. 1H
NMR (300 MHz, CDCl3, 25 °C, TMS), d, ppm: 8.61 (br.s, 1H, OH),
2.3.2. Method 2 (ligand-exchange route)
The synthesis was carried out by gradual and continuous distill-
ing of acetylacetone at 13 hPa and 60 °C off a solution of bis-
(2,4-pentanedionato)-dioxo-molybdenum (VI) (5 mmol, 1.63 g) in
150 ml of dimethylacetamide containing appropriate hydroxamic
acid (10 mmol). The course of the process was monitored and its
exchange mechanism verified by means of a qualitative reaction
of FeCl3 with acetylacetone contained in the distillate. After the
process was complete, the solid residue was held under 12 Pa vac-
uum at 50 °C for 2 h to remove the traces of the solvent; pure com-
plexes were isolated by crystallization.
3.33 (s, 3H, N-CH3), 2.42 (m, 2H, b-CH2), 1.63 (m, 2H,
1.32 (m, 4H, (CH2)2), 0.90 (t, JH–H = 6.8 Hz, 3H, CH3).
c-CH2),
3
2.3.3. Method 3
N-MDHA was prepared in much the same way, taking decanoyl
chloride (0.1 mol, 19.1 g) as a basis. Yield: 10.3 g, 51%, b.p. 132–
135 °C, 12 Pa, m.p. 36–37 °C. Anal. Calc. for C11H23NO2 (MW
201.32): C, 65.63; H, 11.52; N, 6.96. Found: C, 65.48; H, 11.43; N,
7.02%. IR (KBr, cmꢀ1): 3141, 2956, 2919, 2860, 1636, 1601, 1492,
1466, 1445, 1408, 1390, 1191, 1130, 1105, 1066, 1056, 962, 935,
873, 808, 761, 729, 706, 673, 618, 558, 530, 458, 423, 412, 402.
Molybdenum trioxide (15 mmol, 2.16 g) was dissolved under
vigorous stirring in 100 ml of distilled water, adding dropwise a
50% solution of sodium hydroxide up to pH 12. The volume of
the solution was brought to 150 ml by adding water. While stirring
was continued a solution of 30 mmol proligand in 150 ml of etha-
nol was added to the aqueous solution obtained. To the yellowish
reaction mixture was added dropwise a 6 N HCl solution up to pH

2902
V.I. Dzyuba et al. / Polyhedron 29 (2010) 2900–2906
3. After 3 h of stirring the reaction mixture, the intensity of its lem-
on yellow colour increased greatly, and a suspended solid phase
appeared. After holding the reaction mixture at 5 °C for 8 h, the
abundant light yellow precipitate was filtered off, washed on a fil-
ter with three 10 ml portions of cooled ethanol–water mixture, ta-
ken in a 1/1 ratio, and dried under 12 Pa vacuum at 50 °C to remove
the traces of the solvent. Pure complexes were isolated by
recrystallization.
2.3.7. Bis(N-tert-butyl-hexanohydroxamato)-dioxo-molybdenum (VI)
Complex 4 was prepared without intermediate isolation of
proligand, N-tert-butyl-hexanohydroxamic acid.
A mixture of
hexanoyl chloride (100 mmol, 13.36 g), O-benzoyl-N-tert-buty-
lhydroxylamine (100 mmol, 19.33 g) [14] and pyridine (100 mmol,
7.91 g) in 100 ml of dry benzene was stirred at 70 °C for 6 h. Then
the reaction mixture was sequentially washed with dilute HCl, a
saturated solution of NaHCO3 and water and dried over Na2SO4.
Benzene was evaporated in vacuo, and raw N-hexanoyl-O-ben-
zoyl-N-tert-butylhydroxylamine was obtained. To a solution of
the above product in methanol (200 ml) was added a solution of
NaOH (200 mmol, 8.0 g) in 40 ml of water, and the mixture was
stirred vigorously for 2 h; then a solution of 30 mmol (7.26 g) of so-
dium molybdate dihydrate in 150 ml of water was added to the
reaction mixture, and the pH value was adjusted to 12.0. To the al-
most homogeneous yellowish solution was added dropwise a 6 N
HCl solution up to pH 3.0. After subsequent 3 h of stirring the reac-
tion mixture, the yellowish emulsion obtained was diluted twofold
with water and extracted with three 50 ml portions of chloroform.
Combined chloroform extracts were washed with water and dried
over Na2SO4. The solvent was evaporated on a rotary evaporator,
and the semisolid dark-yellow residue was held for 8 h under
12 Pa vacuum at 50 °C to completely remove the volatile organic
impurities. The complex was purified by thrice-repeated crystalli-
zation from an ethanol–water (1/1) mixture at 5 °C. Yield: 7.23 g.
Anal. Calc. for C20H40N2O6Mo (MW 500.49): C, 48.00; H, 8.06; N,
5.60. Found: C, 49.18; H, 7.95; N, 5.43%. M.p. 63–65 °C, decomp.
186–191 °C. IR (KBr, cmꢀ1): 2963, 2928, 2864, 1529, 1466, 1405,
1371, 1312, 1282, 1254, 1234, 1214, 1123, 1113, 1055, 1033,
1022, 978, 952, 916, 883, 808, 755, 718, 651, 623, 572, 542, 490,
435. 1H NMR (300 MHz, CDCl3, 25 °C, TMS), d, ppm: 2.56 (m, 2H,
2.3.4. Bis(decanohydroxamato)-dioxo-molybdenum (VI)
Complex 1 was prepared by the method 1, taking decanohydr-
oxamic acid (30 mmol, 5.62 g) as a basis, and purified by crystalli-
zation from an ethanol–water mixture (2/1) at 5 °C. Yield: 7.21 g,
96%. Anal. Calc. for C20H40N2O6Mo (MW 500.49): C, 48.00; H,
8.06; N, 5.60. Found: C, 49.12; H, 7.91; N, 5.75%. M.p. 159–
160 °C, decomp. 177–180 °C. IR (KBr, cmꢀ1): 3160, 3025, 2945,
2920, 2843, 1565, 1515, 1456, 1424, 1365, 1355, 1295, 1115,
1062, 1002, 988, 945, 925, 890, 765, 725, 702, 608, 555, 545,
485. 1H NMR (300 MHz, DMSO-d6, 25 °C, TMS), d, ppm: 13.23 (s,
1H, NH), 2.26 (m, 2H, b-CH2), 1.49 (m, 2H,
(CH2)6), 0.86 (t, JH–H = 7.0 Hz, 3H, CH3).
c
-CH2), 1.23 (m, 12H,
3
Complex 1 was also prepared by the method 2, taking deca-
nohydroxamic acid (10 mmol, 1.87 g) as a basis, and purified by
crystallization from an ethanol–water mixture (2/1) at 5 °C. Yield:
2.1 g, 84%. The compound was identified by its melting point and
by comparing IR spectra of samples prepared by the methods 1
and 2.
2.3.5. Bis(N-methyl-decanohydroxamato)-dioxo-molybdenum (VI)
Complex 2 was prepared by the method 1, taking N-methyl-
decanohydroxamic acid (30 mmol, 6.04 g) as a basis, and purified
by crystallization from ethanol at ꢀ10 °C. Yield: 7.45 g, 94%. Anal.
Calc. for C22H44N2O6Mo (MW 528.54): C, 49.99; H, 8.39; N, 5.30.
Found: C, 49.86; H, 8.25; N, 5.43%. M.p. 71–72 °C, decomp. 195–
198 °C. IR (KBr, cmꢀ1): 2928, 2848, 1580, 1492, 1471, 1422, 1234,
1156, 1119, 1070, 1014, 993, 963, 922, 884, 812, 790, 756, 729,
710, 673, 651, 605, 582, 524, 504, 458. 1H NMR (300 MHz, CDCl3,
b-CH2), 1.67 (m, 2H,
(CH2)2), 0.90 (t, JH–H = 7.0 Hz, 3H, CH3).
c-CH2), 1.57 (s, 9H, C(CH3)3), 1.32 (m, 4H,
3
2.3.8. Bis(N-methyl-1-adamantanohydroxamato)-dioxo-molybdenum
(VI)
Complex 5 was prepared by the method 1, taking N-methyl-1-
adamantanohydroxamic acid (30 mmol, 6.28 g) as a basis, and
purified by crystallization from ethanol. Yield: 7.52 g, 92%. Anal.
Calc. for C24H36N2O6Mo (MW 544.49): C, 52.94; H, 6.66; N, 5.14.
Found: C, 53.12; H, 6.81; N, 5.08%. Mp (decomp.) 207–208 °C. IR
(KBr, cmꢀ1): 2907, 2850, 1548, 1458, 1405, 1374, 1352, 1320,
1271, 1227, 1193, 1161, 1111, 1068, 998, 970, 942, 916, 894,
831, 799, 759, 702, 656, 635, 599, 540, 466. 1H NMR (300 MHz,
25 °C, TMS), d, ppm: 3.51 (s, 3H, N-CH3), 2.42 (m, 2H, b-CH2),
3
1.67 (m, 2H,
c
-CH2), 1.26 (m, 12H, (CH2)6), 0.88 (t, JH–H = 6.9 Hz,
3H, CH3).
Complex 2 was also prepared by the method 3, taking N-
methyl-decanohydroxamic acid (30 mmol, 6.04 g) as a basis, and
purified by crystallization from ethanol at ꢀ10 °C. Yield: 7.2 g,
91%. The compound was identified by its melting point and by
comparing IR spectra of samples prepared by the methods 1 and 3.
CDCl3, 25 °C, TMS), d, ppm: 3.68 (s, 3H, N-CH3), 2.08 (m, 3H,
CH of Ad), 1.99 (m, 6H, b-CH2 of Ad), 1.73 (m, 6H, d-CH2 of Ad).
c-
Complex 5 was also prepared by the method 3, taking N-
methyl-1-adamantanohydroxamic acid (30 mmol, 6.28 g) as a ba-
sis; and purified by crystallization from ethanol. Yield: 7.03 g,
86%. The compound was identified by its melting point and by
comparing IR spectra of samples prepared by the methods 1 and 3.
2.3.6. Bis(N-methyl-hexanohydroxamato)-dioxo-molybdenum (VI)
Complex 3 was prepared by the method 2, taking N-methyl-
hexanohydroxamic acid (10 mmol, 1.45 g) as a basis, and purified
by crystallization from toluene at 5 °C. Yield: 1.56 g, 75%. Anal. Calc.
for C14H28N2O6Mo (MW 416.32): C, 40.39; H, 6.78; N, 6.73. Found:
C, 40.24; H, 6.86; N, 6.61%. M.p. 85–87 °C, decomp. 197–199 °C. IR
(KBr, cmꢀ1): 2960, 2938, 2865, 1583, 1484, 1422, 1383, 1338, 1280,
1232, 1212, 1156, 1112, 1000, 956, 918, 882, 800, 748, 734, 687,
675, 651, 605, 580, 522, 502, 463, 441, 410. 1H NMR (300 MHz,
2.4. Physical measurements
The IR spectra were recorded in KBr pellets in a range of 4000–
400 cmꢀ1 on a Specord M80 spectrophotometer equipped with an
IBM-compatible operating computing system. The elemental anal-
ysis has been carried out on a Carlo Erba 1106 analyzer. The 1H
NMR spectra were recorded on a Varian VXR-300 spectrometer
in CDCl3 and DMSO-d6 solutions with an internal TMS standard.
CDCl3, 25 °C, TMS), d, ppm: 3.51 (s, 3H, N-CH3), 2.40 (m, 2H, b-
3
CH2), 1.67 (m, 2H,
c
- CH2), 1.32 (m, 4H, (CH2)2), 0.90 (t, JH–H
=
7.1 Hz, 3H, CH3).
Complex
3
was also prepared by the method 3, taking
2.5. X-ray crystallography
N-methyl-hexanohydroxamic acid (30 mmol, 4.36 g) as a basis,
and purified by crystallization from toluene at 5 °C. Yield: 5.56 g,
89%. The compound was identified by its melting point and by
comparing IR spectra of samples prepared by the methods 2 and 3.
The structure of compounds 1–5 was determined by X-ray crys-
tallography. The reflection intensity measurements were per-
formed on a single crystal diffractometer Bruker Smart Apex2

V.I. Dzyuba et al. / Polyhedron 29 (2010) 2900–2906
2903
with Mo K
100 K using
the compounds 1–4 were determined from 3 short
a
radiation (k = 0.71073 Å), graphite monochromator at
- and -scan techniques. The unit cell parameters of
-scans from
molecule in the structure is taken into consideration in experimen-
tal data table (empirical formula, molecular weight, calculated
density, F(0 0 0)). The main experimental data is given in Table 1.
x
u
x
different crystal orientations using RLATT program from SHELXTL pack-
age [15]. Reflection intensities were integrated using SAINT software
[16] and corrected for Lorentz effect, polarization and absorption
by a numerical (1–3) and a semi-empirical (4) technique using SAD-
ABS [17]. In the case of 5, the CELL_NOW program [15] was used for the
determination of cell parameters and independent domain orienta-
tions. As a result, 4 independent orientation matrixes were found,
which indicates nonmerohedral twinning in the crystal. The inten-
sity data of 5 was integrated using four orientation matrixes at the
same time. TWINABS program [18] was used for semi-empirical
absorption correction of the intensity data and produced HKLF4
(for solving structure) and HKLF5 (for anisotropic refinement) files.
The structures 1–4 were solved by direct methods and refined
anisotropically for all non-hydrogen atoms by full matrix least
squares on F2 using SHELXTL program package [15]. All hydrogen
atoms bonded to carbon atoms are positioned geometrically to ide-
alized positions and refined using riding model with constrained
3. Results and discussion
3.1. Synthesis and spectroscopy
Dioxomolybdenum (VI) coordination compounds can be pre-
pared by various methods, depending on which of the precursors:
Na2MoO4, MoO2Cl2 or MoO2(acac)2 is used as the source of the cen-
tral atom of these complexes [20].
The use of MoO2(acac)2 as a precursor in syntheses of com-
plexes with tetradentate [21] and tridentate [22] proligands seems
to us to offer no problems since the ligand-exchange reaction in
these cases is promoted by an entropy effect. At the same time,
the use of the above precursor in syntheses with potentially biden-
tate proligands calls for special conditions for carrying out the
reaction since 2,4-pentanedione in itself is a bidentate proligand,
and there is therefore a certain probability of formation of
mixed-ligand complexes [20]. Both the starting compounds and
complexes of hydroxamic acids with molybdenyl are practically
nonvolatile under low vacuum conditions. Therefore, the removal
of acetylacetone by distilling it off continuously together with
dimethylacetamide allows one to effectively shift the equilibrium
ligand-exchange process towards the formation of desired prod-
ucts with acceptable yield.
isotropic parameters: Uiso(H) = 1.2Uiso(C) for CH2 and Uiso
-
(H) = 1.5Uiso(C) for CH3. The single crystal of 1 poorly scattered at
higher angles (2h > 40°), which causes an effect on Rint value and,
as a result, on precision parameters of the model. During the struc-
ture refinement of 3, half of disordered toluene molecule was
found in the crystal structure near the crystallographic inversion
centre. The use of distance constraints did not give a significant ef-
fect. Therefore, the SQUEEZE routine in PLATON [19] was used to modify
the reflection data for obtaining a reasonable model without sol-
vent molecule. The molar ratio of complex 3 is one toluene mole-
cule per two molecules of complex. The presence of toluene
O
O
CH CON(CH ) ,
60 °C,13 hPa
3
3 2
MoO2
2 HL +
MoO2L2
ð1Þ
2
-2 CH3COCH2COCH3
Table 1
Crystal data and structure refinement for compounds 1–5.
Compound No.
1
2
3
4
5
Empirical formula
Formula weight
Crystal system, space group
Unit cell dimensions
a (Å)
C
20H40MoN2O6
C22H44MoN2O6
528.53
Triclinic, P1
C14H28MoN2O6ꢂ0.5C7H8
461.89
Monoclinic, C2/c
C20H40MoN2O6
500.48
Triclinic, P1
C24H36MoN2O6
544.49
Triclinic, P1
500.48
Orthorhombic, Pbca
ꢀ
ꢀ
ꢀ
17.396(3)
8.9742(16)
30.906(5)
7.8689(2)
11.0538(2)
15.597(1)
100.973(1)
98.442(1)
94.055(1)
1310.60(9)
2
26.0611(10)
7.9568(3)
22.6052(9)
9.2630(2)
10.8550(2)
13.5810(3)
68.063 (1)
77.804(1)
73.530(1)
1206.17(4)
2
13.815(7)
13.862(3)
14.224(3)
91.08(3)
104.26(4)
116.43(2)
2338.0(14)
4
b (Å)
c (Å)
a
(°)
b (°)
(°)
114.499(2)
c
Volume (Å3)
Z
4825.1(14)
8
1.378
4265.5(3)
8
1.438
Dcalc (g/cm3)
1.339
0.536
1.378
0.579
1.547
0.604
l
(mmꢀ1
)
0.579
0.648
F(0 0 0)
2112
560
1924
528
1136
Crystal size (mm)
h Range (°)
0.33 ꢃ 0.11 ꢃ 0.04
0.47 ꢃ 0.29 ꢃ 0.04
0.50 ꢃ 0.33 ꢃ 0.14
0.29 ꢃ 0.13 ꢃ 0.08
0.40 ꢃ 0.35 ꢃ 0.04
1.76–25.00
2.08–26.40
1.72–26.40
1.63–26.40
1.49–26.50
Limiting indices
ꢀ17 6 h 6 20
ꢀ10 6 k 6 10
ꢀ36 6 l 6 21
20814/4238
Rint = 0.1216
99.6
ꢀ9 6 h 6 9
ꢀ13 6 k 6 13
ꢀ19 6 l 6 19
26724/5371
Rint = 0.0994
99.8
ꢀ32 6 h 6 32
ꢀ9 6 k 6 9
ꢀ28 6 l 6 28
19972/4365
Rint = 0.0290
99.9
ꢀ11 6 h 6 11
ꢀ13 6 k 6 13
ꢀ16 6 l 6 16
12900/4920
Rint = 0.0222
99.3
ꢀ17 6 h 6 16
ꢀ17 6 k 6 17
0 6 l 6 17
13649/13649
Rint = 0.0000
89.6
Reflections collected/unique
Completeness to hmax
Absorption correction
Numerical
Numerical
0.9789/0.7866
5371/0/284
0.967
R1 = 0.0379
wR2 = 0.0719
R1 = 0.0551
wR2 = 0.0757
1.048/ꢀ0.671
Numerical
0.9169/0.7415
4365/0/212
1.102
R1 = 0.0312
wR2 = 0.0749
R1 = 0.0333
wR2 = 0.0760
1.594/ꢀ0.889
Semi-empirical
0.9573/0.8492
4920/0/270
1.031
R1 = 0.0226
wR2 = 0.0519
R1 = 0.0258]
wR2 = 0.0537
0.368/ꢀ0.469
Semi-empirical
0.9751/0.7932
13649/0/603
1.063
R1 = 0.0578
wR2 = 0.1376
R1 = 0.0848
wR2 = 0.1591
0.713/ꢀ0.900
Maximum/minimum transmission
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
0.9761/0.8320
4238/2/270
1.058
R1 = 0.0743
wR2 = 0.1675
R1 = 0.1145
wR2 = 0.1823
1.998/ꢀ1.945
Final R indicesa [I > 2
r
(I)]
R indicesa (all data)
Largest difference in peak/hole,
e Åꢀ3
a
2
R1
=
R
|Fo ꢀ Fc|/
R
|Fo| and wR2
=
R
w(Fo ꢀ Fc2)2/
R
(Fo2)2.

2904
V.I. Dzyuba et al. / Polyhedron 29 (2010) 2900–2906
A
number of complexes of molybdenyl with aryl-substituted
in this article is preparative and allows one to use this compound
widely as a precursor in syntheses of Mo (VI) coordination com-
pounds. The used stock is easily removed from the reactor and
regenerated via ammonium paramolybdate to starting MoO3.
hydroxamic acids have been obtained by acidifying an aqueous
solution of a mixture of appropriate proligand and sodium molyb-
date [23]. The increase in the hydrophobicity of proligands as a con-
sequence of lengthening of hydroxamate grouping alkyl
substituents (C5H11) necessitates the use of an aqueous-organic sol-
vent to carry out synthesis analogous to the above one [24]. We
have found that further increase of the mass of alkyl substituents
(C9H19) necessitates considerable updating of the conditions for
the synthesis of complexes in aqueous-organic media. To achieve
acceptable homogenization of the reaction mixture at the beginning
of synthesis, it is necessary to stir it vigorously and to adjust the pH
value to no less than 12.0. When the reaction mixture is acidified to
pH 3.0, an emulsion is formed, which crystallizes into the desired
product only on long standing at 5 °C.
ꢄ
2MoO þ 2NaCl 600 C MoO Cl þ Na MoO
ƒƒ!
ð4Þ
3
2
2
2
4
The method for the preparation of complex 4 differs greatly
from the above procedures 1–3. We failed to isolate N-hexanoyl-
O-benzoyl-N-tert-butylhydroxylamine as a chemically pure com-
pound, though the synthesis was carried out by the procedure de-
scribed for its closest analog [27]. The unpurified product obtained
was used to synthesize hydroxamic acid by the procedure de-
scribed for N-tert-butyl-benzohydroxamic acid [28]. Our attempts
to purify N-tert-butyl-hexanohydroxamic acid by crystallization
or by vacuum distillation (noticeable decomposition of the distil-
late) were unsuccessful. Therefore, the above hydroxamic acid
was used, without isolating it in the pure state, as a precursor in
the synthesis of complex 4. The hydroxamic acids used in the syn-
thesis of complexes 1–3, 5 have been obtained as chemically pure
compounds by known procedures. Since there are no spectroscopic
data on the above compounds in the publications, we have charac-
terized them by IR and 1H NMR spectroscopy.
þ2HCl
2HL þ Na2MoO4 ðpH 12;pH 3Þ MoO2L2 þ 2NaCl þ 2H2O
ƒƒƒƒƒ!
ð2Þ
Larson et al. isolated and described a stable adduct MoO2Cl2ꢂ2DMF,
which is readily soluble in a number of organic solvents [25]. We
have found that 0.2 M solutions of MoO2Cl2 in dimethylformamide
can keep (in absence of moisture, in the dark) several months with-
out losing its suitability for the synthesis of molybdenyl hydroxam-
ato complexes according to the scheme:
The data of the IR spectra of the compounds under investigation
corroborate complex formation. In this case, the vibration frequen-
cies of the C–O groups bonded to metal manifest themselves in the
lower-frequency spectral region in comparison with the stretching
vibration frequencies of the C@O groups of free hydroxamic acid
(1660–1600 cmꢀ1, 1585 cmꢀ1 for N-MAdHA). Besides, two intense
characteristic vibrational bands of the cis-MoO2 group in the range
945–880 cmꢀ1 are observed in the spectra of complexes. In the
high-frequency spectral region, the characteristic absorption band
of stretching vibrations of the OH group (3250–3150 cmꢀ1) disap-
pears on complex formation.
2NðC2H5
Þ
2HL þ MoO2Cl2
3 MoO L þ 2NðC H Þ ꢂ HCl
ð3Þ
ƒƒƒƒ!
2
2
2
5
3
The acceptable solubility of the starting compounds, including
hydroxamic acids, in the above solvent and the effective binding
of hydrogen chloride, which evolves during reaction, by triethyl-
amine ensure high yields of the desired products. The experimental
technique which is employed in the syntheses of organolithium
compounds is quite suitable for this method [26]. The method for
the synthesis of molybdenum dioxydichloride (Eq. (4)) presented
Fig. 1. The thermal ellipsoids plot of molecular structure of compounds 1–5. The ellipsoids are shown at 50% probability level.

V.I. Dzyuba et al. / Polyhedron 29 (2010) 2900–2906
2905
Table 2
Selected bond lengths and angles (Å) and (°).
1
2
3
4
5
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
Mo(1)–O(5)
Mo(1)–O(6)
C(1)–O(1)
C(1)–N(1)
N(1)–O(2)
O(2)–Mo(1)–O(1)
O(4)–Mo(1)–O(3)
O(6)–Mo(1)–O(5)
2.190(5)
2.2003(18)
1.9892(17)
2.1815(18)
1.9916(17)
1.7029(18)
1.6970(19)
1.268(3)
1.312(3)
1.387(3)
73.64(7)
73.78(7)
2.1702(16)
2.0029(16)
2.1924(15)
2.0071(16)
1.7076(17)
1.6958(17)
1.271(3)
1.317(3)
1.377(2)
73.26(6)
73.37(6)
2.1668(12)
1.9959(11)
2.1859(12)
1.9851(11)
1.7081(12)
1.7068(12)
1.274(2)
1.322(2)
1.3865(17)
72.16(4)
2.177(4)
2.007(3)
2.193(4)
2.002(3)
1.705(4)
1.701(4)
1.280(6)
1.337(6)
1.359(5)
1.990(5)
2.208(4)
2.003(5)
1.708(5)
1.693(5)
1.253(8)
1.310(9)
1.381(8)
74.43(18)
73.98(17)
105.8(2)
72.48(14)
72.30(14)
105.26(19)
72.45(4)
104.39(6)
103.89(9)
104.34(9)
Table 3
molecular structure of 5 will be based on the geometrical parame-
ters of one of them. In spite of substituents with different electron
donating properties near the hydroxamic nitrogen atom in the
compounds 1–5, their main values of geometrical parameters are
very close together (Table 2). The Mo(1) atom shows strongly dis-
torted octahedral geometry with the values of O–Mo–O angles in
Mean rms deviation from planes (Å) and dihedral angles between metal cycles (°) in
1–5.
Compound Plane
Rms deviation
(Å)
Dihedral angle
(°)
1
2
3
4
5
Mo(1)O(1)C(1)N(1)O(2)
0.0254
80.05(14)
72.38(6)
76.40(5)
74.31(3)
89.40(11)
2+
Mo(1)O(3)C(11)N(2)O(4) 0.0037
Mo(1)O(1)C(1)N(1)O(2) 0.0367
Mo(1)O(3)C(12)N(2)O(4) 0.0343
the range from 72.16(4)° to 105.8(2)° (Table 2). MoO2 has a cis-
configuration with Mo(1)–O(5) and Mo(1)–O(6) bond lengths
around 1.70 Å in all investigated compounds, which corresponds
to double bond (Mo@O) [30]. The angles O(5)–Mo(1)–O(6) for all
complexes are very close to ideal tetrahedral ones, which can be
Mo(1)O(1)C(1)N(1)O(2)
Mo(1)O(3)C(7)N(2)O(4)
Mo(1)O(1)C(1)N(1)O(2)
0.0130
0.0165
0.0761
Mo(1)O(3)C(11)N(2)O(4) 0.0236
Mo(1)O(1)C(1)N(1)O(2) 0.0714
Mo(1)O(3)C(13)N(2)O(4) 0.0988
accounted for by repulsive
O(6). From the values of C(1)–O(1) and C(1)–N(1) bond lengths
(Table 2) the -delocalization of electron density between the
p–p interaction between O(5) and
p
O(1), C(1) and N(1) atoms can be identified [30]. The oxygen atoms
O(1) and O(3) are in the trans-position to (Mo@O) double bonds,
which cause an elongation of Mo(1)–O(1) and Mo(1)–O(3) relative
to Mo(1)–O(2) and Mo(1)–O(4) of about 0.2 Å (Table 1). This fact is
in good agreement with the previously published structures of re-
lated compounds [24] and can be accounted for by strong trans-ef-
fect of the (Mo@O) group. The metal cycles Mo(1)–O(1)–C(1)–
N(1)–O(2) and Mo(1)–O(3)–C(X)–N(2)–O(4) (X = 11 for 1 and 4;
12 for 2; 7 for 3; 13 for 5) exhibit planar geometry. The mean
rms deviations from planes and dihedral angles between metal cy-
cles are given in Table 3. The dihedral angles for compounds 1–4,
which contain stereochemically labile hydrocarbon chains as a
substituent near the carbonyl carbons, have values from
72.38(6)° to 80.05(14)° [24]. At the same time, the dihedral angle
in 5 is near 90° (Table 3), which can be accounted for by bulky ada-
mantyl substituent with the rigid cage structure near the carbonyl
carbons in the molecule of complex.
The chemical shifts and integral intensities of signals in the 1H
NMR spectra of both molybdenyl complexes and free hydroxamic
acid corroborate the above chemical structure (Scheme 1). In the
solution of decanohydroxamic acid there are trace amounts of tau-
tomeric E-keto form (9.71 ppm (NH) and 8.97 ppm (OH)) like what
was revealed earlier for acetohydroxamic acid (the simplest homo-
logue of DHA) in DMSO-d6 solutions [29].
3.2. X-ray crystallography
The molecular structures of the investigated compounds 1–5
are shown in Fig. 1. The asymmetric units of all compounds, except
5, contain one molecule of complex. Compound 5 crystallized with
2 independent molecules in asymmetric unit, which has similar
geometrical parameters. For this reason, the discussion of the
Fig. 2. Crystal packing projections for the compounds 1 (down the b-axis) and 2 (down the a-axis). The dashed lines indicate H-bonds.

2906
V.I. Dzyuba et al. / Polyhedron 29 (2010) 2900–2906
Table 4
Hydrogen bonds for 1.
According to the data of X-ray diffraction analysis, the crystals
of complexes 1–5 have an almost identical coordination polyhe-
dron structure in spite of noticeable differences in the electron
donating and steric properties of ligand substituents. The latter
have an impact only on the values of dihedral angle between the
planes of chelate rings. The presence of strong intermolecular
hydrogen bonds in the complex MoO2(DH)2 affects greatly the de-
crease in the solubility of this compound in nonpolar organic sol-
vents (decrease in lipophilicity) as compared with N-alkyl-
substituted complexes.
D–Hꢂ ꢂ ꢂA
N(1)–H(1N)ꢂ ꢂ ꢂO(5)#1 0.85(4)
N(2)–H(2N)ꢂ ꢂ ꢂO(3)#2 0.92(4)
d(D–H) (Å) d(Hꢂ ꢂ ꢂA) (Å) d(Dꢂ ꢂ ꢂA) (Å) <(DHA) (°)
2.15(5)
1.85(5)
2.973(8)
2.741(7)
165(7)
161(6)
Symmetry transformations used to generate equivalent atoms: #1 ꢀx, y + 1/2,
ꢀz + 1/2; #2 ꢀx + 1/2, yꢀ1/2, z.
In our view, the complexes described in this article can be use-
ful as prototypes for the development of drugs based on lipophilic
coordination compounds.
Table 5
Hydrogen bonds for 2.
D–Hꢂ ꢂ ꢂA
d(D–H) (Å) d(Hꢂ ꢂ ꢂA) (Å) d(Dꢂ ꢂ ꢂA) (Å) <(DHA) (°)
C(2)–H(2A)ꢂ ꢂ ꢂO(5)#1
0.99
2.35
2.47
2.46
2.42
2.57
3.328(3)
3.349(3)
3.422(4)
3.379(3)
3.413(3)
170.4
148.4
167.3
166.7
143.1
5. Supplementary data
C(11)–H(11A)ꢂ ꢂ ꢂO(5)#1 0.98
C(10)–H(10B)ꢂ ꢂ ꢂO(6)#2 0.98
C(11)–H(11C)ꢂ ꢂ ꢂO(2)#3 0.98
C(13)–H(13B)ꢂ ꢂ ꢂO(6)#4 0.99
CCDC 763211, 763213, 763212, 763210 and 763214 contains
the supplementary crystallographic data for 1, 2, 3, 4 and 5. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Symmetry transformations used to generate equivalent atoms: #1 ꢀx + 1, ꢀy + 1,
ꢀz + 2; #2 x + 1, y, z ꢀ 1; #3 ꢀx, ꢀy + 1, ꢀz + 2; #4 ꢀx, ꢀy, ꢀz + 2.
The crystal structures of 1 and 2 exhibit a similar ‘‘head-to-tail”
packing type in the 0 0 1 direction (Fig. 2). The chain of classical
intermolecular N–Hꢂ ꢂ ꢂO hydrogen bonds was found along 0 1 0
direction of the crystal structure 1 (Fig. 2; Table 4). It should be
noted that even in dilute aqueous solutions, the complex of molyb-
denyl with acetohydroxamic acid (the simplest analog of
MoO2(DH)2) forms intramolecular hydrogen bonds [31]. This may
indicate a stable tendency of complexes of Mo (VI) with nitro-
gen-unsubstituted (N–H) hydroxamic acids to form hydrogen
bonds. In MoO2(DH)2 crystals are formed so strong intermolecular
hydrogen bonds that they make this compound (with long alkyl
substituents) insoluble in ordinary solvents, such as chloroform
and toluene.
In comparison with 1, the crystal structure 2 displays much
weaker intermolecular C–Hꢂ ꢂ ꢂO contacts in the same direction
(Fig. 2; Table 5). The difference in H-bond strength in the above
structures has an effect on interplanar dihedral angles in investi-
gated compounds (Table 2). The stronger N–Hꢂ ꢂ ꢂO hydrogen bonds
in 1 cause a significant increase in the dihedral angle between
metal cycles relative to 2 of up to 7.5° (Table 2).
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Thus, several methods for the synthesis of complexes of molyb-
denyl with hydrophobic alkyl-substituted hydroxamic acids have
been developed. Taking into account the sufficient solubility of all
parent compounds of the reaction mixture in aprotic polar solvent
and the high yield of the desired products, the reaction between
molybdenum dioxodichloride and appropriate hydroxamic acid in
the presence of triethylamine in complete isolation from atmo-
spheric moisture is the most acceptable. The preparative route to
obtain MoO2Cl2 makes the above method readily accessible.