Reactions of H2, silanes, and olefins with superelectrophilic cationic rhenium complexes: Heterolytic cleavage of H2 and relation to the structure and function of hydrogenases

Article abstract of DOI:10.1016/S0020-1693(99)00243-1

The reaction of cis-Re(CO)₄(PR₃)Me (R = Ph, Cy) with the Lewis acid B(C₆F₅)₃ was studied by NMR spectroscopy, and was found to provide an equilibrium mixture of the solvent-coordinated complex [cis-Re(CO)₄(PR₃)(ClCH₂Cl)][MeB(C ₆F₅)₃] and the reactants. Reaction of [cis-Re(CO)₄(PR₃)Me with HX (X = H, SiEt₃) in the presence of B(C₆F₅)₃ at low temperature yielded the σ-bonded complexes [cis-Re(CO)₄(PR₃)(η²-HX)][MeB(C ₆F₅)₃] which decomposed at room temperature via intramolecular heterolytic cleavage of the X-H bond to produce MeX and the hydride-bridged dimer [cis-Re(CO)₄(PR₃)]₂(μ-H)]{MeB(C ₆F₅)₃}. The unstable H₂ binding and cleavage on this and other highly electrophilic organometallic complexes that contain strong π-acceptor CO ligands can be related to the structure and function of metalloenzymes such as Fe-containing hydrogenases that catalyze H₂?2H⁺ + 2e^-. The latter contain dinuclear organometallic-like active sites with CO ligands, which would promote binding and heterolytic cleavage of molecular H₂ in biological systems. The Fe-Fe and Ni-Fe bonds in hydrogenases are likely sites for protonation to form a bridging hydride as the initial step in the mechanism of H₂ formation, and electrophilic fragments such as [Re(CO)₄(L)]⁺ strongly prefer to form bridging rather than terminal hydrides. The reaction of olefins and Et₃SiH with the [cis-Re(CO)₄(PR₃)(ClCH₂Cl)][BAr_F] (BAr_F = B[3,5-(CF₃)₂(C₆H₃)₄ ^-]) system was also investigated, and the X-ray crystal structure of the dimer {[cis-Re(CO)₄(PPh₃)]₂(μ-H)}{BAr_F} was determined.

Full text of DOI:10.1016/S0020-1693(99)00243-1

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Inorganica Chimica Acta 294 (1999) 240–254
Reactions of H2, silanes, and olefins with superelectrophilic
cationic rhenium complexes: heterolytic cleavage of H2 and
relation to the structure and function of hydrogenases
Jean Huhmann-Vincent, Brian L. Scott, Gregory J. Kubas *
Chemical Science and Technology Di6ision, MS J514, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Received 14 January 1999; accepted 23 June 1999
Abstract
The reaction of cis-Re(CO)4(PR3)Me (R=Ph, Cy) with the Lewis acid B(C6F5)3 was studied by NMR spectroscopy, and was
found to provide an equilibrium mixture of the solvent-coordinated complex [cis-Re(CO)4(PR3)(ClCH2Cl)][MeB(C6F5)3] and the
reactants. Reaction of cis-Re(CO)4(PR3)Me with HX (X=H, SiEt3) in the presence of B(C6F5)3 at low temperature yielded the
s-bonded complexes [cis-Re(CO)4(PR3)(h2-HX)][MeB(C6F5)3] which decomposed at room temperature via intramolecular het-
erolytic cleavage of the X–H bond to produce MeX and the hydride-bridged dimer [cis-Re(CO)4(PR3)]2(m-H){MeB(C6F5)3}. The
unstable H2 binding and cleavage on this and other highly electrophilic organometallic complexes that contain strong p-acceptor
CO ligands can be related to the structure and function of metalloenzymes such as Fe-containing hydrogenases that catalyze
H2l2H++2e−. The latter contain dinuclear organometallic-like active sites with CO ligands, which would promote binding and
heterolytic cleavage of molecular H2 in biological systems. The Fe–Fe and Ni–Fe bonds in hydrogenases are likely sites for
protonation to form a bridging hydride as the initial step in the mechanism of H2 formation, and electrophilic fragments such as
[Re(CO)4(L)]+ strongly prefer to form bridging rather than terminal hydrides. The reaction of olefins and Et3SiH with the
[cis-Re(CO)4(PR3)(ClCH2Cl)][BArF] (BArF=B[3,5-(CF3)2(C6H3)4−]) system was also investigated, and the X-ray crystal structure
of the dimer {[cis-Re(CO)4(PPh3)]2(m-H)}{BArF} was determined. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Rhenium complexes; Silane complexes; Dihydrogen complexes; Olefin complexes
1. Introduction
tem contains an electron-rich metal in which the pre-
dominant interaction is Md“HXs* backdonation, ho-
molytic cleavage of the H–X bond can occur via
oxidative addition, as in MoH2(CO)(depe)2 versus less
electron-rich Mo(h2-H2)(CO)(dppe)2 [1k] and in
[ReH2(CO)(PMe3)4][BF4] [4] (Eq. (1)).
We are interested in the reactivity of unsaturated
transition metal complexes LnM towards HX s-bonds,
where X=H, Si and C. The interaction between a
metal complex and a s-bond is governed by both the
extent of p-backdonation from metal d-orbitals to the
s* antibonding HX orbital (Md“HXs*) and the dona-
tion from the HX s-bonding orbital to an empty metal
orbital of appropriate orientation (HXs“M) [1]. Both
interactions weaken the HX bond. If there is a balance
between s-donation and p-backdonation, a stable s-
complex should form, as in W(CO)3(PiPr3)2(h2-H2) [2]
and Mo(CO)(diphosphine)2(h2-HSiPhH2) [3]. If the sys-
(1)
(2)
However, if the system contains an electron-deficient
metal which is a poor p-donor (usually cationic) and
the predominate interaction is HXs“ \M donation,
* Corresponding author. Tel.: +1-505-667 5767; fax: +1-505-667
3314.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00243-1

J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
241
2. Results and discussion
activation of the H–X bond can occur via heterolytic
cleavage (Eq. (2)) because of greatly increased acidity
of the HX ligand [1b–d]. For example, [mer-
Re(CO)3(PPh3)2(h2-H2)]+ is deprotonated by strong
bases such as 1,8-bis(dimethylamino)naphthalene to
form the corresponding neutral [Re]H. As we reported
in a preliminary communication, the analogue with
CO replacing a phosphine, [cis-Re(CO)4(PR3)(h2-H2)]-
[BArF], contains an even more electrophilic Re+
center that lowers the pKa of H2 in the s-complex
considerably [1k]. The H2 ligand is deprotonated by
2.1. Reaction of cis-Re(CO)4(PR3)Me (1) with
B(C6F5)3
We have recently demonstrated that the solvento
complexes [cis-Re(CO)4(PR3)(S)][BArF] (BArF=[B(3,5-
(CF3)2C6H3)4]−; R=Ph, Pr, Cy; S=OEt2, ClCH2Cl,
i
and NC5F5) are prepared from cis-Re(CO)4(PR3)Me (1)
by either methyl abstraction with [Ph3C][BArF] or pro-
tonation with [H(OEt2)2][BArF] in the appropriate sol-
vent [8]. These complexes were characterized
structurally, and a brief study of the reaction of [cis-
Re(CO)4(PR3)(ClCH2Cl)][BArF] with H2 was communi-
cated [1k]. An important feature in this highly elec-
trophilic system is the use of the non-coordinating
anion BArF−, which allows for facile substitution of the
coordinated solvent molecule with other substrates. The
anion-coordinated complexes [cis-Re(CO)4(PPh3)(A)]
have previously been reported by Hope and coworkers
(A=OTeF5−) [9] and Beck and Schweiger (A=
FBF3−) [10].
i
weak bases such as Pr2O, indicating a pKa of 1 to
−2. The Re complex along with Morris and cowork-
ers [5a] very acidic [Ru(h2-H2)(CNH)(dppe)2]2+ and
Bianchini et al. [5b] unstable [IrH2(H2)(triphos)]+ sys-
tems are rare examples of organometallic complexes
that heterolytically activate H2 gas [5]. In these
systems the pKa of H2 is lowered from 37 down to as
low as −5, an increase in acidity of 42 orders of mag-
nitude.
An HX bond can be functionalized through either
route shown in Eqs. (1) and (2). Although there are
many examples of the oxidative addition of H2 and
silanes in electron-rich systems (Eq. (1)) [1,3], well-
defined examples of heterolytic s-bond activation in
electron-poor systems (Eq. (2)) seem generally limited
to reactions of H2, and usually require the presence of
a relatively good base. In this paper, we describe the
reactions of HX (X=H, SiEt3) as well as olefins with
the highly electrophilic cationic system [cis-Re(CO)4-
(PR3)]+ (R=Ph, Cy). The latter contains mainly p-ac-
ceptor CO ligands which enhance the HXs“M s-do-
nation at the expense of the Md“HXs* backdonation,
and thus heterolytic activation pathways are favored
over oxidative addition for both H2 and silane reac-
tions. The presence of CO ligands also appears to be a
critical feature of the active site of hydrogenases [6],
particularly in the recently determined structures of
Fe-only metalloenzymes, Clostridium pasteurianum [6a]
and Desulfo6ibrio desulfuricans [6b] which contain din-
uclear Fe active sites with CO ligands cis and/or trans
to the putative H2 binding site. Until the hydroge-
nases, CO ligands had never been found as intrinsic
constituents of a prosthetic group in biology. Appar-
ently their function is to increase the electrophilicity of
the Fe site, thereby increasing the lability and acidity
of H2, which promotes heterolytic cleavage and trans-
fer of protons to proximal cysteinyl sulfur ligands. It
is also significant that the active site in the crystal
structures is occupied by a labile H2O ligand that has
been shown to be competitively displaced by H2 in
organometallic systems [7]. Organometallic chemistry
thus perfectly models the enzymatic process that inter-
converts protons and H2, as will be discussed in this
paper.
In the current study, cis-Re(CO)4(PR3)Me (R=Ph,
1a; Cy, 1b) was reacted with the Lewis acid borane
B(C6F5)3 in CD2Cl2 solution and the products were
studied by NMR spectroscopy. Here, the generated
ionic complex [cis-Re(CO)4(PR3)(ClCD2Cl)][MeB-
(C6F5)3] (R=Ph, 2a; Cy, 2b) remained in equilibrium
with the starting materials, as shown in Eq. (3).
(3)
For the case where PR3 is PPh3, two broad reso-
nances at −0.56 and 0.48 ppm were observed in the
room temperature 1H NMR spectrum and were as-
signed to the ReMe of 1a and MeB(C6F5)3 of 2a,
respectively. As the solution was cooled, the intensity of
the BMe peak grew and sharpened as the intensity of
the ReMe resonance broadened and decreased, as
shown in Fig. 1.
Fig. 1. 1H NMR (CD2Cl2) of cis-Re(CO)4(PPh3) (1a)+B(C6F5)3,
methyl region.

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J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
The thermodynamic parameters for this equilibrium
were calculated from the integrated ratio of ReMe and
BMe resonances in the 1H NMR spectra as: DH=
−7.190.3 kcal mol−1
,
DS= −24.491.2 e.u.,
DG298=0.290.3 kcal mol−1. The fact that 1 remained
in equilibrium with 2 at room temperature was unex-
pected and indicates that the Lewis acidity of the
[cis-Re(CO)4(PR3)]+ fragment is comparable to that of
B(C6F5)3.
Fig. 2. 13C{1H} NMR (CH2Cl2, −70°C) of cis-Re(CO)4(PPh3)
(1a)+B(C6F5)3, (a) MeB(C6F5)3, (b) free CH2Cl2, (c) Re–ClCH2Cl.
(d) Re–ClCH2Cl, proton coupled, JHC=184.3 Hz.
For the analogous reaction of B(C6F5)3 with the
PCy3 derivative 1b, one very broad resonance was
observed in the H NMR spectrum at room tempera-
(M=Mn [14], Re [1m]). An equilibrium between 1 and
1
either an agostic or zwitterionic complex could lead to
ture at 0.22 ppm, which sharpened and shifted
downfield to 0.42 ppm at low temperature. For com-
parison, the free ReMe complexes 1a and 1b exhibit
doublets at −0.50 ppm (JHP=7.5 Hz) and −0.49
1
broad methyl peaks in the H NMR spectra such as
those shown in Fig. 1.
In order to rule out these possibilities in our Re
system and establish that the product 2 is indeed a
bound dichloromethane complex, low temperature 13C
NMR experiments were conducted. When a CH2Cl2
solution of 1a and B(C6F5)3 was cooled to −70°C and
the 13C{1H} measured, one sharp peak due to bound
dichloromethane in Re(ClCH2Cl)+ (2a) was observed
at 67.0 ppm (Fig. 2). There was also a resonance at 9.4
ppm assigned to Me(B(C6F5)3 which was very broad
due to CB coupling. When the proton decoupler was
turned off, the peak at 67.0 ppm split into a triplet,
with JCH=184.6 Hz, supporting the assignment of this
peak as bound CH2Cl2 (free CH2Cl2: l 54.00 ppm,
1
ppm (JHP=6.4 Hz), respectively in the H NMR spec-
trum in CD2Cl2 at room temperature [8,11]. The room
temperature 31P{1H} NMR spectrum of the equimolar
mixture of 1a and B(C6F5)3 in CD2Cl2 solution showed
broad, overlapping resonances at 11.1 and 10.4 ppm,
and cooling provided one sharp 31P signal at 11.1 ppm.
These chemical shifts are similar to the values we
reported previously for [cis-Re(CO)4(PR3)(ClCD2Cl)]-
[BArF] (11.1 ppm) and unreacted 1a (10.1 ppm) [8]. In
the corresponding room temperature 31P{1H} spectrum
for the PCy3 reaction, one broad resonance was ob-
served at 25.2 ppm, which sharpened and shifted to
25.4 ppm at low temperature. We have observed previ-
ously the 31P resonance for [cis-Re(CO)4(PCy3)-
(ClCD2Cl)][BArF] at 25.5 and 13.1 ppm for 1b [8].
The [cis-Re(CO)4(PR3)(ClCH2Cl)][MeB(C6F5)3] com-
plexes 2a and 2b were not isolated due to the formation
of oils which could not be purified by crystallization
due to the presence of small amounts of impurities
(B5%).
J
CH=179.3 Hz). When the reaction mixture was
warmed to room temperature, the peak assigned to
bound dichloromethane disappeared due to fast
exchange with free CH2Cl2. Similarly, generation of 2b
from 1b and B(C6F5)3 in CH2Cl2 provided a signal
for bound dichloromethane at 67.8 ppm in the 13C{1H}
spectrum (−70°C), which split into a triplet (JCH
=
186.5 ppm) in the proton-coupled experiment. These
results are analogous to those we observed previously
There are reaction pathways other than the equi-
librium between 1 and 2 shown in Eq. (3) which could
account for these observations in the NMR spectra. A
borane Lewis acid can also react with a metal alkyl to
form an ion-pair (zwitterionic) complex with M–Me–B
interactions. This is the case in the solid state for Marks
−
for the related BArF complexes [8]. These obser-
vations in the low temperature 13C spectra are also
very similar to those observed by Gladysz and cowork-
ers [15] with [Cp%Re(NO)(PPh3)(ClCH2Cl)][BF4] in
CH2Cl2 at −85°C (78.3 ppm, JCH=185.5 Hz, JCP
=
3.8 Hz).
There have recently been a number of examples of
isolated complexes with coordinated dichloromethane,
most of which are derived from extremely electron-defi-
cient cationic metal centers with low-interacting anions
[16]. In the complexes characterized by X-ray crystal-
lography, dichloromethane has been found to coordi-
nate bidentate through the chloride atoms in
Ag2(CH2Cl2)4Pd(OTeF5) [17] and [RuH(CO)(CH2Cl2)-
(PtBu2Me)2][BArF] [18] and monodentate through one
of the chloride atoms in [cis-Re(CO)4(PR3)][BArF]
(R=Ph, iPr) [8], [PtAg(CH2Cl2)(C6F5)2(acac)]2 [19],
[Cp*Ir(Me)(PMe3)(ClCH2Cl)][BArF] [20] and [trans-
and coworkers’ Cp2%ZrMe+MeB(C6F5)3
[12] and
−
Schrock and coworkers [NON]ZrMe+MeB(C6F5)3
[13]. Furthermore, the formation of an agostic complex
is possible in which the vacant site on Re+ center is
stabilized by interaction with a C–H bond of a phos-
phine ligand. This type of interaction is very common
and has been observed by X-ray crystallography in the
bis-phosphine derivatives [mer-M(CO)3(PR3)2][BArF]
−

J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
243
PtH(PiPr3)2(ClCH2Cl)][BArF] [21]. The coordinated
dichloromethane in [CpMo(CO)3(ClCH2Cl)][PF6] [22]
and [Cp%Re(NO)(PPh3)(ClCH2Cl)][BF4] [15] has been
assigned on the basis of IR spectral data and low
temperature 13C NMR spectral data, respectively.
2.2. Reaction of [cis-Re(CO)4(PR3)(ClCH2Cl)]-
[MeB(C6FF)3] (2) with H2, HD and D2
Despite the lability of the dichloromethane ligand of
2 in CD2Cl2 solution, only weak binding to H2 was
observed due to fast exchange with free CD2Cl2. A
CD2Cl2 solution of 1 and B(C6F5)3 was frozen in a
J-Young NMR tube at liq. N2 temperature, placed
under approximately 3 atm of H2, and inserted into a
pre-cooled NMR spectrometer probe (−70°C). At low
temperatures (−70 to −40°C), the s-complex [cis-
Re(CO)4(PR3)(h2-H2)][MeB(C6F5)3] (R=Ph, 3a; Cy,
3b) was observed in low concentrations (B5%), iden-
tified by the broad resonance in the 1H NMR spectra at
−4.2 and −4.6 ppm for 3a and 3b, respectively. The
1H and 31P{1H} NMR spectra were comparable to
those we could observe at room temperature for the
Scheme 1. Heterolytic cleavage of H2.
Interestingly, when CD2Cl2 solutions of [cis-
were warmed
Re(CO)4(PR3)(h2-H2)][MeB(C6F5)3]
above −40°C, the formation of CH4 was observed in
1
the H NMR spectra along with an upfield triplet at
−15.6 ppm (R=Ph, JHP=10.0 Hz) or −17.6 ppm
(R=Cy, JHP=7.8 Hz) that was assigned to the
hydride-bridged dimer {[cis-Re(CO)4(PR3)]2(m-H)}-
{MeB(C6F5)3} (R=Ph, 4a; Cy, 4b). At room tempera-
ture the only products observed in the 1H NMR spectra
were the hydride-bridged dimer and methane. There
was only one resonance in the 31P NMR spectra (1.9
ppm, 4a; 13.7 ppm, 4b), indicating complete conversion
to the dimer at room temperature. The reaction of 1
with B(C6F5)3 and H2 was also carried out in C6D5F,
and warming above −20°C also proceeded with the
generation of CH4 and the hydride-bridged dimer 4.
One proposed reaction pathway for an intramolecular
heterolytic cleavage of H2 which involves the borane
anion is shown in Scheme 1 for the CD2Cl2 solution
case.
Direct protonation of the anion MeB(C6F5)3 by the
acidic H2 in 3 would form methane, B(C6F5)3, and the
rhenium hydride cis-Re(CO)4(PR3)H (R=Ph, 5a; Cy,
5b). Compound 5 is not observed in the 1H NMR
spectra, but presumably quickly reacts with unreacted 2
(or 3) to form the hydride-bridged dimer 4, which
appears to be a ‘thermodynamic sink’ in these systems.
Such protonation of a borane anion has precedence as
shown in Eq. (4) where the H2 complex is also unstable
and is generated from H2 gas [5b].
−
more stable analogue with the BArF anion in the less
coordinating solvent C6D5F [1k].
When the analogous reactions are performed with
HD, the upfield resonances at −4.2 and −4.6 ppm in
1
the H NMR spectra split into triplets with JHD=33.9
and 33.6 Hz, respectively, confirming the presence of a
s-complex. The high JHD observed for these complexes
is consistent with a highly electrophilic cationic M(h2-
H2) system [23] and suggests a short H–H bonding
,
distance (0.85–0.87 A) based on Morris and coworkers
and Luther and Heinekey’s correlations [24]. This is
consistent with a bonding picture in which the H2“M
s-bonding interaction is enhanced at the expense of
M“H2 backbonding [23a,b,e]. It is also instructive to
note that the electronic nature of the ligand(s) cis to H2
(PCy3 versus PPh3 in 3) usually has very little effect on
JHD and the H–H distance, especially when H2 is trans
to CO, as chronicled previously [23e]. Fig. 3 shows the
1
Re(h2-HD) region of the H NMR spectrum for the
−
BArF analogue of 3a. No HP coupling is observed
and the broad hump under the triplet is due to the
presence of a small amount of Re(h2-H2).
[IrH2(H2)(triphos)]+[BPh4]−
“IrH3(triphos)+BPh3+benzene
(4)
Another example of anion protonation by an H2 ligand
occurs in [Ru(H2)(CNH)(dppe)2][OTf]2 which elimi-
nates triflic acid, a remarkable example of heterolytic
activation of H2 gas to form a strong acid [5a]. In this
case the acidity of H2 is increased approximately 42
Fig. 3. 1H NMR (C6D5F, −40°C) [cis-Re(CO)4(PPh3)(h2-
HD)][BArF], hydride region, JHD=33.9 Hz.

244
J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
orders of magnitude on binding to Ru (pKa of free H2
is 37 and −5 when bound). Another scenario in Scheme
1 is intermolecular heterolytic cleavage of H2, e.g. proto-
nation of the Me group in equilibrium quantities of 1
by the acidic H2 in 3 to give CH4, 2, and 5.
Although not able to distinguish between the two mech-
anisms, a reaction sequence carried out using D2 instead
such as methanogenic, acetogenic, nitrogen-fixing, pho-
tosynthetic, and sulfate-reducing bacteria. Several
types of hydrogenases have been identified, where the
apparent active site contains either nickel in combina-
tion with iron [Ni–Fe], iron-only (e.g. Clostridium
pasteurianum), or remarkably, no transition metals
at all. The latter, found in methanogenic archaea,
Methanobacterium Thermoautotrophicum, appears to be
a purely organic hydrogenation catalyst, promoting the
reduction of a pterin compound by H2 and also produc-
ing a proton, as a step in methane formation from CO2
and H2 [30]. One proposed mechanism is analogous to
that of Olah et al. [31] for the reversible formation of
carbocations and H2 from alkanes in superacid media,
e.g. Eq. (6):
1
of H2 generates CH3D as observed in the H NMR
spectrum. The room temperature 31P spectra for the H2
and D2 reactions are virtually identical, indicating the
formation of the deuteride-bridged dimer. Finally, no
scrambling of protons into the Re–D–Re dimer was
observed.
This facile heterolytic activation of H2 is indicative of
a very electrophilic Re+ center which lowers the pKa
of H2 in the s-complex considerably. In the analogous
−
BArF
system, the s-complex [cis-Re(CO)4(PR3)-
(h2-H2)][BArF] was stable at room temperature, but
(6)
addition of the weak base iPr2O resulted in the formation
This would indicate that a highly electrophilic site is
critical to H2 activation, which is supported by ab initio
studies [32]. In this context, R3C+ and 16e metal centers
such as [Re(CO)4(PR3)]+ are strongly electrophilic
isolobal fragments. The recent exciting discovery of the
unprecedented biological presence of CO ligands in
metal-containing hydrogenases [6,33] would appear to
relate to the function of p-acceptors in increasing the
electrophilicity of the active site, thereby enhancing
intramolecular heterolysis of H2 as in carbonyl-rich
[Re(CO)4(PR3)(h2-H2)][MeB(C6F5)3] in Scheme 1. Both
experimental [34] and theoretical [35,36] models have
been proposed for the active sites of [NiFe] hydrogenases
based on [Fe(CO)(CN)2] cores since Fe-bound cyanide is
also found in hydrogenases and is difficult to distinguish
from CO crystallographically. It should be emphasized
however that the negatively-charged cyanide ligand is
not a very good p-acceptor but a strong s-donor more
like chloride or thiolate [1n]. Thus, it is undoubtedly the
CO ligand(s) that is crucial in controlling H2 binding and
activation in hydrogenases. Importantly, electrophilic
metal sites with such electron-withdrawing ligands also
can greatly favor H2 binding over N2 binding, particu-
larly when coordinated trans to CO [14].
of [H(OiPr2)2][BArF] and 4. The pKa of Pr2O has not
i
been reported, but the pKa values of Et2OH+ and
Me2OH+ in aqueous sulfuric acid are −2.4 and −2.5,
respectively [25], therefore the pKa of [cis-
Re(CO)4(PR3)(h2-H2)][A] can be estimated as approxi-
mately 1 to −2 [1k].
This type of activation of H2 by an electrophilic
transition metal complex has been observed in the case
of [Cp*Ru(CO)2(h2-H2)][BF4], which protonates Et2O to
form [H(OEt2)][BF4] and the hydride-bridged dimer
{[Cp*Ru(CO)2]2(m-H)}{BF4} [26]. The pKa values of
M(h2-H2) s-complexes have been measured over a range
of approximately −3 to +17 for monocationic com-
plexes, and as low as −6 for dicationic Ru, Os, and Fe
complexes [1c,j,23b,d,24].
2.3. Comparison of heterolytic splitting of H2 on
electrophilic organometallic and hydrogenase sites
As mentioned in Section 1, there is a close relationship
between the heterolytic activation of H2 [27] in CO-con-
taining
organometallic
complexes
such
as
[Re(CO)4(PR3)(h2-H2)]+ and biological enzymes such as
hydrogenases and possibly nitrogenases as well. As
originally noted by Crabtree [28], several properties of
H2 ligands such as their acidity and ability to compete
with N2 ligands clearly is relevant to the structure and
function of these enzymes. Hydrogenases are redox
enzymes where the H2 molecule is split heterolytically,
thus catalyzing the interconversion of H2 and protons to
either utilize H2 as an energy source or dispose excess
electrons as H2 [29].
The recent crystal structures of C. pasteurianum (1.8
,
,
A resolution) and Desulfo6ibrio desulfuricans (1.6 A) are
quite revealing as to the remarkable similarity between
dihydrogen activation on organometallic centers and
biological systems such as hydrogenases. In the former
enzyme, five CO and/or CN ligands were identified to be
bound to a dinuclear iron center, including one in a
bridging position along with two bridging SR ligands
[6a]. An electron-transfer Fe4S4 ‘cubane’ cluster was
attached to Fe(1) via a cysteine thiol bridge as shown
below in 6, which represents one possible structure of the
active site with one CN and 4CO (alternatively replace-
ment of a CO on Fe(2) with CN would give a neutral Fe
center).
H2l2H++2e−
(5)
These enzymes are incredibly efficient (109 turnovers/
h) and crucial to hydrogen metabolism essential to many
microorganisms of immense biotechnological interest

J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
245
Cationic and dicationic Fe–H2 complexes with CN and
CO ligands are known, e.g. [Fe(H2)(CN)(depe)2]+ and
[Fe(H2)(CO)(depe)2]2+, and the CN ligand can be pro-
tonated to CNH [23b,f]. It is reasonably clear that the
bridging diatomic carbon ligand is CO and not CN,
which does not bridge through carbon only (m-CNH
may be possible). This is crucial because it places CO
trans to the aquo ligand located crystallographically on
Fe(2), a rare configuration even in organometallic
chemistry but well exemplified by W(CO)3(PR3)2(H2O)
[7a]. As we have shown, H2 can displace H2O on this
tungsten center, and H2 binding is actually favored at
ambient temperature by 1–2 kcal mol−1 in terms of
DG, indicating that in C. pasteurianum the probable site
for H2 binding/elimination is trans to the m-CO, as also
proposed by Peters et al. [6a]. Importantly, CO labilizes
the ligand trans to it, especially H2 [23e], which is
critical for reversible binding and elimination of H2
from the active site. The more electrophilic cationic Re
fragment [Re(CO)4(PR3)]+ also coordinates H2O trans
to CO, although the aquo ligand is less labile than in
the neutral W complex and is more strongly bound
than ethers or CH2Cl2 [8]. Strong p-acceptors also
greatly increase the acidity of metal-bound H2, particu-
larly when trans but also when cis to H2. Thus, the
[Re–H2]+ complex 3 is much more easily deprotonated
than the W–H2 species, which requires a strong base
[37]. In the hydrogenases, it is likely that the Fe active
site is intermediate in electrophilicity but closer to that
of 3. The active site of D. desulfuricans is similar to that
of C. pasteurianum, but a bridging CO ligand was not
located in the crystal structure. An aquo ligand (or
other monatomic oxygen species such as OH) on Fe(2)
was present, although it is not clear if it is bridging or
terminal, and Fe(2) was proposed to be coordinatively
unsaturated [6b]. A 1,3-propanedithiol ligand was iden-
tified to bridge the irons, similar to the bridging thiols
in 6 (where R could be –CH2CH2CH2–). Assuming the
crystallography is accurate, it is possible that different
enzymes have evolved with stereochemically distinct
active sites. Also, reorientation of a terminal CO to a
bridging position and other ligand rearrangements are
very facile in organometallic systems (equilibrium pro-
cess in many cases), so in the actual mechanistic steps
H2 ligand(s) could be positioned trans to a variety of
Scheme 2.
ligands. The number of structural and mechanistic pos-
sibilities on this remarkably versatile dinuclear site is
immense, especially when other variables such as Fe–
Fe bonding are taken into account. Because the trans
effect is so important in H2 activation, the active site
has a tremendous range of flexibility for either consum-
ing or releasing H2 (Eq. (5) is reversible in most hydro-
genases). The acidity of H2 can be adjusted easily to
suit physiological conditions and the relatively low
redox potentials typical of these active sites.
Scheme 2 presents one (of many) reasonable mecha-
nism for reversible H2 consumption/production on C.
pasteurianum, the basic principles of which could be
applied to other enzymes.
The bridging dithiolate is not shown throughout for
clarity. Although the dinuclear center could be cationic
as shown in 6 for ease of heterolysis of H2 in direct
comparison with organometallic systems such as 3
(Scheme 1), Scheme 2 assumes perhaps more reason-
ably that one CN is present on each Fe, i.e. a neutral
Fe(II) center. It is important to note that such a
variation does not change the oxidation state of the
irons, which is generally believed to be Fe(II) in all
states of the hydrogenases, and a low spin d6 Fe(II)
octahedral configuration is well-known to favor H2
binding [1]. Thus, the function of the unusual CO and
CN ligands must be to maintain this spin state and an
electron-poor metal center.
An Fe–Fe bond is present in both enzymes based on
crystallographic Fe–Fe distances of approximately 2.6
A that are typical for a dithio-bridged Fe–Fe system
[38], as exemplified by the CpFe–CN complex we re-
ported many years ago [38a].
,

246
J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
presumably more likely site for protonation in the
enzyme active sites would be the electrons in the Fe–Fe
and Ni–Fe bonds. The proximal cysteine(299) could
transfer its proton to the M–M bond for example.
Metal–metal bonds can readily be reversibly proto-
nated to form hydride-bridged species [43], and elec-
trophilic systems show a tendency to form m-H
structures as exemplified above by {[Re(CO)4(PR3)]2
(m-H)}+ (4). The Fe–Fe bond in [CpFe(CO)(PR3)(m-
CO)]2 is as basic as weak amines (pKb around 6) and
concomitant shift of bridging CO to terminal positions
occurs (Eq. (8)) [44]. Protonation of the Fe–Fe bond in
[Fe(CO)2(PR3)(m-SR%)]2 takes place in preference to
protonation of the sulfur ligands (Eq. (9)) [45].
The structures in Scheme 2 would give the proper 34e
count for an octahedral Fe–Fe bonded system (assum-
ing a dative 2e interaction between Scubane and Fe(1)).
Transfer of a proton from the cationic Fe–H2 species
could readily occur as in Scheme 1, except here the
proton jumps to a deprotonated cysteine(299) sulfur
,
site located conveniently in close proximity (3 A) to the
H2 binding site (structure 6). Both Crabtree and Morris
and coworkers [39] have demonstrated that such in-
tramolecular heterolytic cleavage occurs readily, as ex-
emplified by Eq. (7).
(8)
(7)
(9)
Although the crystal structure of D. desulfuricans did
not show an adjacent cysteine, a lysine residue is near
to Fe(2). The next steps involve movement of protons
away from the active site to the protein and syn-
chronous or asynchronous electron transfer to the
cubane cluster and then away from the site via other
Fe–S clusters. Thus, the electrons originally present on
H2 could flow through the Fe–Fe bond and, depending
on whether a one- or two-electron transfer process
takes place, one-electron Fe···Fe bonds [38b] may be
present in the intermediates (shown in Scheme 2 for
one-electron transfer steps). These bonds are character-
The basicities of M–M bonds such as in
[CpRu(CO)2]2 were shown to be substantially higher
than that of the metal sites in related 18e mononuclear
complexes and are highly sensitive to the nature of the
ancillary ligands [43]. Theoretical studies of [NiFe] hy-
drogenase mechanisms indicate that hydride-bridged
intermediates are energetically favorable [35,36]. EN-
DOR studies indicate that two types of exchangeable
hydrogen nuclei are present in the vicinity of the Ni
ligands in the Ni–C active form of the [NiFe] enzyme
[46], consistent with the proposal of a m-H ligand [41c].
The bridging hydride could easily revert to a terminal
hydride on electron transfer, displacing the aquo ligand,
as shown in the reverse sequence of Scheme 2. The
terminal hydride could then be protonated to a labile
H2 ligand that is readily displaceable by water, and the
cycle would continue. Formation of a bridging H2
ligand is conceivable although a m-H2 ligand has not
been confirmed structurally.
,
ized by Fe–Fe distances of 2.9–3.1 A, as exemplified by
[Cp2Fe2(m-S2)(m-SEt)2]+ [38,40]. The high flexibility of
,
the Fe–Fe separation (2.6–3.2 A) and the directly-re-
lated degree of Fe–Fe interaction (zero-, one-, or two-
electron bonds) would be expected to facilitate the
electron/proton transfer processes. This may apply to
the [NiFe] hydrogenases as well, which also have bridg-
,
ing SR groups and Ni–Fe separations of 2.55 A in the
,
active site of Desulfo6ibrio 6ulgaris and 2.9 A in the
inactive, oxidized form of Desulfo6ibrio gigas [41].
The reverse reaction, formation of hydrogen gas
from H+ and electrons, is interesting from the stand-
point of the mechanism of initial protonation of the Fe
center to form a metal hydride. Protonation of a 16e
metal center is extremely rare and paradoxical because
it represents reaction of two Lewis acids, the metallo-
Lewis acid and H+. However 16e W(CO)3(PR3)2
(which contains an agostic C–H interaction but effec-
tively has a vacant d-orbital) was found to be proto-
nated by strong acids such as HBF4·Et2O to give
WH(BF4)(CO)3(PR3)2 [42]. Another more basic and
2.4. Preparation and X-ray structure of
{[cis-Re(CO)4(PR3)]2(v-H)}{BArF} (4)
In order to verify the identity of the thermodynami-
cally favored hydride-bridged dimer 4 as the product
from the heterolytic activation of H2 as shown in
Scheme 1, authentic samples of {[cis-Re(CO)4(PR3)]2
(m-H)}{BArF} (R=Ph, 4a%; Cy, 4b%) were prepared.
One equiv. of [H(OEt2)2][BArF] was added to an Et2O
solution of 1a, resulting in the evolution of CH4 and the
formation of the ether complex [cis-Re(CO)4(PPh3)-
(OEt2)][BArF]. Subsequent addition of one equiv. of 5a

J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
247
Table 1
Table 2
Crystal data and structure refinement for 4a% a
Selected bond lengths (A) and angles (°) for 4a%
,
Chemical formula
Formula weight
Temperature (°C)
C76H43BF24O8P2
1985.25
−75
0.71073
P21/c
17.752(3)
22.295(2)
19.268(4)
103.05(2)
7429(2)
4
Bond lengths
Re(1)ꢀRe(2)
Re(1)ꢀH
Re(1)ꢀP(1)
Re(1)ꢀC(1)
Re(1)ꢀC(2)
Re(1)ꢀC(3)
Re(1)ꢀC(4)
3.330
Re(2)ꢀH
1.90(10)
2.500(2)
2.009(11)
1.949(11)
2.010(12)
1.936(10)
1.92(10)
2.504(2)
1.994(10)
1.941(10)
1.994(11)
1.947(11)
Re(2)ꢀP(2)
Re(2)ꢀC(5)
Re(2)ꢀC(6)
Re(2)ꢀC(7)
Re(2)ꢀC(8)
,
Wavelength (A)
Space group
,
a (A)
,
b (A)
,
c (A)
i (°)
Bond angles
3
,
V (A )
Z
Re(1)ꢀHꢀRe(2)
P(1)ꢀRe(1)ꢀH
P(1)ꢀRe(1)ꢀC(4)
C(2)ꢀRe(1)ꢀH
C(1)ꢀRe(1)ꢀC(3)
124(2)
84(3)
172.3(3)
169(3)
174.0(4)
P(2)ꢀRe(2)ꢀH
P(2)ꢀRe(2)ꢀC(6)
C(8)ꢀRe(2)ꢀH
C(5)ꢀRe(2)ꢀC(7)
89(3)
169.0(3)
167(3)
Dcalc (g cm−1
)
1.775
0.3415
R1=0.0539,
wR2=0.0986
v (cm−1
)
174.7(4)
Final R indices
a R1=| ꢀꢀFoꢀ−ꢀFcꢀꢀ/| ꢀFoꢀ and wR2=[S[w(Fo2−Fc2)2]/S[w(Fo2)2]]1/2
.
The parameter w=1/[|2(Fo2)+(0.0348P)2+19.4218P].
solution. Table 1 lists a summary of crystallographic
data, selected bond lengths and angles can be found in
Table 2, the atomic coordinates are in Table 3, and an
ORTEP diagram is shown in Fig. 4. The ORTEP shows
that the cis configuration of phosphine and hydride is
retained, and the environment about each Re center is
octahedral. The position of the bridged hydride was
provided the hydride-bridged dimer 4a%, which was iso-
lated in high yield (92%) as large colorless crystals by
addition of hexanes. The PCy3 analogue 4b% was pre-
pared in
a similar fashion from 1b, 5b, and
[H(OEt2)2][BArF], and isolated in an 82% overall yield
(Eq. (10)).
,
refined and the average Re–H distance is 1.91 A. The
Re–H–Re angle is 124(2)°, which is similar to the angle
we previously observed for the chloride-bridged dimers
{[cis-Re(CO)4(PR3)]2(m-Cl)}{BArF} (R=Ph, 128.2(2)°;
Cy, 123.85(2)°) [8]. The Re–Re separation in 4a% is 3.380
,
A, which is considerably shorter than those observed for
,
the chloride-bridged dimers, 4.333 and 4.460 A, respec-
tively for R=Ph and Cy. This reflects the two-electron,
three-center nature of the bonding in M–m-H–M com-
plexes (denoted by dashed lines). There are no short in-
teractions observed between the cation dimer and the
−
BArF anion.
(10)
2.5. Reaction of [cis-Re(CO)4(PR3)(ClCH2Cl)][A] with
Et3SiH
1
The H NMR spectra of 4a% and 4b% in CD2Cl2 solu-
tion showed upfield triplets at exactly the same chemical
shift as those observed in the H2 heterolytic activation
Approximately 1 equiv. of Et3SiH was added to a
cooled (−40°C) CD2Cl2 solution of 4 and the reaction
was monitored by NMR spectroscopy. Between −40
and −20°C the only product observed was the s-bound
silane complex [cis-Re(CO)4(PR3)(h2-HSiEt3)][MeB-
(C6F5)3] (R=Ph, 6a; Cy, 6b). The assignment of this
product as Re(h2-HSiEt3) was made on the basis of an
upfield doublet at −8.89 ppm (JHP=10.5 Hz, 6a) and
−9.41 ppm (JHP=9.3 Hz, 6b) which had 29Si satellites
(JHSi=60.9 Hz, 6a; 61.6 Hz, 6b). These JHSi fall in the
range of values (20–70 Hz) typically found for known
s-bound SiH complexes [3]. Furthermore, the values of
the JHP coupling constants for these doublets are too
small to be due to a Re–H complex (JHP=21.9 Hz for
5b).
−
experiments described above with either the BArF or
the MeB(C6F5)3 anion. The 31P NMR spectra showed
−
one resonance at 1.9 ppm for 4a% and 13.7 ppm for 4b%,
which are at identical chemical shifts to those observed
−
for the MeB(C6F5)3
derivatives. These hydride-
bridged dimers were stable indefinitely in Et2O solution,
but decomposed slowly in CD2Cl2 solution (approxi-
mately 40% in 5 days) to the chloride-bridged dimers
{[cis-Re(CO)4(PR3)]2(m-Cl)}{BArF}. Addition of a five-
fold excess of [H(OEt2)2][BArF] to a CD2Cl2 solution of
4a% did not lead to protonation of the bridged hydride,
although addition of 5 equiv. of [Ph3C][BArF] did
provide approximately 30% conversion in 15 min to the
dichloromethane complex and Ph3CH.
Crystals of 4a% suitable for X-ray diffraction studies
were obtained by slow diffusion of hexanes into a Et2O
As the NMR sample was warmed to 0°C, the inten-
sity of the s-complex decreased and new resonances

248
J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
Table 3
Table 3 (Continued)
Atomic coordinates (×104) and equivalent isotropic displacement
2
3
a
,
parameters (A ×10 ) for 4a%
x
y
z
Ueq
a
x
y
z
Ueq
C(58)
C(59)
C(60)
C(61)
C(62)
C(63)
C(64)
C(65)
C(66)
C(67)
C(68)
C(47A)
C(49A)
C(53A)
C(55A)
C(59A)
C(61A)
C(65A)
C(67A)
F(1)
F(2)
F(3)
F(4)
F(5)
F(6)
F(7)
F(8)
F(9)
F(10)
F(11)
F(12)
F(13)
F(14)
F(15)
F(16)
F(17)
F(18)
F(19)
F(20)
F(21)
F(22)
F(23)
F(24)
O(1)
3391(5)
7771(4)
8204(4)
8356(4)
8074(4)
7639(4)
7504(4)
7746(4)
8181(4)
8401(4)
8161(4)
7733(4)
5597(5)
5772(5)
5666(4)
5385(4)
8507(5)
8246(5)
8399(5)
8386(6)
5654(3)
5002(3)
5804(3)
5196(3)
5959(5)
5888(6)
6135(2)
5340(3)
5337(3)
4817(3)
5455(5)
809(4)
9093(3)
8408(4)
8320(3)
7792(3)
8563(4)
8562(5)
7997(3)
8546(3)
8879(3)
8343(7)
8214(8)
8947(5)
1441(3)
107(3)
2493(5)
2026(5)
1431(5)
1335(5)
1819(4)
3631(4)
3505(5)
3945(5)
4525(5)
4662(5)
4219(5)
4967(5)
2699(6)
3547(5)
1307(5)
2114(6)
727(5)
3760(6)
5276(6)
5224(3)
4949(3)
5458(3)
2604(5)
2061(4)
2927(5)
3856(3)
4087(3)
3246(3)
1369(4)
1031(5)
5484(5)
2027(4)
2758(4)
1654(5)
408(4)
33(2)
35(2)
38(2)
35(2)
30(2)
28(2)
30(2)
33(2)
41(2)
39(2)
37(2)
45(3)
45(3)
36(2)
39(2)
53(3)
47(3)
52(3)
53(3)
67(2)
57(2)
91(3)
129(4)
150(5)
161(5)
50(2)
57(2)
51(2)
154(5)
172(6)
695(6)
75(2)
121(4)
95(3)
110(3)
122(4)
136(4)
80(2)
74(2)
81(2)
192(7)
247(10)
211(7)
52(2)
60(2)
62(2)
62(2)
59(2)
51(2)
65(2)
67(2)
3535(5)
2954(6)
2246(5)
2124(5)
2302(5)
1559(5)
1356(5)
1887(6)
2631(6)
2829(6)
3996(6)
4830(6)
383(5)
1271(6)
4300(6)
1606(6)
539(6)
3197(7)
3363(4)
4130(3)
4588(5)
4837(6)
4644(6)
5539(5)
104(3)
Re(1)
Re(2)
P(1)
P(2)
B(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
1410(1)
1284(1)
2633(1)
2447(1)
2529(5)
1023(5)
1154(5)
1837(6)
394(6)
1421(1)
2903(1)
1015(1)
3493(1)
7037(4)
1419(4)
593(4)
1509(4)
1634(4)
2794(4)
2593(5)
2935(4)
3675(5)
1487(2)
1945(3)
2302(2)
2202(3)
1744(3)
1386(3)
275(2)
4292(1)
3917(1)
4035(1)
4517(1)
3046(5)
3237(5)
4449(5)
5337(6)
4442(5)
3036(6)
3391(5)
4823(6)
3558(5)
3446(3)
3727(2)
3278(4)
2547(3)
2266(2)
2715(3)
3579(3)
3177(3)
2848(3)
2922(3)
3325(4)
3654(3)
4801(3)
5382(3)
5962(3)
5961(3)
5381(3)
4801(3)
3856(3)
3814(3)
3266(3)
2759(3)
2801(3)
3350(3)
4961(3)
4870(3)
5219(4)
5658(3)
5748(3)
5399(3)
5209(3)
5175(3)
5657(4)
6173(3)
6207(3)
5725(3)
4039(4)
4261(5)
3830(5)
3171(5)
2951(4)
2780(4)
3208(4)
3042(4)
2415(5)
1977(4)
2149(4)
2412(4)
30(1)
30(1)
30(1)
31(1)
23(2)
37(2)
38(2)
43(3)
41(2)
39(2)
42(3)
45(3)
47(3)
31(2)
37(2)
44(3)
61(3)
57(3)
45(2)
37(2)
42(2)
45(3)
48(3)
65(3)
56(3)
34(2)
41(2)
54(3)
59(3)
51(3)
38(2)
29(2)
33(2)
41(2)
42(2)
52(3)
38(2)
35(2)
45(3)
57(3)
56(3)
47(3)
36(2)
34(2)
46(3)
58(3)
61(3)
56(3)
44(3)
32(2)
33(2)
33(2)
31(2)
28(2)
25(2)
28(2)
28(2)
32(2)
24(2)
28(2)
24(2)
1696(6)
292(6)
904(6)
872(6)
C(9)
3048(3)
3557(3)
3878(3)
3690(4)
3181(4)
2860(3)
2503(4)
1790(3)
1716(3)
2355(4)
3069(3)
3143(3)
3419(3)
3213(3)
3783(4)
4558(3)
4764(3)
4194(3)
2720(3)
2505(3)
2647(3)
3004(4)
3219(4)
3077(3)
3321(3)
4054(3)
4692(3)
4597(3)
3864(4)
3226(3)
2295(4)
1567(3)
1470(4)
2101(5)
2828(4)
2925(3)
3387(5)
3933(5)
4414(5)
4321(5)
3777(5)
1855(5)
1362(5)
862(5)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
C(18)
C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
C(25)
C(26)
C(27)
C(28)
C(29)
C(30)
C(31)
C(32)
C(33)
C(34)
C(35)
C(36)
C(37)
C(38)
C(39)
C(40)
C(41)
C(42)
C(43)
C(44)
C(46)
C(47)
C(48)
C(49)
C(50)
C(51)
C(52)
C(53)
C(54)
C(55)
C(56)
C(57)
91(2)
−468(3)
−842(2)
−658(3)
−99(3)
840(3)
795(3)
−231(3)
1258(8)
1837(6)
169(6)
4248(4)
4747(4)
4696(4)
1225(5)
1807(4)
1074(5)
34(4)
569(3)
383(3)
468(3)
738(3)
924(3)
3988(2)
4589(2)
4945(2)
4700(2)
4099(3)
3743(2)
3090(2)
3236(2)
2906(3)
2430(3)
2285(2)
2615(3)
4016(2)
4261(3)
4702(3)
4899(3)
4654(3)
4213(3)
6332(4)
5895(4)
5710(4)
5972(4)
6408(4)
6528(4)
6346(4)
5862(4)
5548(4)
5730(4)
6214(4)
7476(3)
239(4)
925(4)
3879(4)
3081(3)
4138(4)
5857(4)
5339(7)
5272(6)
2635(4)
4538(4)
5929(4)
4499(4)
2511(4)
3061(4)
5329(4)
3328(4)
303(4)
429(4)
3059(6)
3865(6)
3325(9)
822(4)
1008(4)
2048 (5)
−223(4)
1860(5)
−288(4)
686(5)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
O(8)
1579 (3)
1739(3)
2735(3)
2427(3)
2950(3)
4111(3)
599(5)
a Ueq is defined as one third of the trace of the orthogonalized Uij
tensor.
corresponding to the hydride complex 5 and the hy-
dride-bridged dimer 4 began to grow in. At room
temperature the 31P and 1H NMR spectra corresponded
to complete conversion to dimer 4, and resonances in
826(5)
1297(5)
1788(5)
2686(5)
1
the H NMR spectrum corresponded to the formation
of the silane Et3SiMe. GC/MS analysis confirmed

J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
249
room temperature, and if not, what sort of Si contain-
ing products would be formed. When one equiv. of
Et3SiH was injected into a cooled (−40°C) CD2Cl2
solution of [cis-Re(CO)4(PPh3)(ClCD2Cl)][BArF] for-
mation of the s-complex [cis-Re(CO)4(PPh3)(h2-
HSiEt3)][BArF] (6a%) was observed, and the hydride
1
region of the H NMR spectrum is shown in Fig. 5.
However, warming of the sample to room temperature
for 5 min still resulted in the complete conversion to the
hydride-bridged dimer 4a%. GC/MS analysis of the
volatiles from this reaction revealed the presence of the
disiloxane Et3SiOSiEt3, Et3SiF, and 1,3-(CF3)2(C6H4).
This suggests that the Re(h2-HSiEt3) complex is un-
stable at room temperature even with the non-interact-
ing anion BArF−, and that elimination of Et3Si+
probably proceeds with fluoride abstraction from the
−
Fig. 4. ORTEP diagram for {[cis-Re(CO)4(PPh3)]2(m-H)}{BArF} (4a%),
BArF anion as observed for other incipient silylium
−
BArF not shown, 50% probability ellipsoids.
ions [47]. The silicon cation equivalent Et3Si+ which is
eliminated in these reactions probably exists as a highly
reactive solvento complex instead of a bare Si+ cation.
Closely paralleling 6, [IrH2(h2-HSiEt3)2(PPh3)2]SbF6
also forms Et3SiF and a hydride-bridged dimer on
warming to 25°C [48]. As pointed out by Luo et al., the
Si atom of the Si–H bond is highly activated toward
nucleophilic attack because of depletion of the electron
density of the Si–H bond on coordination to cationic
metals. Cationic silane complexes generally cannot
be isolated even if they do not attack the anion.
For example [CpFe(CO)(PEt3)(h2-HSiEt3)]BF4 is read-
ily hydrolyzed by trace water to Et3SiOH and
[CpFe(CO)(PEt3)(h2-H2)]BF4 [49].
Et3SiMe as the major product in the recovered volatiles
from the reaction mixture. The heterolytic cleavage of
SiH in this system occurs very much like the H2 reac-
tions described above. However, a silicon cation equiv-
alent ‘Et3Si+’ (silylium ion [47]) is produced instead of
H+, which then goes on to react with either ReMe in 1,
or the anion MeB(C6F5)3−. There was no evidence for
the formation of either methane or a silicon-bridged
dimer (Eq. (11)).
It is important to note that the s silane complexes 6
were formed in complete conversion at low temperature
in the NMR spectra in CD2Cl2 solution, whereas the H2
complexes 3 were only observed in low concentrations
(B5%) due to fast exchange with free methylene chlo-
ride in CD2Cl2 solution. This indicates that tertiary SiH
is a better ligand than H2 (and CH2Cl2) towards [cis-
Re(CO)4(PR3)]+. This is in agreement with the notion
that the greater basicity of the Si–H bond compared to
the H–H bond makes Si–H a better s donor [1d],
assuming that s donation is the much greater bonding
component in the Re system. Thus silanes appear to be
electronically both better s-donors and p-acceptors in
comparison to H2 (as previously conjectured [1d,f]).
Steric effects are however much greater for silanes
and more congested first-row fragments such as
Mn(CO)3(PCy3)2 and [Mn(CO)(diphosphine)2]+ do not
bind silanes [14,23e]. There are few examples in the
literature of the heterolytic bond cleavage of SiH [48–
50], and we have not found any other examples in
which a s-complex was observed as an intermediate in
the activation. Recently, Bullock and Song [50a] have
shown that Cp(CO)3W(FBF3) reacts with Et3SiH to
produce Cp(CO)3WH (70% conversion, 45 min at
22°C). Fryzuk et al. [50b,c] have shown novel hetero-
(11)
−
The reaction was also carried out using the BArF
derivative to see if the s-complex would be stable at
Fig. 5. 1H NMR (CD2Cl2, −20°C) of [cis-Re(CO)4(PR3)(h2-
HSiEt3)][BArF] (6a%), hydride region.

250
J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
lytic cleavage of H2 and BuSiH3 on a Zr–N2–Zr
dimer, which is calculated to be exothermic by 19.7
kcal mol−1 for cleavage of SiH4 [51] (Eq. (12)).
ethylene is bound quite strongly, there must be consid-
erable s-donation from ethylene to the Re+ center.
The bound cco in 8 also could not be removed under
vacuum and did not exchange with free CD2Cl2 in
1
solution. The H and 13C{1H} spectra for 8 at room
temperature indicate four carbon and five proton envi-
ronments. Finally, attempts to hydrogenate 7 or 8
with H2 were unsuccessful due to the strong binding of
the olefins versus H2 to the Re+ center.
(12)
2.6. Reaction of [cis-Re(CO)4(PR3)(ClCH2Cl)][A] with
olefins
3. Conclusions
The formation of s-complexes and subsequent het-
erolytic activation of HX bonds could be useful in the
functionalization of olefins, such as in ionic hydro-
genation or hydrosilylation. Keeping this in mind, we
examined the reactivity of ethylene and cis-cyclooctene
(cco) towards [cis-Re(CO)4(PR3)]+. Placing a CH2Cl2
solution of [cis-Re(CO)4(PPh3)(S)][BArF] (S=Et2O,
CH2Cl2) under an atmosphere of ethylene and subse-
quent addition of hexanes produced the complex [cis-
Re(CO)4(PPh3)(C2H4)][BArF] (7) in 74% yield (Eq.
(13)). Similarly, the addition of an excess of cco to a
CH2Cl2 solution of [cis-Re(CO)4(PPh3)(S)][BArF] and
The reaction of cis-Re(CO)4(PR3)Me (R=Ph, Cy)
with the Lewis acid B(C6F5)3 was studied by NMR
spectroscopy and was found to provide an equilibrium
mixture of the solvent coordinated complex [cis-
Re(CO)4(PR3)(ClCH2Cl)][MeB(C6F5)3] and the reac-
tants. The coordinated dichloromethane was observed
in the low temperature 13C NMR spectra. Reaction of
cis-Re(CO)4(PR3)Me with H2 in the presence of
B(C6F5)3 at low temperature yielded the s-bonded
[cis-Re(CO)4(PR3)(h2-H2)][MeB(C6F5)3] which decom-
posed at room temperature via heterolytic cleavage of
the H2 to produce methane and the hydride-bridged
dimer {[cis-Re(CO)4(PR3)]2(m-H)}{MeB(C6F5)3} (4).
Similarly, [cis-Re(CO)4(PR3)(ClCH2Cl)][A] reacted
with Et3SiH to form the s-silane complex. The Re(h2-
HSiEt3)+ complex is observed at low temperature, but
decomposes at room temperature to form the thermo-
dynamically-favored hydride-bridged dimer and
‘Et3Si+’, which is not observed, but reacts further to
form methyl, fluoride, and siloxane products via het-
erolytic cleavage of the Si–H bond. The silane com-
plex is somewhat more stable than the H2 complex,
indicating that silanes are better s-donors (in addition
to being better p-acceptors) than H2. Finally, the
olefins reacted in this study bind quite strongly to this
Re+ system, and the rotation about the Re–olefin
bond could not be stopped at low temperature, indi-
cating strong s and only weak p interactions between
the olefin and the Re center.
The H2 binding and heterolytic cleavage on this and
other highly electrophilic organometallic complexes
that contain strong p-acceptor CO ligands can be di-
rectly related to the structure and function of metal-
loenzymes such as Fe-containing hydrogenases that
catalyze H2l2H++2e−. The latter have surprisingly
been found to contain electrophilic dinuclear active
sites that are organometallic in character with CO
ligands which would promote binding and heterolytic
cleavage of molecular H2. The M–M bonds (Fe–Fe
or Fe–Ni) in hydrogenases appear to be an important
structural component and are likely sites for reversible
protonation to form a bridging hydride as the initial
step in one mechanism of H2 formation that is pro-
(13)
subsequent addition of hexanes provided the olefin-co-
ordinated complex [cis-Re(CO)4(PPh3)(C8H14)][BArF]
(8) in 72% yield. These complexes were isolated as
white crystalline solids and satisfactory elemental anal-
ysis were obtained.
The bound ethylene in 7 could not be removed
under vacuum, and did not exchange with CD2Cl2
1
when dissolved for NMR experiments. The H showed
a doublet at 4.11 ppm with JHP=2.4 Hz, shifted up-
field from free ethylene (5.40 ppm). The 13C{1H}
NMR spectrum showed only one ethylene carbon en-
vironment. If the rotation about the Re–(C2H4) axis
could be stopped, then two resonances should be ob-
served in the ethylene region of the 13C NMR spec-
1
trum. However, the low temperature (−70°C) H and
13C{1H} NMR spectra of 7 did not differ from the
spectra taken at room temperature. This result sug-
gests that the rotation about the Re–(C2H4) axis is
not hindered by significant p-interactions between the
Re d-orbitals and ethylene p* orbital, but since the

J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
251
posed here. Even when no other bridging ligands or
metal bonds are present, electrophilic systems appear to
favor formation of hydride-bridged complexes such as 4
rather than mononuclear hydrides. No matter which
mechanism of the many possible for hydrogenase func-
tion ultimately proves to be correct, it is clear that
several principles from organometallic H2 binding and
activation directly come into play. Perhaps a more
proper viewpoint is that once again Nature is billions of
years ahead of us in designing the ideal sites for activa-
tion of H2 and other small molecules.
ReClCH2Cl), 9.4 (BMe). Thermodynamic data were
calculated using the integration between BMe and
ReMe peaks in the H NMR spectra recorded at 23.8,
1
11.1, 2.3, −4.4, −11.3, −19.5 and −27.1°C, with a
CH3OH capillary insert for temperature calibration.
Similarly, 1b (0.021 g, 0.035 mmol) was added to a
CD2Cl2 solution of B(C6F5)3 (0.018 g, 0.035 mmol) in an
1
NMR tube to provide a colorless solution. H NMR
(24°C, CD2Cl2) 1.0–2.2 (m, 33H, Cy), 0.22 (v br, 3H,
BMe+ReMe). 31P{1H} NMR (24°C, CD2Cl2) 25.2 (br).
1H NMR (−70°C, CD2Cl2) 1.0–2.2 (m, 33H, Cy), 0.42
(s, 3H, BMe). 31P{1H} NMR (−70°C, CD2Cl2) 25.4.
4. Experimental
4.3. NMR reactions with H2, HD, and D2
4.1. General considerations
In a typical experiment, a J-Young NMR tube was
charged with CD2Cl2 (0.5 ml) or C6D5F (1) (0.0100 g),
and 1 equiv. of B(C6F5)3. The solution was then frozen
at liquid N2 temperature, backfilled with approximately
3 atm of H2, and closed off. The NMR tube was then
immediately transferred to the pre-cooled NMR probe
(−80°C), and spectra were collected as the sample was
warmed to room temperature in 10 or 20°C increments.
NMR spectra were measured on a Varian UNITY
series 300 MHz spectrometer. FT-IR spectra were ob-
tained on a Nicolet Magna 750 spectrometer. Solvents
were dried either by distillation from P2O5 (CH2Cl2,
NC5F5), Na/benzophenone (Et2O, hexanes), or by elu-
tion from columns of activated alumina and BTS cata-
lyst according to the procedure describe by Grubbs and
coworkers [52]. NMR solvents were dried over either
4.4. Preparation of cis-Re(CO)4(PCy3)(H) (5b)
1
CaH2 or P2O5 and vacuum transferred before use. H
NMR spectra were referenced to either CHDCl2 or
TMS, 31P NMR spectra were referenced to H3PO3, and
13C spectra were referenced to CD2Cl2 or CH2Cl2. The
BAr-F and B(C6F5)3 peaks are not reported in the 13C
NMR data and are virtually identical to those reported
previously, regardless of the cationic fragment [12,21].
All manipulations and reactions with air sensitive com-
pounds were carried out either in a Vacuum Atmo-
spheres He Drybox or under Ar using standard Schlenk
techniques. The B(C6F5)3 [53], 1a [11], 1b [8], cis-
Re(CO)4(PCy3)Cl [8], 5a [54] and [cis-Re(CO)4-
(PR3)(ClCH2Cl)][BArF] [8] were prepared as de-
scribed previously. The H(BEt3)Li, Et3SiH and cis-cy-
clooctene (cco) were purchased from Aldrich.
An aliquot of HBEt3 in Et2O (0.91 ml, 1.0 M) was
added to a stirred and cooled (−78°C) solution of
cis-Re(CO)4(PCy3)Cl (0.556 g, 0.905 mmol) in Et2O (40
ml). The reaction mixture was stirred for 5 min, then
slowly warmed to room temperature and stirred for 30
min. To the resulting pale yellow reaction mixture,
CH2Cl2 (0.50 ml) was added to quench any unreacted
HBEt3. The solvents were removed under vacuum and
the residue was dissolved in benzene (10 ml) and filtered
under Ar through a fine frit. The benzene was removed
under vacuum and the residue chromatographed on
SiO2 (8 g) eluting with hexanes. The first fraction was
collected and removal of volatiles provided 5b as a white
solid (0.23 g, 43% yield). Anal. Calc. for C22H34O4PRe:
C, 45.58; H, 5.91. Found: C, 45.38; H, 5.94%. FT-IR
(cm−1, Nujol, w(CO)) 2073 (m), 1980 (s), 1962 (s), 1951
4.2. Reaction of cis-Re(CO)4(PR3)Me (R=Ph, 1a; Cy,
1b) with B(C6F5)3
1
(s). H NMR (C6D6), 1.82 (m, 6H, Cy), 1.73 (m, 3H,
Cy), 1.64 (m, 6H, Cy), 1.51 (br s, 3H, Cy), 1.38 (m, 6H,
Cy), 1.06 (m, 9H, Cy) −5.48 (d, JHP=21.9, 1H, ReH).
31P{1H} NMR (C6D6) 27.3. 13C{1H} NMR (C6D6)
191.84 (d, JCP=7.4, CO), 191.77 (d, JCP=9.8, CO),
189.34 (d, JCP=38.9, CO), 36.93 (d, JCP=22.8, Cy),
30.43 (s, Cy), 27.97 (d, JCP=10.3, Cy), 26.90 (s, Cy).
The addition of 1a (0.031 g, 0.054 mmol) to a CD2Cl2
solution of B(C6F5)3 (0.028 g, 0.054 mmol) in an NMR
tube provided a colorless solution. 1H NMR (24°C,
CD2Cl2) 7.3–8.0 (m, 15H, Ph), 0.48 (br s, 1.3H, BMe),
−0.56 (br s, 1.7H, ReMe). 31P{1H} NMR (24°C,
CD2Cl2) 11.1, 10.4 (br, overlapping). 1H NMR
(−70°C, CD2Cl2) 7.3–8.0 (m, 15H, Ph), 0.44 (s, 3H,
BMe). 31P{1H} NMR (−70°C, CD2Cl2) 11.1. 13C{1H}
NMR (−70°C, CH2Cl2) 184.09 (d, JCP=8.5, CO),
181.52 (d, JCP=41.8, CO), 180.60 (d, JCP=6.2, CO),
132.77 (d, JCP=11.0, Ph), 132.57 (s, Ph), 129.81 (d,
4.5. Preparation of {[cis-Re(CO)4(PR3)]2(v-H)}{BArF}
(R=Ph, 4a%; Cy, 4b%)
Diethyl ether (3 ml) was added to a vial containing 1a
(0.029 g, 0.050 mmol) and [H(OEt2)2][BArF] (0.051 g,
0.050 mmol). After stirring for 1 min, 5a (0.028 g, 0.050
JCP=11.0, Ph), 127.98 (d, JCP=52.8, Ph), 67.0 (s,

252
J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
mmol) was poured in and the colorless reaction mixture
was stirred for 5 min before the addition of hexanes (3
ml). The solution was stored at room temperature
overnight to provide large, colorless crystals of 4a%
which were washed twice with hexanes and dried in
vacuo (0.091 g, 92% yield). Anal. Calc. for
C76H43BF24O8P2Re2: C, 45.98; H, 2.18. Found: C,
45.57; H, 1.94%. FT-IR (cm−1, Nujol, w(CO)) 2120
(m), 2100 (w), 2035 (s), 2025 (vs), 2017 (s), 2010 (m),
1999 (m), 1979 (s). 1H NMR (CD2Cl2) 7.73 (s, 8H,
cally. The atomic coordinates of the hydride were
2
,
refined with a set isotopic temperature factor (0.08 A ).
The phenyl rings of the triphenylphosphine ligands
were refined as rigid bodies with the C–C distances
,
fixed at 1.39 A. The hydrogen atoms were fixed in
positions of ideal geometry, with a C–H distance of
,
0.93 A and refined using the riding model in the HFIX
facility in SHELX 93. These idealized hydrogen atoms
had their isotropic temperature factors fixed at 1.2
times the equivalent isotropic U of the carbon atom
they were bonded to.
BArF), 7.3–7.6 (m, 34H, Ph+BArF), −15.56 (t, JHP
10.0, m-H). 13P{1H} NMR (CD2Cl2) 1.9. 31C{1H} NMR
(CD2Cl2) 184.90 (d, JCP=8.55, CO), 181.39 (d, JCP
43.26, CO), 179.98 (d, JCP=8.30, CO), 133.73 (d,
CP=11.1, Ph), 132.88 (s, Ph), 131.27 (d, JCP=49.9,
=
=
4.7. NMR reactions with Et3SiH
J
A CD2Cl2 solution of 1a (0.0157 g, 0.027 mmol) and
B(C6F5)3 (0.014g, 0.027 mmol) was transferred to an
NMR tube fitted with a screw-top septa cap and cooled
to −78°C. An aliquot of Et3SiH (5 ml, 0.03 mmol) was
injected and the NMR tube was immediately placed in
Ph), 130.14 (d, JCP=10.4, Ph).
The PCy3 analogue was prepared in a similar manner
from 1b, [H(OEt2)2][BArF], and 5b and isolated in 82%
yield. Anal. Calc. for C76H79BF24O8P2Re2: C, 45.15; H,
3.94. Found: C, 45.04; H, 4.01%. FT-IR (cm−1, Nujol,
w(CO)) 2111 (m), 2095 (m), 2019, 2009, 1999, 1974,
1
the pre-cooled probe. The H and 31P{1H} spectra were
recorded at 10°C intervals between −40 and 20°C.
After warming to room temperature, the volatiles were
vacuum-transferred to a sample tube and analyzed by
GC/MS and showed Et3SiMe (M+ 130) as the major
product and Et3SiOSiEt3 (M+ 246), Et3SiCl (M+ 150)
and (C6H4)(CF3)2 (M+ 214) as minor products. Simi-
larly, Et3SiH was added to a cooled (−78°C) solution
of [cis-Re(CO)4(PPh3)(ClCH2Cl)][BArF] in CD2Cl2 and
1
1968 (s, overlapping). H NMR (CD2Cl2) 7.73 (s, 8H,
BArf), 7.58 (s, 4H, BArF), 0.80–2.25 (m, 66H, Cy),
−17.59 (t, JHP=7.8, m-H). 13P{1H} NMR (CD2Cl2)
13.7. 31C{1H} NMR (CD2Cl2) 186.82 (d, JCP=8.5,
CO), 181.64 (d, JCP=40.0, CO), 180.73 (d, JCP=7.6,
CO), 37.84 (d, JCP=23.5, Cy), 30.13 (s, Cy), 27.88 (d,
JCP=10.4, Cy), 26.39 (s, Cy).
1
the reaction monitored by H and 31P{1H} NMR spec-
4.6. X-ray structure of 4a%
troscopy. 1H NMR (CD2Cl2, −40°C) 7.73 (s, 8H,
BArF), 7.3–7.6 (m, 34H, Ph+BArF), 0.5–1.2 (m, 15H,
SiEt), −8.89 (d, JHP=10.5, JHSi=60.9, 1H, h2-HSi).
31P{1H} (CD2Cl2, −40°C) 7.2. The GC/MS of the
volatiles after warming to room temperature showed
Et3SiOSiEt3 as the major product and Et3SiF (M+ 134)
and (C6H4)(CF3)2 as minor products.
A colorless, triangular shaped crystal was mounted
from a pool of mineral oil under argon gas flow on a
thin glass fiber with silicon grease, and placed under a
liquid nitrogen stream on a Siemens P4/PC diffractome-
ter. The radiation used was graphite monachromatized
Mo Ka radiation (u=0.71069 A). The lattice parame-
ters were optimized from a least-squares calculation on
25 carefully centered reflections of high Bragg angle.
The data were collected using ꢀ scans with a 0.86° scan
range. Three check reflections monitored every 97
reflections showed no systematic variation of intensities.
Lattice determination and data collection were carried
out using XSCANS Version 210b software. All data
reduction, including Lorentz and polarization correc-
tions and structure solution and graphics were per-
formed using SHELXTL PC Version 4.2/360 software.
The structure refinements were performed using SHELX
93 software [55]
In a similar manner, Et3SiH was reacted with either
1b and B(C6F5)3, or [cis-Re(CO)4(PCy3)(ClCH2Cl)]-
[BArF] in CD2Cl2 solution. The resulting NMR spectra
and GC/MS data were similar to those described for
1
the Ph derivative. H NMR (CD2Cl2, −40°C) 7.73 (s,
8H, BArF), 0.8–2.2 (m, 48H, SiEt+PCy), −9.41 (d,
J
HP=9.3, JHSi=61.6, 1H, h2-HSi). 31P{1H} (CD2Cl2,
−40°C) 27.2.
4.8. Preparation of [cis-Re(CO)4(PPh3)(C2H4)][BArF]
(7)
A Schlenk flask was charged with [cis-Re(CO)4-
(PPh3)(OEt2)][BArF] (0.261 g, 0.174 mmol) and placed
under an atmosphere of ethylene. Methylene chloride
(5.0 ml) was added and the colorless solution was
stirred with vortexing for 10 min. After this time, the
volume was reduced by one-third under vacuum and 20
ml of hexanes were added dropwise with constant stir-
ring, resulting in the precipitation of white microcrys-
The structure was solved using Patterson and differ-
ence Fourier techniques. These solutions yielded the
two rhenium atoms and the majority of all other non-
hydrogen atom positions. Subsequent Fourier synthesis
gave all remaining non-hydrogen atom positions. The
bridging hydride was found after all other non-hydro-
gen atoms had been identified and refined anisotropi-

J. Huhmann-Vincent et al. / Inorganica Chimica Acta 294 (1999) 240–254
253
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tals. The solution was syringed off, and the white solid
washed twice with hexanes and dried in vacuo (0.190 g,
74% yield). Anal. Calc. for C56H31BF24O4PRe: C, 46.33;
H, 2.15. Found: C, 46.14; H, 2.15%. FT-IR (cm−1
,
Nujol, w(CO)) 2128 (m), 2054, 2041, 2027, 2115, 1996,
1
1984 (s, overlapping). H NMR (CD2Cl2) 7.73 (s, 8H,
BArF), 7.3–7.6 (m, 34H, Ph+BArF), 4.11 (d, JHP=2.4,
C2H4). 13P{1H} NMR (CD2Cl2) 2.4. 31C{1H} NMR
(CD2Cl2) 182.44 (d, JCP=8.8, CO), 181.98 (d, JCP=8.0,
CO), 178.17 (d, JCP=42.8, CO), 133.4 (d, JCP=2.5,
Ph), 133.08 (d, JCP=10.7, Ph), 130.65 (d, JCP=10.6,
Ph), 129.18 (d, JCP=51.8, Ph), 74.84 (s, C2H4).
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4.9. Preparation of [cis-Re(CO)4(PPh3)(C8H14)][BArF]
(8)
In the dry box, cco (g, mmol) was added to a solution
of 1a (0.021 g, 0.036 mmol) and [Ph3C][BArF] (0.039 g,
0.035 mmol) in CH2Cl2 (5.0 ml). After stirring for 5 min,
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was placed in a −30°C freezer for 1 h. The resulting
pale yellow crystals were collected, washed twice with
hexanes, and dried in vacuo (0.040 g, 72% yield). Anal.
Calc. for C62H41BF24O4PRe: C, 48.55; H, 2.69. Found:
1
C, 48.90; H, 2.68%. FT-IR (cm−1, Nujol, w(CO)). H
NMR (CD2Cl2) 7.73 (s, 8H, BArF), 7.3–7.6 (m, 34H,
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30.01 (s, cco), 25.56 (s, cco).
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5. Supplementary material
X-ray crystallographic files in CIF format for the
structure determination of {[cis-Re(CO)4(PPh3)]2(m-
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Acknowledgements
This work is supported by the Department of Energy,
Office of Basic Energy Sciences, Chemical Sciences
Division. J. Huhmann-Vincent is grateful to the Direc-
tor of the Los Alamos National Laboratory for post-
doctoral funding. We are grateful for helpful discussions
with Dr John Peters at Utah State regarding the struc-
ture and possible mechanisms of Fe hydrogenases.
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