On an unusual shortening of the C(sp3)-C(sp3) bonds in n-propylcobaloximes

Article abstract of DOI:10.1016/S0020-1693(99)00476-4

The structures of two n-propyl cobaloximes, [(n-Pr)Co(Hdmg)₂(L)]·H₂O where L = H₂O (6) and NH₂Ph (7), and Hdmg = monoanion of dimethylglyoxime, are described. The large static disorder undergone by the n-propyl group is attributed to different conformations of n-Pr, differing in the torsional angle around the axial Co-L bond. This result contrasts with those reported for the anhydrous [(n-Pr)Co(Hdmg)₂(H₂O)] (1) derivative and its inclusion compound with α-cyclodextrin (2), where a significant lengthening of 0.16 A? in the Cα-Cβ bond of n-Pr in 2, with respect to an incredibly short bond in 1, was attributed to the interaction of the cobaloxime with α-cyclodextrin. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00476-4

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Inorganica Chimica Acta 299 (2000) 263–267
Note
On an unusual shortening of the C(sp3)–C(sp3) bonds in
n-propylcobaloximes
Giorgio Nardin, Lucio Randaccio *, Giovanni Tauzher
Dipartimento di Scienze Chimiche, Uni6ersita` di Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy
Received 22 July 1999; accepted 19 October 1999
Abstract
The structures of two n-propyl cobaloximes, [(n-Pr)Co(Hdmg)2(L)]·H2O where L=H2O (6) and NH2Ph (7), and Hdmg=
monoanion of dimethylglyoxime, are described. The large static disorder undergone by the n-propyl group is attributed to
different conformations of n-Pr, differing in the torsional angle around the axial Co–L bond. This result contrasts with those
reported for the anhydrous [(n-Pr)Co(Hdmg)2(H2O)] (1) derivative and its inclusion compound with a-cyclodextrin (2), where a
,
significant lengthening of 0.16 A in the Ca–Cb bond of n-Pr in 2, with respect to an incredibly short bond in 1, was attributed
to the interaction of the cobaloxime with a-cyclodextrin. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Cobaloximes complexes; Alkyl complexes
,
to be lengthened by 0.16 A in 2, while the Co–C–C
1. Introduction
angle is widened by 5° in 2. However, no significant
change was observed in the Co–C distances. More
recently, Luo et al. [2] have reported also the structure
of (i-But)Co(Hdmg)2(H2O) (3) and of its inclusion com-
pounds with (a-cd) (4) and (b-cd) (5). Distortions simi-
lar to (a)–(c) were observed, but the i-Bu Ca–Cb
distances in 3–5 are quite normal, whereas the Cb–Me
Recently, a paper describing the preparation and
structure of the cobaloxime (n-Pr)Co(Hdmg)2(H2O) (1),
where Hdmg=monoanion of dimethylglyoxime, and of
its inclusion compound with a-cyclodextrin (a-cd) (2)
has been reported [1]. Interestingly, the structural anal-
ysis showed that the compound 2 is formed by the
inclusion of the cobaloxime n-Pr group into the hydro-
phobic cavity of a-cd. The authors have found by
comparison with the geometry of 1 with that of the
cobaloxime moiety in 2, some ob6ious conformational
changes: (a) the displacement of Co out of the four N
,
distances in 5 were found to be 1.34(1) and 1.24(1) A,
respectively. The authors ascribed [1] the observed dif-
ferences (a)–(d) in 2, to the hydrophobic interaction
between the n-Pr group and the a-cd cavity and to the
steric interaction between the a-cd rim and the cobal-
oxime equatorial moiety. That such interactions are
responsible for the geometrical changes (a)–(c) is con-
vincing, but that they can provoke the dramatic change
,
donor plane towards the alkyl group is 0.008 A in 1
,
and 0.038 A in 2; (b) the two equatorial Hdmg units
make an interplanar angle of 1° in 1 and of 10° in 2; (c)
,
of the Ca–Cb distance from 1.237(13) A in 1 to
,
in 2 the axial Co–OH2 bond is lengthened by 0.02 A,
,
1.394(7) A in 2 is highly questionable. In fact, recent
whereas the C–Co–O angle is narrowed by 3.5°; (d) the
molecular mechanics calculations [3] reproduced satis-
n-Pr Ca–Cb distance, which is unexpectedly short for a
3
3
factorily bond lengths and angles in 1 and 2, except for
,
C(sp )–C(sp ) single bond (1.237(13) A) in 1, is found
,
the Ca–Cb distance calculated to be 1.536 A. The
analysis of the anisotropic thermal factors of the atoms
in the two structures shows high values (about four
times those of the other atoms) of U22 in 1 and of U11
* Corresponding author. Tel.: +39-040-676 3935; fax: +39-040-
676 3903.
E-mail address: randaccio@univ.trieste.it (L. Randaccio)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00476-4

264
G. Nardin et al. / Inorganica Chimica Acta 299 (2000) 263–267
in 2 for the b-carbon atom, and an orientation of the
corresponding axis of the thermal ellipsoids nearly par-
allel to the coordination plane. This observation sug-
gests that the unusual geometry of n-Pr in 1 and 2
might be the result of a mathematical artefact, due to
the refinement, carried out without taking into account
a possible static disorder of the alkyl moiety. In spite of
the large number of structurally characterized alkyl-
cobaloximes [4], to our knowledge no structural data
for an n-propylcobaloxime are available for compari-
son. Therefore, we tried to check the static disorder
hypothesis, redoing the structural analysis of 1. Unfor-
tunately, we were not able to obtain any single crystal
following the method described in Ref. [1]. However,
good quality single crystals of the mono-hydrated form
of 1, [(n-Pr)Co(Hdmg)2(H2O)]·H2O (6), were obtained.
The structures of 6 and that of the [(n-Pr)Co(Hdmg)2-
(PhNH2)]·H2O analogue (7), were determined.
6.90; N, 15.1. Calc. for C11H25N4O6Co: C, 35.8; H,
6.84; N, 15.2%.)
2.1.2. [(n-Pr)Co(Hdmg)2(PhNH2)]·H2O (7)
A water–methanol solution of 6, obtained as above,
was treated with an excess of aniline under stirring at
room temperature (r.t.). Crystals of the complex were
obtained by partial evaporation of the solvent and
collected by filtration. (Found: C, 45.8; H, 6.95; N,
15.5. Calc. for C17H30N5O5Co: C, 46.0; H, 6.82; N,
15.8%.)
2.2. Crystal structure analyses
Intensity data for 6 and 7 were collected on an
Enraf–Nonius CAD4 four-circle automated diffrac-
tometer with graphite-monochromated Mo Ka radia-
tion. Accurate unit cell parameters and orientation
matrix were determined by least-squares refinement of
the setting angles of 25 well-centered reflections in the
range 22B2qB28°. Reflections were collected at r.t. in
ꢀ/2q scan mode. No decay throughout the data collec-
tion was observed. Correction for Lorentz-polarization
and €-scan absorption was applied. The structures
were solved by Patterson and Fourier methods and
refined by full-matrix least-squares method (on F2). The
H atoms were not refined, but included at calculated
positions in the final refinement for all the atomic
species with full occupancy. Crystal, collection and
refinement data are given in Table 1. The programs
used are given in Refs. [6,7].
2. Experimental
2.1. Synthesis
2.1.1. [(n-Pr)Co(Hdmg)2(H2O)]·H2O (6)
The complex (n-Pr)Co(Hdmg)2(Py), prepared as pre-
viously reported [5], was eluted through a column of
acidic resin to obtain the corresponding aquo complex
6. Red–brown crystals, suitable for structure determi-
nation were obtained by evaporation of a water–
methanol solution in the dark. (Found: C, 35.4; H,
Table 1
Crystal data and structure refinement for 6 and 7
Compound
6
7
Empirical formula
Formula weight
T (K)
C11H25CoN4O6
368.28
293(2)
C18H32CoN5O6
473.42
293(2)
,
u (A)
0.7107
monoclinic, C2/c
12.922(3)
12.762(1)
20.982(8)
90
99.83(3)
90
3409(2)
8, 1.435
1.038
1552
0.7107
triclinic, P1
8.465(1)
11.383(1)
12.869(2)
77.08(1)
84.79(1)
70.75(1)
1140.9(2)
2, 1.378
0.794
500
(
Crystal system, space group
,
a (A)
b (A)
c (A)
h (°)
i (°)
k (°)
,
,
3
,
V (A )
Z, zcalc (Mg m−3
)
v(Mo Ka) (mm−1
F(000)
)
Crystal size (mm)
q range (°)
0.5×0.4×0.2
2.26–29.96
0.7×0.7×0.5
2.26–29.98
Reflections collected/unique
Refinement method
5095/4969 [Rint=0.040]
full-matrix least-squares on F2
4969/10/201
6886/6613 [Rint=0.029]
Full-matrix least-squares on F2
6613/14/283
1.054
Data/restraints/parameters
Goodness-of-fit on F2
1.062
Final R indices [I\2|(I)]
R1=0.060, wR2=0.169
R1=0.052, wR2=0.176

G. Nardin et al. / Inorganica Chimica Acta 299 (2000) 263–267
265
Table 2
Comparison of the geometry of the n-Pr–Co–OH2 fragment in 1, 2, 6
and 7 after the anisotropic refinement with all the non-hydrogen
atoms with full occupancy
1
2
6
7
,
Bond distances (A)
Co–OH2(NH2Ph) 2.063(4)
2.083(2)
2.013(4)
1.394(7)
1.550(7)
2.069(3)
2.010(4)
1.335(8)
1.526(8)
2.142(3)
2.017(4)
1.247(13)
1.579(12)
Co–Ca
Ca–Cb
Cb–Cg
2.008(8)
1.237(12)
1.509(13)
Bond angles (°)
Co–Ca–Cb
Ca–Cb–Cg
129.3(7)
126.0(9)
124.1(3)
117.1(4)
125.5(5)
117.9(6)
127.1(7)
116.0(10)
Fig. 1. ORTEP drawing with numbering scheme for the non hydrogen
atoms of (a) [(n-Pr)Co(Hdmg)2(H2O)]·H2O (6), and (b) [(n-
Pr)Co(Hdmg)2(PhNH2)]·H2O (7).
3. Results and discussion
The ORTEP drawings, obtained by anisotropic refine-
ment of all the non-H atomic species of 6 and 7, all
with occupancy factor 1, are given in Fig. 1. The refined
geometry of the Co–n-Pr moiety in 6 (R1=0.057) and
7 (R1=0.052) is very close to that of 1 and 2 (Table 2)
as well as the trend of the thermal factors. The U22
value for Cb in 6 and those of almost all the Uii of Cb
and Cg in 7 are dramatically larger than the others, as
shown also by the Cb thermal ellipsoids of Fig. 1,
whose longer axis is nearly parallel to the Co coordina-
tion plane. Among several tentative models, the most
suitable appeared to be that in which Cb and Cg were
split over two positions C101, C102 and C111, C112,
respectively, with half occupancy factors, corresponding
to two orientations of the n-Pr group with respect to
the equatorial moiety (Fig. 2a). In 7, the disorder of
n-Pr, which is more severe than that in 6, was inter-
preted by assuming three different conformations of
occupancies 0.4, 0.3 and 0.3, respectively (Fig. 2b),
Fig. 2. Sketches showing the static disorder of the n-Pr group in (a)
[(n-Pr)Co(Hdmg)2(H2O)]·H2O (6), and (b) [(n-Pr)Co(Hdmg)2-
(PhNH2)]·H2O (7).

266
G. Nardin et al. / Inorganica Chimica Acta 299 (2000) 263–267
Table 3
174.4(6); −173.1(1.0), −176.7(8); 142.9(9), 179.8(1.1)°
for the three conformations in the n-Pr group in 7. It is
likely that similar results could be obtained by refining
the structures of 1 and 2, using this procedure. On this
basis, it appears clear that the observed abnormal Ca–
Cb shortenings in 1 and 2 are due to a severe static
disorder of the alkyl group and, hence, the observed
change in the C–C distance in 1 and 2 reflects essen-
tially the different degree of conformational disorder.
Therefore, the observed lenghtening (d) should be es-
sentially originated by a minor conformational freedom
of n-Pr in 2, determined by the interaction with the
cavity.
,
Co–C and Co–L axial distances (A) in aquo and aniline alkylcobal-
oximes a
R
L=H2O
L=PhNH2
Co–O
Co–R
Co–N
Co–R
L
1.900(4)
2.058(3)
1.900(4)
1.990(5)
2.009(5)
2.129(1)
2.147(2)
2.139(3)
2.007(5)
1.992(2)
2.030(3)
1.986(3)
Me
Et
b
n-Pr
2.067(3)
2.063(4)
2.052(2)
1.998(4)
2.008(8)
2.011(3)
n-Pr c
i-Bu d
i-Pr
2.177(2)
2.215(4)
2.068(3)
2.159(4)
CH2CMe3
2.056(5)
2.130(2)
2.044(7)
2.129(3)
The disorder affects only the geometry of the alkyl
group. In fact, the geometries of the Co(Hdmg)2(H2O)
moiety and of the H2O–Co–C fragment in 1, 6 and 7
compare well. The butterfly h angle and the displace-
ment d of Co out of four equatorial N-donors in 6 are
Adamantyl e
a Data are from Ref. [4] if not otherwise stated.
b Present work.
c Ref. [1].
d Ref. [2].
,
,
0.9(3)° and −0.001(2) A, and 4.2(3)° and 0.034(1) A in
7, and do not differ significantly from the values re-
ported in 1. Analogously, the Co–C and Co–OH2
(Co–N in 7) axial distances fit well the trends of the
trans and cis influences in the series of aquo and aniline
alkylcobaloximes (Table 3). In both crystals the water
crystallization molecules are hydrogen-bonded to O3
and to an oxygen atom of another symmetry related
water molecule (O2w–O3=2.830(5), O2w–O4%=
e Ref. [8].
Table 4
,
Axial distances (A) in [(R)Co(dmgH)2(PX3)], with R=Cl and Me
and in compounds 1–6 a
Co–Cl
Co–P
Co–Me
Co–P
P(OMe)3
P(Me)3
P(OMe)2Ph
P(OMe)Ph2
P(n-But)3
PPh3
2.280(5)
2.188(4)
2.01(1)
2.256(4)
2.293(1)
2.287(1)
2.352(1)
2.015(3)
2.013(5)
2.019(6)
,
2.866(5) A in 6 and O1w–O3=2.823(5), O1w–O2%=
2.288(2)
2.290(1)
2.294(2)
2.287(2)
2.294(5)
2.213(2)
2.242(1)
2.265(2)
2.330(2)
2.369(5)
,
2.869(5) A in 7). The work presented in Ref. [1], in our
opinion, gives a new and original contribution to cobal-
oxime chemistry. Our aim is only to stress that the
crystallographic results, as obtained by the automatic
refinement of the diffraction pattern, must be carefully
evaluated and interpreted, especially when crystallo-
graphic results are used to formulate conclusions of
chemical interest.
2.026(6)
2.016(5)
2.418(1)
2.463(1)
PChx3
Compound
Co–O
Co–C
b
1
2.063(4)
2.067(3)
2.083(2)
2.052(2)
2.159(3)
2.104(2)
2.008(8)
1.998(4)
2.011(3)
2.011(3)
2.016(4)
2.017(4)
6 c
2 b
3 d
4 d
5 d
In order to interpret the lengthening of the Co–OH2
bond and the constancy of the Co–C bond upon
inclusion of the n-Pr cobaloxime, Luo et al. have
suggested that the ‘capped’ a-cd behaves as a bulky R
group above the Co(Hdmg)2 moiety. In fact, the a-cd
bends the equatorial moiety towards the trans ligand
and provokes a lengthening of the Co–OH2 distance
(steric trans influence), according to the previous find-
ings in cobaloximes [4]. However, the cd-cobaloxime
interactions influence only the Co–OH2 axial distances
in the inclusion compounds, whereas the interactions
between bulky R (or L) ligands and the equatorial
moiety in cobaloximes, when the trans ligand has a
small bulk, affect essentially the Co–R (or Co–L) bond
(steric cis influence). This is shown in Table 4, where
the axial distances in the two series RCo(dmgH)2PX3,
with R=Cl and Me, are reported. In each series the
Co–R distance does not vary appreciably, whereas the
Co–P bond increases significantly with the increase in
bulk of PX3. In 2, the claimed increase in bulk of the
alkyl group, due to the capped cd, does not determine
a Data are from Ref. [4] if not otherwise stated.
b Ref. [1].
c Present work.
d Ref. [2].
essentially differing by the torsion angle around Co–
C9. The refinement was then performed with the Co–
Ca–Cb and Ca–Cb–Cg angles fixed at about 118 and
,
110°, respectively, and the C–C distances about 1.54 A.
All the non-H atomic species were refined anisotropi-
cally, whereas the Cb and Cg fractional species were
treated isotropically. The final R1 values were 0.060 for
6 and 0.052 for 7. The refined conformations in 6 were
characterized by N1–Co–Ca–Cb and Co–Ca–Cb–Cg
torsion angles of 106.6(7) and −174.3(7)°, respectively,
in one conformation and of 144(1) and −17.3(8)° in
the other. The corresponding figures are 95.7(9),

G. Nardin et al. / Inorganica Chimica Acta 299 (2000) 263–267
267
a detectable steric cis influence in the inclusion com-
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the cyclodextrin. More likely, it appears that the cobal-
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the alkyl group without perturbing the Co–C bond,
probably because the Co–C strain requires energy higher
than that necessary to bend the cobaloxime equatorial
moiety and to lengthen the trans Co–OH2 bond.
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Acknowledgements
We thank the Ministero della Ricerca Scientifica e
Tecnologica (Rome) and the Consiglio Nazionale delle
Ricerche (CNR) for financial support.
.