New alkyl-cobalt(III) complexes with tridentate amino-oxime ligands: Synthesis, structure, and reactivity

Article abstract of DOI:10.1002/1099-0682(20011)2001:1<267::AID-EJIC267>3.0.CO;2-Z

The oxidative addition of alkyl halides to the Co^I species generated by the reduction of [Co^III(LNHpy)(HLNHpy)]-(ClO₄)₂ (1), where HLNHpy is the tridentate 2-(2-pyridyl-ethyl)amino-3-butanone oxime ligand and LNHpy is its conjugate base, led to the formation of a new class of organocobalt complexes of general formula [RCo^III(LNHpy)(HLNHpy)]-(ClO₄) [R = Me (2a), Et (2b), CH₂CF₃ (2c), nBu (2d), and CH₂Cl (2e)]. All the complexes were characterised by ¹H and ¹³C NMR spectroscopy. The X-ray structures of 2a, 2b and 2c provide evidence for a pseudo-octahedral configuration, where HLNHpy and LNHpy act as bi- and tridentate ligands, respectively. The axial geometry in 2a is closer to that found in methylcobalamin than that reported for other models, suggesting steric and electronic cis influences of the equatorial ligands close to those of the corrin nucleus. The solution properties and the reactivity show strong analogies with those of the previously known Vitamin B₁₂ models.

Full text of DOI:10.1002/1099-0682(20011)2001:1<267::AID-EJIC267>3.0.CO;2-Z

FULL PAPER
New Alkyl؊Cobalt(III) Complexes with Tridentate Amino؊Oxime Ligands:
Synthesis, Structure, and Reactivity
[
a]
[a]
[a]
[a]
Renata Dreos, Alessandro Felluga, Giorgio Nardin, Lucio Randaccio,*
[a]
[a]
Patrizia Siega, and Giovanni Tauzher*
Keywords: Cobalt / Tridentate ligands / Alkylations / Vitamins / N ligands
I
The oxidative addition of alkyl halides to the Co species
2c provide evidence for a pseudo-octahedral configuration,
III
generated by the reduction of [Co (LNHpy)(HLNHpy)]- where HLNHpy and LNHpy act as bi- and tridentate ligands,
(
ClO
4
)
2
(1), where HLNHpy is the tridentate 2-(2-pyridyl-
respectively. The axial geometry in 2a is closer to that found
ethyl)amino-3-butanone oxime ligand and LNHpy is its con-
in methylcobalamin than that reported for other models, sug-
jugate base, led to the formation of a new class of organocob- gesting steric and electronic cis influences of the equatorial
III
alt complexes of general formula [RCo (LNHpy)(HLNHpy)]-
ClO ) [R = Me (2a), Et (2b), CH CF (2c), nBu (2d), and properties and the reactivity show strong analogies with
CH Cl (2e)]. All the complexes were characterised by H and
C NMR spectroscopy. The X-ray structures of 2a, 2b and
ligands close to those of the corrin nucleus. The solution
(
4
2
3
1
those of the previously known Vitamin B12 models.
2
1
3
Introduction
Previous studies on classical models for coenzyme B ,
12
such as alkylcobaloximes LCo(DH) R (Scheme 1a, DH ϭ
2
monoanion of dimethylglyoxime, R ϭ alkyl, L ϭ neutral
Lewis base), have been aimed at elucidating the factors that
affect the properties of the CoϪC bond. Experimental evid-
ence shows that steric and electronic factors are both im-
portant in this context.[ The recent structural results on
B12 enzymes have shown that in some of the enzyme-bound
B12 coenzymes, the benzimidazole residue is replaced in the
coordination to Co by the imidazole moiety of a histidine
of the protein chain.[ This finding has renewed interest in
1]
2]
[
3,4,5]
the nature of the CoϪL bond in simple models.
The
[
1]
early hypothesis that the CoϪL distance is influenced by
the orientation of the planar L ligand, measured by the tor-
sional angle ϕ around that bond, has been confirmed.[
In orientation A (Scheme 2), when ϕ is close to 0, the
6Ϫ8]
˚
CoϪL distance is found to be shorter by about 0.03 A than
in orientation B (Scheme 2), corresponding to ϕ close to Scheme 1
0°. Typically, orientation A is found in cobaloximes,
9
whereas orientation B is found in iminocobaloxime ana-
logues (Scheme 1b). However, in the lariat-type complexes[
8]
Less definitive indications about the influence of the L
orientation on the CoϪC bond in the ground state has been
found so far. In fact, structural (CoϪR distance) and spec-
troscopic (νCoϪMe) evidences have suggested that orienta-
tion B may correspond to a weaker CoϪC bond than in
orientation A.[ On the other hand, FT-Raman studies did
not detect any change in the CoϪC force constants even
(
Scheme 1c), derived from iminocobaloximes, the values of
ϕ and of the CoϪpy distance are similar to those in the
cobaloxime analogues. In fact, in the lariat-type complexes,
although derived from iminocobaloximes, orientation A is
imposed by the conformational rigidity of the pentadent-
ate ligand.
7]
[9]
in base-on and base-off cobalamins. Furthermore, recent
[
a]
Dipartimento di Scienze Chimiche, Universit a` degli Studi di studies[10] on the interaction of adenosylcobinamides with
Trieste
hindered bases, (1,2-Me Im, 2-Mepy and 2,6-Me py) have
2
2
Via L. Giorgieri 2, 34127 Trieste, Italy
Fax: (internat.) ϩ 39-040/676-3903
E-mail: randaccio@univ.trieste.it
provided evidence for a dominant effect of the axial base
on the transition state of the CoϪC bond-cleavage process.
Eur. J. Inorg. Chem. 2001, 267Ϫ276

WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ1948/01/0101Ϫ0267 $ 17.50ϩ.50/0
267

R. Dreos, A. Felluga, G. Nardin, L. Randaccio, P. Siega, G. Tauzher
FULL PAPER
The process involves the oxidative addition of the appro-
I
priate alkyl halide to the nucleophilic Co species, which
may be generated in situ by the reduction of 1 with NaBH4/
PdCl . Similar synthetic routes have been extensively ex-
2
ploited to obtain cobalt complexes containing highly unsat-
I
urated equatorial ligands. Generally, the Co species are ob-
III
[12]
[11]
tained from Co by the use of NaBH4. We observed
that although the reduction of 1 with NaBH does not pro-
4
II
ceed beyond the formation of a Co species, the complete
reduction to Co could be achieved by the addition of a
I
catalytic amount of PdCl2 to the reaction mixture. The
formation of Co is recognisable by its nucleophilic reactiv-
Scheme 2
I
ity toward alkyl halides, but the dark brown solutions are
extremely sensitive to oxygen and this prevented further
characterisation. It is noteworthy that the Co complex ob-
In order to obtain further insight into the above points,
we have synthesised a new class of organocobalt derivatives
II
tained by reduction with NaBH is unreactive toward alkyl
4
[
RCo(LNHpy)(HLNHpy)]ClO , (2) (HLNHpy ϭ 2-(2-pyri-
4
iodides (see Discussion). Several attempts to synthesise or-
ganometallic derivatives with bulky alkyl groups such as
dylethyl)amino-3-butanone oxime) derived from [Co-
[
11]
(
LNHpy)(HLNHpy)](ClO ) (1),
(Scheme 3). The geo-
4
2
, and adamantyl were carried out, but the CoI
[8]
iC F , iC H
3 7 3 7
metry of 2 is reminiscent of the lariat-type complexes,
species failed to react with the corresponding alkyl halide.
which combine the corrin-like features of the iminocobalox-
imes with an appended axial base. However, they differ
from the latter by having a more crowded equatorial moiety,
owing to the side substituents at the amino CϪN fragments.
Therefore, complexes 2 could potentially mimic the interac-
tions between axial and equatorial moieties in cobalamins
better than previous models.
I
Even after several hours, Co was unchanged and its pres-
ence could be verified by the addition of CH I to the reac-
3
tion mixture, which rapidly led to the formation of the
methyl derivative.
Reactivity of the Methyl Derivative 2a
Like alkylcobalamins[
13]
and synthetic alkylcobalt(III)
chelates,[ complex 2a is light-sensitive. The photolysis of
an aqueous solution of the complex in the presence of oxy-
gen produced formaldehyde and complex 1. This result may
be rationalised by the assumption that the initial step is the
14]
II
homolysis of the CoϪC bond, which produces a Co spe-
cies and methyl radicals. In the presence of oxygen they are
oxidised, giving 1 and formaldehyde, which are the expected
products of aerobic photolysis.[
13]
The methyl-transfer reaction occurs from complex 2a to
II
Co (DH) (H O) . The reaction was carried out in the dark
2
2
2
under nitrogen. After exposure to air, complex 1 and
III
Scheme 3
CH
3 2 2
Co (DH) H O were isolated and identified [Equa-
tion (2)].
The synthesis and NMR characterisation of a series of
complexes of type 2 [Rϭ Me (2a), Et (2b), CH CF (2c),
2
3
nBu (2d), CH Cl (2e)], the crystal structure of 2a, 2b, 2c as
2
well as the reactivity of the Me derivative are reported and
their ability to model the B12 system is discussed.
Results
Since the process occurs in the absence of light, it can be
Synthesis
formulated as a direct transfer of the methyl radical to the
II
The organometallic complexes of type 2 were obtained acceptor. The Co species arising from the dealkylation
according to Equation (1).
gives 1 after oxidation in the air. The complex 2a also
268
Eur. J. Inorg. Chem. 2001, 267Ϫ276

New AlkylϪCobalt(III) Complexes with Tridentate AminoϪOxime Ligands
FULL PAPER
methylates Vitamin B12r under the experimental conditions It seems likely that the reaction proceeds by the stepwise
described above [Equation (3)].
replacement of the two amino oxime ligands by dimethyl-
glyoxime.
X-ray Structures
The ORTEP[15] drawings of 2aϪ2c are given in Figure 1,
crystal data in Table 1. For 2a, only the ORTEP drawing of
The exchange reaction of the chelate moiety of 2a has one of the two crystallographically nonequivalent molecules
been successfully carried out in one case: A methanolic so- (A) is shown. Complexes 2 can formally be described as
lution of 2a, dimethylglyoxime, and pyridine (allowed to being derived from parent compound 1, by substitution of
III
stand overnight) produced CH Co (DH) py as a product. one axial pyridyl moiety with the R group. Complex 1 can
3
2
Figure 1. ORTEP diagram with the numbering scheme for the non-hydrogen atoms of one of the two crystallographic independent
molecules of 2a, 2b, and 2c
Eur. J. Inorg. Chem. 2001, 267Ϫ276
269

R. Dreos, A. Felluga, G. Nardin, L. Randaccio, P. Siega, G. Tauzher
The coordination around N1 is such that the two
FULL PAPER
therefore, to a good approximation, be considered as a
structural reference for complexes 2 (Scheme 3). The pen- CoϪN1ϪC angles differ by up to about 10°, the largest
dant pyridyl group in complexes 2 is anchored through N6 value being on the side of the 2-ethylene chain (Table 2).
to the N5 amino atom by a short intramolecular hydrogen Such an asymmetric coordination of the axial N donor is
˚
[1]
bond in the range of 2.861(8)Ϫ3.020(8) A (Table 2). The typical of lopsided ligands, such as Me Bzm, and of la-
3
two oximic O atoms of LNHpy and HLNHpy are hydro- riat-type complexes of Scheme 1c.[8]
gen-bonded, with O···O distances ranging from 2.401(7) to
The axial distances, together with the torsion angle
˚
2
.432(7) A (Table 2). The average of the values is shorter by around the CoϪN1 bond, ϕ, are given in Table 3. The
˚
˚
about 0.07 A than that of 2.487(2) A, obtained by averaging CoϪN1 distances reflect the notable trans influence of the
[1b]
1
40 values found in cobaloximes.
Analysis of the NϪO alkyl group in complexes 2, relative to that of the pyridyl
bonds in the three complexes and of the Fourier difference group in 1. The CoϪC bond length in complexes 2 appears
maps did not allow for location of the H oxime bridge, to increase with the bulk and to decrease with the electron-
which was assumed to be bonded to O2 (see below), so that withdrawing ability of R, in agreement with previous find-
LNHpy acts as a tridentate and HLNHpy as a bidentate li- ings in cobaloximes.[19]
gand.
Table 1. Crystal data and structure refinement for complexes 2a, 2b and 2c
Compound
2a
2b
2c
Empirical formula
Formula weight
Temperature [K]
C
24.50
H
39ClCoN
6
O
6.50
C
24
605.49
H
38.50ClCoN
6
O
6.25
24.50 2 3 6 6
C H36Cl CoF N O
616.00
293(2)
0.71073
697.42
293(2)
0.71073
293(2)
˚
Wavelength [A]
0.71073
monoclinic, P2
16.868(4)
9.6950(10)
17.218(4)
91.38(2)
2814.9(10)
4, 1.429
0.754
Crystal system, space group
monoclinic, P2
1
/n
1
/n
monoclinic, C2/c
16.090(2)
15.165(3)
25.837(4)
105.220(10)
6083(2)
˚
a [A]
22.581(2)
11.907(2)
22.788(3)
106.370(8)
5878.7(14)
8, 1.392
0.724
˚
b [A]
˚
c [A]
β [°]
˚
3
V [A ]
3
Z, calcd. density [Mg/m ]
Absorption coefficient [mmϪ1]
F(000)
8, 1.523
0.807
2592
1274
2888
Crystal size [mm]
Range for data collect. [°]
Reflections collected/unique
0.3 ϫ 0.6 ϫ 0.9
2.05 to 27.97
7955/7554
0.2 ϫ 0.3 ϫ 0.7
5.01 to 26.02
5675/5494
0.2 ϫ 0.3 ϫ 0.6
5.02 to 26.02
6238/5951
[
R(int) ϭ 0.0232]
[R(int) ϭ 0.0287]
5494/0/347
0.984
[R(int) ϭ 0.0402]
5951/9/354
1.037
Data/restraints/parameters
Goodness-of-fit on F
7554/0/675
2
1.037
Final R indices [I Ͼ 2σ(I)]
R indices (all data)
R1 ϭ 0.0724,
wR2 ϭ 0.2483
R1 ϭ 0.0724,
wR2 ϭ 0.2483
R1 ϭ 0.0747,
wR2 ϭ 0.2044
R1 ϭ 0.1413,
wR2 ϭ 0.2712
0.721 and Ϫ0.804
R1 ϭ 0.0877,
wR2 ϭ 0.2315
R1 ϭ 0.1681,
wR2 ϭ 0.2760
0.934 and Ϫ0.826
Largest diff. peak and hole [e.AϪ3]
˚
0.903 and Ϫ0.930
The bending angle, α, between the two equatorial five- NMR Characterisation
membered rings and the displacement, d, of Co out of the
1
four N equatorial donor plane are given in Table 3. The
The characterisation in solution was performed by H
13
displacement occurs towards the axial pyridyl group in A and
C
NMR spectroscopy in [D ]DMSO. The
6
and B molecules of 2a and towards R in 2b and 2c, and (LNHpy)(HLNHpy) moiety gives rise to strikingly similar
1
reflects the difference in bulk between R and the trans-pyri-
H NMR spectra for all five complexes. The spectrum of 2c
[
16]
dyl group. In all the complexes, the pyridyl ligand is tilted is described as an example. The CH protons give rise to
3
out of the axial direction and the tilt can be measured by two doublets at δ ϭ 0.66 and 1.32 (CH CH) and two sing-
H
3
the dihedral angle, γ, between the mean planes of the pyri- lets at δH ϭ 1.78 and 2.05 (CH CϭN), each signal integrat-
3
dyl ligand and of the four equatorial N donors. The values ing for three protons. Ethylenic CH protons generate four
2
of γ in 1 and 2, (Table 3), are close to those observed in py partially overlapped multiplets in the range δH
ϭ
(
82.3°) and 1,5,6-trimethylbenzimidazole (Me Bzm) (81.0°) 2.88Ϫ3.44, the total integration corresponding to eight pro-
3
iminocobaloximes with methyl substituents on the propy- tons. The signals for the CH protons at δH ϭ 3.75 and 3.82
lene bridge (Scheme 1b, R1 ϭ Me).[
17]
appear as broad multiplets, and those for the NH protons
Equatorial coordination distances and angles in com- at δH ϭ 4.75 and 5.47 as a broad doublet and a broad
plexes 2 are given in Table 2 together with those of 1. In singlet, respectively; each of these signals integrates for one
complexes 2, the CoϪN4 distances are slightly shorter than proton. On addition of D O to the DMSO solution, the
2
those of CoϪN3. This supports the previous assignment of NH signals did not disappear, indicating a slow exchange
the oxime bridge H atom.[18]
on the NMR time scale. In CDCl solution, the NH doub-
3
270
Eur. J. Inorg. Chem. 2001, 267Ϫ276

New AlkylϪCobalt(III) Complexes with Tridentate AminoϪOxime Ligands
FULL PAPER
˚
Table 2. Selected bond lengths [A] and angles [°] for 1 and 2
2
a(A)
2a(B)
2b
2c
1(A)
1(B)
CoϪN1
2.134(5)
1.996(5)
1.888(6)
1.888(5)
2.018(5)
Ϫ
1.960(7)
92.2(2)
89.2(2)
92.1(2)
88.7(2)
177.3(3)
Ϫ
81.9(2)
175.6(2)
98.4(2)
90.4(3)
Ϫ
97.5(3)
177.9(3)
90.5(3)
Ϫ
2.185(5)
1.983(5)
1.900(6)
1.893(5)
2.005(5)
Ϫ
1.983(7)
93.2(2)
88.0(2)
90.3(2)
90.2(2)
175.3(3)
Ϫ
82.3(2)
176.5(2)
97.8(2)
90.6(3)
Ϫ
97.7(2)
178.3(2)
89.8(3)
Ϫ
2.168(5)
1.964(5)
1.893(5)
1.879(5)
2.035(5)
Ϫ
2.015(6)
90.3(2)
89.1(2)
91.1(2)
87.7(2)
177.9(2)
Ϫ
81.6(2)
178.2(2)
98.5(2)
91.8(2)
Ϫ
97.4(2)
176.8(2)
90.9(2)
Ϫ
2.160(6)
1.991(5)
1.903(6)
1.887(6)
2.021(6)
Ϫ
1.998(8)
91.8(2)
89.2(2)
91.6(2)
87.6(2)
178.7(3)
Ϫ
82.2(3)
176.6(3)
98.7(2)
86.9(3)
Ϫ
97.7(3)
176.7(2)
90.7(4)
Ϫ
2.033(4)
1.991(5)
1.912(5)
1.911(5)
1.980(5)
2.017(5)
Ϫ
94.0(2)
85.5(2)
92.7(2)
90.3(2)
Ϫ
176.4(2)
82.0(2)
173.1(2)
99.1(2)
Ϫ
89.1(2)
96.9(2)
175.7(2)
Ϫ
93.2(2)
82.6(2)
Ϫ
84.2(2)
Ϫ
91.0(2)
117.2(3)
126.0(4)
116.3(3)
126.9(3)
2.388(7)
Ϫ
2.045(5)
1.988(5)
1.914(5)
1.902(4)
1.961(5)
1.983(5)
Ϫ
94.4(2)
83.2(2)
91.2(2)
90.6(2)
Ϫ
174.2(2)
81.2(2)
174.3(2)
97.7(2)
Ϫ
89.9(2)
97.7(2)
173.8(2)
Ϫ
93.6(2)
83.1(2)
Ϫ
84.4(2)
Ϫ
92.6(2)
116.7(3)
124.8(3)
116.2(4)
126.1(3)
2.421(6)
Ϫ
CoϪN2
CoϪN3
CoϪN4
CoϪN5
CoϪN6
CoϪC23
N1ϪCoϪN2
N1ϪCoϪN3
N1ϪCoϪN4
N1ϪCoϪN5
N1ϪCoϪC23
N1ϪCoϪN6
N2ϪCoϪN3
N2ϪCoϪN4
N2ϪCoϪN5
N2ϪCoϪC23
N2ϪCoϪN6
N3ϪCoϪN4
N3ϪCoϪN5
N3ϪCoϪC23
N3ϪCoϪN6
N4ϪCoϪN5
N4ϪCoϪC23
N4ϪCoϪN6
N5ϪCoϪC23
N5ϪCoϪN6
CoϪN1ϪC1
CoϪN1ϪC5
CoϪN6ϪC12
CoϪN6ϪC16
O1·····O2
82.4(2)
85.3(3)
Ϫ
91.6(3)
Ϫ
116.9(4)
126.4(5)
Ϫ
82.3(2)
85.9(3)
Ϫ
91.9(3)
Ϫ
118.6(4)
125.0(4)
Ϫ
82.6(2)
86.8(2)
Ϫ
92.4(2)
Ϫ
117.0(4)
125.6(5)
Ϫ
82.0(3)
89.7(3)
Ϫ
92.6(3)
Ϫ
116.6(5)
126.1(5)
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
2.402(7)
2.861(8)
2.401(7)
2.968(8)
2.432(7)
3.020(8)
2.408(8)
2.894(8)
N5·····N6
let (belonging to the bidentate ligand) shifts downfield to and the solvent significantly influences the chemical shift of
δH ϭ 5.74, whereas the NH singlet shifts upfield to δH the Cα protons in complexes 2. The effect is much weaker
.60; the different behaviour could be due to the formation for protons that are more distant from the metal centre.
ϭ
4
1
of an intramolecular hydrogen bond between the proton of For R ϭ Et and nBu, the CH resonances in the H NMR
2
the NH doublet and the nitrogen atom of the noncoordin- spectrum are partially buried; for R ϭ CH CF , the CH
2
3
2
ated pyridine, as found in the solid state. Aromatic protons protons give rise to a quadruplet owing to coupling with
3
generate two series of partially overlapped signals, of cor- fluorine ( J ϭ 15 Hz). It is noteworthy that in the CH Cl
HF
2
2
rect multiplicity and intensity, in the δH range 8.41Ϫ7.30. derivative, these protons generate an AB spectrum ( J
ϭ
HH
The oxime proton resonates at δH ϭ 19.6. This value, very 5 Hz) owing to the coupling between diastereotopic gemi-
close to that found in the lariat-type complexes[ and nal protons.
8]
shifted remarkably downfield with respect to that of coba-
[
20]
[21]
loximes and imino cobaloximes, suggests a strong hy-
drogen bond.
Discussion
The main difficulty in the assignment of H and 13C
1
NMR spectra of the (LNHpy)(HLNHpy) moiety lied in
Complexes 2 are structurally similar to lariat-type com-
distinguishing between the resonances of the tridentate and plexes, but with some additional features. In fact, the equat-
of the bidentate ligands. An almost complete assignment of orial moiety is more crowded with respect to the latter
1
13
H and C signals was performed for 2c using homonuc- model, because of the side substituents on the amino CϪN,
lear and heteronuclear shift correlation 2D NMR (H,H- pointing above and below the equatorial plane. Further-
and H,C-COSY), HMBC and DIFNOE spectroscopy. Shift more, the values of ϕ, found in complexes 1 and 2, make
assignments for the other alkyl complexes were made ac- them ‘‘representatives’’ of the orientation C of Scheme 2,
cording to those of 2c (Table 4 and 5).
as compared with cobaloximes and lariat complexes, which
The protons bonded to the α-carbon atom of the axial exhibit orientation A, and iminocobaloximes, which exhibit
alkyl groups show a remarkable downfield shift with respect orientation B. However, the relative flexibility of the ethyl-
to those of the corresponding cobaloximes (ϩ1.07 ppm for ene chain bearing the coordinated pyridyl group allows for
[20]
Me, ϩ1.25 ppm for Et, and ϩ1.14 ppm for CH CF ) al- a rotation, even if limited, of the axial py residue around
2
3
though a direct comparison is not thoroughly meaningful, the CoϪN1 bond. This rotation is somewhat less restricted
because the spectra of the latter were recorded in CDCl3 than in lariat-type complexes.[ In fact, there is clear evid-
8]
Eur. J. Inorg. Chem. 2001, 267Ϫ276
271

R. Dreos, A. Felluga, G. Nardin, L. Randaccio, P. Siega, G. Tauzher
FULL PAPER
˚
[1]
Table 3. Axial distance (A) in some B12 models, as well as ϕ (°), γ (°), and α (°) values; data are from ref. , unless otherwise stated
(
LNHpy)(HLNHpy)[a]
Compound
1
1
2c
2a
2a
2b
a]
[a]
[b]
[b]
[b]
[b]
R
Py(A)[
py(B)
CH
2
CF
3
Me(A)
Me(B)
Et
˚
˚
CoϪN1/A
2.033(4)
2.017(5)
48
2.045(5)
1.983(5)
44
2.160(6)
2.134(5)
2.185(5)
Ϫ
2.168(5)
CoϪN6/A
Ϫ
44
Ϫ
1.998(8)
0.002
16
84
Ϫ
Ϫ
38
Ϫ
Ϫ
36
Ϫ
ϕ/°
48
Ϫ
ϕЈ/°
30
Ϫ
28
Ϫ
˚
CoϪC/A
1.960(7)
1.983(7)
0.015
14
2.015(6)
Ϫ0.016
˚
d/A
0.022
Ϫ0.004
0.018
9
82
Ϫ
α/°
γ/°
17
13
8
80
Ϫ
81
75
82
Ϫ
γЈ/°
86
89
pyCo(DH)
R
2
R
Cl[
1.956
6
b]
[c]
Me
[c]
Et
[c]
2 3
CH CF
˚
CoϪNax/A
ϕ/°
2.041(4)
2.068(3)
2.081(3)
0
2.035(5)
0.05
9.1
0
1
˚
CoϪR/A
Ϫ
2.010(3)
1.998(5)
0.05
3
˚
d/A
0
0.01
1
α/°
γ/°
2
89
90
89
90
Me
3
Bzm[(DO)(DOH)pn]R[d]
c]
R
Cl[
CH
2
CF
3
Me
Et
˚
CoϪNax/A
ϕ/°
1.993(6)
86
2.060(3)
2.100(3)
115
2.105(3)
119
80
˚
CoϪR/A
Ϫ
2.026(4)
0.06
2.011(3)
0.09
13.8
88
2.041(4)
0.10
16.7
89
˚
d/A
0.06
α/°
γ/°
13
89
16.6
84
(
C1py)h[e]
R
Cl
Me(A)
2.06(1)
0
Me(B)
2.07(1)
0
˚
CoϪNax/A
ϕ/°
1.959(4)
0
˚
CoϪR/A
2.237(2)
0.02
1.6
2.05(2)
0.02
1.99(2)
0.04
˚
d/A
α/°
γ/°
Ϫ1.8
90
Ϫ4.4
90
90
[
a]
This work, excluding 1 from ref. Ϫ [b] S. Geremia, R. Dreos, L. Randaccio, G. Tauzher, L. Antolini, Inorg. Chim. Acta 1994, 216,
[11]
[
c]
[d]
[4]
[e]
ref.[8]
1
25. Ϫ L ϭ py. Ϫ ref.
Ϫ
1
Table 4. H NMR chemical shifts of [RCo(LNHpy)(HLNHpy)]ClO
4
complexes in [D
6
]DMSO
meta[a]
para
NH
CH
CH
ϪCH
CH
[b]
O··H··O axial R
R
ortho
2
2
3
c]
nC
4
H [
9
8.41 (d) 7.69 (d) 7.54 (t) 8.03 (t) 4.74(bs)
.18 (d) 7.31 (d) 7.25 (t) 7.76 (t) 4.50 (bd) 3.73 (bm) (bm, 8 H)
8.41 (d) 7.69 (d) 7.54 (t) 8.03 (t) 4.70 (bs) 3.58 (bm) 3.31Ϫ2.74
.19 (d) 7.31 (d) 7.25 (t) 7.75 (t) 4.44 (bd) 3.72 (bm) (bm, 8 H)
8.42 (d) 7.72 (d) 7.55 (t) 8.04 (t) 4.87 (bs) 3.57 (bm) 3.32Ϫ2.73
.29 (d) 7.33 (d) 7.27 (t) 7.77 (t) 4.44 (bd) 3.67 (bm) (bm, 8 H)
8.45 (d) 7.75 (d) 7.55 (t) 8.05 (t) 5.08(bs) 3.71 (bm) 3.40Ϫ2.84
.34 (d) 7.34 (d) 7.28 (t) 7.78 (t) 4.65 (bd) 3.78 (bm) (bm, 8 H)
8.41 (d) 7.76 (d) 7.55 (t) 8.05 (t) 5.47 (bs) 3.75 (bm) 3.44Ϫ2.88
.36 (d) 7.35 (d) 7.30 (t) 7.80 (t) 4.75 (bd) 3.82 (bm) (bm, 8 H)
3.60 (bm) 3.32Ϫ2.76
1.73 (s) 1.34 (d) 19.40 (s) CH
2.00 (s) 0.84 (d) CH
1.73 (s) 1.35 (d) 19.41 (s) CH
2.00 (s) 0.84 (d) CH
1.72 (s) 1.33 (d) 19.41 (s) 1.89 (s)
1.99 (s) 0.77 (d)
3
2
3
2
0.94 (t)
1.09 (m), 1.44(m), 2.77(m)
0.46 (t)
2.99(q)
8
[
c]
C
2
H
5
8
[
c]
CH
CH
CH
3
2
2
8
Cl[c]
1.77 (s) 1.39 (d) 19.36 (s) 5.02 2
8
2.04 (s) 0.72 (d)
4.98 ( JH-H ϭ 5 Hz)
[c]
CF
3
1.78 (s) 1.32 (d) 19.60 (s) 2.55 (q)
2.05 (s) 0.66 (d)
8
[
a]
[b]
The first column is for meta protons bonded to C4 and C15, the second column for protons bonded to C2 and C13. Ϫ
The first
[c]
column is for methyl groups adjacent to CϭN, the second column for methyl groups adjacent to CH. Ϫ Data in the upper line refer
to the tridentate ligand, data in the lower line refer to the bidentate ligand.
ence that the orientation of the py moiety also appreciably below). Thus, for the same ligand trans to N1 (and N6 in
influences the length of the CoϪN1 bond, so that the ex- 1) larger ϕ values correspond to longer CoϪN1 distances.
pected trend in the trans influence with R is modified (see In fact, the values of ϕ and of the CoϪN1 distance in mole-
272
Eur. J. Inorg. Chem. 2001, 267Ϫ276

New AlkylϪCobalt(III) Complexes with Tridentate AminoϪOxime Ligands
Table 5. 13C NMR chemical shifts of [RCo(LNHpy)(HLNHpy]ClO
complexes in [D
FULL PAPER
4
6
]DMSO
R
C10
C21
C5
C16
C1
C12
C4
C13
C2
C15
C3
C14
C9
C20
C6
C17
C7 [
C8
C19
C11
C22
axial R group
a]
C18
[
b]
nC
4
H
9
156.75 159.57 150.57 126.55 123.26 138.60 60.94 30.21 45.89 17.17 12.74 CH
60.54 159.02 148.01 123.77 121.94 137.13 61.50 32.22 49.64 18.87 14.13 CH
156.68 159.53 150.61 126.53 123.24 138.58 60.89 30.21 45.82 17.13 12.73 CH
60.54 159.00 148.03 123.71 121.92 137.08 61.55 32.25 49.62 18.81 14.12 CH
156.82 159.13 150.78 126.78 123.30 138.74 60.35 30.14 45.06 17.74 12.74
69.50 159.06 148.12 123.73 121.96 137.12 61.58 32.21 50.74 18.53 13.89
157.45 160.32 151.30 127.08 123.46 139.05 59.86 30.30 44.54 16.79 13.15 43.08
60.76 158.96 148.14 123.77 122.07 137.22 61.45 32.02 50.25 18.38 14.32
158.63 160.01 151.10 127.31 123.56 139.13 59.09 30.00 43.61 16.25 13.42 CF
61.52 159.06 148.21 123.80 122.14 137.33 61.40 31.92 50.57 18.52 14.55
3
2
3
2
13.79
1
22.76, 24.30, 33.55
17.06
[
b]
2 5
C H
1
18.05
[
b]
CH
CH
CH
3
2
2
1
Cl [b]
1
[
b]
CF
3
3
133 (q)
1
[
a]
Resonances not assigned separately. Ϫ [b] Data in the upper line refer to the tridentate ligand, data in the lower line refer to the
bidentate ligand.
˚
cule A are smaller than those in molecule B of 2a. Analog- of 2.134(5) A (R ϭ Me, ϕ ϭ 38°) in molecule A of 2a, is
˚
ously, the CoϪN1 bond is longer than the CoϪN6 bond shorter than in 2b [2.168(5) A] (R ϭ Et, ϕ ϭ 36°), as is the
and ϕ is larger than ϕЈ in both molecules A and B of 1 case with the other B12 models. Analogously, the distance
˚
(
Table 3). This suggests that the ϕ values of 36° in 2b and of 2.185(5) A (R ϭ Me ϕ ϭ 48°) in molecule B of 2a is
˚
of 44° in 2c should correspond to a short- and a long-type longer than in 2c [2.160(6) A] (R ϭ CH CF , ϕ ϭ 44°). The
2
3
CoϪN1 bond, respectively. Therefore, it appears that the influence of the torsion angle on the CoϪN1 distance can
CoϪN(py) distance trans to the same donor reaches the be evaluated in 1, where the two axial CoϪN1 distances
largest value when ϕ approaches 50°, which corresponds to (ϕ ഠ 45°) are longer that those of CoϪN6 (ϕ ഠ 30°). There-
orientation C in Scheme 2. That this conformation is the fore, the CoϪN1 distance is the result of a compromise be-
most strained, is also suggested by the increase in the tween the energies involved in the torsion around N1 and
bending α angle when ϕ approaches 50°, as shown in Fig- in the CoϪN1 stretching. These structural features are in
ure 2, where the α/ϕ plot is reported for complexes 1 and 2. agreement with recent molecular mechanics calculations for
LCo(DH) Me (L is a planar N donor ligand) based on a
2
new approach which also takes the contribution of the elec-
tronic trans influence into account.[ These calculations
have furnished profiles of the strain energy against the ϕ
angle, which is characterised by a large maximum centred
at ϕ ϭ 90° and a minimum at ϕ ϭ 0°, the difference in
height ranging from 8 to 40 kJ/mol and increasing with the
L bulk.[ In addition, the large maximum at approximately
ϕ ϭ 90° has a fine structure, consisting of two nearly equal
5]
5]
maxima at about 55° and 145°, separated by a relative min-
Figure 2. Dependence of the torsion angle ϕ around the CoϪN1
bond on the bending angle α between the two equivalent five-mem- imum at ϕ ϭ 90°. This suggested that orientation C
bered rings in complexes 1 and 2; A and B refer to the two inde-
pendent molecules of 1 and 2a
(
Scheme 2) is energetically even less favoured than orienta-
tion B. The corresponding plot of calculated CoϪN(ax) dis-
tances against ϕ shows a profile very similar to that of the
Taking these observations into account, it is of interest strain energy with a CoϪN bond lengthening at ϕ ϭ 50°
˚
to compare the geometry of the axial fragment in complexes which increases from 0.03 to 0.06 A depending on the bulk
with that in the analogous alkylcobaloximes and other of L. The CoϪR distances follow the same trend in all the
2
related B12 models. In the latter series, the CoϪN(ax) dis- series of Table 3, reflecting the effects of the bulk and the
tance increases with an increase in the σ-donating ability of σ-donating ability of R.[
19]
R, for L ligands with very similar ϕ values. Thus, in coba-
Recent accurate CoϪMe and CoϪN axial distances of
˚
loximes and iminocobaloximes, for which ϕ ഠ 0 and 90°, 1.979(4) and 2.162(4) A, respectively, have been reported
respectively, the order of increasing CoϪpy distances is Cl in MeCbl.[ Hence, one of the most interesting structural
ϽϽ CH CF Ͻ Me Ͻ Et. Furthermore, for a given R, the features of model 2 is that the geometry of the axial
22]
2
3
CoϪpy bond is lengthened, when ϕ ഠ 90° (in complexes of NϪCoϪMe fragment is much closer to that of MeCbl
Scheme 1, b) with respect to that observed when ϕ ഠ 0° (in (where the axial N ligand is a benzimidazole residue) than
complexes of Scheme 1, a and c), irrespective of the type of any other previously proposed B12 model, as shown by a
equatorial moiety. For complexes 2, the apparently anomal- comparison of the axial distances in several B12 models and
ous trends in CoϪN(ax) distances with R as well as the MeCbl (Table 6).
differences between the A and B molecules of 2a (and in
The present complexes also show many solution proper-
the same molecule of 1) can easily be interpreted if both ties characteristic of cobalamins and of their synthetic
contributions are considered. In fact, the distance CoϪN1 models. Significant parallels include the observation of the
Eur. J. Inorg. Chem. 2001, 267Ϫ276
273

R. Dreos, A. Felluga, G. Nardin, L. Randaccio, P. Siega, G. Tauzher
FULL PAPER
˚
Table 6. Comparison of the axial distances [A] in MeCbl and in methyl derivatives of several B12 models
CoϪMe
CoϪNax
MeCbl
1.979(4)
2.162(4)
2a
1.960(7), 1.983(7)
2.003(3)
2.134(5), 2.185(5)
2.106(3)
MeCo[(DH)(DOH)pn]py
MeCo[(DH)(DOH)pn] Me
MeCo(C1py)
MeCo(DH)
MeCo(DH)
3
Bzm
2.011(3)
2.100(3)
1.99(2), 2.05(2)
1.998(5)
2.07(1)
2
Me
py
2
2.068(3)
3
Bzm
1.989(2)
2.060(2)
III
II
II
I
Co /Co and Co /Co redox series, the reactions of the homolytic cleavage of the CoϪC bond, thereby providing
I
Co species with alkylating agents to give organocobalt de- clear evidence for the tendency of these complexes to react
rivatives, the photochemically induced CoϪC cleavage and through radical processes. In addition, the majority of the
II
the alkyl-transfer reactions to radical acceptors. On the Co forms of the Vitamin B12 models and Vitamin B12r
other hand, there are some distinctive aspects in the chem- itself have been found to be reactive toward alkyl halides.
ical behaviour of these novel models which deserve atten- On the basis of kinetic studies[ the following stepwise
tion. One is the appreciable influence of the steric factors, free-radical mechanism [Equation (4)] has been proposed
which is clearly indicated by the anomalous trend in the for these series of compounds:
trans influence (see structural results) and by the fact that
28]
it is apparently impossible to prepare derivatives with bulky
alkyl groups, such as isopropyl, adamantyl and perfluoro-
isopropyl, which have been reported for previously known
[
1,23]
models.
(
The observed methyl transfer from 2a to Co-
II
DH) and Vitamin B may also be interpreted in terms
2
12r
of steric factors that lower the stability of CoϪC bond in
these aminoϪoximato complexes.
The electronic factors appear to be less important in de-
termining the stability of the CoϪC bond. In this regard, it
should be noted that both alkylcobalamines and their
simple models, contain highly unsaturated ligands that al-
low for an extensive electronic delocalisation over the equat-
orial plane, while the alkylϪcobalt complexes 2 contain
only two unconjugated CϭN bonds in their chelating sys-
tem. It follows that a high extent of unsaturation of the
Likewise, the lack in reactivity of the CoII form of 1 can
be attributed to its inability to abstract the halogen from
the alkyl halide in the slow step rather than to its low tend-
ency to trap free alkyl radicals.
equatorial ligand is not a necessary prerequisite for the Conclusions
formation of stable alkylϪcobalt complexes. This result is
not unexpected; at least in a few cases, the synthesis of
Tridentate aminoϪoxime ligands are effective in stabilis-
III
alkylϪcobalt compounds containing completely saturated ing the Co ϪC bond, as the analogous tridentate
polyamine (N4) macrocycles has been successfully iminoϪoxime ligands.[
29,30]
The methyl derivative un-
[
24Ϫ26]
achieved.
On the contrary, electronic factors play a dergoes photolytic homolysis and is a better methyl donor
relevant role in stabilising the lowest oxidation state of the than methylcobaloxime. These organometallic complexes
I
metal ion. Thus, both the higher lability of the Co derivat- can therefore be considered as suitable models for the B12
ive of the complex (1) and the more drastic conditions re- system (alkylcobalamin), such as the well-known alkylcoba-
quired for its formation, relative to the formation of the loximes and other related models.[ A distinctive feature of
corresponding cobal(I)oximes, may be ascribed to the lower this new model is the appreciable influence of steric factors
extent of unsaturation present in the equatorial ligand. In on their chemical behaviour. This is clearly indicated by the
1]
II
at least one case it was unequivocally proved that the Co / anomalous trend in the trans influence and by the fact that
I
Co reduction is responsive to the extent of unsaturation it is apparently impossible to prepare derivatives with bulky
present in the ligand; a greater unsaturation causes an an- alkyl groups, such as iPr, adamantyl and perfluoroisopro-
odic shift in the half-wave potentials.[ This trend justifies, pyl, which, on the contrary, have been reported for previ-
at least qualitatively, the experimental observations re- ously known models. In this respect, they more closely par-
27]
ported above.
allel the alkylcobalamin chemistry. Furthermore, the axial
II
The last point to stress is the inability of the Co species distances in the methyl derivative, very close to those re-
derived from 1 to react with alkyl halides to give the corres- cently reported for methylcobalamin, make them a better
ponding alkylϪcobalt species by means of a free radical B12 structural model than any other previously reported.[
process. This behaviour is somewhat unexpected because Structural evidence is given that this behaviour can be re-
both photochemical and alkyl-transfer processes involve lated to the crowded equatorial moiety, which interacts with
22]
274
Eur. J. Inorg. Chem. 2001, 267Ϫ276

New AlkylϪCobalt(III) Complexes with Tridentate AminoϪOxime Ligands
FULL PAPER
the axial ligands more strongly than in cobaloximes. A re- was concentrated to 10 mL, passed through a chromatographic col-
umn of Al O and eluted with CH Cl . A solution of the pure
[
22]
cent comparison
of the structural, spectroscopic and
2
3
2
2
product was obtained (50 mL). The by-products, which were most
strongly bound, could eventually be eluted with methanol. Diiso-
propyl ether was then added until the solution became turbid, and
the suspension was allowed to stand overnight. Crystals of the de-
sired product were recovered by filtration and air-dried. In order
to obtain crystals suitable for X-ray analysis or for a better puri-
fication, the products were treated as follows.
electrochemical properties of alkylcobalamins with those of
alkylcobaloximes suggests that the corrin ligand makes
more electronic charge available on Co than on the (DH)2
ligand (electronic cis influence), and that the longer CoϪN
axial distances in cobalamins are also due to ring electronic
influence. Both the longer CoϪN1 bonds and the increased
difficulty in the Co /Co reduction, relative to alkylcoba-
loximes, suggests a higher charge density on Co in com-
plexes 2. The present models are possibly also electronically
closer to cobalamins. These compounds therefore seem to
offer a better opportunity to study the influence of steric
factors on the formation and cleavage of the CoϪC bond,
than that provided by previous B12 models. Finally, the easy
substitution of the chelating ligand, which occurs without
cleavage of the CoϪMe bond, suggests the possibility of
preparing new series of otherwise inaccessible organometal-
lic compounds.
III
I
[
CH
3
Co(LNHpy)(HLNHpy)]ClO
4
·0.5CH
3
COCH
3
(2a): Some
drops of water were added to a saturated solution of the complex
in acetone, followed by diisopropyl ether, until the solution became
turbid. After standing in the dark in a stoppered vessel for 2 d,
orange crystals were collected by filtration and air-dried. Yield:
0
.31 g (34%). Ϫ C24.5
6
H39ClCoN O6.5 (616.0): calcd. C 47.8, H 6.4,
N 13.6; found C 47.9, H 6.4, N 13.6.
2 5 4
[C H Co(LNHpy)(HLNHpy)]ClO (2b): Recrystallised as above;
red crystals were obtained. Yield: 0.28 g (31%).
38ClCoN
8.0, H 6.3, N 13.7.
Ϫ
C
4
24
H
6 6
O (610.0): calcd. C 47.9, H 6.4, N 14.0; found C
[
CF CH Co(LNHpy)(HLNHpy)]ClO
3
2
4
·0.5CH
2
Cl
2
(2c): Recrys-
Experimental Section
tallised by the diffusion of diisopropyl ether stratified over a satur-
ated solution of the complex in dichloromethane. Yellow crystals
were collected by filtration and air-dried. Yield: 0.37 g (36%). Ϫ
Vitamin B12a and methylcobalamin were purchased from Aldrich.
CH
3
Co(DH)
2
H
2
O, CH
3
Co(DH)
2
py, and Co(DH)
2
(H
2
O)
2
were
C
24.5
H
36Cl
2
3 6 6
CoF N O (697.4): calcd. C 42.2, H 5.2, N 12.1; found
[12]
C 42.5, H 5.2, N 12.2.
prepared by previously reported methods. All other reagents and
chemicals were commercial products and were used without further
[
n-C
red crystals were obtained. Yield: 0.25 g (27%).
42ClCoN
9.4, H 6.7, N 12.8.
4 9 4
H Co[(LNHpy)(HLNHpy)]ClO (2d): Recrystallised as above;
1
13
purification. Ϫ H and C NMR spectra were recorded with a Jeol
Ϫ
1
13
6
EX-400 ( H at 400 MHz and C at 100.4 MHz) from [D ]DMSO
C
4
26
H
6 6
O (629.1): calcd. C 49.6, H 6.75, N 13.4; found C
solutions with TMS as internal standard. Two-dimensional homo-
nuclear and heteronuclear correlated spectra (H,H and H,C COSY)
were obtained using the instrument’s automatic programme. The
[
ClCH
2
Co(LNHpy)(HLNHpy)]ClO
4
(2e): Recrystallised as above;
HMBC spectrum of [CF
]DMSO was obtained as a 2048 ϫ 256 data matrix with 64
scans (preceded by 4 dummy scans) per t value and a PDBS (pulse
delay between scans) of 1.00 s. An 8.7 µs 90° PW (pulse width) of
3 2 4
CH Co(LNHpy)(HLNHpy)]ClO in
yellow crystals were obtained. Yield: 0.42 g (45%).
CoN (621.4): calcd. C 44.5, H 5.7, N 13.5; found C
4.4, H 5.7, N 13.3.
Ϫ
[D
6
23
C H
35Cl
2
6 6
O
1
4
13
C was used. Values of ∆
1
(the delay between the first 90° proton Substitution of the Chelating System by Dimethylglyoxime: Di-
13
pulse and the first 90° C pulse) and ∆
2
(the delay between the first
methylglyoxime (0.114 g, 0.98 mmol) and pyridine (0.5 mL) were
added to a solution of complex 2a (0.300 g, 0.49 mmol) in meth-
anol (20 mL). The solution was allowed to stand for 5 h at room
temperature in the dark. Water (5 mL) was then added to the solu-
tion. After evaporation of the methanol, orange crystals of
and the second 90° 1 C pulse) were 3.6 and 62.5 ms, respectively.
3
Syntheses: The same synthetic procedure was followed for all the
[RCo(LNHpy)(HLNHpy)]ClO
4
complexes (R ϭ Me, Et, nBu,
CH Cl, CH CF ). Ϫ CAUTION: Although no problems were enco-
2
2
3
CH
identified by H NMR (CDCl
5 5
12 H, CH ϪC), 7.33, 7.73, 8.62 (5 H, C H
3 2
Co(DH) py were collected by filtration (0.154 g, 82%) and
untered in the present study, perchlorate salts are potentially explos-
ive and should only be handled in small quantities! Ϫ The reactions
were performed at room temperature, under an inert atmosphere
and in the dark. In a typical experiment, a solution of NaOH
1
3
): δ
H
ϭ 0.82 (3 H, CH
3
ϪCo), 2.13
(
3
N). Following the same
procedure, but with exclusion of pyridine, no reaction was observed
and the starting complex was recovered. After recrystallisation, the
(
0.20 g, 5.00 mmol) in water (2 mL) was added to a suspension of
complex 1 (1.00 g, 1.49 mmol) in methanol (150 mL). The resulting
clear solution was treated with NaBH (0.12 g, 4.70 mmol), dis-
solved in a minimum volume of water, followed by three drops of
PdCl solution, prepared by the addition of conc. HCl to a suspen-
sion of PdCl (1.00 g) in water (20 mL) until complete dissolution
had occurred. When the solution assumed a dark brown colour
complex was identified by elemental analysis. Ϫ C24.5
CoN 6.5 (616.0): calcd. C 47.8, H 6.4, N 13.6; found C47.6, H
.3, N 13.3.
H39Cl
6
O
4
6
2
2 2 2
Transmethylation Reaction on Co(DH) (H O) : A solution of com-
2
plex 2a (0.100 g, 0.17 mmol) in methanol (20 mL) was treated with
II
Co (DH)
2 2 2 2
(H O) (0.055 g, 0.17 mmol, 1 equiv.) under N , in the
(
(
(
after 15 min), a solution of the appropriate alkyl iodide dark at room temperature. After 5 h, the solution was allowed to
7.45 mmol, i.e. 5 equiv. with respect to the complex) in methanol
10 mL) was added. After about 30 min, the dark brown solution
stand in air, water (5 mL) was added and the methanol was re-
moved in a rotary evaporator. The aqueous residue was extracted
turned to bright orange. At this point, the flow of N
pended and the methanol was removed in a rotary evaporator at
2
was sus-
with CH
obtained and identified by H NMR spectroscopy. The aqueous
2 2
Cl (2 ϫ phase, concentrated in air with exclusion of light, gave red crystals
2 2
Cl . By evaporation of the solvent, the complex 1 was
1
4
5
0 °C. The aqueous suspension was extracted with CH
0 mL) and the residual water removed from the organic phase by
SO . After filtration, the solution
1
of CH
H, CH
3
(DH)
2
H
2
O identified by H NMR (CD
ϪC).
3
OD): δ
H
ϭ 0.67 (3
treatment with anhydrous Na
2
4
3
ϪCo), 2.24 (12 H, CH
3
Eur. J. Inorg. Chem. 2001, 267Ϫ276
275

R. Dreos, A. Felluga, G. Nardin, L. Randaccio, P. Siega, G. Tauzher
FULL PAPER
J. T. Drummond, R. G. Mattews, M. L. Ludwig, Science 1994,
Transmethylation of Complex 2a on Vitamin B12r: The reaction was
carried out under nitrogen, in the dark and at room temperature.
A solution of Vitamin B12a (0.050 g) in water (25 mL) was reduced
to Vitamin B12r as described in the literature.[ A solution of the
complex 2a (0.100 g) in methanol (25 mL) was then added and the
reaction mixture was allowed to stand overnight. After the evap-
oration of methanol and the addition of acetone (30 mL), a red
microcrystalline precipitate formed slowly. The product (0.030 g,
[2c]
2
26, 1669Ϫ1670. Ϫ
R. Reitzer, K. Gauber, C. Jogl, U. G.
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[
3]
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cobalamin by comparison of its UV/Vis and H NMR spectra in
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999, 981Ϫ992.
D
2
[
[
6]
7]
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Photolysis of Complex 2a: A solution of [CH
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addition of a few drops of a saturated solution of NaClO . Red
3
Co(LNHpy)-
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[
1
S. Dang, R. Padmakumar, N. Maiti, R. Banerjee, T. G.
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[
12]
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˚
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mined from 25 carefully selected reflections. Data were corrected
for Lorentz polarisation effects and for absorption by the ψ-scan
technique. Detailed crystallographic data are presented in Table 1.
Ϫ The structures were solved by Patterson methods followed by
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conditions are given in Table 1. Crystallographic data (excluding
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ition numbers CCDC-145122 (2a), -145123 (2b), and -145124 (2c).
Copies of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB2 1IZ, UK [Fax: (in-
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Acknowledgments
We thank the Ministero della Ricerca Scientifica e Tecnologica
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[
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(Roma) (PRIN 9803184222) and the Consiglio Nazionale delle
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Eur. J. Inorg. Chem. 2001, 267Ϫ276