Heterobimetallic complexes of rhenium and zinc: Potential catalysts for homogeneous syngas conversion

Article abstract of DOI:10.1021/om200051u

6-(Diphenylphosphino)-2,2′-bipyridine (PNN) coordinates to rhenium carbonyls in both κ¹(P) and κ²(N,N) modes; in the former, the free bpy moiety readily binds to zinc alkyls and halides. [Re(κ¹(P)-PNN)(CO)₅][OTf

Full text of DOI:10.1021/om200051u

ARTICLE
pubs.acs.org/Organometallics
Heterobimetallic Complexes of Rhenium and Zinc: Potential Catalysts
for Homogeneous Syngas Conversion
Nathan M. West,† Jay A. Labinger,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125,
United States
S Supporting Information
b
ABSTRACT:
6-(Diphenylphosphino)-2,20-bipyridine (PNN) coordinates to rhenium carbonyls in both κ1(P) and κ2(N,N) modes; in the
former, the free bpy moiety readily binds to zinc alkyls and halides. [Re(κ1(P)-PNN)(CO)5][OTf] reacts with dialkylzinc
reagents to form [Re(κ1(P)-PNN ZnR)(CO)4(μ2-C(O)R)][OTf] (R = Me, Et, Bn), in which an alkyl group has been
3
transferred to a carbonyl carbon and the resulting monoalkyl Zn is bound both to the bpy nitrogens and the acyl oxygen. ZnCl2
binds readily to the bpy group in Re(κ1(P)-PNN)(CO)4Me, and the resulting adduct undergoes facile migratory insertion,
assisted by the Lewis acidic pendent Zn, to yield Re(κ1(P)-PNN ZnCl)(μ2-Cl)(CO)3(μ2-C(O)Me), in which one of the
3
chlorides occupies the sixth coordination site on Re. Migratory insertion is inhibited by THF or other ethers that can coordinate
to ZnCl2. Migratory insertion is also observed for Re(κ1(P)-PNN)(CO)4(CH2Ph) but not for Re(κ1(P)-PNN)(CO)4-
(CH2OCH3); coordination of the methoxy oxygen to Zn appears to block its ability to coordinate to the carbonyl oxygen
and facilitate migratory insertion. Intramolecular Lewis acid promoted hydride transfer from [(dmpe)2PtH][PF6] to a carbonyl
in [Re(κ1(P)-PNN)(CO)5][OTf] results in formation of a Reꢀformyl species; additional hydride transfer leads to a novel
ReꢀZn-bonded product along with some formaldehyde.
’ INTRODUCTION
(stoichiometric) reductive coupling of two CO molecules to
a derivative of acetic acid.6 In a further elaboration, we
synthesized rhenium carbonyl complexes containing ligands
with a tethered borane and showed that the pendent Lewis acid
facilitates the reductive coupling of two CO ligands;7 the
borane, in combination with a strong base, can also function
as a frustrated Lewis pair capable of cleaving dihydrogen and
delivering a hydride to CO.7c
These boron-linked complexes, while promising, do not
appear viable for catalytic CO reduction, because the inter-
mediate BꢀO bonds are too strong to permit closing a cycle
by releasing the organic product from the metal; a less
oxophilic and more substitutionally labile Lewis acid that is
still capable of promoting the desired reaction steps will be
needed. In order to determine which Lewis acids might be
suitable, we seek to develop versatile ligand architectures that
can support the attachment of a variety of Lewis acidic metals
in proximity to the ReꢀCO reaction site. Our first candidate
The high volatility in the price of petroleum and forecasts of its
decreasing availability have renewed interest in developing
alternative approaches to fuel production. Conversion of
methane- or coal-derived syngas (H2 þ CO) to liquids by the
FischerꢀTropsch reaction currently appears to offer the greatest
promise,1 but the low selectivity of this heterogeneously cata-
lyzed process is a drawback.2 Research on organometallic sys-
tems, both as mechanistic models for the FischerꢀTropsch
reaction and as potentially more selective homogeneous syngas
conversion catalysts, has been active for several decades, but with
only limited success, attributed in large part to the difficult
formation of the first CꢀH bond.3 Some anionic Ru carbonyl
hydride reagents, similar to the active species for homogeneous
CO hydrogenation, have been shown to reduce highly electro-
philic metal carbonyls, such as [CpRe(CO)2(NO)]þ.3g,4 DuBois
has introduced cationic group 9 and 10 transition metal hydrides
that can be synthesized from H2 and are capable of transferring
hydride to a broader range of metal carbonyls.5 We have recently
shown that one of these hydridic reagents, in combination with
a cationic manganese carbonyl complex, can effect the
Received: January 20, 2011
Published: April 26, 2011
r
2011 American Chemical Society
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is 6-(diphenylphosphino)-2,20-bipyridine (PNN), which is
geometrically unlikely to act as a tridentate ligand to a single
metal center; indeed, it has been shown to simultaneously
bind two coppers, one at phosphorus and one through the
nitrogens.8 We anticipated being able to attach the softer P
center to a rhenium carbonyl species, leaving the bpy moiety
available to bind a harder, Lewis acidic metal. We report here
the synthesis of several such compounds, with zinc as the
auxiliary metal, as well as the resulting facilitation of both
migratory insertions and the transfer of a hydride to
coordinated CO.
’ RESULTS AND DISCUSSION
Re(PNN) Complexes. A phosphine-bound Re(PNN) com-
plex can be readily obtained by heating Re(CO)5(OTf) with
PNN at 50 ꢀC for 1 h (eq 1), yielding [Re(κ1(P)-PNN)-
(CO)5][OTf] (1), most clearly indicated by the 31P NMR shift
from ꢀ3.7 ppm for free ligand to 12.2 ppm for the cationic Re
complex. However, this coordination arrangement is not
necessarily the most stable thermodynamically: refluxing Re-
(CO)5Br with PNN in toluene gives fac-Re(κ2(N,N)-PNN)-
(CO)3Br (2) (eq 2). Earlier results on related Re(PN-ligand)
complexes9 suggest that the initial kinetic product might be cis-
Re(κ1(P)-PNN)(CO)4(Br) (3), followed by loss of CO and
isomerization, so that 3 might be obtained under milder
synthetic conditions. Indeed, treatment of the much more
substitutionally labile [Re(CO)4(μ-Br)]2 with PPN at 50 ꢀC in
CH2Cl2 for 1 h gives a roughly 2:1 mixture of 3 and 2 (eq 3).
Pure 3 can be obtained by protonating PPN (at N) with triflic
acid before reaction with [Re(CO)4(μ-Br)]2 at 60 ꢀC for 1 h,
followed by deprotonation with alumina (eq 4). The structures
of both 2 and 3 were confirmed by X-ray crystallography
(Figure 1).
A methyl complex, cis-Re(κ1(P)-PNN)(CO)4(CH3) (4-Me),
was obtained by heating Re(CO)5(CH3) with PNN at 110 ꢀC for
24 h (eq 5); there is no indication of either formation of an N,N-
coordinated species or any migratory insertion to give a rhenium
acetyl complex. Clearly the binding preference of PNN is affected
by the nature of other ligands; perhaps the more strongly
donating methyl group renders the CO’s more substitutionally
inert and makes the P-bound complex kinetically stable. The
analogous alkyl complexes cis-Re(κ1(P)-PNN)(CO)4(CH2OCH3)
(4-MOM) and cis-Re(κ1(P)-PNN)(CO)4(CH2Ph) (4-Bn) were
prepared by reduction of 3 with Na/Hg, followed by treatment
with ClCH2OCH3 or PhCH2Br, respectively (eq 6).
Formation of Zn-Stabilized Acyls by Alkyl Transfer. While
addition of a main-group metal alkyl to MꢀCO is a standard
route for generating an acyl,10 no such reactions of alkyl Zn
reagents with rhenium carbonyls appear to have been reported.
Addition of ZnMe2 to a CD2Cl2 solution of 1 results in rapid
formation of [Re(κ1(P)-PNN ZnMe)(μ2-COMe)(CO)4]-
3
[OTf] (5-Me), in which the Zn is bound to both the pendent
bpy group and the oxygen of the newly formed acyl (eq 7). The
1H NMR of 5-Me shows distinct signals for both the acetyl
(2.45 ppm) and Zn (ꢀ1.31 ppm) methyl groups, the 13C NMR
spectrum includes a doublet (2JPC = 8 Hz) at 285.6 ppm for the
acyl carbon, and the IR spectrum (CD2Cl2) displays peaks at
2100, 2018, and 1997 cmꢀ1 (terminal CO) as well as at
1534 cmꢀ1 (acyl CO). Similar treatment of 1 with ZnEt2 or
ZnBn2 yields the analogous 5-Et and 5-Bn. All were isolated as
sticky solids; no crystals could be obtained, despite multiple
attempts. The methyl compound is stable in solution for several
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Scheme 1
development of a catalytic system. Thermolysis of the neutral Re
acyls 6 at ∼60 ꢀC leads to decarbonylation and formation of the
corresponding Re alkyls 4 as the major products.
In order to determine whether the proximity effect of the
pendent bpy group plays a key role in this reaction, control
reactions were performed on the simple triphenylphosphine
analogue. No reaction was observed between [Re(PPh3)-
(CO)5][OTf] and excess ZnMe2 or ZnEt2 under similar
conditions; however, addition of free bpy to a solution of
[Re(PPh3)(CO)5][OTf] and ZnMe2 results in rapid formation
of the known Re(PPh3)(COMe)(CO)4. The IR spectrum of
Re(PPh3)(COMe)(CO)4 thus prepared is unaffected by water,
with peaks (2088, 2000, 1976, and 1583 cmꢀ1) nearly identical
with those of 6-Me, indicating that Zn does not remain coordi-
nated to the acyl oxygen (Scheme 1).11 Apparently (bpy)ZnMe2
is a stronger methide transfer agent than ZnMe2 (good donor
ligands have been shown to increase the reducing power of
alkylzinc reagents12), which probably is primarily responsible for
the facile reactions of 1 with ZnR2, rather than the attachment of
the Zn-binding site to the Re center.
Figure 1. X-ray crystal structures of (top) fac-Re(κ2(N,N)-PNN)-
(CO)3Br (2) and (bottom) cis-Re(κ1(P)-PNN)(CO)4(Br) (3).
ZnII-Facilitated Migratory Insertion. Migratory insertion for
alkylrhenium carbonyls is rare; in addition to the B-centered
Lewis acid promoted reactions mentioned earlier, there are only
a few examples, all under forcing conditions.13 As noted above,
reaction of Re(CO)5(CH3) with PNN at elevated temperature
gives only CO replacement, not insertion; a similar reaction with
PPh3 gives only Re(PPh3)(CO)4(CH3).14 Presumably this re-
luctance to undergo migratory insertion is a consequence of the
strong MꢀC bonds for the third-row metal Re; analogous Mn
complexes undergo migratory insertion readily even at room
temperature.
weeks, while the other two slowly decompose over the same
period.
A CH2Cl2-soluble form of ZnCl2 was obtained by dissolving
ZnCl2 in THF and drying in vacuo, giving ZnCl2(THF) as a
white powder.15 Addition of ZnCl2(THF) to a solution of 4-Me
in CH2Cl2 rapidly gives a yellow species, whose 1H and 31P NMR
spectra are only slightly altered, suggesting simple formation
of the bpyꢀZn adduct 7-Me. Over a few hours the doublet
at ꢀ0.55 ppm for the Re methyl disappears and is replaced by a
singlet at 2.27 ppm, in the expected region for an acyl methyl,
suggesting insertion to form 8-Me (eq 8). The IR spectrum of
this complex shows three peaks (at 2030, 1951, and 1896 cmꢀ1
)
as expected for a fac-tricarbonyl complex, but no obvious acyl CO
stretch in the IR; probably it is obscured by bpy vibrations in the
mid 1400 cmꢀ1 region (suggesting stronger bonding to Zn than
in 5-Me).
Addition of water removes the coordinated zinc and releases
the free acyl compounds 6-Me, 6-Et, and 6-Bn. 6-Me exhibits IR
peaks at 2087, 2000, 1978, and 1590 cmꢀ1. The modest but
significant increase in the acyl CO stretching frequency
(56 cmꢀ1) provides evidence that Zn is indeed coordinated
to O in 5, but not so strongly that the interaction cannot be easily
disrupted (in contrast to the strong BꢀO bonds of the previ-
ously studied system), which is encouraging for the eventual
The structure of 8-Me was verified by X-ray crystallography;
somewhat unexpectedly, Zn was found to be coordinated to only
one N (the one further from P), the acyl oxygen, and both
chlorides. One Cl occupies the Re site vacated by migratory
insertion, forming a planar five-membered ring containing the Zn
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and Re (Figure 2). The Zn exhibits slightly distorted tetrahedral
coordination; the free bpy N is only 2.7 Å away from Zn, but its
pyridine ring is significantly rotated away from coplanarity with the
coordinated pyr. The acyl CꢀO bond distance of 1.253 Å is
intermediate between typical values for Re acyls (∼1.21 Å)7a,10a,16
and Re alkoxycarbenes (∼1.31 Å),7a,17 which indicates significant
weakening of the double bond due to coordination to Zn (as also
suggested by the absence of a discernible IR stretch); this value is
similar to those observed for other Lewis acid stabilized acyls.6,7a,7b,18
The conversion of 4-Me to 8-Me is analogous to other
migratory insertions of group 7 alkyls facilitated by external
Lewis acids AlX3 (X = Cl, Br),13a,18a,19 although ZnCl2 should be
a considerably milder reagent than AlX3 (and hence potentially
more compatible with catalysis).7b,13a,13c,13d,20 Indeed, in this
system the intramolecular Lewis acid facilitation is essential: no
reaction is observed between Re(PPh3)(CO)4(CH3) and either
ZnCl2(THF) or Zn(bpy)Cl2. Even the (normally much more
facile)21 insertion reaction for Mn(CO)5(CH3) was not effected
by addition of ZnCl2(THF). Furthermore, cis-Re(κ1(P)-
2-(diphenylphosphino)pyridine)(CO)4(CH3), an analogue of
4-Me with only a single “dangling” N center, shows no detect-
able insertion in the presence of ZnCl2(THF). It appears that
bidentate coordination is required to bring the Zn into the
secondary coordination sphere and initiate reaction; perhaps the
steric bulk of the Re group then favors reversible dissociation of
the proximal N, creating a vacant site on Zn that can interact
with the CO oxygen and promote insertion.
The conversion of 7-Me to 8-Me in CH2Cl2 proceeds
smoothly and cleanly, and the reaction may be followed by
in situ IR or NMR spectroscopy, but the kinetics are not entirely
straightforward. At low conversion, the reaction appears to be
roughly first order in 7-Me, but the dependence on
[ZnCl2(THF)] is more complex. The reaction is inhibited by
excess THF and does not proceed at all in pure THF-d8,
indicating an equilibrium between ZnCl2 4-Me (that is, 7-Me)
3
and ZnCl2 THF, so that at high [THF], [7-Me] is very low,
3
completely inhibiting reaction. Kinetics experiments with varying
amounts of THF indicate an inverse order in THF between 1 and
2; the NMR behavior on titration of 4-Me with a 3:1 THF:ZnCl2
mixture is highly complex, suggesting the participation of multi-
ple species depending on the concentration of THF, possibly as
shown in Scheme 2. (In contrast, the NMR of 8-Me does not
change significantly in THF.) Replacement of THF with Et2O or
DME also led to complex inhibitory effects, and ZnCl2 cannot be
used without any ether because of low solubility.
Figure 2. X-ray structure of 8-Me. Selected bond distances (Å) and
angles (deg) for 8-Me: C4ꢀO4, 1.253(3); Zn2ꢀO4, 1.9475(19);
Re1ꢀC4, 2.185(3); Re1ꢀCl2, 2.5406(6); Zn2ꢀCl2, 2.3023(7);
Zn2ꢀCl1, 2.2678(7); Zn2ꢀN1, 2.038(2); Re1ꢀCO(av), 1.938;
Cl2ꢀZn2ꢀO4, 99.78(6); Re1ꢀC4ꢀO4, 125.8(2); Zn2ꢀO4ꢀC4,
125.73(18); Cl2ꢀRe1ꢀC4, 88.81(7).
Scheme 2
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While it is difficult or impossible to separate out individual rate
and equilibrium constants for such a complex scheme, we can
estimate a lower limit for the rate of the insertion step: a solution
of 1 equiv of 4-Me and 10 equiv of a 3:1 mixture of THF
and ZnCl2 smoothly converts to 8-Me with a pseudo-first-order
rate constant of 4.0 ꢁ 10ꢀ5 sꢀ1 at 22 ꢀC. For comparison, the
rate constant of migratory insertion for Re(CO)5Et in CH3CN
(the only reported value) is 3.8 ꢁ 10ꢀ5 sꢀ1 at 46 ꢀC.13c While
the numbers are similar, the difference in temperature, as well as
the facts that the present value is a lower limit and migratory
insertion is typically faster for Et than for Me groups,22 all imply
significant acceleration by the pendent Zn Lewis acid.
Mn alkyls, benzyl was the slowest to migrate, attributed to the
electron-withdrawing nature of the phenyl group.22 In the same
study the migration of CH2OMe was faster than that of benzyl;
here, while 4-MOM rapidly forms an adduct with ZnCl2(THF),
no further reaction is observed: 7-MOM persists over days or
weeks in solution. The 1H NMR signal for the methylene protons
of 7-MOM (which is not affected by added THF) is shifted
considerably from that of 4-MOM upon reaction with Zn; no
such shift is observed for the methyl and benzyl analogues. These
results suggest that an interaction between O of the methox-
ymethyl ligand and Zn (eq 9) may interfere with the ability to
facilitate migratory insertion.
Reactions involving other Lewis acids and alkyl groups were
also briefly investigated. Both ZnBr2/THF and ZnI2/THF
promote formation of analogous insertion products; qualitatively
the reaction is (slightly) fastest with ZnCl2 and slowest with ZnI2,
as expected if the rate enhancement is dependent on Lewis acid
strength. If the reaction were dependent on the nucleophilicity of
the halide and its ability to bind to the Re, then the opposite trend
might be expected. Addition of AgOTf to 4-Me gives no reaction,
but addition of AgOTf to 7-Me results in rapid migratory
insertion, suggesting this is caused by Ag removing Clꢀ from
Zn and generating a more Lewis acidic center. (Direct reaction of
4-Me with Zn(OTf)2 in CH2Cl2 is prohibited by insolubility.)
This observation further supports the proposal that it is the
binding of the carbonyl oxygen to the Zn, rather than coordina-
tion of the bridging Cl, that drives the reaction, with a transition
state such as that shown in Figure 3.
4-Bn undergoes migratory insertion in the presence of ZnCl2-
(THF); the product exhibits two 1H NMR signals (4.20 and 3.71
ppm, 2JHH = 14.6 Hz) for the diastereotopic methylene protons
that result from the conversion of Cs-symmetric 4-Bn to C1-
symmetric 8-Bn. This reaction is qualitatively slower than that of
4-Me, but the rates are within an order of magnitude. In previous
studies on the (non Lewis acid assisted) migratory insertion of
8-Me is unaffected by washing with water, in contrast to the
case for 5-Me, or by exposure to 1 atm of CO (in CD2Cl2 or
THF-d8), in contrast to the case for acyl complexes formed with
the aid of AlX3, where the bridging AlꢀXꢀM linkages are easily
displaced by CO.13a,18a,19,23 Analogous Cl displacement by CO
in 8-Me would lead to Re(κ1(P)-PNN ZnCl2)(CO)4(COMe)
3
(9-Me), which can be independently synthesized by addition of
ZnCl2 to 6-Me; the acyl stretching frequency of 9-Me
(1518 cmꢀ1) is 66 cmꢀ1 lower than that for 6-Me, indicating
that the Zn is coordinated to the acyl oxygen (eq 10). Addition of
water to 9-Me does result in facile removal of the Zn atom and
regeneration of 6-Me. These observations suggest that the
inertness of 8-Me under 1 atm of CO is due to kinetics, not
thermodynamics.
Figure 3. Proposed transition state for Zn-promoted insertion.
Scheme 3
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pyramidal geometry (the sum of angles around Zn is approxi-
mately 641ꢀ; the theoretical values for trigonal-pyramidal and
tetrahedral geometries are 630 and 657ꢀ, respectively), in con-
trast to the Zn atom in 8-Me, which is coordinated to only one N
and exhibits nearly tetrahedral geometry (sum of angles ∼654ꢀ).
The ZnꢀCl distance of 2.3126(3) Å falls within the range
observed for [NH4]2[ZnCl4] (2.11ꢀ2.40 Å).26
Reduction of Re Carbonyls with Hydride Reagents. Addi-
tion of ZnCl2(THF) to a CD2Cl2 solution of 1 gives ZnCl2
adduct 10, which upon addition of [(dmpe)2PtH][PF6] leads to
immediate formation of a Re formyl complex, revealed by the
1
appearance of a downfield H NMR signal (doublet at 15.06
ppm, 3JPH = 1 Hz); some H2 is also liberated. Initially formed in
approximately 80% yield by NMR,24 the formyl species slowly
decomposes over several days to give a red solution, which by
NMR contains some regenerated 1 as well as a new species
exhibiting a downfield 31P NMR shift (47.8 ppm), suggesting
that P may be part of a five-membered ring. This compound was
found by X-ray crystallography to have structure 11 (Scheme 3),
a rare example (only the second structurally characterized25) of
an organometallic complex containing a ReꢀZn bond
(Figure 4). The ReꢀZn bond length of 2.61977(15) Å is about
0.07 Å shorter than those in [PPh4]2[Re7C(CO)21ZnCl].25b
The Zn is coordinated to both N’s and has distorted-trigonal-
Reaction of 1 with Zn(OTf)2 and [(dmpe)2PtH][PF6] in
CD3CN generates a complex closely analogous to 11 (indicated
by the diagnostic downfield 31P NMR resonance) along with
1
some H2, and a new H NMR singlet at 9.57 ppm, which
corresponds well to the literature chemical shifts for formalde-
hyde in CD3CN.27 The latter signal disappears over a few hours
with growth of a new singlet at 3.35 ppm, typical of a CH3OX
species, which would be expected from the reaction of CH2O
with unreacted PtꢀH. The ReꢀZn-bonded species, analogous to
11, might thus arise via sequential transfer of two hydrides to a
CO, giving a zincoxymethyl group, which eliminates formalde-
hyde to generate the ReꢀZn bond (Scheme 4). Such a sequence
would be closely related to the known loss of formaldehyde from
Re(CH2OH) to give ReH.27b It should be noted, however, that
11 is also formed on reaction of Re(κ1(P)-PNN)(CO)4(H) with
ZnCl2; therefore, a route involving decarbonylation of the formyl
group might account for some of the 11 formed by the reaction of
Scheme 3.
Presumably the stability of the ReꢀZn bond allows formation
of formaldehyde (which is highly thermodynamically dis-
favored), but it also appears to preclude catalytic reaction. Bright
red 11 reacts rapidly with 1 equiv of Br2 in CD2Cl2 to cleave the
ReꢀZn bond and form light yellow Re(κ1(P)-PNN Zn(Br)-
3
(Cl))(CO)4(Br) (12). Along with the color change, the 31P
signal for the complex changes from 47.8 ppm for 11 to 10.4
ppm for 12; 10.4 ppm is nearly identical with the chemical shift
for 3 þ ZnCl2(THF) and indicates that the five-membered ring
containing phosphorus has been broken. Milder treatment of 11,
such as exposure to an atmosphere of H2, does not result in any
cleavage of the ReꢀZn bond.
Figure 4. X-ray structure of Re(PNN ZnCl)(CO)4 (11) (cocrystallized
3
CH2Cl2 has been omitted for clarity). Selected bond distances (Å) and
angles (deg): Re1ꢀZn2, 2.61977(15); Re1ꢀP1, 2.4273(3); Re-CO(av),
1.960; Zn2ꢀN1, 2.1259(8); Zn2ꢀN2, 2.1158(8); Zn2ꢀCl1, 2.3126(3);
Re1ꢀZn2ꢀN1, 139.93(2); Re1ꢀZn2ꢀN2, 96.70(2); Re1ꢀZnꢀCl3,
125.978(8); N1ꢀZnꢀN2, 76.55(3).
’ CONCLUSIONS
Re(κ1(P)-PNN) carbonyl complexes constitute an effective
structural framework for positioning a Lewis acidic Zn center to
Scheme 4
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promote CO transformations that can play an essential role in the
ultimate goal of homogeneous catalytic syngas conversion. The
ZnII provides thermodynamic stabilization of the acyl species
derived from either addition of alkylzinc reagents or migratory
insertion, as well as accelerating the latter reaction, presumably
by interacting with the carbonyl oxygen in the transition state.
Migratory insertion is promoted only when the ZnII is intramo-
lecularly anchored, not external. The formation of a formyl
species by addition of PtꢀH to ReꢀCO may be promoted by
the pendent Lewis acid (the analogous reaction of the simple
phosphine complex [Re(CO)5(PPh3)]þ also yields a formyl, but
one that is considerably less stable, decomposing over a period of
hours instead of days7a,b), but the observation of formaldehyde
demonstrates that under some conditions a second hydride
transfer can be effected, a reaction that does not take place with-
out Lewis acid promotion.7a,b These results show that the chelate
effect makes it possible to facilitate difficult carbonylations by
even relatively weak Lewis acids, such as Zn, that could be com-
patible with the requirements of a catalytic system. Ongoing
work on this approach includes exploration of the behavior of
other metals with the PNN ligand as well as development of
additional supporting architectures.
139.3 (d, 1C, 3JPꢀC = 7 Hz, 4-bpy C); 138.1 (s, 1C, 40-bpy C); 133.4 (d,
2C, 4JPꢀC = 3 Hz, p-Ph C); 133.0 (d, 4C, 2JPꢀC = 11 Hz, o-Ph C); 130.6
(d, 4C, 3JPꢀC = 11 Hz, m-Ph C); 129.4 (d, 2C, 1JPꢀC = 52 Hz, i-Ph C);
128.7 (d, 1C, 2JPꢀC = 18 Hz, 5-bpy C); 125.4 (s, 1C, 30-bpy C); 125.1
(d, 1C, 4JPꢀC = 2 Hz, 3-bpy C); 121.9 (s, 1C, 50-bpy C); 121.6 (q, 1C,
1JFꢀC = 321 Hz, OTf C). IR (CH2Cl2, cmꢀ1): 2158 (s), 2055 (vs). Anal.
Calcd for C28H17F3N2O8PReS: C, 41.23; H, 2.10; N, 3.43. Found:
C, 41.44; H, 1.97; N, 3.36.
Re(K2-PNN)(CO)3(Br) (2). PNN (100 mg, 0.294 mmol) was placed
in a bomb containing solid Re(CO)5Br (120 mg, 0.295 mmol) and
20 mL of benzene. The mixture was heated overnight to 100 ꢀC, and the
solvent was removed in vacuo to give the title compound as a bright
yellow solid (649 mg, 0.276 mmol, 94%). Yellow needles suitable for
X-ray structural analysis were grown by slow diffusion of pentane into a
benzene solution of 2 at room temperature. 1H NMR (400 MHz,
CD2Cl2, δ, ppm): 9.09 (d, 1H, J = 5 Hz), 8.23 (d, 1H, J = 8 Hz), 8.19 (d,
1H, J = 8.0 Hz), 8.06 (dt, 2H, J = 8, 1 Hz), 7.90 (t, 1H, J = 8 Hz), 7.8ꢀ7.7
(m, 5H, Ph, bpy), 7.5ꢀ7.3 (m, 8H, Ph, bpy). 31P{1H} NMR (162 MHz,
CD2Cl2, δ, ppm): 5.7 (s). IR (Pet. Ether, cmꢀ1): 2020 (s), 1921 (s),
1896 (s). Anal. Calcd for C25H17BrN2O3PRe C6H6: C, 48.44; H, 3.02;
3
N, 3.64. Found: C, 48.28; H, 2.72; N, 3.63.
Re(K1-PNN)(CO)4(Br) (3). PNN (100 mg, 0.294 mmol) was
protonated with HOTf (26.0 μL, 44.1 mg, 0.294 mmol) in 10 mL of
CH2Cl2, forming a yellow solution; after 5 min the solution was added to
a bomb containing solid [Re(CO)4(μ-Br)]2 (112 mg, 0.148 mmol) and
an additional 10 mL of CH2Cl2 was added. The mixture was heated to
60 ꢀC for 1 h. The solvent was removed in vacuo; the yellow residue was
taken up in benzene, flushed through a plug of alumina, and dried in
vacuo to give the title compound as an off-white powder (166 mg, 0.231
mmol, 79%). An X-ray-quality crystal was obtained as a large clear block
(along with many yellow needles of 2 which formed over time) by slow
diffusion of pentane into a benzene solution of 3 . 1H NMR (400 MHz,
CD2Cl2, δ, ppm): 8.69 (dd, 1H, J = 4.0, 0.7 Hz, 60-bpy), 8.50 (ddd, 1H,
J = 8.1, 2.6, 0.8 Hz, 5-bpy), 8.38 (d, 1H, J = 8.0 Hz, 50-bpy), 7.83 (tt, 2H,
J = 7.9, 2.6 Hz, 4,40-bpy), 7.78ꢀ7.68 (m, 4H, Ph), 7.63ꢀ7.41 (m, 6H,
Ph), 7.36 (ddd, 1H, J = 7.5, 4.7, 1.1 Hz, 3 or 30-bpy), 7.24 (ddd, 1H, J =
7.7, 3.4, 0.8 Hz, 3 or 30-bpy). 31P{1H} NMR (162 MHz, CD2Cl2,
δ, ppm): 5.4 (s). IR (petroleum ether, cmꢀ1): 2106 (m), 2020 (s), 2003
’ EXPERIMENTAL SECTION
Materials and Methods. All air- and moisture-sensitive com-
pounds were manipulated using standard vacuum line or Schlenk
techniques, or in a glovebox under a nitrogen atmosphere. The solvents
for air- and moisture-sensitive reactions were dried with activated
alumina by the method of Grubbs.28 All NMR solvents were purchased
from Cambridge Isotopes Laboratories, Inc. Benzene-d6 was distilled
from sodium benzophenone ketyl or titanocene. Dichloromethane-d2
and acetonitrile-d3 were distilled from calcium hydride and run through a
small column of activated alumina. Tetrahydrofuran-d8 was purchased in
a sealed ampule and dried by passage through activated alumina. Unless
noted, other materials were used as received. Re(CO)5Br was purchased
from Strem Chemicals, Inc. 6-Ph2P-2,20-bpy (PNN),8a [Re(CO)4(μ-
Br)]2,29 Re(CO)5OTf,30 and [HPt(dmpe)2][PF6]31 were synthesized
according to literature procedures. Elemental analyses were performed
(s), 1951 (m). Anal. Calcd for C26H17BrN2O4PRe CH2Cl2: C, 40.36;
3
H, 2.38; N, 3.49. Found: C, 40.57; H, 2.37; N, 3.54.
1
by Robertson Microlit, Madison, NJ. H and 13C NMR spectra were
Re(PNN)(CO)4(Me) (4-Me). PNN (100 mg, 0.294 mmol) and
Re(CO)5(CH3) (100 mg, 0.293 mmol) were placed in a bomb and
dissolved in 20 mL of toluene. The bomb was heated to 120 ꢀC for 24 h.
The solvent was removed under vacuum, and the resulting dark oily
product was extracted several times with ∼20 mL of hexanes. The
hexanes were removed under vacuum to give 8-Me as a sticky oil (65 mg,
0.100 mmol, 34%). Alternatively, reduction of 3 (200 mg, 0.290 mmol)
with Na/Hg (33.0 mg of Na/3.3 g of Hg, 1.17 mmol) and methylation
with MeI (36 μL, 82 mg, 0.580 mmol) gives 4-Me (121 mg, 0.186 mmol,
64%), as does thermolysis of 6-Me (80 mg, 0.117) at 100 ꢀC in benzene
recorded on Varian Mercury 300 MHz and Varian 400 MHz spectro-
meters at room temperature, unless indicated otherwise. Chemical shifts
are reported with respect to residual internal protio solvent for 1H and
13C{1H} spectra. 31P{1H} NMR spectra were referenced to external
85% H3PO4. X-ray crystallography was carried out by Dr. Michael W.
Day and Lawrence M. Henling using a Bruker KAPPA APEXII X-ray
diffractometer.
[Re(K1(P)-PNN)(CO)5][OTf] (1). A solution of Re(CO)5(OTf)
(140 mg, 0.295 mmol) and PNN (100 mg, 0.294 mmol) in 20 mL of
CH2Cl2 was heated in a bomb to 50 ꢀC for 1 h. The solvent was removed
in vacuo, and 10 mL of THF was added to the light yellow sticky product
and evaporated in vacuo to remove any residual CH2Cl2. The solid was
washed with 2 ꢁ 10 mL of THF to give a white powder (206 mg, 0.253
mmol, 86%). 1H NMR (300 MHz, CD2Cl2, δ, ppm): 8.75 (ddd, 1H, J =
4.8, 1.8, 1.0 Hz, 60-bpy H); 8.66 (ddd, 1H, J = 8.1, 3.2, 1.0 Hz, 3-bpy H);
8.31 (dt, 1H, J = 8.0, 1.1 Hz, 50-bpy H); 8.01 (td, 1H, J = 7.7, 1.8 Hz, 40-
bpy H); 7.96 (td, 1H, J = 7.8, 4.0 Hz, 4-bpy H); 7.66 (m, 6H, Ph), 7.56
(m, 4H, Ph); 7.45 (ddd, 1H, J = 7.6, 4.8, 1.2 Hz, 30-bpy H); 7.33 (ddd,
1H, J = 7.8, 3.5, 1.0 Hz, 5-bpy H). 31P{1H} NMR (121.5 MHz, CD2Cl2,
δ, ppm): 12.2 (s). 13C{1H} NMR (100.54 MHz, CD2Cl2, δ, ppm):
179.3 (br d, 4C, 2JPꢀC = 7 Hz, cis-CO’s); 176.0 (br d, 1C, 2JPꢀC = 39 Hz,
1
(20 mL) overnight (54 mg, 68%). H NMR (300 MHz, CD2Cl2, δ,
ppm): 8.67 (ddd, 1H, J = 4.8, 1.8, 0.9 Hz, bpy H); 8.47 (ddd, 1H, J = 8.0,
2.5, 1.0 Hz, bpy H); 8.41 (td, 1H, J = 8.0, 1.1 Hz, bpy H); 7.83 (ddd, 1H,
J = 8.0, 7.2, 1.8 Hz, bpy H); 7.80 (td, 1H, J = 7.9, 3.6 Hz, bpy H); 7.59 (m,
4H, Ph); 7.47 (m, 6H, Ph); 7.35 (ddd, 1H, J = 7.5, 4.8, 1.2 Hz, bpy H);
7.22 (ddd, 1H, J = 7.7, 3.5, 1.0 Hz, bpy H); ꢀ0.53 (d, 3JPꢀH = 7.7 Hz, 3H,
ReCH3). 31P{1H} NMR (121.5 MHz, CD2Cl2, δ, ppm): 12.5 (s). Anal.
Calcd for C27H20N2O4PRe: C, 49.61; H, 3.08; N, 4.29. Found: C, 51.02;
H, 3.63; N, 3.97.
Re(PNN)(CO)4(CH2OMe) (4-MOM). In the glovebox, compound
3 (250 mg, 0.362 mmol) was dissolved in 15 mL of THF and added
dropwise to a flask containing a 1% Na/Hg amalgam (40.5 mg of Na/4.1
g of Hg, 1.45 mmol). The solution turned dark purple after stirring for 4
h. The solution was decanted from the Na/Hg into a flask containing
3
trans-CO); 158.7 (d, 1C, JPꢀC = 20 Hz, 2-bpy C); 154.4 (s, 1C,
20-bpy C); 153.4 (d, 1JPꢀC = 83 Hz, 6-bpy C); 150.3 (s, 1C, 60-bpy C);
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Organometallics
ARTICLE
ClCH2OCH3 (41.2 μL, 0.543 mmol) dissolved in 10 mL of THF.
Immediately the solution lightened in color and a cloudy precipitate
formed. After it was stirred overnight, the solution was evaporated; the
residue was taken up in 15 mL of benzene and filtered through a plug of
silica gel. The benzene was then evaporated, leaving 4-MOM as a clear
oil (52 mg, 0.079 mmol, 0.22%). Solid, analytically pure samples could
not be obtained. 1H NMR (300 MHz, CD2Cl2, δ, ppm): 8.66 (d, 1H, J =
4.5 Hz, bpy H); 8.47 (dd, 1H, J = 7.7, 2.0 Hz, bpy H); 8.41 (d, 1H, J = 8.4
Hz, bpy H); 7.81 (m, 2H, bpy H); 7.6ꢀ7.5 (m, 4H, Ph); 7.49 (m, 6H,
Ph); 7.34 (dd, 1H, J = 7.5, 5.1 Hz, bpy H); 7.25 (dd, 1H, J = 7.5, 3.3 Hz,
bpy H); 3.72 (d, 2H, J = 6.0 Hz, CH2OCH3); 2.94 (s, 3H, CH2OCH3).
quantitative by 1H NMR. 1H NMR (500 MHz, CD2Cl2, δ, ppm): 8.77
(m, 3H, 3,50,60-bpy H); 8.37 (m, 1H, 40-bpy H); 8.31 (td, 1H, J = 8.1, 2.6
Hz, 40-bpy H); 7.83 (ddd, 1H, J = 7.6, 5.1, 1.0 Hz, 30-bpy H); 7.62 (m,
6H, Ph); 7.47 (m, 4H, Ph); 7.29 (ddd, 1H, J = 7.8, 4.9, 0.9 Hz, 5-bpy H);
2.76 (q, 2H, 3JHꢀH = 7.1 Hz, ReCOCH2CH3); 0.87 (t, 3H, 3JHꢀH = 8.1
Hz, ZnCH2CH3); 0.32 (t, 3H, 3JHꢀH = 7.1 Hz, ReCOCH2CH3); ꢀ0.55
3
(q, 2H, JHꢀH = 7.1 Hz, ZnCH2CH3). 31P{1H} NMR (121.5 MHz,
CD2Cl2, δ, ppm): 9.9 (s). 13C{1H} NMR (125.71 MHz, CD2Cl2, δ,
2
ppm): 285.8 (d, 1C, JPꢀC = 8 Hz, C(OZn)CH2CH3); 186.1 (d, 1C,
2JPꢀC = 8 Hz, cis-P-trans-COEt-CO); 185.2 (d, 1C, 2JPꢀC = 45 Hz, trans-
1
P-trans-COEt-CO); 156.9 (d, 1C, JPꢀC = 56 Hz, 6-bpy C); 153.5 (d,
31P{1H} NMR (121.5 MHz, CD2Cl2, δ, ppm): 14.4 (s). IR (ν, cmꢀ1
,
3JPꢀC = 11 Hz, 2-bpy C); 149.7 (s, 1C, 20-bpy C); 149.2 (s, 1C, 60-bpy
C); 143.1 (d, 1C, 3JPꢀC = 6 Hz, 4-bpy C); 142.7 (s, 1C, 40-bpy C); 134.4
(d, 4C, 2JPꢀC = 11 Hz, o-Ph C); 133.4 (d, 2C, 4JPꢀC = 2 Hz, p-Ph C);
131.5 (d, 1C, 2JPꢀC = 15 Hz, 5-bpy C); 130.7 (d, 4C, 3JPꢀC = 11 Hz, m-
Ph C); 128.2 (s, 1C, 30-bpy C); 125.6 (s, 1C, 3-bpy C); 124.8 (s, 1C, 50-
bpy C); 63.1 (s, 1C, C(OZn)CH2CH3); 12.9 (s, 1C, ZnCH2CH3); 8.4
(s, 1C, C(OZn)CH2CH3); ꢀ0.182 (s, 1C, ZnCH2CH3).
hexanes): 2085, 1993, 1978, 1941.
Re(PNN)(CO)4(Bn) (4-Bn). A procedure analogous to that for 4-
MOM was used, with PhCH2Br (64.6 μL, 0.543 mmol) instead of
ClCH2OCH3. 4-Bn was isolated as a light brown oil (113 mg, 0.161
mmol, 45%). Alternatively, 6-Bn (45 mg, 0.062 mmol) could be heated
to 80 ꢀC overnight in 15 mL of benzene to yield crude 4-Bn, which can
be purified by flushing the solution through a plug of silica (32 mg, 0.046
mmol, 71%). Solid, analytically pure samples could not be obtained. 1H
NMR (300 MHz, CD2Cl2, δ, ppm): 8.69 (ddd, 1H, J = 4.8, 1.8, 0.9 Hz,
bpy H); 8.51 (ddd, 1H, J = 8.0, 2.5, 1.0 Hz, bpy H); 8.48 (td, 1H, J = 8.0,
1.0 Hz, bpy H); 7.86 (dt, 1H, J = 7.8, 1.8 Hz, bpy H); 7.83 (td, 1H, J = 7.9,
3.6 Hz, bpy H); 7.66 (m, 4H, Ph); 7.52 (m, 6H, Ph); 7.37 (ddd, 1H, J =
7.6, 4.8, 1.2 Hz, bpy H); 7.23 (ddd, 1H, J = 7.7, 3.4, 1.0 Hz, bpy H); 6.98
(m, 1H, Bn H); 6.76 (dd, 2H, J = 8.2, 1.2 Hz, Bn H); 6.68 (t, 2H, J = 7.3
[Re(PNN ZnEt)(CO)4(COBn)][OTf] (5-Bn). The same proce-
3
dure as for 5-Me was used, but with ZnBn2 (1 drop); formation of 5-
Bn was quantitative by 1H NMR. ZnBn2 was formed by stirring ZnCl2
(100 mg, 0.733 mmol) and KBn (191 mg, 1.47 mmol) in 10 mL of
benzene for 30 min and then filtering the solution through Celite and
removing the solvent under vacuum. 1H NMR (300 MHz, CD2Cl2, δ,
ppm): 8.45 (m, 2H, bpy H); 8.7 (m, 2H, 4,40-bpy H); 7.65 (m, 6H, Ph);
7.52 (m, 4H, Ph); 7.31 (m, 2H, bpy H); 7.1ꢀ6.7 (m, 9H, Ph); 6.47 (d,
2H, o-Bn); 4.03 (br s, 2H, ReCOCH2Ph); 1.91 (br s, 2H, ZnCH2Ph).
31P{1H} NMR (121.5 MHz, CD2Cl2, δ, ppm): 10.8 (s).
3
Hz, Bn H); 1.94 (d, JPꢀH = 6.1 Hz, 3H, ReCH2Ph). 31P{1H} NMR
(121.5 MHz, CD2Cl2, δ, ppm): 13.4 (s).
Re(Ph2Ppy)(CO)4(Me). A procedure analogous to the synthesis of
4-Me was used, but with Ph2Ppy (77 mg, 0.294 mmol) instead of PNN.
The reaction was much cleaner than the PNN reaction and gave 152 mg
(0.252 mmol, 86% yield) of white powder. 1H NMR (300 MHz,
CD2Cl2, δ, ppm): 8.72 (d, 1H, J = 4.8 Hz, 6-H, py); 7.7ꢀ7.3 (m,
13H, Ph, py); ꢀ0.5 (d, 3H, J = 7 Hz, ReꢀCH3). 31P{1H} NMR (121.5
MHz, CD2Cl2, δ, ppm): 9.8 (s). IR (CH2Cl2, cmꢀ1): 2075, 2091,
1969, 1930.
Re(PNN)(CO)4(COMe) (6-Me). A CD2Cl2 solution of 5-Me,
prepared as described above, was washed with 10 mL of water in air
and dried over MgSO4. After filtration the CH2Cl2 was removed in
vacuo to yield 6-Me (11 mg, 0.016 mmol, 90%). 1H NMR (300 MHz,
CD2Cl2, δ, ppm): 8.68 (ddd, 1H, J = 4.8, 1.8, 1.0 Hz, bpy H); 8.49 (ddd,
1H, J = 8.0, 2.7, 1.0 Hz, bpy H); 8.41 (td, 1H, J = 8.0, 1.1 Hz, bpy H); 7.82
(m, 2H, bpy H); 7.62 (m, 6H, Ph); 7.49 (m, 4H, Ph); 7.35 (ddd, 1H, J =
4.8, 2.7, 1.1 Hz, bpy H); 7.24 (ddd, 1H, J = 4.3, 3.4, 1.1 Hz, bpy H); 2.16
(s, 3H, ReCOCH3). 31P{1H} NMR (121.5 MHz, CD2Cl2, δ, ppm):
13.8 (s). 13C{1H} NMR (100.54 MHz, CD2Cl2, δ, ppm): 257.5 (d, 1C,
[Re(PNN ZnMe)(CO)4(COMe)][OTf] (5-Me). To a CD2Cl2
3
solution (0.6 mL) of 1 (15 mg, 0.018 mmol) in a J. Young NMR tube
was added 1 drop of ZnMe2. The tube was capped and shaken; the
reaction was quantitatively complete by the time the 1H NMR spectrum
could be recorded. Removal of the solvent from the reaction by vacuum
and trituration with 5 mL of pentane yielded 5-Me as a sticky clear oil
(16 mg, 0.018 mmol, 100%). 1H NMR (300 MHz, CD2Cl2, δ, ppm):
8.77 (dd, 1H, J = 8.3, 1.1 Hz, 3-bpy H); 8.70 (m, 2H, 50,60-bpy H); 8.36
(td, 1H, J = 7.8, 1.8 Hz, 40-bpy H); 8.30 (td, 1H, J = 8.2, 2.6 Hz, 4-bpy H);
7.82 (ddd, 1H, J = 7.7, 5.0, 1.0 Hz, 30-bpy H); 7.62 (m, 6H, Ph); 7.48 (m,
4H, Ph); 7.27 (ddd, 1H, J = 7.9, 5.0, 0.9 Hz, 5-bpy H); 2.45 (s, 3H,
ReCOCH3); ꢀ1.31 (s, 3H, ZnCH3). 31P{1H} NMR (121.5 MHz,
CD2Cl2, δ, ppm): 9.4 (s). 13C{1H} NMR (100.54 MHz, CD2Cl2, δ,
ppm): 283.1 (br s, 1C, C(OZn)CH3); 188.2 (br s, 2C, cis-P-cis-COMe-
2
2JPꢀC = 10 Hz, C(O)CH3); 190.2 (d, 2C, JPꢀC = 10 Hz, cis-P-cis-
COMe-CO’s); 189.3 (d, 1C, 2JPꢀC = 46 Hz, trans-P-trans-COMe-CO);
2
1
189.1 (d, 1C, JPꢀC = 7 Hz, cis-P-trans-COMe-CO); 158.0 (d, 1C,
JPꢀC = 74 Hz, 6-bpy C); 157.3 (d, 3JPꢀC = 17 Hz, 2-bpy C); 155.5 (s, 1C,
20-bpy C); 149.8 (s, 1C, 60-bpy C); 137.8 (d, 1C, 3JPꢀC = 6 Hz, 4-bpy C);
137.5 (s, 1C, 40 bpy C); 134.3 (d, 4C, 2JPꢀC = 11 Hz, o-Ph C); 133.1 (br
d, 2C, 1JPꢀC = 47 Hz, i-Ph C); 131.5 (d, 2C, 4JPꢀC = 2 Hz, p-Ph C); 129.2
(d, 4C, 3JPꢀC = 10 Hz, m-Ph C); 128.1 (d, 1C, 2JPꢀC = 18 Hz, 5-bpy C);
124.8 (s, 1C, 30-bpy C); 122.3 (d, 1C, 4JPꢀC = 2 Hz, 3-bpy C); 122.0 (s,
1C, 50-bpy C); 56.4 (d, 1C, 3JPꢀC = 1 Hz, C(O)CH3). IR (petroleum
ether, cmꢀ1): 2087 (m), 1997 (s), 1978 (s), 1952 (s).
Re(PNN)(CO)4(COEt) (6-Et). The same procedure as for 6-Me was
used, but with 5-Et (10 mg, 0.016 mmol, 80%). 1H NMR (300 MHz,
C6D6, δ, ppm): 8.61 (td, 1H, J = 8.0, 1.1 Hz, bpy H); 8.55 (ddd, 1H, J =
7.9, 2.7, 1.0 Hz, bpy H); 8.47 (ddd, 1H, J = 4.8, 1.8, 0.9 Hz, bpy H); 7.72
(m, 4H, bpy H, Ph); 7.24 (td, 1H, J = 7.7, 1.8 Hz, bpy H); 7.1ꢀ6.9 (m,
8H, Ph); 6.66 (ddd, 1H, J = 7.6, 4.8, 1.6 Hz, bpy H); 2.64 (q, 3JHꢀH = 7.3
Hz, 2H, ReCOCH2CH3); 0.76 (t, 3JHꢀH = 7.3 Hz, 3H, ReCOCH2CH3).
31P{1H} NMR (121.5 MHz, C6D6, δ, ppm): 13.1 (s). IR (petroleum
ether, cmꢀ1): 2084 (m), 1993 (s), 1975 (s), 1948 (s).
2
CO’s); 186.0 (d, 1C, JPꢀC = 9 Hz, cis-P-trans-COMe-CO); 185.0 (d,
1C, 2JPꢀC = 45 Hz, trans-P-trans-COMe-CO); 156.7 (d, 1C, 1JPꢀC = 56
Hz, 6-bpy C); 153.4 (d, 3JPꢀC = 10 Hz, 2-bpy C); 149.6 (s, 1C, 20-bpy C);
3
148.9 (s, 1C, 60-bpy C); 143.0 (d, 1C, JPꢀC = 6 Hz, 4-bpy C); 142.8
(s, 1C, 40-bpy C); 134.5 (d, 4C, 2JPꢀC = 11 Hz, o-Ph C); 133.4 (d, 2C,
4JPꢀC = 2 Hz, p-Ph C); 131.5 (d, 1C, 2JPꢀC = 15 Hz, 5-bpy C); 130.7 (d,
4C, 3JPꢀC = 11 Hz, m-Ph C); 121.1 (br d, 2C, 1JPꢀC = 48 Hz, i-Ph C);
128.4 (s, 1C, 30-bpy C); 125.5 (d, 1C, 4JPꢀC = 1 Hz, 3-bpy C); 124.6 (s,
1
1C, 50-bpy C); 121.3 (q, 1C, JFꢀC = 321 Hz, OTf C); 57.6 (s, 1C,
Re(PNN)(CO)4(COBn) (6-Bn). The same procedure as for 6-Me
C(OZn)CH3); ꢀ15.0 (br s, 1C, ZnCH3). IR (CH2Cl2, cmꢀ1): 2100
(m), 2018 (s), 1997 (s), 1535 (w). Anal. Calcd for C30H23F3N2O8-
PReSZn: C, 39.55; H, 2.54; N, 3.07. Found: C, 39.27; H, 2.43; N, 2.97.
1
was used, but with 5-Bn (12 mg, 0.016 mmol, 88%). H NMR (300
MHz, CD2Cl2, δ, ppm): 8.69 (d, 1H, J = 4.2 Hz, bpy H); 8.51 (d, 1H, J =
8.0 Hz, bpy H); 8.43 (d, 1H, J = 8.0 Hz, bpy H); 7.83 (m, 2H, bpy H);
7.63 (m, 6H, Ph); 7.49 (m, 4H, Ph, Bn H); 7.36 (m, 2H, Hz, bpy H, Bn
H); 7.26 (dd, 1H, J = 7.4, 3.0 Hz, bpy H); 7.16 (m, 2H, Bn H); 6.80 (d,
[Re(PNN ZnEt)(CO)4(COEt)][OTf] (5-Et). The same procedure
3
as for 5-Me was used, but with ZnEt2 (1 drop); formation of 5-Et was
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Organometallics
ARTICLE
2H, J = 6.6 Hz, Bn H); 3.75 (s, 3H, ReCOCH2Ph). 31P{1H} NMR
(121.5 MHz, CD2Cl2, δ, ppm): 13.2 (s).
0.024 mmol); the solution was shaken and transferred to a J. Young
NMR tube. Compound 9 was the only species present in solution by the
time the first NMR spectrum could be taken; 9 was not isolated but was
observed in situ. A small amount of the solution was transferred to a
solution IR cell in the glovebox, and the IR was recorded. Washing the
solution of 9 with 10 mL of water resulted in the removal of Zn and re-
Re(PPh3)(CO)4(COMe). To a CD2Cl2 solution (0.6 mL) of
[Re(PPh3)(CO)5][OTf] (15 mg, 0.020 mmol) and 2,20-bipyridine
(4.0 mg, 0.026 mmol) in a J. Young NMR tube was added 1 drop of
ZnMe2. The tube was capped and shaken; the resulting 1H NMR
spectrum showed quantitative conversion to the acyl. The sample can be
washed with water in air in a manner analogous to the preparation of 6;
the resulting product is a mixture of Re(PPh3)(CO)4(COMe) and 2,20-
bipyridine by 1H NMR. 1H NMR (300 MHz, CD2Cl2, δ, ppm): 8.70 (d,
3H, J = 4.8 Hz, Ph); 8.29 (d, 3H, J = 8.1 Hz, Ph); 8.14 (td, 3H, J = 7.9, 1.6
Hz, Ph); 7.64 (dd, 3H, J = 7.1, 5.6 Hz, Ph); 7.47 (m, 3H, Ph); 2.28 (s, 3H,
ReCOCH3). 31P NMR (121.5 MHz, CD2Cl2, δ, ppm): 9.6 (s).
Re(PNN ZnCl2)(CO)4(Me) (7-Me) and Re(PNN Zn(Cl)(μ-
1
formation of 6-Me, as observed by IR. H NMR (400 MHz, CD2Cl2,
δ, ppm): 8.90 (d, 1H, J = 5.0 Hz, bpy H); 8.68 (dd, 1H, J = 8.0, 1.8 Hz, bpy
H); 8.63 (d, 1H, J = 8.1 Hz, bpy H); 8.45 (td, 2H, J = 8.0, 2.9 Hz, bpy H);
7.85 (dt, 1H, J= 7.3, 5.6 Hz, bpyH);7.61(m, 7H, Ph, bpyH); 7.47 (m, 4H,
Ph); 2.75 (s, 3H, ReCOCH3). 31P{1H} NMR (121.5 MHz, CD2Cl2, δ,
ppm): 13.4 (s). 13C{1H} NMR (100.54 MHz, CD2Cl2, δ, ppm): 294.1 (s,
1C, C(OZn)CH3); 187.4 (d, 2C, J = 9 Hz, cis-P-cis-COMe-CO’s); 186.1
(d, 1C, 2JPꢀC = 7 Hz, cis-P-trans-COMe-CO); 185.0 (d, 1C, 2JPꢀC = 45 Hz,
trans-P-trans-COMe-CO); 157.3 (d, 1C, 1JPꢀC = 58 Hz, 6-bpy C); 153.4
(d, 3JPꢀC = 13 Hz, 2-bpy C); 150.1 (s, 1C, 20-bpy C); 149.7 (s, 1C, 60-bpy
C); 143.7 (br s, 2C, 40,4-bpy C); 134.5 (d, 4C, 2JPꢀC = 11 Hz, o-Ph C);
133.5 (s, 2C, p-Ph C); 131.9 (d, 1C, 2JPꢀC = 14 Hz, 5-bpyC); 130.7 (d, 4C,
3JPꢀC = 11 Hz, m-Ph C); 129.6 (d, 2C, 1JPꢀC = 49 Hz, i-Ph C); 128.8 (s,
1C, 30-bpy C); 126.4 (s, 1C, 3-bpy C); 125.6 (s, 1C, 50-bpy C); 58.0 (s, 1C,
C(OZn)CH3). IR (CD2Cl2, cmꢀ1): 2103, 2023, 1998, 1971, 1518.
3
3
Cl))(CO)3(COMe) (8-Me). Solid ZnCl2(THF) (4.0 mg, 0.019 mmol)
was added to a CD2Cl2 solution (0.6 mL) of 4-Me (12.4 mg, 0.0190
mmol) in an NMR tube which was capped and shaken, giving a yellow
solution of 7-Me, which was not isolated but was observed in situ. 1H
NMR (300 MHz, CD2Cl2, δ, ppm): 8.69 (d, 1H, J = 4.1 Hz, 60-bpy H);
8.36 (d, 1H, J = 7.0 Hz, bpy H); 8.29 (d, 1H, J = 8.0 Hz, bpy H); 8.04 (m,
2H, bpy H); 7.67 (m, 4H, Ph); 7.53 (m, 8H, Ph, bpy H); ꢀ0.55 (d, 3H,
3JPꢀH = 7.5 Hz, ReCH3). 31P{1H} NMR (121.5 MHz, CD2Cl2, δ, ppm):
13.3 (s). Some chemical shifts are dependent on the concentration of
THF in solution. IR (CH2Cl2, cmꢀ1): 2078, 2018, 1945.
[Re(PNN ZnCl2)(CO)5][OTf] (10). Solid ZnCl2(THF) (4.0 mg,
3
0.019 mmol) and 1 (15 mg, 0.018 mmol) were combined in a vial and
dissolved in CD2Cl2 (0.6 mL), and the solution was transferred to a
J. Young NMR tube, yielding 10, which was observed in situ. 1H NMR
(300MHz, CD2Cl2, δ, ppm): 9.00 (ddd, 1H, J= 5.3, 1.6, 0.8 Hz, 60 bpyH);
8.40 (ddd, 1H, J = 7.9, 3.3, 0.9 Hz, bpy H); 8.21 (td, 1H, J = 7.8, 1.6 Hz,
bpy H); 8.05 (d, 1H, J = 7.7 Hz, bpy H); 8.01 (td, 1H, J = 7.9, 3.9 Hz, bpy
H); 7.62 (m, 11H, Ph, bpy H); 7.39 (ddd, 1H, J = 7.9, 3.5, 0.9 Hz, bpy H).
31P{1H} NMR (121.5 MHz, CD2Cl2, δ, ppm): 13.8 (s). Some chemical
shifts are dependent on the concentration of THF in solution.
Over the course of several hours 7-Me smoothly underwent migra-
tory insertion to form 8-Me, which was isolated by pouring the solution
into a vial and drying in vacuo (15 mg, 0.019 mmol, 100%). Small X-ray-
quality crystals were obtained by slow diffusion of pentane into a
1
CH2Cl2 solution. H NMR (300 MHz, CD2Cl2, δ, ppm): 9.73 (d,
1H, J = 5 Hz, 60-bpy H); 8.25 (m, 1H, bpy H); 8.14 (d, 1H, J = 8 Hz,
bpy H); 7.99 (d, 1H, J = 8 Hz, bpy H); 7.92 (td, 1H, J = 8 Hz, 2 Hz, bpy
H); 7.82 (m, 1H, bpy H); 7.62 (m, 3H, Ph); 7.43 (m, 7H, Ph); 2.26 (s,
3H, Re(COCH3)). 31P{1H} NMR (121.5 MHz, CD2Cl2, δ, ppm): 16.8
(s). IR (CH2Cl2, cmꢀ1): 2030 (s), 1952 (s), 1896 (s), 1492 (m), 1435
(m). 13C{1H} NMR (100.54 MHz, CD2Cl2, δ, ppm): 309.8 (s, 1C,
C(OZn)CH3); 196.5 (s, 1C, CO’s); 192.7 (s, 1C, CO’s); 192.2 (s, 1C,
CO’s); 159.2 (d, 1C, 1JPꢀC = 58 Hz, 6-bpy C); 152.4 (d, 3JPꢀC = 13 Hz,
2-bpy C); 152.1 (s, 1C, 20-bpy C); 151.9 (s, 1C, 60-bpy C); 143.0 (s, 1C,
40-bpy C); 141.0 (s, 1C, 4-bpy C); 134.4 (d, 4C, 2JPꢀC = 11 Hz, o-Ph C);
Reaction of 10 with [(dmpe)2PtH][PF6]. To a CD2Cl2 solution
(0.6 mL) of 10 in an NMR tube was added [(dmpe)2PtH][PF6] (12 mg,
0.018 mmol); the NMR showed the formation of some H2 and 1 in
addition to formation of Re(PNN ZnCl)(CO)4 (11) in increasing
3
amounts as the amount of PtH used was increased. (Alternatively,
11 was obtained from the reaction of CD3CN solution (0.6 mL) of 1
(15 mg, 0.018 mmol) and Zn(OTf)2 (7.0 mg, 0.019 mmol) with
[(dmpe)2PtH][PF6].) After several days X-ray-quality orange crystals
of 11 formed in the bottom of the NMR tube. Characterization data for
11 are as follows. 1H NMR (300 MHz, CD2Cl2, δ, ppm): 8.98 (ddd, 1H,
J = 5.1, 1.6, 0.9 Hz, 60-bpy H); 8.28 (dt, 1H, J = 8.1, 1.1 Hz, bpy H); 8.19
(dd, 1H, J = 7.6, 1.7 Hz, bpy H); 8.14 (ddd, 1H, J = 8.0, 2.5, 0.8 Hz, bpy
H); 7.96 (td, 1H, J = 7.8, 2.5 Hz, bpy H); 7.74 (ddd, 1H, J = 7.5, 1.5, 1.2
Hz, bpy H); 7.48 (m, 10H, Ph); 7.33 (ddd, 1H, J = 7.7, 2.0, 0.7 Hz, bpy
H). 31P{1H} NMR (121.5 MHz, CD2Cl2, δ, ppm): 47.8 (s). Anal. Calcd
3
1
133.2 (d, 4C, JPꢀC = 11 Hz, m-Ph C); 131.8 (d, 2C, JPꢀC = 49 Hz,
i-Ph C); 127.5 (s, 1C, 30-bpy C); 124.9 (s, 1C, 3-bpy C); 124.3 (s, 1C,
50-bpy C); 129.5 (m, 3C, p-Ph C, 5-bpy C); 52.8 (s, 1C, C(OZn)CH3).
Re(PNN Zn(Cl)(μ-Cl))(CO)3(COBn) (8-Bn). 4-Bn (10.5 mg,
3
0.0144 mmol) and ZnCl2(THF) (3.0 mg, 0.014 mmol) were combined
1
by the same procedure as for 8-Me; 8-Bn was not isolated. H NMR
(300 MHz, CD2Cl2, δ, ppm): 9.27 (d, 1H, J = 4.6 Hz, 60-bpy H); 8.43 (t,
2H, J=8.2,bpyH);8.36(m, 2H, bpyH); 8.11 (d, 1H, J=8.0,1.8Hz,bpyH);
7.62 (m, 4H, Ph); 7.49 (m, 6H, Ph); 7.28 (m, 1H, Ph); 7.16 (m, 2H, Ph);
6.93 (dd, 2H, J = 7.8, 1.7, Bn H); 4.20 (d, 1H, J = 14.6 Hz,
Re(COCH2Ph)); 3.71 (d, 1H, J = 14.6 Hz, Re(COCH2Ph)). 31P{1H}
NMR (121.5 MHz, CD2Cl2, δ, ppm): 16.1 (s).
for C26H17Cl1N2O4PReZn CH2Cl2: C, 39.34; H, 2.32; N, 3.40. Found:
3
C, 39.11; H, 2.51; N, 3.32.
Re(PNN Zn(Br)(Cl))(CO)4(Br) (12) in Situ from 11. To a
3
CD2Cl2 (0.6 mL) slurry of several crystals of 11 in an NMR tube was
added 1 drop of Br2, and the tube was shaken to yield an orange solution
(the product is yellow; the solution was tinted orange by unreacted Br2)
of 12. 1H NMR (300 MHz, CD2Cl2, δ, ppm): 8.86 (d, 1H, J = 4.7 Hz, 60-
bpy H); 8.49 (m, 2H, bpy H); 8.29 (m, 1H, bpy H); 8.16 (td, 1H, J = 8.2,
2.6 Hz, bpy H); 7.69 (m, 4H, Ph); 7.59 (m, 7H, Ph, bpy H); 7.48 (dd,
1H, J = 7.4, 3.8 Hz, bpy H). 31P{1H} NMR (121.5 MHz, CD2Cl2, δ,
ppm): 10.4 (s).
Reaction of Re(PNN)(CO)4(CH2OMe) (4-MOM) with ZnCl2-
(THF). 4-MOM (12.8 mg, 0.0187 mmol) and ZnCl2(THF) (4.0 mg,
0.019 mmol) were combined by the same procedure as for 8-Me. 7-
1
MOM forms rapidly and can be observed by H NMR (400 MHz,
C2D2Cl4, δ, ppm): 9.48 (br s, 1H, 60-bpy H); 8.15 (m, 1H, bpy H); 7.84
(br m, 2H); 7.52 (m, 12H), 7.25 (br d, 1H, J = 6 Hz, bpy H); 4.81 (br s,
2H, CH2OCH3); 2.90 (br s, 3H, CH2OCH3). Continued monitoring of
the sample shows no change, indicating that migratory insertion does
not occur. Evaporating the solvent from the NMR tube and redissolving
in THF-free CD2Cl2 results in no significant change in the chemical
shifts of 7-MOM. 7-MOM was not isolated.
NMR Kinetics. Because 4-Me and 4-Bn could not be isolated as
solids, standard solutions of the reagents were used: a sample of 4 was
dissolved in a measured amount of C2D2Cl4 along with a known amount
of a standard, such as ferrocene, a measured volume aliquot of the
1
solution was removed and diluted with C2D2Cl4, and the H NMR
Re(PNN ZnCl2)(CO)4(COMe) (9). To a CD2Cl2 solution (0.6 mL)
of 6-Me (15 mg, 0.022 mmol) was added ZnCl2(THF) (5.0 mg,
spectrum was recorded. From this the molarity of the solution could be
determined by comparison of the integrals of 4 to those of the standard.
3
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Organometallics
ARTICLE
A standard solution of 3:1 THF:ZnCl2 in C2D2Cl4 was also prepared.
The solution of 4 was combined with 10 equiv (based on Zn) of the Zn
solution and additional THF as needed in a J. Young NMR tube and then
diluted to a total volume of 0.8 mL with C2D2Cl4. The reaction progress
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1
was monitored by H NMR spectroscopy. Kinetics experiments using
ZnCl2(DME) (DME = 1,2-dimethoxyethane), ZnBr2(THF), and ZnI2-
(THF) instead of ZnCl2(THF) were also conducted using the same
procedure.
ZnCl2(THF). Anhydrous ZnCl2 (500 mg, 3.66 mmol) was dissolved
in 10 mL of dry THF, and the solvent was evaporated after all of the
ZnCl2 had dissolved. After it was dried under vacuum for 1 h, the vial was
weighed; 769 mg of white powdery contents were obtained (100.1%
yield based on ZnCl2(THF)). X-ray-quality clear hygroscopic needles
were grown from CH2Cl2/pentane, but data collection was stopped
when the preliminary structure was found to be the same as that
published for [Zn(Cl)(μ-Cl)(THF)]¥.15a
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’ ASSOCIATED CONTENT
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S
Supporting Information. CIF files, figures, and tables
b
giving crystal structure data for 2, 3, 8-Me, and 11. This material
is available free of charge via the Internet at http://pubs.acs.org.
Crystallographic data have also been deposited at the CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K., and copies can be
obtained on request, free of charge, by quoting the publication
citation and the deposition numbers 793163 (2), 793162 (3),
757498 (8-Me), an 757305 (11).
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: jal@caltech.edu (J.A.L.); bercaw@caltech.edu (J.E.B.).
Present Addresses
†Department of Chemistry and Biochemistry, University of the
Sciences, 600 S 43rd St., Philadelphia, PA 19104.
(15) (a) Bottomley, F.; Ferris, E. C.; White, P. S. Acta Cry-
stallogr., Sect. C: Cryst. Struct. Commun. 1989, C45, 816. (b) Dashti, A.;
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’ ACKNOWLEDGMENT
We acknowledge Larry Henling and Dr. Mike Day for their
assistance with crystallography, Dr. David VanderVelde for his
assistance with NMR spectroscopy, Alex Miller for helpful
discussions, and the continued generous financial support of
BP through the MC2 program. The Bruker KAPPA APEXII
X-ray diffractometer was purchased via an NSF CRIF:MU award
to the California Institute of Technology (No. CHE-0639094).
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