Reduction of cationic P-coordinated (diphenylphosphino)alkyne iron complexes with sodium borohydride

Article abstract of DOI:10.1016/s0020-1693(99)00102-4

The reaction of cationic complexes [(η⁵-C₅H₅)Fe(CO)₂(PPh ₂CCR)]⁺ (R=H, Me, ^tBu, Ph, Tol) with NaBH₄ at -78°C in THF yielded the neutral complexes [(η⁴-C

Full text of DOI:10.1016/s0020-1693(99)00102-4

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 291 (1999) 207–211
Reduction of cationic P-coordinated (diphenylphosphino)alkyne
iron complexes with sodium borohydride
Elmostafa Louattani, Joan Suades *
Departament de Qu´ımica, Edifici C, Uni6ersitat Auto`noma de Barcelona, 08193 Bellaterra, Spain
Received 23 October 1998; accepted 3 February 1999
Abstract
t
The reaction of cationic complexes [(h5-C5H5)Fe(CO)2(PPh2CꢀCR)]+ (R=H, Me, Bu, Ph, Tol) with NaBH4 at −78°C in
THF yielded the neutral complexes [(h4-C5H6)Fe(CO)2(PPh2CꢀCR)], which were characterized by elemental analysis, and NMR
and IR spectroscopy. This result is consistent with the idea that the electrophilic character of the cation [(h5-
C5H5)Fe(CO)2(PPh2CꢀCR)]+ is mainly located on the [(h5-C5H5)Fe(CO)2] fragment and the polarization of the alkyne does not
change the direction of the nucleophilic attack with respect to the cation [(h5-C5H5)Fe(CO)2(PPh3)]+. The comparison between
spectroscopic data of cationic and neutral complexes supplies new information about the influence of metal on the alkyne function
in P-coordinated (diphenylphosphino)alkynes. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Phosphinoalkyne complexes; Iron complexes; Cyclopentadienyl complexes
aim of studying the influence of phosphorus coordina-
tion on the alkyne function in (diphenylphos-
phino)alkynes, we previously prepared and studied
anionic [5] and cationic [6] iron P-coordinated
(diphenylphosphino)alkynes. The cationic P-coordi-
nated complexes [(h5-C5H5)Fe(CO)2(PPh2CꢀCR)]+ are
considerably attractive, since they are easily prepared
(Scheme 1) and are very stable and versatile compounds
which display an uncoordinated and polarized alkyne
function. This uncoordinated alkyne can react with
other substrates like [Co2(CO)8], leading to heteroge-
neous iron–cobalt complexes [7]. Recently, we have
shown that an intramolecular reaction between the
uncoordinated alkyne and the iron atom coordinated to
phosphorus may be possible after reaction with
Me3NO, which transforms the eighteen-electron coordi-
natively saturated iron atom into a sixteen-electron
unsaturated center [8].
1. Introduction
Phosphinoalkynes X2PCꢀCR are a class of ligands
which have been extensively used in the synthesis of
new polynuclear transition metal complexes [1]. Their
reaction with neutral polynuclear iron and ruthenium
carbonyl compounds under mild conditions usually in-
volves breaking of the phosphorus–alkynyl bond to
yield h2-acetylide phosphido-bridged clusters [2]. We
studied the same reaction with the anionic iron car-
bonyl cluster [HFe3(CO)11]−, which led to similar phos-
phorus–alkyne bond breaking but in addition, we
observed the insertion of the CꢀC in the Fe–H bond
and there were interesting rearrangements depending
on the nature of R group and the reaction conditions
[3].
Although phosphinoalkynes exhibit two functional
groups (phosphine and alkyne) capable of reacting with
transition metals, there is some evidence which indi-
cates that their reaction with transition metals starts
with the formation of P-coordinated (diphenlphos-
phino)alkyne complexes, although only a limited num-
ber of these compounds have been isolated [4]. With the
In this present work, we examine the reactivity of the
cationic
iron
P-coordinated
complexes
[(h5-
C5H5)Fe(CO)2(PPh2CꢀCR)]+ towards the nucleophile
NaBH4 in order to obtain chemical evidence of CꢀC
bond polarization revealed by a possible reaction be-
tween the nucleophile and the alkyne function. Spectro-
scopic and theoretical studies suggest that the alkyne
* Corresponding author. Fax: +34-93-5813101.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00102-4

208
E. Louattani, J. Suades / Inorganica Chimica Acta 291 (1999) 207–211
Scheme 1.
polarization is a consequence of the cationic charge
of the phosphorus atom after coordination to iron
(Scheme 1). The study of the reaction with sodium
borohydride could supply information about a possi-
ble transfer of the electrophilic character of the [(h5-
C5H5)Fe(CO)2]+ fragment to the alkyne function.
Yield of 1, 47%. Anal. Calc. for C21H17FeO2P: C,
64.98; H, 4.41. Found: C, 65.54; H, 4.57%. IR (KBr):
3270 (w(ꢀCH)), 2757 (w(CHexo)), 1967, 1904 (w(CO))
cm−1 1H NMR (C6D6): 2.30 (t, J=10.8 Hz, Hexo
.
(C5H6 group)), 2.50 (s, H1H(4C H )), 2.66 (d, J=6.4
5
6
Hz, ꢀCH), 2.81 (d, J=10.0 Hz, Hendo(C H )), 5.12 (s,
5
6
H2H3(C H )), 6.9–7.2 (m, Ph) ppm. 31P NMR (C6D6):
5
6
52.5 ppm.
Yield of 2, 49%. Anal. Calc. for C22H19FeO2P: C,
65.70; H, 4.76. Found: C, 65.90; H, 4.73%. IR (KBr):
2763 (w(CHexo)), 2195 (w(CꢀC)), 1967, 1906 (w(CO))
2. Experimental
2.1. General
1
cm−1. H NMR (C6D6): 1.39 (s, CH3), 2.35 (t, J=
10.8 Hz, Hexo(C H )), 2.52 (s, H1H(4C H )), 2.86 (d, J=
All reactions were performed under nitrogen by
standard Schlenk tube techniques. IR spectra were
recorded with a Perkin–Elmer 1710 FT spectrometer
using KBr pellets. The NMR spectra were recorded
by the Servei de Ressona`ncia Magne`tica Nuclear de
la Universitat Auto`noma de Barcelona on a Bruker
AM400 instrument. The 31P chemical shifts are re-
ported in ppm upfield from external 85% H3PO4. The
1H and 13C chemical shifts are expressed in ppm upfi-
eld from TMS.
5
5
6
6
11.2 Hz, Hendo(C H )), 5.14 (s, H2H(3C H )), 6.9–7.2 (m,
5
5
Ph) ppm. 31P NMR (C6D6): 50.7 ppm.6
6
Yield of 3, 53%. Anal. Calc. for C25H25FeO2P: C,
67.58; H, 5.67. Found: C, 67.67; H, 5.75%. IR (KBr):
2747 (w(CHexo)), 2203, 2164 (w(CꢀC)), 1968, 1910
(w(CO)) cm−1 1H NMR (C6D6): 1.12 (s, C(CH3)3),
.
2.39 (t, J=10.4 Hz, Hexo(C H )), 2.57 (s, H1H(4C H )),
5
5
6
2.89 (d, J=11.2 Hz, Hendo(C H )), 5.27 (s, H2H(3C H6)),
5
5
6
6
7.0–7.2 (m, Ph) ppm. 31P NMR (C6D6): 49.5 ppm.
The complexes [(h5-C5H5)Fe(CO)2(PPh2CꢀCR)][BF4]
were prepared by published procedures [6]. Micro-
analyses were performed in Servei d’Ana`lisi Qu´ımica
de la Universitat Auto`noma de Barcelona.
Yield of 4, 58%. Anal. Calc. for C27H21FeO2P: C,
69.85; H, 4.56. Found: C, 69.74; H, 4.76%. IR (KBr):
2755 (w(CHexo)), 2171 (w(CꢀC)), 1956, 1897 (w(CO))
cm−1
.
1H NMR (C6D6): 2.34 (t, J=11.2 Hz,
Hexo(C H )), 2.61 (s, H1H(4C H )), 2.87 (d, J=11.2 Hz,
5
5
Hendo(C H )), 5.20 (s, H2H(3C H6)), 6.8–7.3 (m, Ph) ppm.
6
2.2. Synthesis of 1–5 (R=H (1), CH3 (2), C(CH3)3
(3), Ph (4), Tol (5))
5
5
6
6
13C NMR (C6D6; except phenyl resonances): 45.1 (s,
C5(C H )), 51.4 (s, C1C(4C H )), 85.0 (s, C2C3(C H )), 85.1
5
5
5
6
6
(d, J=110.8 Hz, ꢀCP), 109.0 (d, J=10.4 Hz,6 ꢀCPh),
219.1 (d, J=12.7 Hz, FeCO). 31P NMR (C6D6): 51.4
ppm.
In
a
typical procedure,
a
solution of [(h5-
C5H5)Fe(CO)2(PPh2CꢀCR)][BF4] (0.9 mmol) in te-
trahydrofurane (20 ml) was cooled to −78°C,
NaBH4 (0.17 g, 4.4 mmol) was added and the mix-
ture was stirred for 15 min. Next, the cooling bath
was removed and the resulting mixture was stirred at
room temperature for an additional 3 h. The resulting
solution was evaporated to dryness and the residual
oil was extracted with pentane (3×15 ml) and the
extracted fraction was cooled at −78°C and filtered.
The pentane solution obtained was concentrated to a
few millilitres and cooled at −20°C. The titled com-
plexes crystallize as yellow crystals, which were col-
lected and dried in vacuum.
Yield of 5, 54%. Anal. Calc. for C28H23FeO2P: C,
70.31; H, 4.85. Found: C, 65.54; H, 4.57%. IR (KBr):
2754 cm−1 (w(CHexo)), 2169 (w(CꢀC)), 1957, 1895
1
(w(CO)) cm−1. H NMR (C6D6): 1.95 (s, CH3), 2.36
(t, J=9.6 Hz, Hexo(C H )), 2.62 (s, H1H(4C H )), 2.88 (d,
5
5
6
J=9.6 Hz, Hendo(C H )), 5.31 (s, H2H(3C H6)), 6.7–7.3
5
5
6
(m, Ph) ppm. 13C NMR (C6D6; except p6henyl reso-
nances): 21.5 (s, CH3), 45.3 (s, C5(C H )), 51.6 (s,
5
6
C1C4(C H )), 84.4 (d, J=103.2 Hz, ꢀCP), 85.2 (s,
5
C2C3(C H6)), 109.6 (d, J=10.4 Hz, ꢀCPh), 219.1 (d,
5
J=13.16 Hz, FeCO). 31P NMR (C6D6): 51.2 ppm.

E. Louattani, J. Suades / Inorganica Chimica Acta 291 (1999) 207–211
209
Scheme 2.
respect to the cationic complexes [(h5-C5H5)Fe-
(CO)2(PPh2CꢀCR)]+. For instance, it is noteworthy
that the w(CꢀO) bands in the neutral complexes 1–5 are
shifted :100 cm−1 to lower frequencies with respect
to the cationic complexes and the position observed is
very similar to the values obtained in related neutral
Fe(0) complexes like [(h4-C8H6)Fe(CO)2(PPh3)] (1970,
1920 cm−1) [14]. This is another example of the well-
known influence between metal atom charge and metal
to carbonyl backbonding in accordance with the change
in oxidation state of the iron atom from Fe(+2) to
Fe(0) in the reaction (Scheme 1). This shift observed in
the w(CꢀO) bands contrasts strongly with the very small
changes observed in the position of the w(CꢀC) bands
when going from the cationic iron complexes [(h5-
C5H5)Fe(CO)2(PPh2CꢀCR)]+ to the neutral iron com-
plexes [(h4-C5H6)Fe(CO)2(PPh2CꢀCR)] (Table 1).
Although is evident that the CO ligands are directly
bonded to the metal atom rather than to the CꢀC
fragment, some years ago it was suggested that the
increase observed in the w(CꢀC) band of (diphenylphos-
phino)alkynes after coordination to metal atoms could
be explained by the backbonding of the transition metal
to the phosphorus atom [13,15]. This retrodative bond-
ing will lower the ability of the phosphorus d-orbitals
to attract the electron density of the filled p-orbitals of
the CꢀC bond, so the frequency of CꢀC increases in
phosphinoalkynes after coordination of phosphorus to
a transition metal. In contrast to this hypothesis, the
similarity between the w(CꢀC) bands of iron complexes
with different oxidation states shown in Table 1 is
consistent with the following previously proposed idea
[6]: in free phosphinoalkynes the occupancy of the p*
CꢀC bonds is higher than in the P-coordinated com-
plexes as a consequence of the delocalization of the
phosphorus lone pair. Consequently, when this lone
pair forms a bond, the w(CꢀC) should increase and the
3. Results and discussion
The reaction between tetrahydrofurane solutions of
the
cationic
complexes
[(h5-C5H5)Fe(CO)2-
(PPh2CꢀCR)][BF4] cooled at −78°C with an excess of
NaBH4 afforded neutral complexes which could be
extracted with pentane, and on crystallization yielded
yellow crystals of complexes 1–5 (Scheme 2).
The analysis of the spectroscopic data of complexes
1–5 clearly indicates that the hydride addition to the
cyclopentadienyl ligand has taken place, leading to the
formation of h4-cyclopentadiene iron complexes as
shown in Scheme 2.
The IR spectrum of all the complexes prepared ex-
hibits one intense band at :2750 cm−1, which has
been attributed to the C–H(exo) and is considered a
common characteristic of h4-cyclopentadiene complexes
[9–11]. In addition, complexes 1–5 show a group of
1
similar signals in the H NMR spectra at 2.3–2.4 ppm
(t), 2.5–2.6 ppm (s), 2.8–2.9 ppm (d) and 5.1–5.3 ppm
(s), which are analogous to those reported for the
complexes [(h4-C5H6)Fe(CO)2(PPh3)] [9,12] and [(h4-
C5H6)Fe(CO)(PPh2CH2CH2PPh2)] [10], and have been
previously assigned to the protons of the h4-cyclopenta-
diene ligand [11]. Furthermore, the presence of the
uncoordinated alkyne is clearly revealed in the IR
spectra of complexes 2–51 by a band of medium inten-
sity observed between 2200 and 2170 cm−1 which is
assigned to the n(CꢀC) of the uncoordinated alkyne
[6,13]. In addition to supplying structural information,
the IR spectra furnish supplementary data about the
electronic changes performed on complexes 1–5 with
1 The presence of the CꢀCH group in complex 1 is revealed by the
characteristic signal of w(C–H) at 3270 cm−1 and NMR results. A
similar behavior was observed in the cationic complex [(h5-
C5H5)Fe(CO)2(PPh2CꢀCH)]+ [6].

210
E. Louattani, J. Suades / Inorganica Chimica Acta 291 (1999) 207–211
Table 1
w(CꢀC) data (cm−1) for (diphenylphosphino)alkynes and iron P-coordinated complexes
R
Ph2PCꢀCR
[(h5-C5H5)Fe(CO)2(PPh2CꢀCR)]+
[(h4-C5H6)Fe(CO)2(PPh2CꢀCR)]
Me
tBu
Ph
2158
2200, 2170
2156
2200
2202, 2161
2173
2195
2203, 2164
2171
a
Tol
2156
2171
2169
a The existence of two bands in substituted tert-butylalkynes has been previously reported and assigned to a Fermi resonance [16].
Table 2
influence of the metal electron density on this effect
13C Chemical shifts (l, ppm) for the acetylenic atoms
should be small, as shown in Table 1 with the Fe(+2)
and Fe(0) complexes2. Finally, we wish to emphasize
l (C1) l (C2)
l (C2)−l (C1)
that although the positions of the w(CꢀC) bands are
similar between cationic and neutral complexes in Table
1, the intensity of this band increases in the cationic
complexes [(h5-C5H5)Fe(CO)2(PPh2CꢀCR)]+ in com-
parison with free (diphenylphosphino)alkynes Ph2
PCꢀCR and decreases in the neutral complexes [(h4-
C5H6)Fe(CO)2(PPh2CꢀCR)] with respect to the cationic
complexes. These changes may be related with varia-
tions in the triple bond dipole.
[(h5-C5H5)Fe(CO)2-
(PPh2CꢀCPh)]+
[(h4-C5H6)Fe(CO)2-
(PPh2CꢀCPh)] (4)
80.3 a
115.7 a 35.4
85.1
109.0
23.9
Ph2PCꢀCPh
86.5 a
80.0 a
109.4 a 22.9
116.5 a 36.5
[(h5-C5H5)Fe(CO)2-
(PPh2CꢀCTol)]+
[(h4-C5H6)Fe(CO)2-
(PPh2CꢀCTol)] (5)
84.4
109.6
24.6
Previous studies have shown a correlation between
13C NMR chemical shift differences of acetylenic car-
bons and the charge polarization of the CꢀC bond
[6,18]. Therefore, this approach could indicate changes
in the CꢀC bond polarization after reduction of the
cationic complexes [(h5-C5H5)Fe(CO)2(PPh2CꢀCR)]+
by sodium borohydride. Unfortunately, only the 13C
NMR spectra of the arylalkyne complexes 4 and 5
could be registered but in both compounds, the reso-
nances of acetylenic carbons l (C1) and l (C2) were
shifted in the same direction, downfield and upfield,
respectively, in reference to the corresponding cationic
complexes (Table 2). This variation is in agreement
with the previous hypothesis that alkyne polarization in
P-coordinated phosphinoalkyne compounds can be re-
lated to the charge on the phosphorus atom [6]. We
should also remark that acetylenic resonances of com-
plexes 4 and 5 are very similar to those of the corre-
sponding free phosphinoalkynes (Table 2), which
implies that alkyne polarization on these neutral com-
plexes and free ligands is comparable.
Ph2PCꢀCTol
85.3 a
108.9 a 23.6
a Ref. [6].
takes place on the cyclopentadienyl ring and no reac-
tion has been detected on the alkyne fragment. This
result supports previous theoretical studies, which indi-
cate that the charge in the cationic complexes [(h5-
C5H5)Fe(CO)2(PPh2CꢀCR)]+ is mainly located on the
iron atom [6]. Thus we confirm that the polarization of
the CꢀC bond observed by spectroscopy is not enough
to alter the direction of the nucleophilic attack to the
cyclopentadienyl ring.
Acknowledgements
The authors thank the DGICYT for financial
support.
In conclusion, in the study of the reactivity between
the cationic complexes [(h5-C5H5)Fe(CO)2(PPh2Cꢀ
CR)]+ and the nucleophile [BH4]−, we have observed
that nucleophilic addition to the organometallic cation
References
[1] Some examples: (a) A.J. Carty, H.N. Paik, G.J. Palenik, Inorg.
Chem. 16 (1977) 300. (b) D.F. Jones, P.H. Dixneuf, T.G. South-
ern, J. Marouille, D. Grandjean, P. Guenot, Inorg. Chem. 20
(1981) 3247. (c) H. Lang, L. Zsolnai, J. Organomet. Chem.
(1989) 131. (d) W. Imhof, B. Eber, G. Huttner, C. Emmerich, J.
Organomet. Chem. 447 (1993) 21. (e) D.K. Johnson, T.
Rukachaisirikul, Y. Sun, N.J. Taylor, A.J. Canty, A.J. Carty,
Inorg. Chem. 32 (1993) 5544. (f) J. Fornies, E. Lalinde, A.
Martin, M.T. Moreno, A.J. Welch, J. Chem. Soc. Dalton Trans.
2 The position of the w(CꢀC) band in the [Ph3PCꢀCMe]+ cation
(2200 cm−1
)
[17] is also similar to those of [(h5-C5H5)-
Fe(CO)2(PPh2CꢀCMe)]+ and [(h4-C5H6)Fe(CO)2(PPh2CꢀCMe)].
This result supports the second hypothesis, since in the triphenylphos-
phonium cation there is no transition metal bonded to phosphorus
and the loss of the lone pair leads to a similar position in the n(CꢀC)
band.

E. Louattani, J. Suades / Inorganica Chimica Acta 291 (1999) 207–211
211
(1995) 1333. (g) R.S. Dickson, T. Desimone, R.J. Parker, G.D.
Fallon, Organometallics 16, (1997) 1531.
[7] E. Louattani, J. Suades, A. Alvarez-Larena, J.F. Piniella, G.
Germain, J. Organomet. Chem. 506 (1996) 121.
[8] E. Louattani, I. Moldes, J. Suades, A. Alvarez-Larena, J.F.
Piniella, Organometallics 17 (1998) 3394.
[9] A. Davison, M.L.H. Green, G. Wilkinson, J. Chem. Soc. (1961)
3172.
[10] S.G. Davies, J. Hibberd, S.J. Simpson, J. Organomet. Chem. 246
(1983) C16.
[11] S.G. Davies, S.D. Moon, S.J. Simpson, S.E. Thomas, J. Chem.
Soc., Dalton Trans. (1983) 1805.
[2] Some examples: (a) A.J. Carty, Adv. Chem. Ser. 196 (1981) 163.
(b) M.I. Bruce, M.L. Williams, J.M. Patrick, A.H. White, J.
Chem. Soc., Dalton Trans. (1985) 1229. (c) D. Nucciarone, S.A.
MacLaughlin, N.J. Taylor, A.J. Carty, Organometallics 7 (1988)
106. (d) E. Sappa, G. Pasquinelli, A. Tiripicchio, M. Tiripicchio-
Camellini, J. Chem. Soc., Dalton Trans. (1989) 601. (e) A.J.
Carty, G. Hogarth, G. Enright, G. Frapper, Chem. Commun.
(1997) 1883.
[3] (a) J. Suades, F. Dahan, R. Mathieu, Organometallics 8 (1989)
842. (b) D. Montllo´, J. Suades, M.R. Torres, A. Perales, R.
Mathieu, J. Chem. Soc., Chem. Commun. (1989) 97. (c) D.
Montllo´, J. Suades, F. Dahan, R. Mathieu, Organometallics 9
(1990) 2933.
[4] Some examples: (a) Y.S. Wong, S. Jacobson, P.C. Chieh, A.J.
Carty, Inorg. Chem. 13 (1974) 284. (b) E. Sappa, G. Predieri, A.
Tiripicchio, M. Tiripicchio Camellini, J. Organomet. Chem. 297
(1985) 103. (c) C.J. Adams, M.I. Bruce, E. Horn, E.R.T.
Tiekink, J. Chem. Soc., Dalton Trans. (1992) 1157. (d) I.
Moldes, J. Ros, Inorg. Chim. Acta 232 (1995) 75.
[12] C. Lapinte, D. Catheline, D. Astruc, Organometallics 7 (1988)
1683.
[13] R.B. King, A. Efraty, Inorg. Chim. Acta 4 (1970) 319.
[14] P.E. Riley, R.E. Davis, Organometallics 2 (1983) 286.
[15] A.J. Carty, A. Efraty, Inorg. Chem. 8 (1969) 543.
[16] T.B. Grindley, K.F. Johnson, A.R. Katritzky, H.J. Keogh, C.
Thirkettle, R.D. Topsom, J. Chem. Soc., Perkin Trans. II, (1974)
282.
[17] E.E. Schweizer, S.D. Goff, W.P. Murray, J. Org. Chem. 42
(1977) 200.
[18] (a) D. Rossenberg, W. Drenth, Tetrahedron 27 (1971) 3893. (b)
T.A. Albright, W.J. Freeman, E.E. Schweizer, J. Am. Chem.
Soc. 97 (1975) 2946. (c) A.J. Carty, Pure Appl. Chem. 54 (1982)
113.
[5] E. Louattani, J. Suades, R. Mathieu, J. Organomet. Chem. 421
(1991) 335.
[6] E. Louattani, A. Lledo´s, J. Suades, A. Alvarez-Larena, J.F.
Piniella, Organometallics 14 (1995) 1053.
.