β-Diketones derived from cyclopentadienyl rhenium tricarbonyl

Article abstract of DOI:10.1016/j.inoche.2007.05.027

The acetyl complex (η⁵-C₅H₄COCH₃)Re(CO)₃ reacts with KO^tBu and an excess of appropriate ester to provide cyrhetrenyl-β-diketones complexes (η⁵-C₅H₄COCH₂COR)Re(CO)₃ (R = CF₃, 1a; CH₃, 1b; Ph, 1c). These new 1,3-diketones exist predominantly as enol tautomer, although a enol/keto mixture of approx. 10:1 is present in complexes 1b-c, in chloroform-d solution. The complexes have been characterized by spectroscopic techniques IR, ¹H and ¹³C NMR and mass spectrometry. X-ray crystallography of complex 1b shows that only the enol form occurs in the solid state. The O-C-C-C-O fragment of the molecule is planar with asymmetric enolisation in the direction furthest from the cyrhetrenyl group.

Full text of DOI:10.1016/j.inoche.2007.05.027

Inorganic Chemistry Communications 10 (2007) 1031–1034
www.elsevier.com/locate/inoche
b-Diketones derived from cyclopentadienyl rhenium tricarbonyl
a
a
a,
a
Teresa Cautivo , Fernando Godoy , A. Hugo Klahn *, Gonzalo E. Buono-Core ,
a
b
b
Diego Sierra , Mauricio Fuentealba , M. Teresa Garland
a
Instituto de Quı´mica, P. Universidad Cato´lica de Valpara´ıso, Casilla 4059, Valparaı´so, Chile
Facultad de Ciencias Fı´sicas y Matema´ticas, Universidad de Chile, Casilla 653, Santiago, Chile
b
Received 25 April 2007; accepted 17 May 2007
Available online 5 June 2007
Abstract
The acetyl complex (g5–C5H4COCH3)Re(CO)3 reacts with KOtBu and an excess of appropriate ester to provide cyrhetrenyl-b-dike-
tones complexes (g5–C5H4COCH2COR)Re(CO)3 (R = CF3, 1a; CH3, 1b; Ph, 1c). These new 1,3-diketones exist predominantly as enol
tautomer, although a enol/keto mixture of approx. 10:1 is present in complexes 1b–c, in chloroform-d solution. The complexes have been
characterized by spectroscopic techniques IR, 1H and 13C NMR and mass spectrometry. X-ray crystallography of complex 1b shows that
only the enol form occurs in the solid state. The O–C–C–C–O fragment of the molecule is planar with asymmetric enolisation in the
direction furthest from the cyrhetrenyl group.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Cyrhetrenyl-1,3-diketone; Rhenium; Organometallic ligand
Ferrocene containing b-diketones were first prepared in
1958 [1]. More recently new synthetic routes have been
developed to obtain these compounds in a much better
yield [2]. Owing to the coordination capability of these
ligands, several transition metal complexes of the type
M(ferrocene-diketonate)n (M = VO, Mn, Co, Ni, Cu, Zn.
n = 2; M = Al, Cr, Mn, Fe. n = 3) have been studied with
regards to their structural, electrochemical and kinetic
aspects [3].
As far as we know, no other cyclopentadienyl complexes
containing b-diketones as a side arm have been reported in
the literature. This encouraged us to find a method to syn-
thesize cyrhetrenyl-b-diketones of the type (g5–
C5H4COCH2COR)Re(CO)3. These new compounds can
be prepared, by following a similar procedure to that
reported for the ferrocenyl-b-diketones, that is by using
the known acetyl complex (g5–C5H4COCH3)Re(CO)3 in
reaction with KOtBu in THF solution, followed by the
addition of an excess of the corresponding ester
(RCO2CH2CH3) and further acidification with aqueous
HCl solution [4a] (Scheme 1).
The use of KOtBu, rather than other recommended
bases (NaNH2 or LiNPri Þ, to deprotonate acetylcyrhetrene
2
provides a much more reproducible synthesis of 1a–c.
Cyrhetrenyl b-diketones complexes 1, could be isolated
as yellow pale microcrystals in moderate to low yields:
1-cyrhetrenyl-4,4,4-trifluorobutane-1,3-dione 1a (R = 48%)
[4a], 1-cyrhetrenyl-butane-1,3-dione 1b (R = 42%) [4b],
1-cyrhetrenyl-3-phenylpropane-1,3-dione 1c (R = 26%) [4c].
The lower yield of 1c probably is associated to the less
electrophilic character of the carbonyl carbon of ethyl ben-
zoate compared to ethyl acetate and ethyl trifluoroacetate.
This can also explain the presence of acetylcyrhetrene after
the reaction workup.
The presence of the b-diketone group in 1a–c, was
1
unequivocally established by IR, H and13C NMR spec-
troscopies. For all cases the IR spectra, in addition to the
absorption bands for the CO groups bound to rhenium,
showed the presence of a new CO band at 1606 cmꢀ1
(CH2Cl2 solution) which is shifted to lower wavenumber
compared to their acetyl precursor (mCO = 1688 cmꢀ1) [5].
*
Corresponding author. Fax: +56 (32) 2273422.
E-mail address: hklahn@ucv.cl (A.H. Klahn).
1387-7003/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.05.027

1032
T. Cautivo et al. / Inorganic Chemistry Communications 10 (2007) 1031–1034
O
O
O
CO groups (Re–CO) occurred at almost the same d to that
reported for the acetylcyrhetrene [5].
R
CH3
Each of 1a–c shows a strong molecular ion in the mass
spectrum. In addition, the successive loss of three CO are
the most relevant fragments observed in the spectra of
the compounds.
1) t-BuOK / THF
2) RCO2CH2CH3
Re
Re
+
C
O
3) H / H2O
C
C
C
C
O
C
O
O
O
O
Despite the considerable number of b-diketones
described in the literature, crystallographic information
for those containing one or two metal fragments as substit-
uents is still scarce [7b]. With the aim to provide typical
metrical data for this type of compound the X-ray structure
analysis of 1b was conducted [9].
1a, R = CF3
1b, R = CH3
1c, R = Ph.
Scheme 1.
1
The H NMR spectrum for 1a, showed only one of two
possible tautomers, since the expected resonance at about d
4.0 for the keto form was not detected even in a saturated
deuterobenzene solution, whereas for 1b–c a mixture of
enol (major) and keto (minor) forms were established in
about 10:1 ratio. Similar tautomeric mixtures have been
observed in b-diketones containing a ferrocenyl group:
(g5–C5H5)Fe(g5–C5H4COCH2COR), R = CF3 [2,6], CH3
[7] and C6H5 [2,6].
Single crystals of 1b were obtained by slow diffusion of
hexane into an ether solution of 1b. Fig. 1 shows an
ORTEP view of the structure of 1b, together with some
selected bond distances and angles. All the heavy atoms
in the side-chain C(6), C(8), O(1) and O(2) are coplanar,
with the two oxygen atoms on the same side of the chains,
˚
and separated by 2.482 A. This is the same overall confor-
mation as adopted in ferrocenyl-butane-1,3-dione [7b] and
in other enolized 1,3-diketones [10]. The bond length C(6)–
The enol tautomers were easily identified by a broad low
field resonance (d ꢁ 15) characteristic for a b-diketones
hydrogen-bond and a singlet at about 6 ppm assigned to
the olefinic hydrogen (CH). The latter occurs at similar d
to those reported for ferrocene derivatives mentioned
above. On the other hand, the 1H NMR spectra of 1b
and 1c but no 1a, showed a singlet at about 4 ppm,
assigned to the CH2 group of the keto form, as well as
two triplets for the cyrhetrenyl group. The two observed
˚
˚
C(7) of 1.399(10) A and C(7)–C(8) of 1.355(10) A are
slightly shorter than those measured for the distinct C–C
bonds in asymmetric enolized 1,3-diones [7b,10]. Similarly
˚
the C(6)–O(1) and C(8)–O(2) distances 1.261(8) A and
1.307(7) A, lie in the low limit of the ranges observed in
˚
such enols for the two types of C–O distances (1.269–
˚
˚
1.283 A and 1.306–1.337 A, respectively). It is important
to mention at this point that C–C and C@O bond distances
measured in non-enolized 1,3-diketones lie in the ranges
1
signals for the methyl groups found in the H NMR spec-
˚
˚
trum of 1b (d 2.10 and d 2.29) are also indicative of the
1.507–1.537 A and 1.212–1.221 A, respectively [11], wholly
different from the values found in 1b. By considering the
above results and the co-planarity of the 1,3-butanedione
fragment, we conclude that 1b, in the solid state, possesses
an asymmetric enolisation in the direction away from the
cyrethrenyl side group. Further more, the dihedral angle
of 4.20ꢁ (0.25) between the pseudo-aromatic b-diketone
presence of the tautomers in solution.
The presence of a single resonance at d ꢀ69.2 in the 19
F
NMR spectrum of 1a (assigned to the fluorine atoms of the
CF3 group) is a further evidence for the existence of the
enol tautomer for this compound. This value is very similar
to the corresponding shift found for trifluoroacetylacetone
(d: ꢀ70.1) and trifluorophenylbutane-1,3-dione (d: ꢀ69.8)
in CDCl3 [8]. It is important to note that Swart et al. on
1
the basis of H NMR determined over 99% enolisation in
several b-diketones containing a CF3 group [6].
The 13C NMR spectrum, also in CDCl3 solution,
showed only the expected resonances for the enol tauto-
mer. For 1a, the resonances at d 117.3 (1JFC = 281.0 Hz)
and d 172.2 (2JFC = 36.7 Hz) are almost identical to those
reported for the complex (g5–C5H5)Fe(g5–C5H4COCH2-
COCF3) [2,6] (assigned to CF3 and COCF3 groups,
respectively).
In all cases, the 13C shift of the side-chain (CH) and
cyclopentadienyl Cipso occurring in the range 92–96 ppm,
were unequivocally assigned by using DEPT 135 and
1H–13C-HSQC. The two CO shifts of the diketone frag-
ment of 1b and 1c (d 181–189) are close to the CO shift
reported for pentane-2,4-dione and the analogous ferroce-
nyl-diketone [2,7]. It is not possible to determine from the
chemical shift data alone the sense in which the enolization
of these compounds occur. The resonances for the terminal
˚
Fig. 1. Molecular structure of 1b. Selected bond lengths (A) and bond
angles (ꢁ): C(1)–C(6) 1.469(9), C(6)–C(7) 1.399(10), C(7)–C(8) 1.355(10),
Cp(centroid)–Re 1.945(4), Re–(CO) 1.912, O(1)–C(6)–C(7) 120.2(7), C(6)–
C(7)–C(8) 121.1(7), C(7)–C(8)–O(2) 122.4(8). Dihedral angle 4.20ꢁ(0.25),
C(O)–Re–C(O) 89.13.

T. Cautivo et al. / Inorganic Chemistry Communications 10 (2007) 1031–1034
1033
C5H4); 93.1 (s, CH); 118.1 (q, J = 281.0 Hz; CF3); 171.8 (q,
J = 36.6 Hz; COCF3); 184.9 (s, CO); 191.7 (s, Re–CO). 19F NMR
(C2D6CO) d: ꢀ69.2 (s, CF3). Mass spectrum (based on 187Re) m/z:
474 [M+]; 455 [M+–F]; 446 [M+–CO]; 418 [M+–2CO]; 405 [M+–CF3];
390 [M+–3CO]. Anal. Calc. for C12H6O5F3Re: C 30.45, H 1.28%;
Found: C 30.76, H 1.32%;
(b) Synthesis of (g5–C5H4COCH2COCH3)Re(CO)3 (1b): Complex 1b
was obtained following the same procedure for 1a, but using
CH3CO2CH2CH3. It was isolated as a pale yellow solid 47 mg
(42%). IR (CH2Cl2, m(CO), cmꢀ1): 2030 (s), 1937 (vs), 1606 (w). 1H
NMR (CDCl3), enol/keto ratio ꢁ10:1. Enol tautomer; d: 2.10 (s, 3H,
CH3); 5.44 (t, 2H, J = 2.4 Hz; C5H4); 5.69 (s, H, CH); 6,03 (t, 2H,
J = 2.4 Hz; C5H4); 15.39 (s, an, H, OH). Keto tautomer; d: 2.29 (s,
3H, CH3); 3.71 (s, 2H, CH2); 5.43 (t, 2H, J = 2.4 Hz; C5H4); 6.00 (t,
2H, J = 2.4 Hz; C5H4). 13C {1H}RMN (CDCl3) d: 24.1 (s, CH3); 85.0
(s, C5H4); 86.7 (s, C5H4), 95.7 (s, Cipso C5H4); 96.2 (s, CH); 182.8 (s,
COCH3); 188.5 (s, CO); 192.0 (s, Re–CO). Mass spectrum (IE, based
on 187Re) m/z : 420 [M+]; 392 [M+–CO]; 364 [M+–2CO]; 336 [M+–
3CO]. Anal. Calc. for C12H9O5Re: C 34.37, H 2.16%; Found: C 34.69,
H 2.07%;
plane and the planar cyclopentadienyl ring attached to the
b-diketone skeleton indicates appreciable conjugation
between the two groups. On the other hand, within the
cyrhetrenyl group, the average Re–C(O) distance and the
Re–C–O angle are concordant with related tricarbonyl
cyclopentadienyl rhenium (I) complexes [12].
Supplementary material
CCDC 644403 contains the supplementary crystallo-
graphic data for 1b. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk.
(c) Synthesis of (g5–C5H4COCH2COC6H5)Re(CO)3 (1c): The com-
plex 1c was prepared by a similar procedure to that described for 1a,
but using C6H5CO2CH2CH3. However, an additional purification
was needed to separate 1c from the starting acetylcyrhetrene and the
excess of ethylbenzoate. The reaction mixture was stirred with a
saturated solution of CuAc2 in ethanol. After 1 h, water 10 mL was
added and then the copper complex of 1c was extracted with CH2Cl2
(2 · 30 mL). The solvent of the organic layer was pumped off and the
green solid formed was washed with hexane and redissolved in
CH2Cl2, and treated with an aqueous HCl solution 6 M (20 mL)
under stirring for 1 h. The organic layer was dried over sodium
sulfate, filtered through Celite and the solvent removed under reduced
pressure, affording a yellow pale solid. Crystallization from hexane at
ꢀ18 ꢁC gave 1c as white microcrystals. 13 mg, 26% yield. IR (CH2Cl2,
m(CO), cmꢀ1): 2030 (s), 1937 (vs), 1606 (w). 1H NMR (CDCl3) enol/
keto ratio ꢁ10:1. Enol form; d: 5.44 (t, 2H, J = 2.5 Hz; C5H4); 6.09 (t,
2H, J = 2.5 Hz; C5H4); 6.29 (s, CH); 7.48 (t, 2H, J = 7.3 Hz; C6H5);
7.55 (t, H, J = 7.3 Hz; C6H5); 7.88 (d, 2H, J = 7.8 Hz; C6H5) 16.03 (s,
br, H, OH). Keto form; d: 4.25 (s, 2H, CH2); 5.41 (t, 2H, J = 2.5 Hz;
C5H4); 6.07 (t, 2H, J = 2.5 Hz; C5H4). 13C {1H}RMN (CDCl3)d: 85.0
(s, C5H4); 86.9 (s, C5H4); 92.8 (s, Cipso C5H4); 96.1 (s, CH), 126.9 (s,
C6H5); 128.7 (s, C6H5); 132.6 (s, C6H5); 134.2 (s, Cipso C6H5); 181.8 (s,
COC6H5); 184.2 (s, CO); 192.1 (s, Re–CO). Mass spectrum (IE, based
on 187Re): m/z 482 [M+]; 454 [M+–CO]; 398 [M+–3CO]. Anal. Calc.
for C17H11O5Re: C 42.41, H 2.30%; Found: C 42.67, H 2.40%.
[5] S.S. Jones, M.D. Rausch, T. Bitterwolf, J. Organomet. Chem. 396
(1990) 279.
Acknowledgement
A.H.K. acknowledges FONDECYT (Project 1060487)
´
and D.G.I.P. Pontificia Universidad Catolica de Val-
´
paraıso (DI-125710/99). T.C. acknowledges CONICYT
for Doctoral scholarship. We also appreciate the financial
support of MECESUP-Chile (Project UCH 0116) for a
MS instrument and CONICYT-FONDAP (Grant
11980002).
References
[1] C.R. Hauser, C.E. Caim, J. Org. Chem. 23 (1958) 1142.
[2] (a) C.E. Cain’t, A. Mashburn, C.R. Hauser, J. Org. Chem. 26 (1961)
1030;
(b) W.R. Cullen, S.J. Rettig, E.B. Wickenheiser, J. Mol. Catal. 66
(1991) 251;
(c) C.M. Zakaria, C.A. Morrison, D. McAndrew, W. Bell, C.
Glidewell, J. Organomet. Chem. 485 (1995) 201.
[3] (a) P. Zanello, F. Fabrizi de Biani, C. Glidewell, J. Koening, J.
Marsh, Polyhedron 17 (1998) 1795;
´
(b) I. Imani, T. Ota, Bull. Chem. Soc. Jpn. 47 (1974) 2497;
(c) P.D.W. Boyd, P.M. Johns, C.E.F. Rickard, Acta Cryst. C 62
(2006) m590;
(d) C.-G. Yan, L. Liu, J. Han, Acta Cryst. E 63 (2007) m683.
[4] (a) Synthesis of (g5-C5 H4COCH2COCF3)Re(CO)3 (1a). To a solution
of complex (g5–C5H4COCH3)Re(CO)3 (100 mg, 0.26 mmol) in THF
(10 mL) was added t-BuOK (32 mg, 0.29 mmol). After 30 min of
stirring at room temperature, an excess of CF3CO2CH2CH3 (2.62 mL,
26.0 mmol, d = 1.273 g/mL) was used to ensure complete reaction,
and the resulting suspension was stirred overnight. After this time, the
diketonate salt was converted to the diketone by shaking it with 50%
aqueous HCl solution (20 mL) and the product was extracted from
the mixture with ether (2 · 20 mL). The combined ether extracts were
dried over sodium sulfate, filtered through Celite and the solvent
evaporated under reduced pressure, affording a pale yellow solid.
Extraction with hexanes (2 · 10 mL) yielded a white–yellow solid,
59 mg, 48% yield. IR (CH2Cl2, m(CO), cmꢀ1): 2033 (s), 1942 (vs), 1606
(w). 1H NMR (CDCl3) d: (enol tautomer P 99): 5.47 (t, 2H,
J = 4.4 Hz, C5H4); 6.01 (s, H, CH); 6.06 (t, 2H, J = 4.4 Hz, C5H4);
14.30 (s, br., 1H, OH). 1H RMN (C6D6) d: 4.15 (t, 2H, J = 2.0 Hz;
C5H4); 4.88 (t, 2H, J = 2.0 Hz; C5H4); 5.59 (s, H, CH). 13C
{1H}RMN (CDCl3)d: 85.8 (s, C5H4); 87.6 (s, C5H4); 92.1 (s, Cipso
C5H4); 92.9 (s, CH), 117.3 (q, J = 281.0 Hz; CF3); 172.2 (q,
J = 36.7 Hz; COCF3); 184.6 (s, CO); 191.1 (s, Re–CO). 13C
{1H}RMN (C6D6) d: 85.3 (s, C5H4); 87.4 (s, C5H4); 92.1 (s, Cipso
[6] W.C. Du Plessis, T.G. Vosloo, J. C Swarts, J. Chem. Soc. Dalton
Trans. (1998) 2507.
[7] (a) W.R. Cullen, E.B. Wickenheiser, J. Organomet. Chem. 370 (1989)
141;
(b) W. Bell, J.A. Crayston, C. Glidewell, M.A. Mazid, M.B.
Hursthouse, J. Organomet. Chem. 434 (1992) 115.
[8] T.S. Everett on ‘‘Chemistry of Fluorine Compounds II: A Critical
Review’’, M. Hudlicky, A.E. Pavlath (Eds), ACS Monograph 187,
1995, p. 1056.
[9] Crystal data for 1b C12H9O5Re, M = 419.39, tetragonal, space group
˚
˚
P43212, T = 298 K, a = 10.0029(7) A, b = 10.0029(7) A, c =
3
˚
˚
24.966(2) A, V = 2498.1(3) A , Z = 8, F000 = 1568, GOF = 1.077,
Dcalcd = 2.230 g cmꢀ3, crystal size 0.24 · 0.19 · 0.18 mm. Data were
collected on a Bruker Smart Apex diffractometer equipped with a
CCD area detector, using graphite monochromated Mo Ka radiation
˚
(k = 0.71073 A). A total of 21106 reflections were collected, of which
2908 reflections are independent [R(int) = 0.0491]. Absorption cor-
rection was performed using the multi-scan method. The structures
were solved by Patterson method and refined by full-matrix least
squares on F2. The final R1(wR2) factors were 0.0330(0.0735) for
reflections I > 2r(I) and 0.0418(0.0766) for all the reflections.

1034
T. Cautivo et al. / Inorganic Chemistry Communications 10 (2007) 1031–1034
[10] (a) A. Mostad, U. Pedersen, P.B. Rasmussen, S.-O. Lawesson, Acta
Chem. Scand. B 37 (1983) 901;
(h) R.D.G. Jones, Acta Cryst. B 32 (1976) 1224;
(i) M.C. Etter, D. A Jahn, E. Urbanczyk-Lipkowska, Acta Cryst. C
(b) R.D.G. Jones, Acta Cryst. B 32 (1976) 2133;
(c) R.D.G. Jones, Acta Cryst. B 32 (1976) 1807;
(d) L.A.M. Baxter, A.J. Blake, G.A. Heath, T.A. Stephenson, Acta
Cryst. C 46 (1990) 508;
43 (1987) 260.
[11] (a) E. Gyepes, T. Glowiak, S. Toma, J. Soldanova, J. Organomet.
Chem. 276 (1984) 209;
(b) E. Gyepes, T. Glowiak, S. Toma, J. Organomet. Chem. 316 (1989)
(e) H.H. Tonnesen, J. Karlsen, A. Mostad, U. Pedersen, P.B.
Rasmussen, S.-O. Lawesson, Acta Chem. Scand. B 37 (1983) 79;
(f) H.H. Tonnesen, J. Karlsen, A. Mostad, Acta Chem. Scand. B 36
(1982) 475;
141;
(c) D.F. Mullica, J.W. Karban, D.A. Grossie, Acta Cryst. C 43 (1987)
748.
[12] P.J. Fitzpatrick, Y. Le Page, I.A. Butler, Acta Cryst. B 37 (1981)
1052.
(g) K. Kato, Acta Cryst. B 27 (1971) 2028;