Oxygen Abstraction Reactions of N-Substituted Hydroxamic Acids with Molybdenum(V) and Vanadium(III) and -(IV) Compounds

Article abstract of DOI:10.1021/ic950819r

A wide range of N-substituted mono- and dihydroxamic acids undergo oxygen abstraction on reaction with V(III), V(IV), and Mo(V) compounds to form hydroxamates of V(V) and Mo(VI) respectively together with the corresponding amides and diamides. The molybdenyl and vanadyl hydroxamates form metal - oxygen clusters under FABMS conditions. The X-ray crystal structures of [MoO₂{CH₃(CH₂)_nC(O)N(C ₆H₅)O}₂ (1 and 2) (n = 4, 5) show monomeric structures with structural trans effects and consequent weakening of the Mo-O(ligand) bonds which may account for the tendency to form clusters in FABMS. In constrast, the electrospray MS of the vanadyl dihydroxamates, VO(OH)[PhN(O)C(O)(CH₂)_nC(O)N(O)Ph] (n = 3, 5) and VO(OH)[p-CH₃C₆H₄N(O)C-(O) (CH₂)_nC(O)N(O)C₆H₄-CH₃) (n = 2, 4) show the presence of dimers in solution.

Full text of DOI:10.1021/ic950819r

1674
Inorg. Chem. 1996, 35, 1674-1679
Oxygen Abstraction Reactions of N-Substituted Hydroxamic Acids with Molybdenum(V)
and Vanadium(III) and -(IV) Compounds
David A. Brown,*,† H. Bo1gge,‡ R. Coogan,† D. Doocey,†,‡ T. J. Kemp,§ A. Mu1ller,‡ and
B. Neumann‡
Department of Chemistry, University College, Belfield, Dublin 4, Ireland, Lehrstuhl fu¨r Anorganische
Chemie 1, Fakulta¨t fu¨r Chemie, Universita¨t Bielefeld, Universita¨trasse 25, Postfach 8640,
4800 Bielefeld 1, Germany, and Department of Chemistry, University of Warwick,
Coventry CV4 7AL, U.K.
ReceiVed June 30, 1995X
A wide range of N-substituted mono- and dihydroxamic acids undergo oxygen abstraction on reaction with V(III),
V(IV), and Mo(V) compounds to form hydroxamates of V(V) and Mo(VI) respectively together with the
corresponding amides and diamides. The molybdenyl and vanadyl hydroxamates form metal-oxygen clusters
under FABMS conditions. The X-ray crystal structures of [MoO2{CH3(CH2)nC(O)N(C6H5)O}2 (1 and 2) (n )
4, 5) show monomeric structures with structural trans effects and consequent weakening of the Mo-O(ligand)
bonds which may account for the tendency to form clusters in FABMS. In constrast, the electrospray MS of the
vanadyl dihydroxamates, VO(OH)[PhN(O)C(O)(CH2)nC(O)N(O)Ph] (n ) 3, 5) and VO(OH)[p-CH3C6H4N(O)C-
(O) (CH2)nC(O)N(O)C6H4-CH3) (n ) 2, 4) show the presence of dimers in solution.
Introduction
evidence for a number of aminohydroxamic acids showing
different coordination behavior, that is (N,N), (N,O), and (O,O),
depending on the metal being complexed and on the pH of
solution.12
Hydroxamic acids are important bioligands.1 Naturally
occurring hydroxamic acids (siderophores) are involved in the
microbial transport of iron and consequently have therapeutic
uses in iron-related conditions.2 They are also inhibitors of
urease activity3 and have been used therapeutically in the
treatment of hepatic coma.4 Their biological activity is related
to their ability to form very stable chelates with a range of metals
and most especially with iron.5 In the majority of metal chelates
formed by hydroxamic acids, coordination occurs by deproto-
nation of the OH group and subsequent O,O coordination of
the carbonyl oxygen and deprotonated OH as in, for example
Fe(PhCONHO)3.6 Although this mode of bonding has been
confirmed by X-ray crystallography in a wide range of metal
complexes, no examples have yet been observed of N,O
coordination by normal hydroxamic acids which would involve
deprotonation of the NH group despite recent experimental and
theoretical studies7,8 which, at least for RCONHOH, R ) H,
CH3 suggest that these monohydroxamic acids are N-acids in
the gas phase. However, aminohydroxamic acids such as
glycinehydroxamic acid (GHA), NH2CH2CONHOH, provided
the first example of N,N-coordination in [(Ni(NH2CH2-
CONHO)2],9 and subsequent examples include the GHA
complexes of Co(III)10 and Cu (II).11 There is now good
In most cases of metal complexation by hydroxamic acids
(HA), redox reactions do not occur and there is no change in
oxidation number of the metal. In contrast, hydroxamates of
V(IV), V(V), Mo(V), and Mo(VI) are well established13 and
their spectroscopic properties are consistent with O,O coordina-
tion by the hydroxamate ligands although relatively few X-ray
crystallographic studies have been reported. Recently the crystal
structures of oxochlorobis(benzohydroxamate)vanadium(V) and
oxoisopropoxy(N,N ′-dihydroxy-N,N-du¨sopropylheptanedi-
amido)vanadium(V) were both shown to involve hydroxamate,
O,O, coordination to vanadium.14 To the best of our knowledge,
the only analogous molybdenum structures are those of the
hydroxamic acids (HA) derived from two analgesics, phenacetin
and acetanilide, MoO2A2. In both cases the ligand arrangement
about the Mo atom is distorted octahedral with the dioxo
oxygens bonded cis to each other.15
However, in many cases, attempts to prepare V(IV) or Mo-
(V) hydroxamates from appropriate starting materials such as
VOSO4‚xH2O and [NH4]2[MoOCl5] give hydroxamates of the
metals in higher oxidation states, i.e., VO(OH)A2 and MoO2A2
(where HA ) hydroxamic acid) and this has been generally
attributed to the ease of oxidation of the hydroxamates of the
metal in lower oxidation states.13 Analogous uranyl hydrox-
amates are generally prepared from uranyl salts, e.g. [UVIO2-
(NO3)2] and so metal oxidation is not observed on complexation
† University College.
‡ Universita¨t Bielefeld.
§ University of Warwick.
X Abstract published in AdVance ACS Abstracts, February 1, 1996.
(1) Kehl, H., Ed. Chemistry and Biology of Hydroxamic Acids; S.
Karger: New York, 1982.
(2) Brown, D. A.; Chidambaram, M. V. In Metal Ions in Biological
Systems; Sigel, H., Ed.; Marcel Dekker Inc.: New York, 1982; Vol.
14.
(3) Hase, J.; Kobashi, K. J. Biochemistry (Tokyo) 1967, 62, 293.
(4) Fishbein, W. N.; Strecter, C. L.; Daly, J. E. J. Pharmacol. Exp. Ther.
1973, 186, 173.
(5) Nielands, J. B. J. Am. Chem. Soc. 1952, 74, 4846.
(6) Lindner, V. H. J.; Gottlicher, S Acta Crystallogr., Sect. B 1969, 25,
632.
(9) Brown, D. A.; Roche, A. L.; Pakkanen, T. A.; Pakkanen, T. K.;
Smolander, K. J. Chem. Soc., Chem. Commun. 1982, 676.
(10) Pakkanen, T. K.; Pakkanen, T. A.; Smolander, K.; Brown, D. A.; Glass,
W. K.; Roche, A. L. J. Mol. Struct. 1987, 162, 313.
(11) De Miranda-Pinto, C. O. B.; Paniago, E. B.; Carvalho, S.; Tabak, M.;
Mascarenhas, Y. P. Inorg. Chem. Acta 1987, 137, 145.
(12) Farkas, E.; Brown, D. A.; Cittaro, R.; Glass W. K. J. Chem. Soc.,
Dalton Trans. 1993, 2803.
(7) Decouzon, M.; Exner, O.; Gal. J. F.; Nana, P. C. J. Org. Chem. 1990,
55, 3980.
(8) Ventura, O. N.; Rama, J. R.; Turi, L.; Dannenberg, J. J. J. Am. Chem.
Soc. 1993, 115, 5754.
(13) Chatterjee, B. Coord. Chem. ReV. 1978, 26, 281.
(14) Fisher, D. C.; Barclay-Peet, S. J.; Balfe, C. A.; Raymond, K. N. Inorg.
Chem. 1989, 28, 4399.
(15) Brewer, G. A.; Sinn, E. Inorg. Chem. 1981, 20, 1823.
0020-1669/96/1335-1674$12.00/0 © 1996 American Chemical Society

Reactions of N-Substituted Hydroxamic Acids
Inorganic Chemistry, Vol. 35, No. 6, 1996 1675
with hydroxamic acids.16 However, in one noteworthy case
involving the reaction of uranium tetrachloride with the anion
of N-phenylbenzohydroxamic acid (PBHA) in THF as solvent
resulted in oxidation of U(IV) to U(VI) and formation of UO2-
Cl(PBA)(THF)2 accompanied by oxygen abstraction from some
of the PBHA to form benzanilide.17
In the present paper, it will be shown that formation of
hydroxamates of vanadium and molybdenum in higher oxida-
tion states than the starter compounds occurs even under
anaerobic conditions for a wide range of N-substituted mono
and dihydroxamic acids and is accompanied by oxygen abstrac-
tion from the hydroxamic acid with formation of the corre-
sponding amide, a reaction which has hitherto been unrecog-
nized except in the case of U(IV) mentioned above.17
Results and Discussion
Figure 1. X-ray crystal structure of MoO2[CH3(CH2)4C(O)N(O)C6H5]2
(1).
(A) Molybdenum Hydroxamates. Reaction of a series of
N-arylmonohydroxamic acids (HA), CH3(CH2)nC(O)N(R)OH
(R ) C6H5, n ) 4, 5, 6, 8; R ) p-CH3C6H4, n ) 5) and N,N ′-
disubstituted dihydroxamic acids (H2A′), HON(R′)C(O)(CH2)nC-
(O)N(R′)OH (R′ ) C6H5, n ) 2, 3, 5, 7; R′ ) p-CH3C6H4, n )
2, 3, 4, 7; and R′ ) p-C2H5C6H4, n ) 5) with both the
molybdenum(V) reagent (NH4)2[MoOCl5] (method A) and the
molybdenum(VI) reagent Na2MoO42H2O (method B) gave the
corresponding molybdenyl, Mo(VI), hydroxamates, MoVIO2A2
and MoVIO2A′, respectively. Formation of the Mo(VI) com-
plexes thus occurs irrespective of the oxidation state (V or VI)
of the molybdenum in the starting materials.
The two series of complexes obtained by methods A and B
were shown to be identical by comparison of their microana-
lytical and infrared and 1H NMR spectral data (see Supporting
Information). The carbonyl infrared stretching frequencies
which occur around 1590-1640 cm-1 in the free ligands occur
in the complexes at about 1540 cm-1, consistent with coordina-
tion of the carbonyl oxygen atom to the metal which, taken in
conjunction with the disappearance of the OH band of the free
ligand on complexation, supports O,O coordination for the
hydroxamate ligand, and the appearance of a strong doublet in
the 900-930 cm-1 region indicates a cis-MoO2 group in all
cases (Supporting Information).
Figure 2. X-ray crystal structure of MoO2[CH3(CH2)5C(O)N(O)C6H5]2
(2).
Table 1. Crystallographic Data for 1 and 2
1
2
FABMS. Satisfactory FABMS could only be obtained for
[MoO2{CH3(CH2)nC(O)N(C6H5)O}2], n ) 4 and 5, and [MoO2-
{O(R)N(O)C(CH2)nC(O)N(R)O}], R ) C6H5, n ) 3 and 7. As
for the analogous uranyl complexes,16 a series of oligomers of
formula [(MoO2)n]+ and [(MoO2)nOn]+ occur with clusters
containing up to nine molybdenum atoms. The occurrence of
oligomeric ions retaining a coordinated hydroxamate ligand is
much rarer than that of the metal-oxygen clusters. Selected
cluster peaks are given in the Supporting Information.
X-ray Crystal Structures. The crystal structures of the two
molybdenyl monohydroxamates [MoO2{CH3(CH2)nC(O)N
(C6H5)O}2] (1 and 2) (n ) 4 and 5) are shown in Figures 1 and
2. Selected bond distances and angles are given in Tables 2
and 3 for 1 and in Tables 5 and 6 for 2 with crystallographic
data in Table 1. They confirm the O,O mode of coordination
of the hydroxamate ligands and cis-arrangement of the MoO2
grouping as concluded from the spectroscopic data. The
widespread occurrence of the cis-MoO2 arrangement is in accord
with extended Hu¨ckel calculations by Tatsumi and Hoffmann18
and arises primarily from the maximum utilization of vacant
“d” orbitals in π-bonding with oxygen lone pairs in the cis case.
Clearly both complexes are monomeric so cluster formation in
formula
mol wt
cryst size, mm
space group (No.)
a, Å
b, Å
c, Å
C24H32MoN2O6
540.5
0.7 × 0.2 × 0.1
P21/c (14)
15.616(7)
10.408(4)
16.613(8)
C26H36MoN2O6
568.5
0.5 × 0.3 × 0.1
Pbca (61)
10.406(6)
16.611(6)
33.027(7)
R, deg
â, deg
100.69(4)
γ, deg
V, Å3
2653(2)
4
1.353
1120
0.532
0.184/0.428
0.111, 0.099
5709(4)
8
1.323
2368
0.484
0.221/0.227
0.077, 0.049
Z
D
calc, g/cm3
F(000)
µ, mm-1
transm coeff
a
R, Rw
a For all structures Mo KR radiation was used at T ) 294 K.
SHELXTL-Plus: 1/w ) σ2(F) + 0.0001F2, R ) ∑||Fo| - |Fc||/∑|Fo|,
2
Rw ) (∑w(|Fo| - |Fc|)2/∑wFo )1/2
.
FABMS must arise under the conditions of the method. In both
1 and 2, the molybdenum atoms lie in a distorted octahedral
environment similar to those in the molybdenum complexes of
phenacetin and acetanilide.15 Thus the Mo-O(ligand) bonds
trans to the terminal Mo-O bonds are elongated compared to
the cis M-O(ligand) bonds, e.g. in 1, n ) 4, trans-Mo-O(ligand)
) 2.19Å, which compares closely with those in ref 15; e.g.
(16) Brown, D. A.; Ismail, S. Inorg. Chem. Acta 1990, 171, 41.
(17) Raymond, K. N.; Smith, R. L. J. Inorg. Nucl. Chem. 1979, 41, 1431.
(18) Tatsumi, K.; Hoffmann, R. Inorg. Chem. 1980, 19, 2656.

1676 Inorganic Chemistry, Vol. 35, No. 6, 1996
Table 2. Selected Bond Lengths (Å) for 1
Brown et al.
Table 4. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Coefficients (Å2 × 103) for 1
Mo-O(1)
2.191(7)
2.163(8)
1.651(7)
1.254(16)
1.233(17)
1.465(16)
1.408(18)
1.532(16)
Mo-O(2)
2.019(8)
1.998(9)
1.725(9)
1.350(12)
1.387(14)
1.310(15)
1.382(17)
1.461(20)
Mo-O(3)
Mo-O(4)
x
y
z
U(eq)a
Mo-O(5)
Mo-O(6)
Mo
6408(1)
7478(4)
5895(4)
6373(5)
7048(5)
5399(5)
6850(5)
6422(6)
6943(6)
6140(8)
5712(9)
5140(8)
4949(7)
5392(7)
5977(7)
8079(9)
8307(10)
7694(12)
6856(11)
6569(8)
7188(8)
7251(8)
7929(7)
8249(8)
8746(10)
9066(13)
9401(16)
6604(8)
6512(8)
7017(12)
8005(15)
8451(26)
9377(20)
1948(1)
2761(8)
3336(8)
794(8)
324(8)
1582(8)
2643(9)
5137(1)
6033(5)
5751(5)
6213(5)
4999(5)
4705(5)
4365(4)
6392(6)
5508(7)
7729(7)
8174(8)
7776(8)
6920(8)
6495(7)
6876(7)
5378(9)
5181(11)
4924(9)
4847(9)
5016(7)
5286(7)
6493(7)
7196(8)
7842(8)
8644(10)
9272(13)
10037(19)
6185(7)
6806(9)
7638(9)
7680(12)
7824(20)
7801(27)
47(1)
57(3)
52(3)
53(3)
55(3)
57(3)
65(3)
46(4)
59(4)
57(5)
65(5)
61(5)
60(5)
50(5)
41(4)
73(6)
91(8)
77(7)
72(6)
55(5)
49(4)
49(5)
60(5)
79(6)
101(8)
159(14)
218(21)
51(5)
70(6)
100(8)
134(12)
271(31)
406(37)
O(1)-C(13)
O(3)-C(19)
N(1)-C(6)
N(2)-C(12)
C(13)-C(14)
O(2)-N(1)
O(4)-N(2)
N(1)-C(13)
N(2)-C(19)
C(19)-C(20)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
N(1)
N(2)
C(1)
C(2)
C(3)
C(4)
C(5)
Table 3. Selected Bond Angles (deg) for 1
3892(10)
-698(10)
4631(13)
5503(14)
6359(14)
6416(13)
5580(12)
4755(11)
-2180(14)
-3426(18)
-4335(15)
-4023(16)
-2801(14)
-1931(13)
3599(12)
4146(14)
3127(17)
3791(20)
2675(29)
3294(30)
-336(14)
-1295(14)
-894(19)
-803(25)
-1900(32)
-1512(39)
O(1)-Mo-O(2)
O(2)-Mo-O(3)
O(2)-Mo-O(4)
O(1)-Mo-O(5)
O(3)-Mo-O(5)
O(1)-Mo-O(6)
O(3)-Mo-O(6)
O(5)-Mo-O(6)
Mo-O(2)-N(1)
Mo-O(4)-N(2)
O(2)-N(1)-C(13)
O(4)-N(2)-C(12)
72.8(3)
84.7(3)
156.6(3)
158.6(3)
93.6(3)
O(1)-Mo-O(3)
O(1)-Mo-O(4)
O(3)-Mo-O(4)
O(2)-Mo-O(5)
O(4)-Mo-O(5)
O(2)-Mo-O(6)
O(4)-Mo-O(6)
Mo-O(1)-C(13)
Mo-O(3)-C(19)
O(2)-N(1)-C(6)
77.7(3)
94.0(3)
73.4(3)
87.0(3)
102.3(4)
109.1(4)
89.5(4)
114.6(7)
116.5(8)
114.4(8)
129.1(10)
113.2(10)
117.9(10)
89.0(3)
157.3(3)
104.8(4)
117.6(6)
117.4(7)
C(6)
C(7)
C(8)
C(9)
116.5(10) C(6)-N(1)-C(13)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
C(18)
C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
118.1(10) O(4)-N(2)-C(19)
C(12)-N(2)-C(19) 128.7(11) O(1)-C(13)-N(1)
O(1)-C(13)-C(14) 119.4(10) N(1)-C(13)-C(14) 122.5(11)
O(3)-C(19)-N(2)
116.7(11) O(3)-C(19)-C(20) 123.8(12)
N(2)-C(19)-C(20) 119.4(12)
trans-Mo-O(average) ) 2.20 Å over cis-Mo-O(average) 2.02
Å. This structural trans effect may lead to ready cleavage of
the MosO(ligand) bond trans to the ModO terminal, thereby
aiding the formation of molybdenum-oxygen bridges and hence
clusters in the FABMS of these complexes:
MoO2L2 f MoO2L+ + L
a Equivalent isotropic U defined as one-third of the trace of the
orthogonalized Uij tensor.
+
MoO2L2 + MoO2L+ f Mo2O4L3
Table 5. Selected Bond Lengths (Å) for 2
Mo(1)-O(1)
Mo(1)-O(3)
Mo(1)-O(5)
O(1)-C(13)
O(3)-C(20)
N(1)-C(6)
2.206(9)
2.163(9)
1.699(9)
1.267(17)
1.244(17)
1.425(14)
1.400(13)
1.516(21)
Mo(1)-O(2)
Mo(1)-O(4)
Mo(1)-O(6)
O(2)-N(1)
2.002(9)
2.006(9)
1.693(9)
1.388(15)
1.377(14)
1.290(21)
1.323(18)
1.525(19)
Ion-neutral molecule reactions have been suggested previously
as an explanation for the formation of clusters in the FABMS
of related complexes.19,20
O(4)-N(2)
Oxygen Abstraction Reactions. In view of the previous
report by Raymond and co-workers on the formation of a uranyl
hydroxamate and benzanilide from the reaction of UCl4 and
N-phenylbenzohydroxamate,17 we reinvestigated the above
reactions with (NH4)2[MoOCl5] as starter under anaerobic
conditions when both the MoVI hydroxamate and the corre-
sponding amide or diamide formed; the latter were fully
N(1)-C(13)
N(2)-C(20)
C(20)-C(21)
N(2)-C(12)
C(13)-C(14)
Table 6. Selected Bond Angles (deg) for 2
O(1)-Mo(1)-O(2)
O(2)-Mo(1)-O(3)
73.5(4)
83.9(3)
O(1)-Mo(1)-O(3)
O(1)-Mo(1)-O(4)
O(3)-Mo(1)-O(4)
O(2)-Mo(1)-O(5)
O(4)-Mo(1)-O(5)
O(2)-Mo(1)-O(6)
O(4)-Mo(1)-O(6)
77.6(4)
93.0(3)
74.3(3)
85.3(4)
104.2(4)
109.0(4)
89.4(4)
O(2)-Mo(1)-O(4) 156.5(3)
1
characterized by microanalysis, infrared and H NMR spec-
O(1)-Mo(1)-O(5) 157.5(3)
troscopy, and EIMS (Supporting Information). Oxygen abstrac-
tion thus occurs from the hydroxamic acid to form the
corresponding amide with concomitant oxidation of Mo(V) to
Mo(VI) and formation of the molybdenyl complexes MoO2A2
and MoO2A′ (see Scheme 1). In a few cases, on addition of
the ligand (in a water/methanol mixture) to (NH4)2[MoOCl5],
it was noted that the initial orange/yellow color turned a deeper
shade of orange which may be due to initial formation to the
MoV hydroxamate prior to oxygen transfer occurring.
O(3)-Mo(1)-O(5)
O(1)-Mo(1)-O(6)
93.1(4)
88.7(4)
O(3)-Mo(1)-O(6) 157.9(4)
O(5)-Mo(1)-O(6) 105.6(4)
Mo(1)-O(2)-N(1) 117.0(8)
Mo(1)-O(4)-N(2) 115.5(7)
Mo(1)-O(1)-C(13) 113.8(9)
Mo(1)-O(3)-C(20) 113.2(8)
O(2)-N(1)-C(6)
114.1(11)
129.0(11)
114.6(10)
118.4(13)
O(2)-N(1)-C(13)
O(4)-N(2)-C(12)
116.9(11) O(6)-N(1)-C(13)
116.9(9) O(4)-N(2)-C(20)
C(12)-N(2)-C(20) 128.4(11) O(1)-C(13)-N(1)
O(1)-C(13)-C(14) 117.4(13) N(1)-C(13)-C(14) 124.2(13)
O(3)-C(20)-N(2)
119.7(13) O(3)-C(20)-C(21) 118.6(12)
N(2)-C(20)-C(21) 121.7(12)
Scheme 1
(NH4)2[MoVOCl5] + 3CH3(CH2)nC(O)N(Ph)OH f
MoVIO2[CH3(CH2)nCONPhO]2 + CH3(CH2)nC(O)N(Ph)H
Vanadium Complexes. Oxygen abstraction from hydrox-
amic acids (HA and H2A′) and formation of the corresponding
amides and diamides also occurs with both V(III) and V(IV)
compounds and concomitant formation of vanadium(V) hy-
droxamates, V(O)(OH)(A)2, V(O)OH(A′), and, in a number of
cases, VOClA′, e.g. VOCl(O(R)N(O)C(CH2)nC(O)N(R)O) (R
) C6H5, n ) 3; R ) C6H4CH3, n ) 7).
(19) Ashton, P. R.; Fenton, D. E. Inorg. Chem. Acta 1988, 146, 99.
(20) Jennings, K. R.; Kemp, T. J.; Read, P. A. Inorg. Chim. Acta 1989,
157, 157.

Reactions of N-Substituted Hydroxamic Acids
Inorganic Chemistry, Vol. 35, No. 6, 1996 1677
Thus reaction of the above monohydroxamic and dihydrox-
amic acids with both the vanadium(IV) reagent VO(SO4)
(method C), and the vanadium(V) reagent NH4VO3 (Method
D) gave two identical series of complexes as confirmed by
analysis, IR, NMR spectroscopy (Supporting Information).
Similar structural arguments apply to the vanadium(V) com-
plexes as to the above molybdenum(VI) series; for example in
all cases the infrared spectra show a lowering of about 50 cm-1
of ν(CO) from the free ligand to the corresponding complex,
disappearance of ν(OH) of the free ligand, and a sharp intense
band at around 950-990 cm-1 assigned to the ν(VdO)
stretching vibration consistent with O,O-hydroxamate coordina-
tion. Even when the reaction was carried out under anaerobic
conditions, formation of the VV complex occurred together with
the appropriate mono or diamide, which again were fully
characterized. The FABMS showed cluster formation of general
formula [(VO)nOm]+ containing up to 12 vanadium atoms being
observed with the instrumentation available. The FABMS of
VO(OH){C6H5(O)N(O)C(CH2)4C(O)N(O)C6H5)} and the as-
signments of the major cluster peaks are given in the Supporting
Information. There was no evidence for retention of the V(OH)
group in the clusters, suggesting that this group is very easily
abstracted under FABMS conditions. Unfortunately, we were
not able to obtain crystals suitable for X-ray crystal structural
studies for any of the above series but known X-ray crystal
structures, e.g. that of VOCl(PhCONHO)2,14 suggest that again
cluster formation occurs by molecule-ion reactions in the
FABMS experiments. A number of V(IV) primary hydroxam-
VCl3 in chloroform where yields of more than 80% have been
obtained with the stoichiometry shown in Scheme 2, method
E.
Dimerization of V(V) Dihydroxamates. Electrospray
Spectra of VO(OH)[C6H5N(O) C(O)(CH2)nC(O)N(O)C6H5]
(n ) 3, 5) (10a,b) and VO(OH)[p-CH3C6H4N(O)C(O)-
(CH2)nC(O)N(O)C6H4-p-Me) (n ) 2, 4), (10c,d). Satisfactory
NMR spectra could not be obtained for the dihydroxamate
complexes of V(V) due to their poor solubility in a range of
NMR solvents suggesting that they are dimeric in solution as
observed in the crystal structure of VO(OiC3H7)(ON(i-C3H7)C-
(O)(CH2)nC(O)N(i-C3H7)O) (n ) 5) and also indicated from
molecular weight measurements for n ) 3. Only in the case
of n ) 10 was a monomer indicated.14 The ready formation of
metal clusters in FABMS above also means that parent ion peaks
due to dimeric structures could not be identified thereby
precluding this technique as a means of structure determination.
However the electrospray mass spectra of the above series gave
clear evidence for the occurrence of dimers in the four
compounds, 10a-d, (n ) 3, 5, 2, 4), in methanol solution and
provides a striking example of the advantages of this most gentle
ionisation method.
Compound 10a, n ) 3 gave a spectrum with a strong peak
at m/z 789 with fairly intense lines at 790-792 signifying
additional protons, together with weaker peaks at 682, 586, 511
and some others. The peaks at 789-792 correspond to the
+
dimeric formulation (where A′ ) ligand dianion) A′2V2O4Hn
(m/z ) 790 + n), although why the peak corresponding to n )
-1 is the most intense is unclear. The peak at m/z 682
corresponds to loss of PhNO from one ligand of the dimer, while
that at m/z 586 refers to (A′V2O4HPhNO)+ (m/z ) 586).
Chemically-induced decomposition of the ion of mass 790 gave
daughter ions at m/z ) 682 and 493; the latter is attributable to
(A′V2O5)+.
13
ates have been prepared from VOSO4 suggesting that N-
substitution favors oxygen abstraction. It is hoped that mecha-
nistic studies currently in progress21 will clarify this point.
Reactions with VCl3 (Method E). In this case, oxidation
and oxygen abstraction again occurred on reaction with the
above hydroxamic acids (in a 1:2 ratio of V/HA) but a number
of the dihydroxamic acids (H2A′) gave the corresponding oxo-
chloro complexes of V(V) as well as the corresponding diamides
(Scheme 2).
Compound 10c, n ) 2, gave a dominant peak at m/z ) 817.24
attributed to the dimer (A′V2O4)+ (m/z ) 818). No vestige of
the monomer ion was present. Compound 10d, n ) 4, gave a
main peak at m/z ) 873.26: the dimer (A′V2O4)+ has m/z )
874 with high-mass satellites at 888.68 (A′V2O5)+ and 905.34
(A′V2O6)+. Compound 10b, n ) 5, gave a main peak at m/z
845.25 attributed to the dimer (A′V2O4)+ (m/z ) 846) and
Scheme 2. Methods C-E
Method C
+
+
+
-2H N2
satellites at m/z 861 (A′V2O5 ) 862) and 877 (A′V2O6 )
VIVSO4‚xH2O + 2H2A′ H O/acetone8
878). Collision-induced decomposition of the mass 845 ion
gave a spectrum dominated by fragments at m/z ) 738
(attributed to loss of PhNO) and 521 due possibly to (A′V2O5)+
(m/z ) 522); a smaller peak at m/z ) 813 signifies loss of O2
to give (A′V2O2)+ (Supporting Information).
2
VVO(OH)A′ + H(R)N(O)C(CH2)nC(O)N(R)H
Method D
+
These results parallel quite closely those obtained recently
by Crumbliss et al.22 for the ESMS of 1:1 ferric dihydroxamates
in acetonitrile/water mixtures where dimer peaks occurred for
hydroxamic acids with n ) 2 and 4, and only for the long chain
acid (n ) 8) did the corresponding monomer predominate.
Clearly steric constraints in the shorter chain dihydroxamic acids
results in dimer formation as illustrated by the crystal structure
of VO[i-C3H7N(O)C(O)(CH2)5C(O)N(O)-i-C3H7].14
-2H N2
NH4VVO3 + H2A′ water/acetone8 VVO(OH)A′
Method E
+
-2H N2
VCl3 + H2A′ water/acetone8
VVO(Cl)A′ + H(R)N(O)C(CH3)nC(O)N(R)H
Conclusions
Schemes 1 and 2 both involve stoichiometric ratios of
hydroxamic acid to starter metal compound of 3:1 as employed
in this work; however, because of the difficult separation and
extensive purification of the products required, resulting in yields
of not more than 30%, it was not possible to verify the
stoichiometry of the products. However, mechanistic studies
in progress21 have confirmed the stoichiometry in the case of
the reaction between N-phenylhexanohydroxamic acid and
Oxygen abstraction from N-substituted mono- and dihydrox-
amic acids by V(III), V(IV), and Mo(V) compounds and their
concomitant oxidation to V(V) and Mo(VI) occurs for a range
of hydroxamic acids. Reduction of hydroxamic acids to amides
occurs with phosphines23 and nitroso compounds24 to form
phosphine oxides and nitro compounds. In the former case an
(22) Caudle, M.; Stevens, R. D.; Crumbliss, A. L. Inorg. Chem. 1994, 33,
(21) Cittaro, R. Private communication.
6111.

1678 Inorganic Chemistry, Vol. 35, No. 6, 1996
Brown et al.
Table 7. Atomic Coordinates (x104) and Equivalent Isotropic
intramoleculear mechanism is indicated by double labeling
experiments.23
Displacement Coefficients (Å2 × 103) for 2
x
y
z
U(eq)a
Metal-centered oxygen atom transfer reactions have been
comprehensively reviewed by Holm25 with particular emphasis
on molybdenum centered reactions where µ-oxo dimer formation
often occurs. The detailed mechanism of the oxygen abstraction
reaction reported in this paper is not clear but may involve prior
formation of the lower valence state complex, e.g. Mo(IV), prior
to oxygen abstraction from the hydroxamic acid and the
observation of transient colors in some reactions with the Mo-
(IV) starter supports this idea. We have obtained no evidence
so far for the formation of a µ-oxo-dimer. Further mechanistic
studies are in progress.21
Mo(1)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
N(1)
N(2)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
1961(1)
2765(8)
3319(8)
806(8)
4893(1)
5617(6)
5595(5)
5975(5)
4639(5)
4637(4)
4070(5)
6166(6)
5171(7)
7538(5)
8054
7746
6921
6405
6713
4838(5)
4611
4461
4539
4766
653(1)
1152(3)
409(3)
629(3)
961(3)
167(3)
857(3)
659(4)
908(3)
534(2)
332
52
-27
175
456
1444(2)
1569
1282
870
45(1)
54(4)
50(4)
57(4)
48(4)
49(4)
54(4)
41(5)
50(5)
50(4)
50(4)
52(5)
48(4)
44(4)
39(4)
66(4)
90(6)
66(5)
58(5)
57(4)
43(3)
39(4)
59(5)
63(5)
98(7)
108(6)
172(10)
291(16)
44(4)
63(5)
76(5)
90(6)
118(7)
189(11)
340(20)
350(9)
1608(8)
2666(8)
3879(10)
-649(10)
4679(7)
5520
6392
6425
5584
4712
Experimental Section
Solvents were freshly dried by standard methods. Reagents were
used directly. Reactions studied under anaerobic conditions used
solvents which were degassed under N2 and carried out under dry
nitrogen using standard septum techniques. Infrared spectra were
measured, either using a 0.1 mm CaF2 cell or as a KBr disk on a Perkin
-2143(8)
-3371
C(9)
-4326
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
C(18)
C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
C(25)
C(26)
-4052
-2823
746
1033
1
-1869
4916
Elmer 1720 FT spectrometer linked to a Model 3700 data station. H
and 13C NMR spectra were recorded on a JEOL GX27O spectrometer.
Analyses were performed by the Microanalytical Unit of the Chemical
Services Unit of University College, Dublin. EI Mass Spectra were
recorded on a Finnegan Matt Incos 50 mass spectrometer with an
integrated FCMS system. FABMS were run in a glycerol matrix on a
VG7OE mass spectrometer with argon as bombardment gas at 8 kV.
ESMS were run on a Fisons “Quattro II” triple quadrupole mass
spectrometer at the University of Warwick, U.K.
3580(14)
4127(13)
3134(14)
3719(15)
2644(15)
3415(19)
2273(22)
-316(14)
-1286(12)
-811(14)
-675(16)
-1938(18)
-1695(22)
-2992(25)
6136(9)
6695(8)
7318(8)
8001(9)
8585(9)
9311(11)
9808(12)
5870(9)
6549(7)
7294(8)
7220(10)
7171(11)
6985(13)
6957(14)
1038(5)
1358(4)
1503(4)
1752(5)
1900(5)
2124(7)
2230(7)
747(5)
690(5)
942(4)
1391(5)
1589(5)
2077(7)
2215(8)
Preparation of Hydroxamic Acids. The N-phenyl and N-tolyl-
substituted hydroxamic acids were prepared by reaction of N-phenyl-
or N-tolylhydroxylamine with the appropriate acid (or diacid) chloride.26
Preparation of N-Arylhydroxylamines. These were prepared by
a modified version of that reported by Brink and Crumbliss;27 for
example, 20 mL of nitrobenzene was added to a stirred solution of
zinc dust in a 1:1 ethanol/water mixture (200 mL) followed by dropwise
addition of saturated ammonium chloride until a temperature of 65 °C
was reached and maintained for 3 h by intermittent cooling (NH4Cl/
ice bath). After filtration, addition of sodium chloride (150 g) and
cooling in an ice bath gave the N-arylhydroxylamine which was
separated from NaCl by addition of ether (75 mL), filtration and
subsequent reduction of solvent volume to give light yellow crystals.
In the case of N-phenylhydroxamine, prompt use is essential since the
compound decomposes in air and light.
a Equivalent isotropic U defined as one-third of the trace of the
orthogonalized Uij tensor.
To a warm deaerated aqueous solution (25 mL) of ammonium
oxopentachloromolybdate (0.34 g, 1.9 mmol) was added under nitrogen
a deaerated methanolic solution (25 mls) of N-phenylhexanohydroxamic
acid (1.2 g, 5.7 mmol) and heated on a steam bath for 1 h whereupon
the color changed from orange to a greenish/yellow. After refrigeration
overnight, pale yellow crystals of MoO2{CH3(CH2)4CONPhO}2 formed,
which were dried over CaCl2. The filtrate was reduced in volume and
on cooling overnight deposited a gray powder which on recrystallization
analyzed as CH3(CH2)4CONHPh (Supporting Information).
Preparation of Hydroxamic Acids. A typical preparation is given
for N,N ′-diphenylpimelohydroxamic acid, HON(Ph)C(O)(CH2)5C(O)N-
(Ph)OH.
Method B from the Mo(VI) Reagent, Na2MoO4‚2H2O. An
ethanolic solution (20 mL) of N-phenylhexanohydroxamic acid (0.42
g, 2 mmol) was added with stirring to an aqueous solution (20 mL) of
sodium molybdate (0.19 g, 1 mmol) to give immediately a light yellow
colored solution from which, after addition of 6 N HCl to adjust the
pH to 3, yellow crystals precipitated. Recrystallization from hot ethanol
gave shining yellow crystals (96% yield) suitable for X-ray analysis.
To a suspension of freshly crystallized N-phenylhydroxylamine
(19.8 g, 0.18 mmol) in 500 mL of ether was added a suspension of
NaHCO3(25.0 g, 0.3 mol) in water (40 mL) and the mixture cooled to
-10 °C followed by dropwise addition of pimeloyl chloride (10.9 g,
0.06 mol) over 1 h with stirring. The product was filtered and saturated
with a 10% solution of NaHCO3, filtered, washed with cold water and
recrystallized from hot acetone to give white needles of N,N ′-
diphenylpimelohydroxamic acid.
Preparation of Molybdenyl Hydroxamates MoO2(A)2 and MoO2-
(A′) from the Corresponding Monohydroxamic Acid (HA) and
Dihydroxamic Acids (H2A′). Method A from the Mo(V) Reagent,
(NH4)2[MoOCl5]. A typical preparation is given for MoO2{CH3(CH2)4
C(O)N(Ph)O}2. (NH4)2[MoOCl5], was prepared by the method of
Simon and Souchay.28
Preparation of Vanadium Hydroxamates VO(OH)(A2) and VO-
(OH)(A′). Method C from the V(IV) Reagent, VOSO4. A hot
deaerated aqueous solution (15 mL) of VOSO4 (0.16 g, 1 mmol) was
added dropwise under nitrogen to a hot acetone solution (15 mL) of
N-phenylhexanohydroxamic acid (0.42 g, 2 mmol) and heated on a
water bath for 30 min. When the mixture was cooled overnight, violet
crystals of VO(OH){CH3(CH2)4CONPhO}2 formed. Removal of
solvent from the filtrate gave a gray powder which on recrystallization
from CHCl3 gave colorless crystals of CH3(CH2)4CONHPh.
Method D from the V(V) Reagent, NH4VO3. A hot aqueous
solution (20 mL) of ammonium vanadate (0.12 g, 1 mmol) was added
dropwise with stirring to a solution of N-phenylhexanohydroxamic acid
(0.42 g, 2 mmol) in acetone (20 mL). Adjusting the pH to 2.3 by
addition of 2M H2SO4 and standing overnight at 3 °C gave a precipitate,
which on washing and drying over CaCl2 gave violet crystals of
VO(OH){CH3(CH2)4CONPhO}2.
(23) Forrester, A. R.; Ogiling, M. M.; Thomson, R. H. J. Chem. Soc. B
1970, 1081.
(24) Kurtzweil, M. L.; Loo, D.; Beak, P. J. Am. Chem. Soc. 1993, 115,
421.
(25) Holm, R. H. Chem. ReV. 1987, 87, 1401.
(26) Gupta, V. K.; Tandon, S. J. J. Ind. Chem. Soc. 1969, 46, 9.
(27) Crumbliss, A. L.; Brink, C. P. J. Org. Chem. 1982, 47, 1171.
(28) Simon, J. P.; Souchay, P. Bull. Soc. Chim. Fr. 1956, 1402.

Reactions of N-Substituted Hydroxamic Acids
Inorganic Chemistry, Vol. 35, No. 6, 1996 1679
Method E from the V(III) Reagent, VCl3. VCl3 gave in most
cases the vanadium(V) VO(OH)(A2) and VO(OH)(A′) series; however,
for HONRCO(CH2)nCONROH), where R ) C6H5, n ) 3, and R )
C6H4CH3, n ) 7, the chloro complexes VOCl(A′) were obtained. For
example, VCl3 (0.16 g, 1 mmol) in dry deoxygenated acetone (15 mL)
was added dropwise to a hot acetone solution (20 mL) of N,N ′-
diphenylglutarohydroxamic acid (0.32 g, 1 mmol). After the solution
was refluxed for 45 min under N2, a black precipitate of VOCl[OPh-
NOC(CH2)3CONPhO] formed. Removal of solvent under vacuum from
the filtrate gave a grayish powder which on recrystallization gave the
diamide, PhNHOC(CH2)3CONHPh. Satisfactory analyses were ob-
tained for all the above compounds (see Supporting Information).
Structure Determination of [MoO2{CH3(CH2)4C(O)N(C6H5)O}2]
(1) and [MoO2{CH3 (CH2)5C(O)N(C6H5)O}] (2). Lattice parameters
were refined from 32 (1) and 35 (2) reflections with 10° e 2θ e 30°.
Data were collected at room temperature on a Siemens R3 m/V
diffractometer, with ω-scan, 4 e 2θ e 50° for 1 and 3 e 2θ e 40° for
2, 5355 (1) and 6207 (2) reflections measured, three check reflections
recorded every 100, and 4% decay for 1 corrected. Lp and empirical
absorption corrections were done (ψ scans). The structures were solved
by direct methods and refined by full-matrix least-squares techniques
(SHELXTL-Plus).29 All non-hydrogen atoms except C24 for 1 and
the carbon atoms in 2 were refined anisotropically; hydrogen atoms
were refined in calculated positions (C-H ) 0.96 Å) riding on the
corresponding carbon atoms. The phenyl rings in 2 were refined as
regular hexagons (C-C ) 1.395 Å). The residuals in the final ∆F
map were 1.6 (1) and 0.8 (2) e/Å3 close to the Mo atoms. Scattering
factors were taken from standard sources.30 Further details are given
in Tables 1, 4, and 7.
were carried out in a Fisons’ “Quattro II” triple quadrupole mass
spectrometer (VG Biotech, Altrincham, U.K.) equipped with an
atmospheric pressure ionisation (API) source operated in the nebulizer-
assisted electrospray mode. The potential on the electrospray needle
was set at 3.6 kV, and the extraction cone voltage was normally set at
30V. Vanadium(V) hydroxamates at a concentration of 1.0 µg/µL were
dissolved in methanol by sonication followed by removal of excess
solid by centrifugation. Aliquots of 10 µL were introduced into the
ion source at a flow rate of 5 µL/min. Mass spectra were acquired
over the range m/z 1500 to m/z 80 during a 10 s scan, and by operating
the data system in the multichannel acquisition (MCA) mode, several
scans were summed to produce the final spectrum. Calibration was
carried out using a solution of sodium iodide. Dimeric ions of selected
m/z passed from the first quadrupole mass analyzer into the rf-only
quadrupole collision cell containing argon at 3.8 × 10-3 mbar at a
translation energy of 15 eV. Fragment ion spectra were obtained by
scanning the final quadrupole mass analyser over the m/z range from
the mass of the precursor ion down to m/z 40 in 10 s using the MCA
mode.
Acknowledgment. We thank Mr. Peter Caplan of the Mass
Spectrometry Laboratory, Chemical Services Unit, UCD, for
the FABMS and Dr. Armelle Buzy of the Department of
Chemistry, University of Warwick, for assistance in obtaining
FABMS and ESI spectra, respectively.
Supporting Information Available: Tables of analytical data for
1
complexes, hydroxamic acids, and amides, infrared data, H and 13C
Electrospray Ionization Mass Spectrometry. The electrospray
ionization-collision-induced decomposition (ESI/CID) experiments
NMR data, cluster peaks in selected FABMS, and the ESMS of
compound 10a in methanol, and figures showing the collision-induced
decomposition spectra of m/z ) 845 peak from the mass spectrum of
VO(OH)[O(C6H5)N(O)C(CH2)5C(O)N(C6H5)O] (34 pages). Ordering
information is given on any current masthead page.
(29) Sheldrick, G. M. SHELXTL-Plus. Siemens Analytical Instruments Inc.,
Madison, WI, 1991.
(30) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, England, 1974; Vol. IV.
IC950819R