Electron transfer versus nucleophilic pathways in the ion-pair annihilation of organoborate anions by carbonylmanganese(I) cations

Article abstract of DOI:10.1016/S0022-328X(98)01166-8

Substituted carbonylmanganese cations [Mn(CO)₅L]⁺, where L = py, PPh₃ and PPh₂Me, readily react with various organoborate anions (tetramethylborate, methyltriphenylborate and tetraphenylborate) in THF solution to afford a mixture of dimanganese carbonyls, hydridomanganese carbonyls and alkylmanganese carbonyls. The formation of the dimanganese carbonyl dimers as well as the hydridomanganese carbonyls suggests the involvement of 19-electron carbonylmanganese radicals that stem from an initial electron transfer. On the other hand, the acetonitrile-substituted analogue [Mn(CO)₅(CH₃CN)]⁺ reacts with the same borate anions to afford the alkylated RMn(CO)₅, where R = CH₃ and C₆H₅, as the sole carbonylmanganese product. As such, this alkylative annihilation is best formulated as a direct attack on the carbonyl carbon by the borate nucleophile. The two different pathways can be understood in terms of the balance between the electrophilicity of the carbonyl ligand and the electron affinity of the carbonylmanganese cation.

Full text of DOI:10.1016/S0022-328X(98)01166-8

Journal of Organometallic Chemistry 580 (1999) 295–303
Electron transfer versus nucleophilic pathways in the ion-pair
annihilation of organoborate anions by carbonylmanganese(I) cations
Dunming Zhu, Jay K. Kochi *
Department of Chemistry, Uni6ersity of Houston, Houston, TX 77204-5641, USA
Received 21 September 1998
Abstract
Substituted carbonylmanganese cations [Mn(CO)5L]+, where L=py, PPh3 and PPh2Me, readily react with various organobo-
rate anions (tetramethylborate, methyltriphenylborate and tetraphenylborate) in THF solution to afford a mixture of dimanganese
carbonyls, hydridomanganese carbonyls and alkylmanganese carbonyls. The formation of the dimanganese carbonyl dimers as
well as the hydridomanganese carbonyls suggests the involvement of 19-electron carbonylmanganese radicals that stem from an
initial electron transfer. On the other hand, the acetonitrile-substituted analogue [Mn(CO)5(CH3CN)]+ reacts with the same
borate anions to afford the alkylated RMn(CO)5, where R=CH3 and C6H5, as the sole carbonylmanganese product. As such, this
alkylative annihilation is best formulated as a direct attack on the carbonyl carbon by the borate nucleophile. The two different
pathways can be understood in terms of the balance between the electrophilicity of the carbonyl ligand and the electron affinity
of the carbonylmanganese cation. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Electron transfer; Nucleophilic attack; Ion-pair annihilation; Carbonylmanganese cations; Organoborate anions;
Tetramethylborate
induced electron transfer from alkylborate anions [6].
Can this photochemical process have a thermal coun-
1. Introduction
terpart? In order to answer this question, we follow the
Nucleophilic addition to the coordinated ligands of
mechanistic dichotomy previously established between
transition metal carbonyls is a useful strategy in the
nucleophilic attack and single electron transfer with
organometallic synthesis and considered to be the key
metal carbonyl anions as nucleophiles [7], and now
step in a variety of metal carbonyl-catalyzed reactions
inquire whether the thermal nucleophilic alkylation of
[1]. Thus various alkyllithium and Grignard reagents,
metal carbonyls such as [Mn(CO)5L]+ by alkylborate
borohydrides or amines add to the carbonylmanganese
anions is actually preceded by an electron-transfer step
cation, [Mn(CO)5L]+, to yield the corresponding acyl,
in Eq. (1), i.e.
alkyl, formyl or carbamoyl derivatives via nucleophilic
[Mn(CO)5L]+ +BR4− “BR4 +[Mn(CO)5L]
’
’
attack on the carbonyl carbon atom [2–4]. However,
the same carbonylmanganese cations are also readily
convertible to the neutral 19-electron radicals by chem-
ical or electrochemical reduction [5], and there is a
recent report that the alkylation of carbonylrhenium(I)
cation [(bpy)Re(CO)3(py)]+ can be effected by photo-
“RMn(CO)5, etc.
(1)
To probe this question, we chose the anionic
organoborates: tetramethylborate, methyltriphenylbo-
rate, and tetraphenylborate, as mild alkylating reagents
with varied nucleophilic properties [8]. For example,
tetraalkylborates is known to react with acyl chlorides
such as benzoyl chloride to selectively give ketones, and
* Corresponding author. Fax: +1-713-7432709.
E-mail address: jkochi@pop.uh.edu (J. K. Kochi)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01166-8

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D. Zhu, J. K. Kochi / Journal of Organometallic Chemistry 580 (1999) 295–303
the reaction is considered to proceed via an SN2 mecha-
nism. Furthermore, these organoborates are also electron
donors in photoinduced electron transfer to the cationic
cyanine dye, N-(p-benzoylbenzyl)-N,N,N-tri-n-butylam-
monium ion [9], as well as in the thermal nucleophilic
alkylation of pyridinium cations [10]. For these studies,
we analyzed the carbonylmanganese products to provide
the mechanistic basis for the intermediacy of the 19-elec-
tron carbonylmanganese radical via an initial transfer of
a single electron from the borate anion (electron donor)
to the manganese carbonyl cation (electron acceptor), as
presented in Eq. (1). We hope that further time-resolved
spectroscopic and electrochemical studies of reactive
intermediates [11] will establish the mechanistic validity
of the product studies.
(CH3CN)]+PF6− at −78°C, and the mixture was al-
lowed to slowly warm to 0°C. Periodic IR analysis
indicated that the reaction occurred mainly at −20°C to
give methylmanganese pentacarbonyl CH3Mn(CO)5 as
the sole product (Table 1). When the reaction was carried
out in THF-d8, the solvate (CH3)3B(OC4D8) was ob-
served by H-NMR (Eq. (2)) [15].
[Mn(CO)5(CH3CN)]+ +B(CH3)4−T“HFCH3Mn(CO)5
1
+(CH3)3B(OC4H8)
(2)
Interestingly, when the same reaction was carried out
at 23°C, a mixture of carbonylmanganese products was
obtained—with pentacarbonylmanganate Mn(CO)5− as
the major component [16].
When two equivalents of tetramethylborate was
treated with one equivalent of [Mn(CO)5(CH3CN)]+ at
−20°C, CH3Mn(CO)5 was observed initially. However,
as the mixture was allowed to warm to room tempera-
ture, CH3Mn(CO)5 was rapidly converted to
[Mn(CO)5]− as monitored by infrared spectroscopy. In
a control experiment, CH3Mn(CO)5 was found to rapidly
react with tetramethylborate in THF at room tempera-
ture to generate pentacarbonylmanganate [Mn(CO)5]−
in quantitative yield [17]. As such, we consider the
interaction of tetramethylborate with [Mn(CO)5(CH3-
CN)]+ at room temperature to proceed in two steps, viz.
Eq. (2) followed by Eq. (3).
2. Results
The manganese cations, [Mn(CO)5(L)]+ where L=
CH3CN, py, PPh3 and PPh2Me, readily reacted with
tetramethylborate, methyltriphenylborate, and te-
traphenylborate in tetrahydrofuran (in the dark) to give
a mixture of alkylmanganese carbonyls and/or diman-
ganese decacarbonyls and hydridomanganese carbonyls.
In more polar solvents such as acetonitrile, no thermal
reaction was observed; and the large solvent effect on
reactivity suggested that ion pairing was important [12].
The reactivity of different borates decreased in the
order: BMe4− \BMePh3− \BPh4−. Thus tetramethyl-
borate readily reacted with the carbonylmanganese
cations at −20°C, whereas methyltriphenylborate was
less reactive, and yielded a similar mixture in a few hours
at room temperature. Tetraphenylborate was the least
reactive, and the completion of the reaction required 20
h at room temperature (or more rapidly at elevated
temperatures). Such a reactivity pattern follows the
general order of the oxidation potentials of these te-
traalkylborates [13]. By the same token, the cationic
CH3Mn(CO)5+B(CH3)4−T“HF[Mn(CO)5]− +C2H6
+Me3B(THF)
(3)
Table 1
Annihilation of carbonylmanganese cations [Mn(CO)5L]+ with
tetramethylborate anion in tetrahydrofurana
Product (yield %)b
manganesecarbonyls
[Mn(CO)5(CH3CN)]+
and
Entry
L
[Mn(CO)5(Py)]+ were more reactive than the phosphine-
1
2
3
4
5
6
CH3CN
CH3CNc
CH3CNe
CH3Mn(CO)5 (88)
substituted
analogues
[Mn(CO)5(PPh3)]+
and
d
[Mn(CO)5]−
[Mn(CO)5(PPh2Me)]+ in accord with the relative values
of their reduction potentials [14].
[Mn(CO)5]− (85)
c,g
–
[Mn(CO)5]− (100)
The redox products (and their distribution) were
dependent on the ligand L of the substituted carbonyl-
manganese cations as well as on the organoborates. Since
the nature of the carbonylmanganese products is more
informative, particular attention was paid on their iden-
tification and analysis as follows.
Pyridine
PPh3
CH3Mn(CO)5 (54), Mn2(CO)10 (10)
CH3Mn(CO)4L (15)f, HMn(CO)4L
(12)f, Mn2(CO)8L2 (58)f
7
PPh2Me
CH3Mn(CO)4L (5)f, HMn(CO)4L
(11)f, Mn2(CO)8L2 (61)f
a Reaction carried out from −78 to 0°C in the period of 2 h unless
indicated otherwise.
b Products quantified by IR analysis unless indicated otherwise.
2.1. Tetramethylborate
c At 23°C.
d Not quantified.
Typically, a THF solution of tetra-n-butylammonium
tetramethylborate was mixed with the acetonitrile-
substituted carbonylmanganese cation [Mn(CO)5-
e Two equivalents of TBA+BMe−4 used.
f Isolated yield.
g CH3Mn(CO)5 used instead of [Mn(CO)L]+
.

D. Zhu, J. K. Kochi / Journal of Organometallic Chemistry 580 (1999) 295–303
297
Table 2
trile-substituted manganese cation cleanly afforded
phenylmanganese pentacarbonyl in high yield, as
quantified by infrared spectroscopy. When the volatile
components of the reaction mixture were condensed in
vacuo at −196°C, a small amount of benzene and 98%
yield of acetonitrile were found by GC analysis. How-
ever, the pyridine derivative [Mn(CO)5(py)]+ reacted
with tetraphenylborate to afford dimanganese decacar-
bonyl. The volatile component of the reaction mixture
was benzene (81%), but no pyridine was detected by
GC analysis [The pyridine is presumed to form the
adduct Ph3B(NC5H5) with triphenylborane [18].] Te-
traphenylborate reacted with phosphine-substituted
[Mn(CO)5(PPh3)]+ and [Mn(CO)5(PPh2Me)]+ at room
temperature at such a slow rate that almost no reaction
was observed after 24 h. The reactions were subse-
quently effected by heating the mixture to 60°C for 24
h; and the carbonylmanganese products were character-
ized as a mixture of C6H5Mn(CO)4(L), Mn2(CO)8(L)2
and HMn(CO)4(L), where L=PPh3 and PPh2Me as
summarized in Table 3.
Annihilation of carbonylmanganese cations [Mn(CO)5L]+ with
methyltriphenylborate anion in tetrahydrofurana
L
Product (yield %)b
1
2
3
CH3CN
Pyridine
PPh3
CH3Mn(CO)5 (82)c
Mn2(CO)10 (33)c
CH3Mn(CO)4L (24), HMn(CO)4L
(37), Mn2(CO)8L2 (28)
CH3Mn(CO)4L (10), HMn(CO)4L
(65), Mn2(CO)8L2 (5)
4
PPh2Me
a Reaction carried out at 23°C for 6 h.
b Isolated yield unless indicated otherwise.
c Products quantified by IR analysis.
When tetramethylborate was treated with the
pyridine derivative [Mn(CO)5(Py)]+ under the same
conditions, it afforded methylmanganese pentacarbonyl
as the major carbonyl product. In addition, a small
amount of Mn2(CO)10 was detected by TLC. On the
other hand, when tetramethylborate was treated with
the phosphine-substituted carbonylmanganese cation
[Mn(CO)5(PPh3)]+ in THF solution, dimanganese car-
bonyl Mn2(CO)8(PPh3)2 was generated as the major
carbonyl product, together with the alkylated product
CH3Mn(CO)4(PPh3) and the substituted hydridoman-
ganese complex HMn(CO)4(PPh3) in small amounts.
Essentially the same results were obtained with te-
tramethylborate and [Mn(CO)5(PPh2Me)]+ in THF so-
lution, as shown in Table 1. In the latter, the
characterization of the boron-containing products indi-
cated the presence of trimethylborane [as the pyridine
adduct Me3B(NC5H5)] in 60% yield.
3. Discussion
In order to account for the various carbonylman-
ganese products obtained from the treatment of various
[Mn(CO)5(L)]+ cations with BMe4−, BMePh3−, BPh4−
in Tables 1–3, let us first summarize the characteristic
behavior of the corresponding 19- and 17-electron car-
bonylmanganese radicals as follows.
3.1. Fate of 19- and 17-electron carbonylmanganese
radicals
2.2. Methyltriphenylborate
Cathodic reduction of the substituted carbonylman-
ganese cations [Mn(CO)5(L)]+ (L=CO, CH3CN, py,
PPh3, PPh2Me, etc.) produces transient carbonylman-
Methyltriphenylborate reacted with the acetonitrile-
substituted manganese cation to give methylmanganese
pentacarbonyl cleanly as the sole carbonyl product.
However, the reaction of methyltriphenylborate with
the pyridine-substituted manganese cation was compli-
cated, and afforded a mixture of dimanganese decacar-
bonyl and other unidentified carbonyl products in low
yields. In contrast to the reaction with [Mn-
(CO)5(CH3CN)]+, no methylmanganese pentacarbonyl
was detected. Methyltriphenylborate behaved similarly
to tetramethylborate, and reacted with the phosphine-
substituted carbonylmanganese cations [Mn(CO)5-
(PPh3)]+ and [Mn(CO)5(PPh2Me)]+ in THF at room
temperature to afford a mixture of CH3Mn(CO)4(L),
Mn2(CO)8(L)2 and HMn(CO)4(L) where L=PPh3 and
PPh2Me, respectively (Table 2).
’
ganese radicals [Mn(CO)5(L)] [5a,b]. As supersaturated
’
19-electron species, [Mn(CO)5(L)] is known to undergo
Table 3
Annihilation of carbonylmanganese cations [Mn(CO)5L]+ with te-
traphenylborate anion in tetrahydrofurana
L
Product (yield %)b
1
2
3
CH3CN
Pyridine
PPh3
C6H5Mn(CO)5 (86)c
Mn2(CO)10 (65)c
d
C6H5Mn(CO)4L (35), HMn(CO)4L
(21), Mn2(CO)8L2 (32)
4
PPh2Med C6H5Mn(CO)4L (23), HMn(CO)4L
(41), Mn2(CO)8L2 (22)
a Reaction carried out at 23°C for 18 h.
b Isolated yield unless indicated otherwise.
c Products quantified by IR analysis.
d At 60°C for 24 h.
2.3. Tetraphenylborate
The annihilation of tetraphenylborate by the acetoni-

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D. Zhu, J. K. Kochi / Journal of Organometallic Chemistry 580 (1999) 295–303
two basic transformations, namely, (a) ligand dissocia-
tion to afford the corresponding 17-electron radicals
’
’
[Mn(CO)5] and/or [Mn(CO)4(L)] (Eqs. (4) and (5)) [5],
(4,5)
Scheme 1.
ET
and (b) hydrogen abstraction to produce the hydri-
domanganese complexes HMn(CO)4(L) (Eq. (6)) [5],
[Mn(CO)5L]+ +BRR% − “[Mn(CO)5L] +BRR%
’
’
3
+e
HS
[Mn(CO)5L]+ “[Mn(CO)5L] “HMn(CO)4L+CO
3
’
(10)
(6)
where the ligand L is either py, PPh3 or PPh2Me and
where HS is the hydrogen-donor solvent. The competi-
tion between these two pathways is highly dependent
on the nature of ligand L in the following way. In the
case of L=CH3CN and py, acetonitrile or pyridine
dissociation is much faster than either hydrogen ab-
straction or CO dissociation [5], and the 17-e carbonyl-
manganese radical [Mn(CO)5] then formed is subject to
ready dimerization to afford dimanganese decacarbonyl
(Eq. (7)) [5c], i.e.
−
the alkylborate BRR%
is either B(CH3)4−,
3
B(CH3)(C6H5)3− or B(C6H5)4−.
For L=pyridine, the 19-electron carbonylmanganese
’
radicals [Mn(CO)5(py)] rapidly lose pyridine to yield
’
17-electron radical [Mn(CO)5] , the coupling of which
leads to dimanganese decacarbonyl Mn2(CO)10 [5]. The
tetramethylboranyl, methyltriphenylboranyl and te-
traphenylboranyl radicals are also labile and subject to
boron–carbon bond cleavage to yield (a) methyl radical
and trimethylborane, (b) methyl radical and triphenylb-
orane, or (c) phenyl radical and triphenylborane
[6,9,10], respectively. Methylmanganese pentacarbonyl
CH3Mn(CO)5 can derive from either the coupling of
methyl radical and the 17-e carbonylmanganese radical
’
−2L
’
’
2[Mn(CO)5L] “ 2[Mn(CO)5] “Mn2(CO)10
(7)
where L=CH3CN and pyridine. When L is a phos-
phine ligand such as PPh3 and PPh2Me, the loss of CO
is preferred over the dissociation of phosphine. As a
result, dimanganese decacarbonyl is not an important
reduction product of phosphine-substituted carbonyl-
manganese cations. However, the rate of hydrogen
abstraction from the solvent HS is comparable to that
of the ligand dissociation (i.e. loss of carbon monoxide)
[5]. Hydrogen abstraction of 19-electron carbonylman-
ganese radicals from the solvent forms the formylman-
ganese intermediate HCOMn(CO)4(L), which is readily
converted to hydridomanganese carbonyl HM-
n(CO)4(L) via the rapid extrusion of carbon monoxide
[19]. Loss of CO from the 19-e carbonylmanganese
’
[Mn(CO)5] or the coupling of methyl radical and the
’
19-e radical [Mn(CO)5(py)] followed by the expulsion
of pyridine (Scheme 1).
When L is either PPh3 or PPh2Me, the dissociation of
’
CO from the 19-e radicals [Mn(CO)5(PPh3)] or
’
[Mn(CO)5(PPh2Me)] competes with hydrogen abstrac-
tion [5,19]. Dissociation of CO produces 17-e carbonyl-
’
manganese radicals [Mn(CO)4(PPh3)] or [Mn(CO)4-
’
(PPh2Me)] , which subsequently undergo (a) dimeriza-
tion to afford Mn2(CO)8(L)2 with L=phosphines, or
(b) coupling with methyl radical or phenyl radical to
form CH3Mn(CO)4(L) or C6H5Mn(CO)4(L) where L=
’
radicals [Mn(CO)5(L)] produces 17-e radicals; and the
’
’
PPh3 and PPh2Me. The 19-e radicals [Mn(CO)5(PPh3)]
[Mn(CO)4(L)] readily dimerize to yield Mn2(CO)8(L)2
’
and [Mn(CO)5(PPh2Me)] can also abstract hydrogen
where L=phosphines (Eqs. (8) and (9)) [5].
atom from the solvent (HS) to generate formylman-
ganese complexes HCOMn(CO)4(PPh3) and HCOM-
n(CO)4(PPh2Me), respectively, which subsequently yield
the hydridomanganese complexes HMn(CO)4(PPh3)
and HMn(CO)4(PPh2Me) by ready extrusion of carbon
monoxide [5,19] (Scheme 2).
−CO
fast
’
’
[Mn(CO)5L] “ [Mn(CO)4L] “1/2 Mn2(CO)8L2
(8)
(9)
HS
−CO
’
[Mn(CO)5L] “HCOMn(CO)4L “ HMn(CO)4L
fast
3.2. Comments on the electron-transfer mechanism
The comparison of the carbonylmanganese products
in Tables 1–3 with the known behavior of the 19-e
carbonylmanganese radicals supports their intermedi-
acy in the nucleophilic interaction of [Mn(CO)5(L)]+
(L=py, PPh3 and PPh2Me) with tetramethylborate,
methyltriphenylborate or tetraphenylborate via an ini-
tial electron transfer (Eq. (10)) [5,7,11], i.e.
Scheme 2.

D. Zhu, J. K. Kochi / Journal of Organometallic Chemistry 580 (1999) 295–303
299
Scheme 3.
Scheme 4.
3.3. Comments on the nucleophilic mechanism
The acetonitrile-substituted carbonylmanganese
Another
route
to
CH3Mn(CO)4(L)
or
C6H5Mn(CO)4(L) for L=PPh3 and PPh2Me involves
the coupling of the 19-e radicals [Mn(CO)5(PPh3)] and
[Mn(CO)5(PPh2Me)] with methyl radical (or phenyl
radical), followed by decarbonylation of CH3CO-
Mn(CO)4(L) or C6H5COMn(CO)4(L) [20] (Eq. (11)),
i.e.
−CO
’
cation reacts with tetramethylborate, methyltriphenyl-
borate and tetraphenylborate to produce either
CH3Mn(CO)5 or C6H5Mn(CO)5 as the only carbonyl-
manganese product. There are two pathways possible
for the formation of CH3Mn(CO)5 and C6H5Mn(CO)5.
One is the electron-transfer mechanism as discussed
above for L=Py, PPh3 and PPh2Me, which simulta-
neously generates the 19-e carbonyl radical
’
’
’
[Mn(CO)5L] +R “RCOMn(CO)4L “ RMn(CO)4L
(11)
’
[Mn(CO)5(CH3CN)] and the boranyl radicals in
where L=PPh3, PPh2Me and R=CH3, C6H5.
CH3Mn(CO)5 reacts rapidly with tetramethylborate
to afford pentacarbonylmanganate Mn(CO)5− as de-
scribed in Eq. (3). Since [Mn(CO)5(PPh3)]+ has been
reported to react with [Mn(CO)4(PPh3)]− to generate
Mn2(CO)8(PPh3)2 and HMn(CO)4(PPh3) (Eq. (12)) [5c],
i.e.
Scheme 4.
The subsequent cleavage of the carbon–boron bond
leads to methyl or phenyl radical. The 19-e radical
’
[Mn(CO)5(CH3CN)] then couples with either methyl or
phenyl radical to yield CH3COMn(CO)4(CH3CN) or
C6H5COMn(CO)4(CH3CN), which immediately form
CH3Mn(CO)5 or C6H5Mn(CO)5 by expulsion of ace-
tonitrile [22,23].
[Mn(CO)5(PPh3)]+ +[Mn(CO)4(PPh3)]−
An alternative, direct pathway involves tetramethyl-
borate, methyltriphenylborate and tetraphenylborate
acting as nucleophiles by their direct attack on the
carbonyl group of [Mn(CO)5(CH3CN)]+ as outlined in
Scheme 5 [2–4].
“Mn2(CO)8(PPh3)2+HMn(CO)4(PPh3)
(12)
There is an alternative possibility for the formation
of Mn2(CO)8(L)2 and HMn(CO)4(L) (where L=PPh3
and PPh2Me) in the reaction of [Mn(CO)5(L)]+ with
tetramethylborate. Thus [Mn(CO)5(L)]+ may react
with tetramethylborate via nucleophilic attack of te-
tramethylborate on the carbonyl ligand followed by
decarbonylation to give CH3Mn(CO)4(L), which then
reacts with the unreacted tetramethylborate to afford
[Mn(CO)4(L)]−. The [Mn(CO)4(L)]− subsequently re-
acts with [Mn(CO)5(L)]+ to produce Mn2(CO)8(L)2
and HMn(CO)4(L) (where L=PPh3 and PPh2Me) as
described in Scheme 3.
In order to test this possibility, we carried out a
control experiment of the reaction of CH3Mn(CO)4-
(PPh3) with tetramethylborate, and found that
CH3Mn(CO)4(PPh3) does not react with tetramethylbo-
rate under identical reaction conditions. Another path-
way to form [Mn(CO)4(PPh3)]− via the interaction of
CH3COMn(CO)4(PPh3) with tetramethylborate [21] is
also unlikely, as indicated by the control experiment.
We conclude therefore that Mn2(CO)8(L)2 and HM-
n(CO)4(L) where L=PPh3 and PPh2Me is unlikely to
be derived from the direct ion-pair annihilation of
manganate(-I) anion and manganese(I) cation.
The
alkylated
CH3COMn(CO)4(CH3CN)
or
C6H5COMn(CO)4(CH3CN) is then converted to
CH3Mn(CO)5 or C6H5Mn(CO)5 by rapid expulsion of
CH3CN [22]. The singular absence of dimanganese
decacarbonyl in the carbonylmanganese product sug-
gests that the electron-transfer pathway is unlikely
[5,7,24,25]. As a result, we believe the more reasonable
pathway involves the direct nucleophilic attack of te-
traalkylborates on the acetonitrile-substituted carbonyl-
manganese cation [Mn(CO)5(CH3CN)]+.
A question is then raised as to why the behavior of
the acetonitrile-substituted carbonylmanganese cation
[Mn(CO)5(CH3CN)]+ is so different from that of the
pyridine- and phosphine-substituted analogues in the
reaction with tetraalkylborates. The interaction of car-
Scheme 5.

300
D. Zhu, J. K. Kochi / Journal of Organometallic Chemistry 580 (1999) 295–303
Table 4
depending on the electrophilicity of the carbonyl lig-
ands and the electron affinity of the carbonylmanganese
cations (which in turn can be readily modulated by
varying the ligand L). The organoborate anions can
thus be used to probe the electrophilicity of the car-
bonyl carbons and the overall electron affinities of the
carbonylmanganese cations.
Infrared CO stretching frequencies and CO stretching force constants
of Mn(CO)5(L)+
a
wCO (cm−1
CH3CN 2161(w), 2074(s), 2047m
)
Force constant (mdyn/A)
˚
L
17.05, 17.83
16.97, 17.67
17.37, 17.46
17.36, 17.45
py
PPh3
2154(w), 2063(s), 2042m
2142(w), 2063(sh), 2052s
PPh2Me 2144(w), 2062(sh), 2051s
a From Drew et al. in Ref. [2b].
5. Experimental
bonylmanganese cations with tetraalkylborates may re-
sult in either single electron transfer from tetraalkylbo-
rates to the carbonylmanganese cations or nucleophilic
attack of tetraalkylborates on the carbonyl ligand of
the carbonylmanganese cations. These two processes
can compete and the predominant pathway will be
dependent on the electrophilicity of the carbonyl
groups as well as the electron affinity of the carbonyl-
manganese cations [5,26]. Since acetonitrile is a poor
s-donor and relatively good p-acceptor [27], the elec-
5.1. Materials and instrumentation
Dimanganese decacarbonyl from Strem was sub-
limed, and triphenylphosphine was recrystallized from
ethanol prior to use. Diphenylmethylphosphine and
pyridine were redistilled. The carbonylmanganese
cations [Mn(CO)5(L)]+PF6− (L=CH3CN, py, PPh3
and PPh2Me) [2,5], the tetrabutylammonium salts of
tetramethylborate, methyltriphenylborate and n-
butyltrimethylborate [30,31], and the authentic samples
tron
density
on
the
carbonyl
group
in
of
CH3Mn(CO)4(PPh3) [20b], CH3Mn(CO)4(PPh2Me)
[20b], C6H5Mn(CO)4(PPh3) [20a], C6H5Mn-
CH3Mn(CO)5
[32],
C6H5Mn(CO)5
[33]
[Mn(CO)5(CH3CN)]+ is withdrawn onto the CH3CN
ligand. Thus the electrophilicity of the carbonyl ligand
is enhanced to an extent that the nucleophilic attack of
tetraalkylborates on the carbonyl ligand should pre-
dominate over electron transfer. By contrast, pyridine,
triphenylphosphine and diphenylmethylphosphine are
relatively good s-donors and relatively poor p-accep-
tors [27]. As a result, the electron density on the
pyridine or phosphine ligand is partially transferred to
the carbonyl groups, and the carbonyl ligands in car-
bonylmanganese cations [Mn(CO)5(L)]+ (L=py, PPh3
and PPh2Me) are not sufficiently electrophilic to en-
courage nucleophilic attack of borates. Indeed, the
electron density residing on the carbonyl group of these
carbonylmanganese cations is reflected in the CO
stretching frequencies [28] in Table 4, which lists the
CO stretching frequencies of [Mn(CO)5(L)]+PF6− (L=
CH3CN, py, PPh3 and PPh2Me) and their calculated
force constants derived from the spectroscopic data
[2b]. It can be seen that [Mn(CO)5(CH3CN)]+ has the
highest CO stretching frequencies and force constants,
suggesting that the carbonyl carbon atoms in
[Mn(CO)5(CH3CN)]+ are more positively charged and
thus more electrophilic. As a result, nucleophilic attack
of BR4− on the carbonyl ligand of [Mn(CO)5(CH3-
CN)]+ is preferred over the electron-transfer process
[29].
(CO)4(PPh2Me) [20a] and HMn(CO)4(PPh3) [34] were
prepared according to the previously reported methods.
Sodium tetraphenylborate from Aldrich was used as
received. Tetrahydrofuran was distilled from sodium
benzophenone and stored in a Schlenk flask under
argon. Acetonitrile was refluxed over 0.1% KMnO4 for
1 h and distilled into another flask. The purified solvent
was then redistilled serially from P2O5 and CaH2 and
stored in a Schlenk flask under argon. Chloroform-d,
anhydrous THF-d8 and benzene-d6 from Aldrich (in
small ampoules) were used as received. All the reactions
were carried out under an atmosphere of argon using
1
standard Schlenk techniques or in a drybox. H- and
13C-NMR spectra were recorded on a General Electric
QE-300 spectrometer. IR spectra were obtained with
NaCl cells (0.1 mm) using a Nicolet 10 DX FT spec-
trometer with 4 cm−1 resolution.
5.2. Reaction of [Mn(CO)5L]+PF−6 with the borates
5.2.1. Bu4N+BMe−4
Typically 5 ml of 0.01 M solution of Bu4NBMe4 in
THF was added to 5 ml of 0.01 M solution of
[Mn(CO)5L]+PF6− in THF at −78°C. The resulting
mixture was slowly warmed to 0°C over the course of 2
h. The reaction was monitored (by periodically extract-
ing a sample for IR analysis) until the starting material
was completely consumed. The products were identified
by spectral comparisons with those of the authentic
samples or those reported in the literature.
CH3Mn(CO)5 and Mn2(CO)10 were quantified by com-
parison with calibration curves constructed from the IR
4. Conclusion
The annihilation of carbonylmanganese cations
[Mn(CO)5(L)]+ by organoborate anions proceeds via
either an electron-transfer or a nucleophilic pathway,

D. Zhu, J. K. Kochi / Journal of Organometallic Chemistry 580 (1999) 295–303
301
absorbance of the unique carbonyl stretching band
versus the concentration of the solution. For L=
CH3CN, CH3Mn(CO)5 was observed as the only man-
ganese carbonyl product by the IR analysis of the
reaction mixture. The carbonyl stretching bands with
wCO=2114 (vw), 2045 (vw), 2008 (s), 1984 (m) cm−1
were in good agreement with an authentic sample of
CH3Mn(CO)5. CH3Mn(CO)5 was quantified by the
principal carbonyl band at 2008 cm−1. In a NMR scale
experiment of this reaction in THF-d8, Me3B(THF) was
observed (l=0.20 ppm [10], not quantified). In the
case of [Mn(CO)5(Py)]+PF6−, in addition to
CH3Mn(CO)5, a small amount of Mn2(CO)10 was de-
tected by TLC using a 4:1 mixture of hexane and THF
as the eluent. A yellow band on TLC had the same Rf
value as the authentic Mn2(CO)10 sample. The yield of
Mn2(CO)10 was estimated by the IR band at 2045
cm−1. For L=PPh3 and PPh3Me, the carbonyl stretch-
ing bands of the products were severely overlapped,
and quantitative IR analysis could not be performed.
Therefore, a larger scale of reaction was carried out for
L=PPh3 and PPh3Me with 0.2 mmol of
[Mn(CO)5L]+PF6− and Bu4NBMe4. After the reactions
were completed (as monitored by IR analysis), the
volatile was removed in vacuo and the residue was
chromatographed on a deactivated alumina column
(hexane as eluent) to give the following products: For
L=PPh2Me, Mn2(CO)8(PPh2Me)2 [5]: 45 mg, 61%
temperature and the resulting mixture was stirred at
this temperature for a few hours. The products were
identified and quantified as described above, and the
results are summarized in Table 2.
5.2.3. NaBPh4
Typically 5 ml of 0.01 M solution of NaBPh4 in THF
and 5 ml of 0.01 M solution of [Mn(CO)5L]+PF6− in
THF were mixed at room temperature. For L=
CH3CN and C5H5N, the resulting mixture was stirred
at this temperature for 20 h. For L=PPh3 and
PPh3Me, the resulting mixture was heated to 60°C and
kept at this temperature for a day. The products were
identified and quantified as described above, and the
results are summarized in Table 3.
5.2.4. Bu4N+B(Bun)Me−3
A
volume of 10 ml of 0.01 M solution of
Bu4N+B(Bun)Me3− in THF was added to 10 ml of 0.01
M solution of [Mn(CO)5L]+PF6− in THF at −78°C.
When the solution was warmed to −60°C, IR analysis
indicated that the starting material was completely con-
sumed. Only four carbonyl stretching bands with wCO
=
2114 (vw), 2045 (vw), 2008 (s), 1984 (m) cm−1
appeared and were in good agreement with those re-
ported for CH3Mn(CO)5 in the literature [36]. Further-
more, when the solution was kept at room temperature
for 2 days, no change in shape and intensity of the
carbonyl stretching bands was observed. Since n-
C4H9Mn(CO)5 has been reported to be very thermally
unstable [36], this suggested that the product was
CH3Mn(CO)5 and no n-C4H9Mn(CO)5 was observable
(if any n-C4H9Mn(CO)5, only very small amount of it
existed). In another run of this reaction, when IR
analysis indicated that the starting material was com-
pletely consumed at −60°C, an equivalent of PPh2Me
was added. The mixture was then warmed to 0°C. IR
analysis indicated that RMn(CO)5 was converted to
RCOMn(CO)4(PPh2Me) [20], which was isolated by
filtration through a layer of deactivated alumina and
identified as CH3COMn(CO)4(PPh2Me) [20]: 28.0 mg,
71% yield. wCO 2066 (w), 1989 (m), 1963 (s), 1952 (s)
yield, wCO
,
1981 (vw), 1952 (s) cm−1
;
HMn-
(CO)4(PPh2Me) [5,34]: 8 mg, 11% yield. wCO 2062 (m),
1
1971 (sh), 1965 (s), 1949 (m) cm−1. H-NMR (C6D6), l
−7.34 ppm (JPH=39.0 Hz); Only
a trace of
CH3Mn(CO)4(PPh2Me) [20a] was detected by TLC us-
ing a 4:1 mixture of hexane and THF as the eluent and
IR comparison with an authentic sample. In this exper-
iment, the volatile was condensed into another flask at
−196°C. After a drop of pyridine was added, the
solvent was removed at −30°C to give a liquid, which
was identified by NMR as a mixture of Me3B(NC5H5)
[35] and THF (30 mg). The ratio of Me3B(NC5H5) to
1
THF was determined to be 3:2 by H-NMR, and the
1
yield of Me3B(NC5H5) was calculated to be 60%. H-
1
cm−1. H-NMR (CDCl3), l ppm 2.13 (s, 3H, PMe),
NMR (C6D6), l ppm 0.434 (s, 9H, BMe3), 6.45 (m, 2H,
py), 6.77 (m, 1H, py), 8.32 (m, 2H, py). For L=PPh3,
Mn2(CO)8(PPh3)2 [5]: 49 mg, 58% yield. wCO, 1983 (vw),
1954 (s) cm−1; HMn(CO)4(PPh3) [5]: 10 mg, 12% yield.
2.40 (s, 3H, MeCO), 7.5 (s, 10H, Ph).
Acknowledgements
wCO, 2062 (m), 1970 (sh), 1965 (s), 1952 (m) cm−1
.
1H-NMR (C6D6), l −6.90 ppm (JPH=34.4 Hz);
CH3Mn(CO)4(PPh3) [20a]: 13 mg, 15% yield. wCO, 2055
The authors would like to thank the National Science
Foundation and the R.A. Welch Foundation for finan-
cial support.
(w), 1970 (sh), 1965 (s), 1934 (m) cm−1 1H-NMR
.
(CDCl3), l −0.50 ppm (JPH=7.6 Hz).
5.2.2. Bu4N+BMePh−3
References
Typically
5
ml of 0.01
M
solution of
Bu4N+BMePh3− in THF and 5 ml of 0.01 M solution
of [Mn(CO)5L]+PF6− in THF were mixed at room
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D. Zhu, J. K. Kochi / Journal of Organometallic Chemistry 580 (1999) 295–303
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[22] No CH3COMn(CO)4(CH3CN) was observed in the mixture of
CH3Mn(CO)5 and acetonitrile, so the reverse dissociation of
acetonitrile should be very fast.
’
[23] Alternatively, the 19-e radical [Mn(CO)5(CH3CN)] first loses
’
acetonitrile to form 17-electron radical [Mn(CO)5] , which then
couples with methyl or phenyl radical to yield CH3Mn(CO)5 or
C6H5Mn(CO)5, respectively.
[24] Furthermore, if the reaction proceeded via an electron-transfer
mechanism, n-butyltrimethylborate is expected to generate
butyltrimethylboranyl radical, which would preferentially rup-
ture the boron–butyl bond to give butyl radical and trimethylb-
orane (Note the boron–butyl bond is weaker than the
boron–methyl bond [9,10], and butyl radical is more stable than
methyl [25]). Butylmanganese pentacarbonyl C4H9Mn(CO)5
should thus be the major alkylmanganese product. However, the
reaction of n-butyltrimethylborate with [Mn(CO)5(CH3CN)]+
produces methylmanganese pentacarbonyl, and no butylman-
ganese pentacarbonyl is observed.
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methyltriphenylborate and tetraphenylborate are 0.575, 0.957
and 0.99 V vs SCE, respectively, in THF (measured by cyclic
voltammetry at a scan rate of 100 mV s−1).
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,
[Mn(CO)5(py)]+, Mn(CO)5(PPh3)]+ and [Mn(CO)5(PPh2Me)]+
in THF are −0.84, −0.81, −1.05 and −1.16 V vs SCE,
respectively, measured by cyclic voltammetry at the scan rate of
500 mV s−1. see Ref. [5].
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˚
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when the force constant k\15.3 mdyn A−1. See D.J. Darens-
˚
bourg, M.Y. Darensbourg, Inorg. Chem. 9 (1970) 1691. (b) A

D. Zhu, J. K. Kochi / Journal of Organometallic Chemistry 580 (1999) 295–303
303
similar mechanistic dichotomy has been presented in the ion-pair
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