Monophospholyl analogs of cobaltocenium, rhodocenium, and iridocenium cations. Molecular structures of [MCp*(η5-PC4H2tBu2)] +BPh4- (M = Co, Rh)

Article abstract of DOI:10.1021/om990820o

Monophosphametallocenium cations of the cobalt group are easily prepared through metathesis of bis(η⁵-2,5-di-tert-butylphospholyl)lead (1) with [MCp*X₂]₂ (M = Co, Rh, Ir). Crystal structure analyses confirm their metallocene structures, and electrochemical studies show that [CoCp*(PC₄H₂tBu₂)]⁺ (2) undergoes reversible reduction to a long-lived product at a potential of -0.74 V (vs SCE).

Full text of DOI:10.1021/om990820o

954
Organometallics 2000, 19, 954-956
Mon op h osp h olyl An a logs of Coba ltocen iu m ,
Rh od ocen iu m , a n d Ir id ocen iu m Ca tion s. Molecu la r
-
Str u ctu r es of [MCp *(η5-P C4H2tBu 2)]+BP h 4 (M ) Co, Rh )
Kareen Forissier, Louis Ricard, Duncan Carmichael,* and Franc¸ois Mathey*
Laboratoire “He´te´roe´le´ments et Coordination”, CNRS UMR 7653, DCPH Ecole Polytechnique,
91128 Palaiseau Cedex, France
Received October 18, 1999
Ch a r t 1
Summary: Monophosphametallocenium cations of the
cobalt group are easily prepared through metathesis of
bis(η5-2,5-di-tert-butylphospholyl)lead (1) with [MCp*X2]2
(M ) Co, Rh, Ir). Crystal structure analyses confirm their
metallocene structures, and electrochemical studies show
that [CoCp*(PC4H2tBu2)]+ (2) undergoes reversible re-
duction to a long-lived product at a potential of -0.74
V (vs SCE).
We have recently been exploring the specific prepara-
tion of η5-phospholyl complexes by using a sterically
hindered phosphole which incorporates tert-butyl groups
to minimize redox reactions and η1-coordination. This
ligand has allowed the preparation of a number of
previously inaccessible transition-metal1 and p-block2
derivatives. The p-block complexes should, in principle,
be better phospholyl ligand transfer reagents for redox-
sensitive metal centers than the corresponding alkali-
metal salts,3 and the 1,1′-diphosphaplumbocene 1 seems
to be particularly attractive, because of the ease with
which any PbX2 byproducts can be removed.2a As part
of a project directed toward the eventual synthesis of
paramagnetic phosphametallocenes, we evaluated the
reactions of 1 with [MCp*X2]2 (M ) Co, X ) I; M ) Rh,
Ir; X ) Cl).
Stirring dichloromethane solutions of [Pb(PC4tBu2H2)2]
(1) with [CoCp*I2]2 at room temperature followed by
filtration, metathesis of the crude [CoCp*(PC4H2tBu2)]+I-
with KPF6, NaBF4, or NaBPh4 in MeOH, and crystal-
lization from chloroform/methanol to remove traces of
2,2′,5,5′-tetra-tert-butyl-1,1′-biphosphole gave yellow,
essentially air-stable, crystals of [CoCp*(PC4H2tBu2)]+X-
(2-X) in good yield. The corresponding rhodium and
iridium complexes [MCp*Cl2]2 were converted into
colorless, air-stable [MCp*(PC4H2tBu2)]+X- (3, M ) Rh;
4, M ) Ir) under similar conditions and purified
analogously.4
Thus, in being recovered unchanged from neat io-
domethane after 7 days at 25 °C, [2-BPh4] shows the
classically low nucleophilicity expected from sp2-hybrid-
ized phosphorus.5 In addition, the complexes are soluble
and stable in polar media (MeOH, MeCN) and show 31P
and 13C NMR parameters which are insensitive to the
1
nature of X (Cl, I, BF4, BPh4, PF6). The large J PC
coupling constants observed in each of the compounds
are typical of aromatic phospholyl ligands,6 and the
isoelectronic rhodocenium (3) and ruthenocene (5)1a
In principle, two structural forms may be written for
2-4 (Chart 1) but all available solution data indicate
that these complexes exist as metallocene cations rather
than cyclopentadienyl(η4-phosphole)metal(I) complexes.
complexes have 13C NMR spectra which are very similar
in form. The differing charge upon moving from 5 to 3
induces low-field shifts at each of the rhodium-bound
carbon atoms whose magnitudes (phospholyl, PCdC
+24.7 ppm, PCdC +15.4 ppm; Cp*, MeC +16.4 ppm)
suggest that both rings perceive the metal electron
density to a similar degree.
* To whom correspondence should be addressed. E-mail:
D.C., DuncanC@mars.polytechnique.fr; F.M., francois.mathey@
polytechnique.fr.
(1) (a) Carmichael, D.; Ricard, L.; Mathey, F. J . Chem. Soc., Chem.
Commun. 1994, 1167. (b) Carmichael, D.; Ricard, L.; Mathey, F. J .
Chem. Soc., Chem. Commun. 1994, 2459. (c) Caffyn, A. J . M.;
Carmichael, D.; Mathey, F.; Ricard, L. Organometallics 1997, 16, 2049.
(2) (a) Forissier, K.; Ricard, L.; Carmichael, D.; Mathey, F. Chem.
Commun. 1999, 1273. (b) Schnepf, A.; Sto¨sser, G.; Carmichael, D.;
Mathey, F.; Schno¨ckel, H. Angew. Chem., Int. Ed. 1999, 38, 1646.
(3) Nief, F.; Mathey, F. J . Chem. Soc., Chem. Commun. 1988, 770.
A solid-state cationic metallocene formulation was
confirmed by X-ray diffraction studies on 2-BPh4 and
10.1021/om990820o CCC: $19.00 © 2000 American Chemical Society
Publication on Web 02/16/2000

Communications
Organometallics, Vol. 19, No. 6, 2000 955
F igu r e 2. Structure of the cation in 3-BPh4. Selected bond
lengths (Å) and angles (deg): P(1)-C(1), 1.788(2); P(1)-
C(4), 1.788(2); C(1)-C(2), 1.421(3); C(2)-C(3), 1.433(2);
C(3)-C(4), 1.425(3); C(13)-C(14), 1.438(3); C(14)-C(15),
1.443(3); C(15)-C(16), 1.430(3); C(16)-C(17), 1.438(3);
C(13)-C(17), 1.431(3); Rh(1)-P(1), 2.4243(5); Rh(1)-C(1),
2.259(2); Rh(1)-C(2), 2.200(2); Rh(1)-C(3), 2.1948(2); Rh-
(1)-C(4), 2.260(2); C(1)-P(1)-C(4), 90.5(1); P(1)-C(1)-
C(2), 111.8(1); C(1)-C(2)-C(3), 113.1(2); C(2)-C(3)-C(4),
112.7(2); C(3)-C(4)-P(1), 111.9(1).
F igu r e 1. Structure of the cation in 2-BPh4. Selected bond
lengths (Å) and angles (deg): P(1)-C(1), 1.788(2); P(1)-
C(4), 1.784(2); C(1)-C(2), 1.411(2); C(2)-C(3), 1.418(2);
C(3)-C(4), 1.415(2); C(13)-C(14), 1.429(2); C(14)-C(15),
1.423(3); C(15)-C(16), 1.436(2); C(16)-C(17) 1.424(2);
C(13)-C(17), 1.437(2); Co(1)-P(1), 2.3055(5); Co(1)-C(1),
2.152(2); Co(1)-C(2), 2.061(2); Co(1)-C(3), 2.053(2); Co-
(1)-C(4), 2.133(2); C(1)-P(1)-C(4), 90.35(8); P(1)-C(1)-
C(2), 111.5(1); C(1)-C(2)-C(3), 113.3(2); C(2)-C(3)-C(4),
113.2(2); C(3)-C(4)-P(1), 111.6(1).
those of other (2,5-di-tert-butylphospholyl)cobalt com-
plexes and fall within the normal ranges.1c It is note-
worthy that there is slight evidence for greater bond
length equalization in the CC phospholyl framework for
the Co than for the Rh compound and also that all bonds
in the phospholyl ligand of the rhodium compound are
slightly elongated by comparison with those of the cobalt
complex; this effect is mirrored in the Cp* portion of
the molecule. Predictably9 the M-Cp distances in 3 and
the neutral ruthenocene analogue 51a are similar, but
the M-phospholyl distance is slightly elongated in 3.
Electrochemical studies on the cobaltocenium complex
2-PF6 reveal a first-reduction half-wave potential at
-0.74 V (vs SCE in n-Bu4NBF4-THF) which is es-
sentially reversible at 50 mV s-1 and insensitive to
doping with water. A second reduction observed at
-1.95 V is irreversible even at scan rates of 250 mV
s-1. No oxidation wave could be found within the THF
window. The diminution of approximately 0.45 V in the
reduction potential upon replacement of Cp by the
3-BPh4, which both show discrete well-separated ions7
(Figures 1 and 2).
The cations have planar phospholyl rings and well-
equalized C-C bond lengths indicative of high aroma-
ticity;8 lengths and angles in 2 are comparable with
(4) Synthesis of 2c: finely ground [CoCp*I2]2 (0.150 g, 0.170 mmol)
was added at room temperature to a dichloromethane (10 mL) solution
of bis(η5-2,5-di-tert-butylphospholyl)lead (1; 0.100 g, 0.167 mmol). After
15 min of stirring, the green reaction mixture was evaporated to
dryness and redissolved in methanol (35 mL) and the solution was
filtered through Celite and treated with NaBPh4 (0.115 g, 0.336 mmol).
The precipitate was collected on
a frit and recrystallized from
chloroform/methanol to give yellow crystals of 2c. Yield: 0.101 g, 77%.
1H NMR (CDCl3): δ 7.42 (s, br, 8H, CH), 7.04 (t, 3J (H-H) ) 7.1 Hz,
8H, CH), 6.91 (d, 3J (H-H) ) 7.1 Hz, 4H, CH), 5.45 (d, 3J (P-H) ) 4.0
Hz, 2H, PCCH), 1.87 (s, br, 15H, CCH3), 1.14 (s, br, 18H, C(CH3)3),
13C NMR (CDCl3): δ 164.7 (q, 1J (B-C) ) 49.7 Hz, BC), 139.8 (d, 1J (P-
C) ) 67.1 Hz, PC), 136.9 (s, Ph), 126.2 (s, Ph), 122.4 (s, Ph), 99.4, (s,
PC), 92.2 (d, 2J (P-C) ) 7.6 Hz, MeC), 94.1 9d, 2J (P-C) ) 6.1 Hz, PCC),
35.8 (d, 2J (P-C) ) 13.4 Hz, Me3C), 32.4 (d, 3J (P-C) ) 6.1 Hz, Me3C),
12.5 (s, MeC). 31P NMR (CDCl3): δ -22.3. EI-MS: m/z 389 (MH+
-
HBPh4; 100%), 164 (tBuC4H2tBu+; 82%). Synthesis of 3c: similarly,
powdered [RhCp*Cl2]2 (0.150 g, 0.241 mmol) and bis(η5-2,5-di-tert-
butylphospholyl)lead (1; 0.145 g, 0.243 mmol) in dichloromethane (15
mL) were stirred at room temperature for 20 min, redissolved in
methanol, filtered, and treated with NaBPh4 (0.166 g, 0.485 mmol) to
give colorless 3c upon recrystallization from chloroform/methanol.
Yield: 0.204 g, 56%. 1H NMR (CDCl3): δ 7.42 (s, br, 8H, CH), δ 7.04
(t, 3J (H-H) ) 7.1 Hz, 8H, CH), 6.91 (d, 3J (H-H) ) 7.1 Hz, 4H, CH),
5.45 (d, 3J (P-H) ) 4.0 Hz, 2H, PCCH), 1.87 (s, br, 15H, CCH3), 1.14
(s, br, 18H, C(CH3)3). 13C NMR (CDCl3): δ 164.7 (q, 1J (B-C) ) 49.7
Hz, BC), 141.4 (dd, 1J (Rh-C) ) 7.8 Hz, 1J (P-C) ) 69.6 Hz, PC), 136.9
(s, Ph), 126.1 (s, Ph), 122.4 (s, Ph), 104.1 (d, 1J (Rh-C) ) 7.6 Hz, MeC),
94.1 (pseudo-t, 1J (Rh-C) ) 2J (P-C) ) 7.6 Hz, PCC), 35.2 (d, 2J (P-C)
) 12.6 Hz, Me3C), 32.4 (d, 3J (P-C) ) 6.1 Hz, Me3C), 12.3 (s, MeC).
31P{1H} NMR (CDCl3): δ -6.7 (d, 1J (Rh-P) ) 18.3 Hz). EI-MS: m/z
433 (MH+ - HBPh4; 100%), 164 (tBuC4H2tBu+; 94%). Synthesis of 4c:
(5) For example: Mathey, F.; Mitschler, A.; Weiss, R. J . Am. Chem.
Soc. 1978, 100, 5748.
(6) Mathey, F. Coord. Chem. Rev. 1994, 37, 1.
(7) Crystal data for 2c: C46H55BCoP; Mr ) 708.61; triclinic; space
group P1h; a ) 11.1692(5) Å, b ) 11.9323(5) Å, c ) 16.2789(7) Å, R )
78.111(2)°, â ) 71.694(2)°, γ ) 72.959(2)°, V ) 1953.75(15) Å3; Z ) 2;
D ) 1.205 g cm-3; µ ) 0.511 cm-1; F(000) ) 756; crystal dimensions
0.22 × 0.10 × 0.06 mm; 14 399 reflections collected, 6862 of which with
I > 2σ(I); goodness of fit on F2 1.021; R1 ) 0.0408 (I > 2σ(I)); wR2 )
0.0894 (all data); maximum/minimum residual density 0.331(0.053)/-
0.380(0.053) e Å-3. Crystal data for 3c: C47H56BCl3PRh; Mr ) 869.94;
triclinic; space group P1h; a ) 10.8262(2) Å, b ) 12.1038(3) Å, c )
17.5262(3) Å, R ) 88.2620(1)°, â ) 86.9715(1)°, γ ) 72.9101(1)°, V )
2191.87(8) Å3; Z ) 2; D ) 1.318 g cm-3; µ ) 0.640 cm-1; F(000) ) 904;
crystal dimensions 0.18 × 0.18 × 0.14 mm; 14733 reflections collected,
9182 of which with I > 2σ(I); goodness of fit on F2 1.103; R1 ) 0.0310
(I > 2σ(I)); wR2 ) 0.0869 (all data); maximum/minimum residual
density 0.953(0.088)/-0.874(0.088) e Å-3. All data were collected on a
KappaCCD diffractometer at 150.0(1) K using Mo KR radiation (λ )
0.710 73 Å). Full details are given in the Supporting Information.
(8) Schleyer, P. v. R.; Freeman, P. K.; J iao, H.; Goldfuss, B. Angew.
[IrCp*Cl2]2 (0.050 g, 0.06 mmol) and
1 (0.040 g, 0.07 mmol) in
dichloromethane (15 mL) were likewise converted by NaBPh4 (0.046
g, 0.13 mmol) into colorless needlelike crystals of [Cp*Ir(PC4H2But2)]+-
[BPh4]- (4c) from chloroform/methanol. Yield: 0.050 g, 50%. 1H NMR
(CDCl3): δ 7.45 (m, br, 8H), 7.00 (m, 12H), 5.48 (d, 3J (P-H) ) 4.1 Hz,
2H, PCCH), 2.07 (s, 15H, CCH3), 1.17 (s, 18H, C(CH3)3). 13C NMR
(CDCl3): δ 164.8 (q, 1J (B-C) ) 48.9 Hz, BC), 136.9 (s, Ph), 129.0 (d,
1J (P-C) ) 68.7 Hz, PC), 126.2 (s, Ph), 122.4 (s, Ph), 97.8 (s, MeC),
87.4 (d, 2J (P-C) ) 6.1 Hz, PCC), 34.5 (d, 2J (P-C) ) 10.7 Hz, Me3C),
32.2 (d, 3J (P-C) ) 7.6 Hz, Me3C), 11.9 (s, MeC). 31P NMR (C6D6): δ
-40.3. EI-MS: m/z 523 (MH+ - HBPh4; 79%), 164 (tBuC4H2tBu+;
100%).
Chem., Int. Ed. Engl. 1995, 34, 337.
(9) For instance, no difference between Fe(Cp*)2 and Co(Cp*)2
+
:
Dixon, D. A.; Miller, J . S. J . Am. Chem. Soc. 1987, 109, 3656.

956 Organometallics, Vol. 19, No. 6, 2000
Communications
phospholyl ligand (for [Cp*CpCo] E1 ) -1.17 V; DMF,
n-Bu4NBF4)10 is broadly in accord with earlier data
obtained by Lemoine et al. indicating a 0.6 V difference
in reduction potential between ferrocene and the diphos-
phaferrocene derivative 6.11a-c It also concurs with more
recent results showing that replacement of a tBuC group
by P in the multiphosphaferrocene couple 7 and 8 lowers
the reduction potential by ca. 350 mV.11d
In an important recent contribution, Zenneck and
colleagues have reported a neutral triphosphacobal-
(10) Gloaguen, B.; Astruc, D. J . Am. Chem. Soc. 1990, 112, 4607.
(11) (a) Lemoine, P.; Gross, M.; Braunstein, P.; Mathey, F.; Des-
champs, B.; Nelson, J . H. Organometallics 1984, 3, 1303, (b) Lemoine,
P.; Gross, M.; Braunstein, P.; Mathey, F.; Deschamps, B.; Nelson, J .
H. J . Organomet. Chem. 1985, 295, 189. Roman, E.; Leiva, A. M.;
Casasempere, M. A.; Charrier, C.; Mathey, F.; Garland, M. T.; Le
Marouille, J . Y. J . Organomet. Chem. 1986, 309, 323. (d) Bartsch, R.;
Datsenko, S.; Ignatiev, N. V.; Mu¨ller, C.; Nixon, J . F.; Pickett, C. J . J .
Organomet. Chem. 1997, 529, 375. (e) Weber, L.; Sommer, O.; Stam-
mler, H.-G.; Neumann, B.; Ko¨lle, U. Chem. Ber. 1995, 128, 665. (f) For
further electrochemistry of phospholyl complexes, see also: Feher, R.;
Kohler, F. H.; Nief, F.; Ricard, L.; Rossmayer, S. Organometallics 1997,
16, 4610 and references therein.
(12) (a) Envers, A.; Heinemann, F.; Kummer, S.; Wrackmeyer, B.;
Zenneck, U. Lecture LF3-3 to XIVth ICPC, Cincinnati, OH, J uly 12-
17, 1998. After submission, two fuller descriptions have appeared: (b)
Envers, A.; Heinemann, F.; Wrackmeyer, B.; Zenneck, U. Chem. Eur.
J . 1999, 5, 3143. (c) Envers, A.; Heinemann, F.; Kummer, S.; Wrack-
meyer, B.; Zeller, M.; Zenneck, U. Phosphorus, Sulfur Silicon Relat.
Elem. 1999, 144-6, 725.
(13) (a) An undetected hexaphosphacobaltocene was suggested as
a conceivable intermediate in the synthesis of a triphospholyl(tri-
phosphadiene)cobalt(I) compound: Bartsch, R.; Hitchcock, P. B.; Nixon,
J . F. J . Chem. Soc., Chem. Commun. 1988, 819. (b) Other reactions of
triphospholides with Co and Rh(III) precursors have given complex
redox products: Nixon, J . F. Coord. Chem. Rev. 1995, 145, 201. (c)
Cobalt triple-decker monophospholyl compounds: Chase, K. J .; Bryan,
R. F.; Woods, M. K.; Grimes, R. N. Organometallics 1991, 10, 2631.
(d) Diazacobaltocenium: Kuhn, N.; Ko¨ckerling, M.; Stubenrauch, S.;
Bla¨ser, D.; Boese, R. J . Chem. Soc., Chem. Commun. 1988, 819. See
also: (e) Kuhn, N.; Ko¨ckerling, M.; Stubenrauch, S.; Bla¨ser, D.; Boese,
R. Z. Naturforsch., B 1999, 54B, 424. (f) Borole analogues of cobalto-
cenium salts: Herberich, G. E. In Comprehensive Organometallic
Chemistry II; Wilkinson, G., Stone, F. G. A.; Abel, E. W., Eds.;
Pergamon: Oxford, U.K., 1995; Vol. 1, p 197.
tocene derivative 9,12 but no other simple phospholyl-
containing metallocene complexes of the group IX
elements appear to be known.13 Our studies confirm
that monophosphametallocenes of the cobalt group are
also viable synthetic targets and provide a straightfor-
ward access to their precursors. Full details concerning
the chemical reduction of these phosphametallocenium
ions and the structures and properties of mono- and
diphosphacobaltocenes will be presented in due course.
Ack n ow led gm en t. We thank Dr P. le Floch (Ecole
Polytechnique) for some technical assistance with the
cyclic voltammetry measurements and Ecole Polytech-
nique (grant to K.F.) and the CNRS for funding.
Su p p or tin g In for m a tion Ava ila ble: Crystal structure
data for 2-BPh4 and 3-BPh4. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM990820O