η2(3e)-vinyl complexes and one-electron-transfer reactions: Tris(pentafluorophenyl)borane as a one-electron oxidant

Article abstract of DOI:10.1021/om0007668

The η²(3e)-vinyl complex [Mo{ double bond C(Ph)CHPh}-{P(OMe)₃}₂Cp] is oxidized by [FeCp₂]⁺, [CPh₃]⁺, or B(C₆F₅)₃ to form the 17-electron cation [Mo{ double bond C(Ph)CHPh}{P(OMe)₃}₂Cp]⁺, which on warming loses H to form the cationic η²(4e)-alkyne complex [Mo(η²-PhC triple bond CPh){P(OMe)₃}₂Cp]⁺. In the case of the borane there is evidence for a competing reaction between the η²-vinyl complex and the acid (H₂O)B(C₆F₅)₃, resulting in the formation of a labile trans-stilbene complex.

Full text of DOI:10.1021/om0007668

© Copyright 2001
Volume 20, Number 2, January 22, 2001
American Chemical Society
Communications
η2(3e)-Vin yl Com p lexes a n d On e-Electr on -Tr a n sfer
Rea ction s: Tr is(p en ta flu or op h en yl)bor a n e a s a
On e-Electr on Oxid a n t
Claire J . Beddows,† Andrew D. Burrows,† Neil G. Connelly,‡ Michael Green,*,†,‡
J ason M. Lynam,†,‡ and Timothy J . Paget‡
Department of Chemistry, University of Bath, Bath BA2 7AY, U.K., and School of Chemistry,
University of Bristol, Bristol BS8 1TS, U.K.
Received September 5, 2000
propenes and show a relationship between the bonding
Summary: The η2(3e)-vinyl complex [Mo{dC(Ph)CHPh}-
capabilities of η2(4e)-alkyne and η2(3e)-vinyl ligands.
{P(OMe)3}2Cp] is oxidized by [FeCp2]+, [CPh3]+, or
Given that the metal-alkyne moiety can act as an
B(C6F5)3 to form the 17-electron cation [Mo{dC(Ph)C-
HPh}{P(OMe)3}2Cp]+, which on warming loses H to
form the cationic η2(4e)-alkyne complex [Mo(η2-PhCt
CPh){P(OMe)3}2Cp]+. In the case of the borane there is
evidence for a competing reaction between the η2-vinyl
complex and the acid (H2O)B(C6F5)3, resulting in the
formation of a labile trans-stilbene complex.
electron sink in the d4-d5 redox-related pair [Mo(CO)2-
(η2-PhCtCPh)Tp′][PF6] (Tp′ ) HB(3,5-dimethylpyra-
zolyl)) and [Mo(CO)2(η2-PhCtCPh)Tp′],5 and a previous
indication of a one-electron-transfer reaction involving
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K. W. J . Chem. Soc., Chem. Commun. 1979, 30. (b) Carlton, L.;
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M. A.; Fechtenko¨tter, A.; Arif, A. M.; Richmond, T. G. Organometallics
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The last two decades have seen the development of
-
the transition-metal chemistry of coordinated CHCH2
fragments where both carbons are bonded to the metal
center.1-4 Computational studies1d,3d led to the descrip-
tion of these species as η2(3e)-vinyls or metallacyclo-
† University of Bath.
‡ University of Bristol.
(1) Mo: (a) Green, M.; Norman, N. C.; Orpen, A. G. J . Am. Chem.
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10.1021/om0007668 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 12/15/2000

232 Organometallics, Vol. 20, No. 2, 2001
Communications
Sch em e 1a
F igu r e 1. ESR spectrum of the cation [Mo{dC(Ph)CHPh}-
{P(OMe)3}2Cp]+ (1+).
an η2(3e)-vinyl,6 we have explored the possibility of a
related d3-d4 redox chemistry for the η2(3e)-vinyl
complex [Mo{dC(Ph)CHPh}{P(OMe)3}2Cp] (1).1a,1d In
a
L ) P(OMe)3. Legend: (i) +[FeCp2][PF6] or +[CPh3][BF4];
this investigation we have also discovered that the
(ii) -H or -D.
7
Lewis acid B(C6F5)3 can act as a one-electron oxidant
toward an η2(3e)-vinyl complex.
Addition (195 K) of 1 equiv of [FeCp2][PF6] to a
solution of 1 in CH2Cl2 led to a rapid change in color
and, on warming to room temperature, the formation
in quantitative yield of the [PF6]- salt of the four-
electron-donor alkyne cation [Mo(η2-PhCtCPh)-
{P(OMe)3}2Cp]+ (2+). An insight into the processes
involved in this reaction was obtained by electrochemi-
cal and ESR spectroscopic studies. The cyclic voltam-
mogram of 1, in CH2Cl2 at a platinum electrode, shows
a reversible oxidation wave at -0.11 V, implying the
ready formation of 1+, followed by a second wave at ca.
0.1 V. When a mixture of 1 and [FeCp2][PF6] at 195 K
in CH2Cl2/thf (1:2) was warmed to 260 K, the well-
resolved isotropic ESR spectrum observed, a doublet of
doublets of doublets with Mo satellites (A(1H) ) 8.0 G,
A(31P1) ) 23.7 G, A(31P2) ) 37.8 G; A(95,97Mo) ) 17.0 G;
giso ) 2.011), was consistent with the formation of 1+
(Figure 1). When the reaction mixture was warmed
further, to room temperature, this signal disappeared.
These results suggest that the formation of 2+ involves
initial one-electron oxidation of 1 to form the 17-electron
cation 1+, stabilized by delocalization of unpaired spin
density onto the η2-vinyl fragment. Cation 1+ then loses
a hydrogen atom8 from the â-carbon of the η2-vinyl to
form 2+ (Scheme 1). When the reaction was repeated
(260 K, CH2Cl2) with the deuterated η2(3e)-vinyl com-
F igu r e 2. ESR spectrum of the cation [Mo{dC(Ph)CDPh}-
{P(OMe)3}2Cp] (3+).
As noted earlier, η2(3e)-vinyl complexes can be viewed
as metallacyclopropenes and, because of the isolobal
relationship CH ¶ MoL2Cp, 1 might be expected to
show reactivity patterns similar to those of cyclopro-
penes. Indeed, addition (190 K, CH2Cl2) of [CPh3][BF4]
to a solution of 1 followed by warming the mixture to
room temperature led to the formation in high yield of
CHPh3 and the η2(4e)-alkyne cation 2+, a complex
isoelectronic with a cyclopropenium cation (Scheme 1).
However, ESR spectroscopy showed that one-electron
oxidation by CPh3+ rather than simple hydride abstrac-
tion was involved. Thus, warming a mixture of 1 and
[CPh3][BF4] in CH2Cl2 from 195 to 260 K led to the same
ESR spectrum as observed when [FeCp2]+ was used as
oxidant: i.e., the spectrum of 1+. In addition there was
the characteristic signal of the CPh3 radical.9 These
signals disappear when the mixture is warmed to room
temperature.
plex [Mo{dC(Ph)CDPh}{P(OMe)3}2Cp] (3), the corre-
sponding ESR signal of 3+ appeared (Figure 2) as a
doublet of doublets with Mo satellites (A(P1) ) 23.8 G,
A(P2) ) 37.9 G, A(Mo) ) 16.9 G; giso ) 2.011). An EHMO
calculation, based on the established bond parameters
for 1,1d is consistent with removal of the electron from
2
a metal-centered HOMO (32% dz , 42% dxy).
The facility with which the d4 η2(3e)-vinyl complex 1
undergoes one-electron oxidation suggested the pos-
(6) Clegg, W.; Green, M.; Hall, C. A.; Hockless, D. C. R.; Norman,
N. C.; Woolhouse, C. M. J . Chem. Soc., Chem. Commun. 1990, 1330.
(7) Review: Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345.
(8) For recent examples of hydrogen atom loss from a 17-electron
cation see: (a) Bleeke, J . R.; Hinkle, P. V.; Rath, N. P. J . Am. Chem.
Soc. 1999, 121, 595. (b) Burrows, A. D.; Green, M.; J effery, J . C.;
Lynam, J . M.; Mahon, M. F. Angew. Chem., Int. Ed. 1999, 38, 3043.
(9) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(10) Galakhov, M. V.; Heinz, G.; Royo, P. Chem. Commun. 1998,
17.

Communications
Organometallics, Vol. 20, No. 2, 2001 233
Sch em e 2a
sibility that it might also undergo electron transfer with
B(C6F5)3, which is isoelectronic with [CPh3]+. This
prediction proved to be correct. When solid 1 was added
to a frozen solution of B(C6F5)3 in CH2Cl2/1,2-dichloro-
ethane (1:1) at 77 K and the reaction mixture warmed
to 260 K, a strong, well-resolved signal of 1+ appeared
in the ESR spectrum (together with a signal due to a
minor unidentified molybdenum-centered radical spe-
cies); there was no evidence for the formation of a boron-
centered radical. Similarly, when B(C6F5)3 was reacted
with the deuterated complex 3 the ESR signal for 3+
was observed. In the context of the known7,10,11 reactions
of B(C6F5)3 with organometallic complexes, this is an
especially interesting observation, since it has recently
been reported12 that B(C6F5)3 acts as a one-electron
oxidant toward an azazirconacyclobutene.
Interestingly, when 1 was reacted (CD2Cl2, 190 K f
room temperature) on a preparative scale (0.1 mmol),
the yield of species containing the 2+ cation was
observed (1H, 31P, 11B NMR) to vary (80 f 50%). These
findings were rationalized when it was found that
bubbling CO through the reaction mixture (warmed to
room temperature) gave the complex 2+ together with
trans-stilbene (MS, NMR) and the dicarbonyl species
[Mo(CO)2{P(OMe)3}2Cp]+ (4+) (MS, NMR, ν(CO) (1998
and 1927 cm-1)). This suggested that competing reac-
tions were occurring in which the η2(3e)-vinyl complex
1 underwent either a one-electron-oxidation reaction (1
f 2+) with B(C6F5)3 or proton transfer (1 f trans-
stilbene complex) with the acid (H2O)B(C6F5)311b formed
in varying amounts by the reaction of traces of H2O with
B(C6F5)3.
a
L ) P(OMe)3. Legend: (i) +(H2O)B(C6F5)3, X- ) [B(OH)-
(C6F5)3]-; (ii) +[PhNHMe2]BF4, -C6H5NMe2, X- ) [BF4]-; (iii)
+HBF4‚OEt2, X- ) [BF4]-; (iv) +CO; (v) +Ph2C2, -trans-
stilbene, X- ) [B(OH)(C6F5)3]- or [BF4]-.
Strong support for this rationale was obtained when
it was observed that 1 reacted (CD2Cl2, 190 K f room
temperature) with [PhNHMe2]BF4 or HBF4‚OEt2, i.e.
[HOEt2]BF4, to form in quantitative yield the cationic
trans-stilbene complex 5+ (X- ) BF4-) (Scheme 2),
which was fully characterized by NMR spectroscopy16
(1H, 31P{1H}, and 13C{1H}).17 When 1 was treated with
(H2O)B(C6F5)3, the cation 5+ with the counteranion
Therefore, in the reaction of 1 with a mixture of B(C6F5)3
and (H2O)B(C6F5)3 any 2+, i.e. η2(4e)-alkyne substituted
cation, which is formed must arise as a result of B(C6F5)3
acting as a one-electron oxidant.
In summary, the η2(3e)-vinyl complex 1 undergoes
one-electron oxidation with [FeCp2]+ or [CPh3]+ to form
the relatively stable 17-electron species 1+, which can
then lose a hydrogen atom to form the cationic η2(4e)-
alkyne complex 2+. Especially interesting is the obser-
vation that 1+ is formed on reaction of B(C6F5)3 with 1
and that (H2O)B(C6F5)3, present in varying15 amounts,
reacts with 1 by a different reaction pathway involving
formal addition of a proton to the R-carbon of the η2-
(3e)-vinyl ligand to form a trans-stilbene complex.
[B(OH)(C6F5)3]- (11B NMR (CD2Cl2) δ -3.8 (lit.13,14
δ
-3.8, CDCl3)) was formed. Moreover, when the cationic
species 5+ (X- ) [BF4]- or [B(OH)(C6F5)3]-) was reacted
(room temperature) with either carbon monoxide or
diphenylacetylene, the cis-dicarbonyl species 4+ (Scheme
2) or the η2(4e)-alkyne cation 2+ was formed, respec-
tively, in quantitative yield. It is important to note that
(H2O)B(C6F5)3 has recently11b been shown to act as a
one-electron oxidant toward [MCp2] (M ) Cr, Fe, Co).
However, in the reaction of 1 with [PhNHMe2]BF4 and
[HOEt2]BF4 only the trans-stilbene complex 4+ is formed.
Ack n ow led gm en t . We thank the EPSRC for
support.
OM0007668
(15) Although all glassware was flame-dried prior to use and CH2-
Cl2 was distilled under N2 from CaH2, traces of water could not be
excluded. A similar difficulty was observed by Norton and co-workers.12
Preparative reactions were carried out on a 0.1 mmol scale in 10 cm3
of solvent. ESR spectra were measured on a 0.01 mmol scale in 0.2
cm3 of solvent.
(11) (a) Danopoulos, A. A.; Galsworthy, J . R.; Green, M. L. H.;
Cafferkey, S.; Doerrer, L. H.; Hursthouse, M. B. Chem. Commun. 1998,
2529. (b) Doerrer, L. H.; Green, M. L. H. J . Chem. Soc., Dalton Trans.
1999, 4325.
(12) Harlan, C. J .; Hascall, T.; Fujita, E.; Norton, J . R. J . Am. Chem.
Soc. 1999, 121, 7274. In this important study, evidence was presented
for the facile one-electron oxidation of an azazirconacyclobutene and
cobaltocene by B(C6F5)3. As in the case of the formation of the
molybdenum species 1+ the structural identity of the boron-containing
counteranions in the zirconium reaction was not established. As noted
by Norton et al., in many reactions with Lewis acids that generate
radical cations the structural identity of the counteranion has not been
established.
(16) Selected NMR spectroscopic data for complex 5 recorded in CD2-
3
Cl2 at ambient temperature: 1H, δ 7.6-7.0 (Ar), δ 6.35 (ddd, J PH
)
)
3
3
3
8.5 Hz, J HH ) 8.5 Hz, J PH ) 4.7 Hz, 1H, PhCHd), δ 5.17 (dd, J PH
1.4 Hz, 3J PH ) 0.9 Hz, 5H, C5H5), δ 3.44 (dd, 3J PH ) 9.3 Hz, 5J PH ) 1.1
3
5
Hz, 9H, P{OMe}3), δ 3.29 (dd, J PH ) 9.5 Hz, J PH ) 1.0 Hz, 9H,
P{OMe}3), δ 2.51 (dd, 3J PH ) 7.2 Hz, 3J HH ) 8.5 Hz, 1H, PhCHd); 31P-
{1H}, δ 172.1 (d, J PP ) 104.0 Hz, P{OMe}3), δ 170.6 (d, J PP ) 104.0
2
2
Hz, P{OMe}3); 13C{1H}, δ 90.5 (s, C5H5), δ 79.7 (dd, J PC ) 6.0 Hz,
2
2
2
(13) Siedle, A. R.; Newmark, R. A.; Lamanna, W. M.; Huffman, J .
C. Organometallics 1993, 12, 1491.
2J PC ) 4.0 Hz, PhCHd), δ 57.1 (dd, J PC ) 5.5 Hz, J PC ) 2.5 Hz,
PhCHd).
(14) (a) Hill, G. S.; Manojlovic-Muir, L.; Muir, K. W.; Puddephatt,
R. J . Organometallics 1997, 16, 525. (b) Hill, G. S.; Rendina, L. M.;
Puddephatt, R. J . J . Chem. Soc., Dalton Trans. 1996, 1809.
(17) An NMR spectroscopic study of the reaction of 1 with [PhNHMe2]-
BF4 showed smooth conversion to 5+ and uncoordinated PhNMe2
between -50 and -25 °C.