Generation and structural characterization of a gold(III) alkene complex

Article abstract of DOI:10.1002/anie.201209140

A Missing Gold Link: An Au^III alkene complex has been prepared and characterized by NMR spectroscopy and X-ray crystallography. Its bonding features have been analyzed by DFT calculations and natural bond orbital (NBO) analysis. In [(cod)AuMe₂]⁺, the unequal Au-C bond lengths result from the domination of the preference of 1,5-cyclooctadiene (cod) for nonparallel double bonds over back donation from the metal which favors parallel double bonds. Copyright

Full text of DOI:10.1002/anie.201209140

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DOI: 10.1002/anie.201209140
Gold Complexes
Generation and Structural Characterization of a Gold(III) Alkene
Complex**
Eirin Langseth, Margaret L. Scheuermann, David Balcells, Werner Kaminsky,
Karen I. Goldberg, Odile Eisenstein, Richard H. Heyn, and Mats Tilset*
Gold complexes are effective catalysts for a range of organic
transformations including the coupling of alkenes and alkynes
to other organic fragments.[1–10] A key step in these catalytic
the stability of this unusual molecule, in which the Au center
is directly bonded solely to carbon atoms.
The cyclometalated tolylpyridine (tpyH) complex
[(tpy)AuMe2] (1, Scheme 1) was prepared as recently de-
scribed.[38] Addition of triflic acid (HOTf, ca. 2.2 equiv) to
ꢀ
reactions is assumed to be the coordination of a C C multiple
bond to the gold center.[1,2,6] Although various catalytic cycles
involving AuIII p complexes have been proposed,[2,6,11,12] no
AuIII alkene,[13] alkyne, allene, or arene complexes have been
conclusively detected and characterized.[1,6,14,15] In 1964,
Chalk proposed the generation of [C8H12·AuCl3],[16] based
solely on an IR spectrum and elemental analysis of mixtures
containing two species.[14,15] Since then several groups have
attempted to prepare olefin complexes of AuIII chlorides and
bromides, but clear identification of an AuIII alkene complex
has been elusive.[14,15,17–21] Gas phase calculations of AuCl3 and
ethylene suggest that the alkene binding is exothermic.[22,23] In
contrast, AuI alkene complexes have been known for some
time with several examples reported[16,24–32] and in part
reviewed.[1–3,33–35]
Scheme 1. Protonation of [(tpy)AuMe2] 1 at low temperature followed
by addition of cod to generate [(cod)AuMe2+][Xꢀ] (3-X; X=OTf or
BArf). BArf=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate.
Recent interest in the characterization of potential AuIII
intermediates has resulted in the isolation of both an AuIII
hydride[36] and an AuIII vinyl[37] complex. Herein, we describe
the preparation of the first crystallographically characterized
AuIII alkene complex and discuss the factors that contribute to
a CD2Cl2 solution of 1 at ꢀ788C followed by warming to
ꢀ408C yielded the AuIII dimethyl species [(tpyH)AuMe2-
2
(OTf)] (2).[39] Selective protonolysis of the Au C(sp ) bond,
ꢀ
as has been observed in other AuIII complexes,[40] is confirmed
by the two inequivalent AuMe groups, the absence of
methane, and the symmetry of the tolyl group. NOE
interactions between the protons of one AuMe group and
aromatic protons of tpyH suggest that tpyH is coordinated to
Au as a monodentate ligand in 2 through the N atom. Triflate
presumably occupies the fourth site of the square plane
around the AuIII center.
[*] Dr. D. Balcells, Prof. M. Tilset
Centre for Theoretical and Computational Chemistry (CTCC)
Department of Chemistry, University of Oslo
P.O. Box 1033 Blindern, 0315 Oslo (Norway)
E-mail: mats.tilset@kjemi.uio.no
E. Langseth[+]
Addition of 1,5-cyclooctadiene (cod, 1 equiv) to the
solution of 2 at ꢀ788C followed by slow warming to 08C
resulted in the gradual consumption of cod with concomitant
Department of Chemistry, University of Oslo, Oslo (Norway)
M. L. Scheuermann,[+] Prof. W. Kaminsky, Prof. K. I. Goldberg
Department of Chemistry, University of Washington, Seattle (USA)
+
formation of the AuIII alkene complex [(cod)AuMe2 ][OTfꢀ]
Dr. R. H. Heyn
(3-OTf; Scheme 1) in 85% yield from 1 (NMR, internal
standard). The byproduct is tolylpyridinium triflate,
tpyH2OTf, formed by reaction of HOTf with tpyH liberated
from the Au center upon coordination of cod.
SINTEF Materials and Chemistry, Oslo (Norway)
Prof. O. Eisenstein
Institut Charles Gerhardt, UMR 5253 CNRS Universitꢀ Montpellier 2
Montpellier (France)
The 1H NMR spectrum of 3-OTf at 08C consists of a single
AuMe resonance at d = 1.71, a pair of multiplets for the CH2
groups in the backbone of the bound cod at d = 2.75 and 2.99,
and a broad singlet for the vinylic protons of the bound cod at
d = 6.39, which is 0.8 ppm higher than the signal of free cod in
CD2Cl2 at d = 5.56. Although vinylic protons typically shift to
lower d values on binding to a metal center,[41] as seen in the
AuI complex [(HB{3,5-(CF3)2pz}3)Au(C2H4)],[31] there are also
examples of shifts to higher d values with especially electron-
deficient metals, such as AgI in [(HB{3,5-(CF3)2tz}3)Ag-
and
Centre for Theoretical and Computational Chemistry (CTCC)
Department of Chemistry, University of Oslo, Oslo (Norway)
[+] These authors contributed equally to this work.
[**] This work was supported by the National Science Foundation under
grants no. DGE-0718124 and CHE-1012045, the Research Council of
Norway, grants no. 185513/I30 and 212030/F11 (travel grant to
M.S.), and the University of Oslo with a Kristine Bonnevie travel
grant to E.L.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209140.
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(C2H4)] (pz = pyrazolyl, tz = triazolyl).[42] The signal of the
ethylene protons of this Ag complex appears 0.3 ppm higher
than that of free ethylene. Notably, the 13C resonance for the
bound ethylene shifts to lower values by 13.6 ppm. In 3-OTf
the 13C signal of the cod sp2 carbon atoms is also shifted to
a higher d value, 134 ppm when compared to free cod at d =
129. Further support for the binding of cod to gold is
a through-space NOE interaction (700 MHz, 233 K) between
the vinylic protons of the bound cod and the AuMe groups.
The stability of 3-OTf at ambient temperature in solution
is somewhat limited. After 12 h at 258C, the concentration of
3-OTf (initially ca. 14 mm) decreased by approximately 50%
even with a cod concentration of 40 mm in CD2Cl2. Attempts
to obtain crystals from reaction mixtures containing 3-OTf
and excess HOTf invariably led to isolation of tpyH2OTf.
In an effort to isolate crystals of the AuIII alkene complex,
the reaction was repeated using 0.9 equivalent of HOTf to
suppress the formation of tpyH2OTf in the reaction mixture.
However, the subsequent addition of as much as 50 equiv-
alents of cod resulted in only approximately 30% yield of the
desired 3-OTf (NMR, internal standard). In addition, the
reaction mixture contained 2 (ca. 20%) and a new species (ca.
Figure 1. Two ORTEP views of the AuIII cation in the 100 K solid-state
structure of 3-BArf with 50% probability ellipsoids. Hydrogen atoms
are omitted for clarity. Selected bond lengths [ꢁ] and angles [8]: Au(1)–
C(1) 2.049(4), Au(1)–C(2) 2.055(4), Au(1)–C(3) 2.371(4), Au(1)–C(4)
2.415(4), Au(1)–C(7) 2.362(4), Au(1)–C(8) 2.406(4), C(3)–C(4)
1.348(5), C(7)–C(8) 1.364(5); C(3)-Au(1)-C(4) 32.71(13), C(7)-Au(1)-
C(8) 33.23(12), C(7)-Au(1)-C(4) 77.86(13), C(3)-Au(1)-C(8) 77.87(13),
C(1)-Au(1)-C(2) 85.0(2).
than the other (2.362(4) vs. 2.406(4) and 2.371(4) vs.
=
2.415(4) ꢀ, respectively). The C C bond lengths in 3-BArf
are 1.348(5) and 1.364(5) ꢀ. Several cationic AuI alkene
complexes have been recently structurally character-
ized.[24–30,32] Their Au Calkene bond lengths are in the range
ꢀ
of 2.098(5)[24]–2.37(1)[30] ꢀ, and the C C distances are in the
+
=
20%) that was subsequently identified as [(tpyH)2AuMe2 ]-
[OTfꢀ] (4-OTf) by comparison to an independently prepared
and crystallographically characterized sample.[39,43] Upon
addition of tpyH (ca. 6 equiv) to the equilibrium mixture
above, the composition of the reaction mixture changed to
< 5% yield of 3-OTf, approximately 20% of 2, and approx-
imately 60% of 4-OTf. These observations suggest that 2,
3-OTf, 4-OTf, free cod, and free tpyH are in equilibrium, that
is, cod and tpyH compete for coordination sites at the AuIII
moiety. Importantly, the 1H NMR resonances of 3-OTf
remain unchanged regardless of whether the other species
present in the reaction mixture are tpyH2OTf (resulting from
excess acid) or 2 and 4-OTf (less than 1 equiv of acid). From
this observation we infer that 3-OTf does not have an
associated tpyH ligand in solution. All attempts to obtain
crystals from reaction mixtures without excess HOTf resulted
only in crystals of 4-OTf.[39]
range of 1.319[26]–1.409(4)[25] ꢀ. It is noteworthy that the two
long Au Ccod distances in the cationic AuIII complex 3-BArf
ꢀ
ꢀ
are longer than the longest Au Calkene distances in the
structurally characterized, cationic AuI complexes.[24–30,32]
=
Furthermore, the C C distances in 3-BArf are essentially in
the middle of the range seen for these AuI complexes, thus
indicating some back-bonding from AuIII to the C C bonds,
=
despite the strong trans influence of the two methyl groups.
ꢀ
The Au Ccod bonds in 3-BArf are also considerably longer
than the Pt Ccod bonds in the isoelectronic PtII complex
ꢀ
=
[(cod)PtMe2], and the C C distances are significantly shorter
in the Au complex than in the Pt analogue (Table 1), thereby
hinting at reduced back-bonding from d(M) to p*(cod)
II
orbitals when AuIII is compared to Pt . The Au CMe bond
ꢀ
lengths in 3-BArf are 2.049(4) and 2.055(4) ꢀ, quite typical
for cationic AuIIIMe2 fragments with relatively weak donor
ligands trans to the methyl groups.[45,46]
When approximately one equivalent of [{3,5-(CF3)2-
C6H3}4Bꢀ][(Et2O)2H+] (HBArf·2 OEt2)[44] was used for the
protonolysis of 1 in CD2Cl2, followed by addition of 15
+
The structures of [(cod)AuMe2 ] and [(cod)PtMe2],
optimized at the DFT level with the hybrid PBE0 functional
and quasi relativistic effective core potential (ECP) for Au
(see computational details[47]), indispensable for representing
the structures and reactivity of Au complexes,[57,58] are in
excellent agreement with the solid-state structures (Table 1
and the Supporting Information). In particular, the calcula-
tions reproduce very well the nonequivalence of the two
+
equivalents of cod, [(cod)AuMe2 ][BArfꢀ] (3-BArf) was
generated in approximately 70% yield (NMR, internal
standard). The 1H NMR chemical shifts for the AuMe
groups and the vinylic and CH2 protons of bound cod in
3-BArf appear within 0.2 ppm of the corresponding resonan-
ces in a spectrum of 3-OTf. The solution containing 3-BArf
was layered with pentane and left in a freezer at ꢀ358C,
resulting in small crystalline demispheres suitable for X-ray
diffraction. The structure of the cation of 3-BArf is shown in
Figure 1.[43]
ꢀ
=
M Ccod bond lengths to the same C C, and the larger
difference between these distances for Au versus Pt. For
+
[(cod)AuMe2 ], the distances differ by 0.052 ꢀ, but for
[(cod)PtMe2] they differ by only 0.024 ꢀ. In relation to this,
the olefinic carbon atoms C3, C4, C7, and C8 are not
coplanar; the calculated dihedral angles C3-C4-C7-C8 are
13.78 and 11.68 for AuIII and PtII, respectively. These
calculated values are in excellent agreement with the solid-
state values of 13.48 and 12.78, which show that these
structural features are not due to crystal packing or to the
counteranion in the case of AuIII, since the geometries were
Compound 3-BArf crystallized in the monoclinic space
group P21/c with the Au cation sitting in a pocket formed by
the aryl groups of several BArfꢀ anions.[43] The Au Ccod bonds
ꢀ
are rather long for a metal alkene complex, 2.389 ꢀ (avg.).
The cation is nearly C2 symmetric (not crystallographically
=
imposed). Interestingly, each cod C C bond is asymmetrically
ꢀ
bonded to Au such that one Au C bond is significantly longer
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Table 1: Selected geometrical parameters for [(cod)AuMe2+],
[(cod)PtMe2], and free cod, and distortion energies of the cod ligand.[a]
See Figure 1 for atom labels.
distortion energy is the back donation from the metal to the
two p* orbitals as shown by an NBO (natural bond orbital)
analysis. The four orbitals containing the 8 d electrons were
identified amongst the NLMOs (natural localized molecular
orbitals). The dz2 and dx2ꢀy2 orbitals appear as lone pairs
located on the metal center, with no significant contribution
from the ligands (see the Supporting Information). In
contrast, the dxz and dyz orbitals are involved in the d!p*
[(cod)AuMe2+
Exp.[b] Theo.[c] Exp.[d]
]
[(cod)PtMe2]
cod
Theo.[c] Exp.[e]
Theo.[c]
Distances [ꢁ]
M–C1[f]
2.049(4) 2.055 2.040(11) 2.057
2.055(4) 2.055 2.072(11) 2.057
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
M–C2[f]
M–C3 (d1)[f] 2.362(4) 2.390 2.204(11) 2.211
=
back donation from the metal center to the C C bond of cod.
M–C4 (d2)[f] 2.406(4) 2.442 2.255(12) 2.235
+
This interaction is weaker in [(cod)AuMe2 ], where the
D(d2ꢀd1)
0.044
0.052 0.051
0.024
=
contribution from the p*(C C) (see the Supporting Informa-
M–C7 (d3)[f] 2.371(4) 2.390 2.224(11) 2.211
M–C8 (d4)[f] 2.415(4) 2.442 2.230(11) 2.235
tion) is 2.2% with the dxz and 2.8% with the dyz. The
corresponding numbers for [(cod)PtMe2] are 9.5% with the
D(d4ꢀd3)
D(avg.)[g]
C3–C4
0.044
0.044
0.052 0.006
0.052 0.029
0.024
0.024
d
xz and 12% with the dyz. The difference between Au and Pt is
1.364(5) 1.366 1.348(17) 1.385 1.340(3) 1.335
1.348(5) 1.366 1.361(17) 1.385 1.340(3) 1.335
apparent in the graphical representation of these NLMOs
(Figure 2) and is consistent with the greater distortion (more
C7–C8
Angles [8]
C3-C4-C7-C8 13.4
C4-C5-C6-C7 46.5
13.7
44.4
44.4
12.7
34.5
33.1
11.6
36.7
36.7
20.2
63.8
63.8
23.6
61.2
61.2
C8-C9-C10-
C3
42.6
E [kcalmolꢀ1
Distortion
E[h]
]
–
4.7
–
12.7
–
0.0
[a] Distances [ꢁ], angles [8], energies (kcalmolꢀ1). [b] X-ray structure
reported in this work. [c] DFT (PBE0) calculations;[47] [d] Data from X-ray
structure.[54] An earlier determined structure, for which the cif file is not
available, gives a D(avg.) of 0.024 ꢁ;[55] [e] Data from gas-phase electron
diffraction structure;[56] [f] M=Au, Pt; [g] Mean value of D(d2ꢀd1) and
D(d4ꢀd3); [h] Energy difference, E(bound)ꢀE(free), between optimized
free cod and cod in the confirmations calculated in [(cod)AuMe2+] and
[(cod)PtMe2], respectively.
Figure 2. NLMOs (with orbital phases in green/red) associated with
the metal d!p*(cod) back donation.
fully optimized in the gas phase and in the absence of
a counteranion. To determine the origin of these features, free
cod was optimized at the same level of theory. In free cod, the
C3-C4-C7-C8 dihedral angle is 23.68, showing that the two
eclipsed) of the -CH2CH2- backbones of cod in the Pt complex
=
and the increasing elongation of the C C bonds (1.335 ꢀ in
+
free cod, 1.366 ꢀ in [(cod)AuMe2 ], and 1.385 ꢀ in
=
C C bonds are distinctively nonparallel. It thus appears that
[(cod)PtMe2]). These results show that, compared to Pt, the
=
=
the two C C bonds become increasingly parallel upon
metal d!p*(C C) back donation is weaker, but still signifi-
coordination and more so for PtII than AuIII. This trend is
attributed to the preference for a staggered conformation of
the -CH2CH2- backbone of cod. In free cod where no
constraints are present, the two -CH2CH2- units are fully
staggered as indicated by the dihedral angles C4-C5-C6-C7
and C8-C9-C10-C3 of 61.28, associated with the dihedral
angle C3-C4-C7-C8 of 23.68. Coordination of the metal to the
cant, in [(cod)AuMe2 ], thus stabilizing this unprecedented
+
AuIII bis(alkene) complex.
In summary, we have successfully characterized the first
AuIII alkene complex, including the crystal structure of
+
[(cod)AuMe2 ][BArfꢀ]. This unambiguous demonstration of
the existence of AuIII alkene complexes validates their
inclusion in mechanistic proposals and will allow for further
work to better understand mechanisms that potentially
involve AuIII alkene complexes. Reactivity studies of 3-OTf
along with improved synthesis of this and other AuIII
complexes with p-bound ligands are currently under inves-
tigation.
=
two C C bonds, which forces the two double bonds to be
more parallel, also forces the -CH2CH2- linkers to assume
a more eclipsed conformation. The effect is smaller for AuIII
than PtII as shown by the C4-C5-C6-C7 and C8-C9-C10-C3
dihedral angles of 458 and 358 (avg.), respectively. The
-CH2CH2- backbone never reaches a fully eclipsed confor-
=
Received: November 14, 2012
Published online: January 2, 2013
mation and the two C C bonds never become coplanar, which
ꢀ
leads to nonequivalent M C bond lengths.
Distorting the cod ligand from its preferred conformation
to its conformations in the metal complexes has an energy
cost of 4.7 kcalmolꢀ1 and 12.7 kcalmolꢀ1 for AuIII and PtII,
respectively. The electronic effect that compensates for this
Keywords: density functional calculations · gold ·
.
gold(III) alkene · homogeneous catalysis · reactive intermediates
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