Syntheses, x-ray crystal structures, and solution behavior of cationic, two-coordinate gold(I) η2-diene complexes

Article abstract of DOI:10.1021/om200291s

A family of cationic gold p-diene complexes of the form {[P(t-Bu) ₂o-biphenyl]Au(η²-diene)}+SbF₆^- were isolated in >80% yield from reaction of various dienes with a mixture of [P(t-Bu)2o-biphenyl]AuCl and AgSbF6 and were characterized by spectroscopy and, in four cases, by X-ray crystallography. Solution and solid-state analysis of these complexes established selective binding of gold to the less substituted CdC bond of the diene in the case of dienes that possessed differentially substituted CdC bonds. ¹³C NMR analysis and evaluation of the relative binding affinities of substituted dienes point to a bonding model in which the gold-alkene interaction is stabilized via donation of electron density from the uncomplexed CdC bond to the complexed CdC bond of the diene. Variable-temperature NMR analysis of gold p-diene complexes revealed fluxional behavior consistent with facile (δG = 9.6-11.9 kcal mol-1) intramolecular exchange of the complexed and uncomplexed CdC bonds of the diene ligand.

Full text of DOI:10.1021/om200291s

ARTICLE
pubs.acs.org/Organometallics
Syntheses, X-ray Crystal Structures, and Solution Behavior of Cationic,
Two-Coordinate Gold(I) η2-Diene Complexes
Rachel E. M. Brooner and Ross A. Widenhoefer*
French Family Science Center, Duke University, Durham, North Carolina 27708-0346, United States
S Supporting Information
b
ABSTRACT: A family of cationic gold π-diene compꢀlexes of
the form {[P(t-Bu)2o-biphenyl]Au(η2-diene)}þSbF6 were
isolated in >80% yield from reaction of various dienes with a
mixture of [P(t-Bu)2o-biphenyl]AuCl and AgSbF6 and were
characterized by spectroscopy and, in four cases, by X-ray
crystallography. Solution and solid-state analysis of these com-
plexes established selective binding of gold to the less substituted CdC bond of the diene in the case of dienes that possessed
differentially substituted CdC bonds. 13C NMR analysis and evaluation of the relative binding affinities of substituted dienes point
to a bonding model in which the goldꢀalkene interaction is stabilized via donation of electron density from the uncomplexed CdC
bond to the complexed CdC bond of the diene. Variable-temperature NMR analysis of gold π-diene complexes revealed fluxional
behavior consistent with facile (ΔG‡ = 9.6ꢀ11.9 kcal molꢀ1) intramolecular exchange of the complexed and uncomplexed CdC
bonds of the diene ligand.
’ INTRODUCTION
complexes.18,19 Here we report the synthesis, solution and solid-
state structure, and fluxional behavior of cationic, two-coordinate
gold(I) η2-diene compounds that contain a di-tert-butyl
o-biphenylphosphine ligand.
In recent years, considerable effort has been directed toward
the development of cationic gold(I) complexes as catalysts for
the π-activation of CꢀC multiple bonds toward addition of
carbon and heteroatom nucleophiles.1 Included in this family of
transformations is the gold(I)-catalyzed intermolecular 1,2-addi-
tion of the XꢀH bond of a sulfonamide,2 carbamate,3 thiol,4 or
indole5 to the less substituted CdC bond of a conjugated diene
and also the gold(I)-catalyzed intramolecular 1,4-addition of the
NꢀH bond of a sulfonamide to a conjugated diene.6 The
selective 1,2-addition of the nucleophile across the less substi-
tuted CdC bond in the gold(I)-catalyzed intermolecular hydro-
functionalization of conjugated dienes distinguishes these
transformations from other transition metal-catalyzed diene
hydrofunctionalization processes, which typically occur with
net 1,4-addition7,8 and/or telomerization.8,9
’ RESULTS AND DISCUSSION
Synthesis of Gold η2-Diene Complexes. Cationic, two-
coordinate gold(I) η2-diene complexes were synthesized em-
ploying a procedure similar to that used to prepare cationic
gold(I) η2-alkene complexes.12 For example, treatment of a
methylene chloride suspension of [P(t-Bu)2o-biphenyl]AuCl
and AgSbF6 (1:1) with trans-1,3-pentadiene (10 equiv) for 6 h
at room temperature followed by filtration and removal of
volatile materials led to the isolation of {[P(t-Bu)2o-biphenyl]-
Au[η2-(E)-H2CdC(H)C(H)dC(H)Me]}þSbF6 (1) in 93%
ꢀ
Gold(I)-catalyzed diene hydrofunctionalization has been pro-
posed to occur via initial π-coordination of the diene to gold,
either through the less substituted CdC bond3ꢀ6 or to both
CdC bonds,2 followed by outer-sphere addition of the nucleo-
phile and subsequent protodeauration. An outer-sphere pathway
involving attack of the nucleophile on a gold η2-diene inter-
mediate was further supported by computational analysis of the
gold(I)-catalyzed intramolecular hydroamination of 3-methyl-
1,3-pentadiene with benzyl carbamate.10 Despite the potential
role of cationic gold(I) π-diene complexes in the gold(I)-
catalyzed hydrofunctionalization of conjugated dienes, no well-
characterized gold π-diene complexes have been documented.11
As such, information regarding the structure, bonding, and
fluxional behavior of cationic gold(I) π-diene complexes is limited
to that provided by computational analysis7 or that gleaned from
related gold π-alkene,12ꢀ14 alkyne,14ꢀ16 arene,16,17 and allene
yield as an air and thermally stable white solid (Scheme 1).
Likewise, the cationic gold(I) η2-diene complexes {[P(t-Bu)2o-
biphenyl]Au(η2-diene)}þSbF6 [diene = 4-methyl-1,3-penta-
ꢀ
diene (2), 2,4-dimethyl-1,3-pentadiene (3), 1,3-butadiene (4),
2,3-dimethyl-2-butene (5), 1,3-cyclohexadiene (6), and 1,4-cyclo-
hexadiene (7) were isolated in >80% yield as air- and thermally
stable white solids. Although stable indefinitely in the solid state,
these gold π-diene complexes were only modestly stable in
solution and tended to decompose when concentrated. This
behavior complicated purification, and solutions of these com-
plexes typically contained traces (∼5%) of the free diene. More
specifically, complexes 2 and 3 were effectively crystallized from
concentrated CH2Cl2 solutions, which, in the case of 2, also
Received: April 4, 2011
Published: May 04, 2011
r
2011 American Chemical Society
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ARTICLE
Scheme 1
Figure 1. ORTEP drawing of 1. Ellipsoids are shown at the 50%
probability level. Counterion and hydrogen atoms are omitted for clarity.
Selectedbonddistances(Å), bondangles(deg), andtorsionalangles(deg):
AuꢀC1 = 2.226(7), AuꢀC2 = 2.332(5), C1ꢀC2 = 1.363(9), C2ꢀC3 =
produced X-ray quality crystals. Conversely, crystallization of 1,
4, and 7 from concentrated CH2Cl2 solutions produced X-ray
quality crystals but also formed amorphous material that de-
graded the bulk purity of the sample. Attempted crystallization of
5 and 6 led to degradation of the sample without crystal
formation. Nevertheless, complexes 1ꢀ7 displayed satisfactory
combustion analysis, were unambiguously characterized in solution
by 1H, 13C, and 31P NMR spectroscopy, and, in cases of complexes
1, 2, 4, and 7, were characterized by X-ray crystallography.
X-ray Crystal Structure of 1. Slow diffusion of hexanes into a
CH2Cl2 solution of 1 at 4 °C gave colorless crystals of 1 suitable
for single-crystal X-ray analysis (Figure 1). The crystal structure
of 1 established a monomeric complex with the trans-1,
3-pentadiene ligand η2-bonded to gold through the less sub-
stituted C1dC2 bond. Complex 1 adopts a distorted linear
conformation with a PꢀAuꢀalkene(cent) angle of 169.3° and
with the diene ligand unsymmetrically bonded to gold with a
shorter AuꢀC1 and longer AuꢀC2 interaction (Δd d 0.113 Å).
The coordinated C1dC2 bond of the diene is elongated relative
to the uncomplexed C3dC4 bond (Δd = 0.049 Å), and the diene
moiety adopts a near planar s-trans configuration with a
C1ꢀC2ꢀC3ꢀC4 dihedral angle of 169.9°. The diene ligand is
positioned such that the angle defined by AuꢀC1ꢀC2 is rotated
by ∼12° relative to the plane defined by AuꢀPꢀC14 with the
CdC(H)Me group directed away from the protruding phenyl
ring and along the AuꢀPꢀC14 plane. The protruding phenyl
group of the o-biphenyl phosphine moiety is not perpendicular to
the P-bound aryl ring, but instead is rotated toward the C6
quaternary carbon atom with a C14ꢀC19ꢀC20ꢀC21 dihedral
angle of 70.6°. The distance between the gold atom and the plane
of the protruding phenyl group is 2.88 Å with a gold to Cipso
distance of AuꢀC20 = 3.018 Å, suggesting the presence of a weak
Arꢀarene interaction. Similar Auꢀarene interactions have been
observed for gold dialkyl(o-biphenyl)phosphine17,20,21 and related
gold complexes,22 although the Auꢀarene distance of 1 is shorter
than is typically observed (Auꢀarene(plane) = 3.0ꢀ3.2 Å,17,20
AuꢀCipso = 3.16ꢀ3.36 Å).21
1.394(7), C3ꢀC4 = 1.313(10), C4ꢀC5 = 1.492(12), Auꢀarene(plane)
=
2.88, AuꢀC20 = 3.018, PꢀAuꢀC1dC2(cent) = 163.3, C1ꢀC2ꢀC3 =
128.4(5), C2ꢀC3ꢀC4 = 126.8(5), C1ꢀC2ꢀC3ꢀC4 = 169.9(8),
C14ꢀC19ꢀC20ꢀC21 = 70.6(9), C14ꢀC19ꢀC20ꢀC25 = 120.0(8).
strong preference toward η4-coordination of the diene.24 Exam-
ples of transition metal η2-diene complexes include three-co-
ordinate palladium, platinum, and nickel complexes that contain
both an η2-1,3-butadiene ligand and a chelating bis(phosphine)
ligand.25 Thermolysis of (η5-C5Me5)W(σ-alkyl)(π-allyl) com-
plexes generates 16-electron η2-diene intermediates that can be
trapped as their 18-electron PMe3 adducts.26 Photolysis of
tungsten,27,28 molybdenum,28,29 rhenium,30 and cobalt31 carbonyl
compounds in the presence of diene also forms η2-diene com-
pounds. Similarly, treatment of molybdenum η4-diene compounds
with a tertiary phosphine or CO forms the corresponding
molybdenum η2-diene compound.32 Iron η2-diene complexes
have been isolated from rearrangements of iron-allylidene
compounds33 and from the reaction of iron σ-alkenyl com-
pounds with triphenylcarbenium-hexafluorophosphate.34
Solution-State Structure and Fluxional Behavior of 1. The
solution-state structure and fluxional behavior of trans-1,3-pen-
tadiene complex 1 was probed spectroscopically. In the 13C
NMR spectrum of 1 at ꢀ80 °C,35 the resonance corresponding
to the C1 carbon atom of bound trans-1,3-pentadiene (δ 90.6, d,
JCP = 10.1 Hz) was shifted ∼24 ppm to lower frequency relative
to that of the free diene (δ 114.4), while the C2 (δ 137.1) and C3
(δ 129) diene resonances of 1 were only slightly perturbed (Δδ
<4 ppm) relative to the resonances of free trans-1,3-pentadiene
(Table 1). In comparison, the C4 diene resonance of 1 (δ 144.3)
was shifted to higher frequency by ∼14 ppm relative to that of
free diene (δ 129.8). The 1H NMR spectrum of 1 mirrored that
of the 13C NMR spectrum. For example, the C1 diene protons
of 1 [δ 4.17, 4.11] were shifted ∼0.8 ppm to lower frequency
relative to those of free trans-1,3-pentadiene (δ 5.06, 4.93),
while the C2 (δ 6.51) and C3 (δ 5.99) diene resonances of
1 differed by e0.2 ppm relative to those of free trans-1,
3-pentadiene. In comparison, the C4 proton of 1 (δ 6.26)
was shifted to higher frequency by ∼0.5 ppm relative to that of
free diene.
The η2-coordination of the trans-1,3-pentadiene ligand in
complex 1, as opposed to η4-coordiation, is not surprising owing
to the pronounced tendency of gold(I) to form two-coordinate
14-electron complexes, rather than three-coordinate, 16-electron
complexes.23 Nevertheless, η2-complexes of conjugated dienes
are rare, as most transition metal diene complexes display a
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Table 1. 13C NMR Chemical Shifts and JPC of the Diene sp2 Carbon Atoms of Complexes 1ꢀ6 in CD2Cl2 and Chemical Shift
Difference (Δδ) Relative to Free Diene
C1
C2
C3
Δδ (ppm)a
C4
Δδ (ppm)a
complex
temp (°C)
δ (ppm)
J
PC (Hz)
Δδ (ppm)a
δ (ppm)
JPC (Hz)
Δδ (ppm)a
δ (ppm)
δ (ppm)
1
ꢀ80
25
90.6
90.6
91.7
94.9
91.9
114.7
10.0
15.0
16.3
12.6
15.0
8.3
ꢀ23.8
ꢀ23.8
ꢀ21.9
ꢀ22.6
ꢀ21.2
ꢀ11.6
137.1
134.3
158.7
133.5
154.7
117.4
ꢀ0.1
0.8
129b
ꢀ3.6
ꢀ2.6
ꢀ2.3
ꢀ4.8
ꢀ3.4
ꢀ3.3
144.3
153.1
149.4
128.6
124.2
136.4
14.5
17.5
15.5
11.1
11.1
10.1
2
123.5
125.1c
133.3
140.2
121.1
3
25
16.3
ꢀ4.6
10.9
ꢀ7.0
4d
5d
6d
ꢀ90
ꢀ60
ꢀ70
2.5
6.2
a Negative Δδ corresponds to a shift of the complex to lower frequency relative to free diene; positive Δδ corresponds to a shift of the complex to higher
frequency relative to free diene. b C3 diene resonance overlapped with an aromatic o-biphenyl resonance. c JPC = 5.2 Hz. d C1 and C2 are defined as the
gold-bound CdC bond.
The spectroscopy of the cationic gold 1-hexene complex
ꢀ
{[P(t-Bu)2o-biphenyl]Au[η2-H2CdC(H)(CH2)3CH3]}þSbF6
(8) serves as a point of comparison for the spectroscopy of diene
complex 1.12 The 13C NMR spectrum of 8 displays a large shift
(Δδ≈ ꢀ16) of the C1alkenecarbon atom to lower frequency and
small shift of the C2 carbon atom to higher frequency (Δδ ≈ ∼2)
relative to the corresponding resonances of free 1-hexene. Like-
wise, the terminal alkene protons of 8 are shifted ∼0.5 ppm to
lower frequency, while the internal alkenes protons are shifted
∼0.3 ppm to higher frequency. We have attributed this behavior to
unsymmetric binding of the alkene to gold with stronger interac-
tion between AuꢀC1 relative to AuꢀC2, a contention that was
supported by X-ray crystallographic studies.12 Therefore, the
significant shifts of the C1 diene carbon atom and C1 diene
Figure 2. Temperature dependence of the quaternary tert-butyl 13C
NMR resonance of 1 from ꢀ50 to ꢀ80 °C in CD2Cl2.
protons of 1 to higher frequency indicate that the diene binds to
gold through the C1dC2 bond, as was observed in the solid state.
The most notable features of the NMR spectroscopy of 1 were the
pronounced shifts of the C4 carbon resonance and C4 vinyl
protons to higher frequency relative to free diene. These observa-
tions point to stabilization of the goldꢀπ interaction of 1 through
donation of electron density from the uncomplexed C3dC4 bond
to the complexed C1dC2 bond, leading to depletion of electron
density at the C4 diene carbon atom of 1.
a more likely mechanism for racemization of 1 involves intra-
molecular exchange of the complexed and uncomplexed CdC
bonds of the trans-1,3-pentadienyl ligand of 1 through s-cis and s-
trans η4-diene transition states and intermediates (Scheme 2).
For example, intramolecular ligand exchange of (R)-1 via the s-cis
η4-diene intermediate s-cis-I would form the undetected
C3dC4-bound isomer 1a (Scheme 2). A second intramolecular
ligand exchange via the s-trans η4-diene intermediate s-trans-I
would form (S)-1. Note that both s-cis and s-trans η4-diene
intermediates are required for racemization of 1; a mechanism
involving either s-cis or s-trans η4-diene intermediates would lead
to exchange of the complexed and uncomplexed diene CdC
bonds of 1 without racemization (see below).
X-ray Crystal Structures of 2, 4, and 7. In addition to trans-
1,3-pentadiene complex 1, 4-methyl-1,3-butadiene complex 2,
1,3-butadiene complex 4, and 1,4-cyclohexadiene complex 7
were analyzed by X-ray crystallography (Figures 3ꢀ5). Refine-
ment of 2 was complicated by an orientational disorder of the
4-methyl-1,3-pentadiene ligand, which precluded meaningful
analysis of the bond distances and angles within the 4-methyl-
1,3-pentadiene ligand. Nevertheless, the crystal structure of 2
revealed a distorted linear η2-diene complex with the gold atom
bound unsymmetrically to the less substituted C1dC2 bond of
the diene with a shorter AuꢀC1 and longer AuꢀC2 interaction,
as was observed for complex 1 (Figure 3). In the major con-
former of 2 (2a, 57% occupancy), the diene ligand is positioned
so that the AuꢀC1ꢀC2 plane is rotated by 77.4° relative to the
AuꢀPꢀC15 plane with the C3 carbon atom directed away from
Because the C1dC2 bond of trans-1,3-pentadiene is prochiral,
a static structure for 1 possesses diastereotopic tert-butyl groups.
Indeed, at ꢀ80 °C the 13C NMR spectrum of 1 displayed a pair of
phosphorus-coupled doublets at δ 37.0 and 36.9 (JCP = 24 Hz),
corresponding to the quaternary tert-butyl carbon atoms
(Figure 2). As the temperature was raised, these doublets both
shifted to higher frequency and broadened, forming a single
phosphorus-coupled doublet at δ 38.2 (JCP = 24 Hz) at ꢀ50 °C
that sharpened further upon warming to room temperature.
The temperature-dependent behavior of the quaternary tert-
butyl 13C resonances of 1 was independent of free trans-1,3-
pentadiene (150 mM), which argues against an intermolecular
pathway for interconversion of the phosphorus-bound tert-butyl
groups. We were unable to satisfactorily simulate the tempera-
ture-dependent 13C NMR spectra of 1, perhaps due in part to the
temperature-dependent chemical shifts of the quaternary tert-butyl
carbon atoms.
The presence of equivalent tert-butyl groups in the 13C NMR
spectrum of 1 at eꢀ50 °C points to a facile fluxional process that
exchanges the prochiral faces of the trans-1,3-pentadiene ligand.
Although exchange of alkene π-faces without displacement has
been documented in the case of a cationic rhenium complex,36
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the protruding phenyl ring of the o-biphenylphosphine ligand. In
the minor conformer (2b, 43% occupancy), the coordinated
C1dC2 bond is bound through the opposite π-face relative to 2a
and is positioned such that the AuꢀC1ꢀC2 plane rotated by
66.8° relative to the AuꢀPꢀC15 plane with the C3 carbon atom
directed toward the protruding phenyl group. In both 2a and 2b,
the protruding phenyl group of the o-biphenyl phosphine moiety
is rotated toward the C7 quaternary carbon atom with
C15ꢀC20ꢀC21ꢀC22 = 80.2° and with an Auꢀarene(plane)
distance of 3.02 Å.
Similar to 2, complex 4 suffered from an orientational disorder
of the 1,3-butadiene ligand. In both conformers of 4, gold adopts
a distorted linear conformation [PꢀAuꢀalkene(cent) = 166ꢀ
168°] with the η2-butadiene ligand bound unsymmetrically to
gold through a shorter AuꢀC1 and longer AuꢀC2 interaction
(Δd = 0.9ꢀ0.10 Å; Figure 4). In the major conformer of 4 (4a,
64% occupancy), the coordinated C1dC2 bond of the diene (as
defined by the AuꢀC1ꢀC2 plane) is rotated ∼38° relative to the
AuꢀPꢀC14 plane with the unbound C3dC4 group directed
away from the o-phenyl ring and along the AuꢀPꢀC14 plane. In
the minor conformer of 4 (4b, 36% occupancy), the diene ligand is
bound through the opposite π-face relative to 4a and is posi-
tioned such that the AuꢀC1ꢀC2 plane is rotated ∼20° relative
to the AuꢀPꢀC14 plane with the unbound C3dC4 moiety
directed away from the proximal phenyl group. In both con-
formers, the protruding phenyl group of the o-biphenyl phos-
phine moiety is rotated toward the C9 quaternary carbon atom
with a C13ꢀC14ꢀC19ꢀC24 dihedral angle of 63.5° and with an
Auꢀarene(plane) distance of 2.96 Å.
Complex 7, which contains a nonconjugated 1,4-cyclohexa-
diene ligand, adopts a slightly distorted linear conformation
(PꢀAuꢀalkene(cent) angle of 171.3°) with the diene ligand
bound more symmetrically to gold (Δd AuꢀC1/AuꢀC2 =
0.031 Å; Figure 5) than was observed for acyclic 1,3-diene
complexes 1, 2, and 4. The coordinated C1dC2 bond of the
cyclohexadiene ligand is positioned roughly parallel to the pro-
truding phenyl ring of the o-biphenyl moiety with the remainder
of the cyclohexadiene ligand directed away from the proximal
phenyl group. The two aryl rings of the o-biphenylphosphine
ligand are perpendicular to one another with a C15ꢀC20ꢀ
C21ꢀC22 dihedral angle of 91.0°. Within the 1,4-cyclohexa-
diene moiety, the coordinated C1dC2 bond is elongated by
0.038 Å relative to the uncomplexed C3dC4 bond, and the
CꢀCdCꢀC groupings associated with both the complexed and
uncomplexed CdC bonds are planar, with C6ꢀC1ꢀ
C2ꢀC3 and C3ꢀC4ꢀC5ꢀC6 dihedral angles of <2°. Interest-
ingly, the 1,4-cyclohexadiene ligand adopts a boat-like conforma-
tion with an angle of ∼18° between the C6ꢀC1ꢀC2ꢀC3 and
C3ꢀC4ꢀC5ꢀC6 planes with the concave face directed toward
the tert-butyl groups.
Scheme 2
Spectroscopy of Complexes 2ꢀ6. The vinylic protons and
sp2 diene carbon atoms of complexes 3, 4, and 6 were unam-
biguously assigned from the 1H and 13C NMR spectra supported
1
by various combinations of 1Hꢀ H COSY, HMQC, HMBC, and
variable-temperature 13C NMR spectroscopy. The vinylic protons
and sp2 diene carbon atoms of complexes 2 and 5 were confidently
assigned from the 1H and 13C NMR spectra and by analogy to the
spectroscopy of complexes 1, 3, 4, and 6. As was the case with 1,
assignment of 13C resonances was aided by the presence of
phosphorus coupling to the C1 diene carbon atom with JCP ranging
from 12.6 to 16.3 Hz for acyclic diene complexes 2ꢀ5 and 8.3 Hz
for cyclohexadiene complex 6 (Table 1). Details regarding these 1H
and 13C assignments are provided in the Supporting Information.
Several points regarding the 13C NMR spectroscopy of com-
plexes 1ꢀ6 are worth noting (Table 1). Each of the complexes
Figure 3. ORTEP drawings of 2a (left structure) and 2b (right structure). Ellipsoids are shown at the 50% probability level. Counterion and hydrogen
atoms are omitted for clarity. Selected bond distances (Å) and angles (deg) for 2a: AuꢀC1 = 2.245(4), AuꢀC2 = 2.292(8), Auꢀarene(plane) = 3.02,
AuꢀC21 = 3.036, PꢀAuꢀC1dC2(cent) = 169.06, C15ꢀC20ꢀC21ꢀC22 = 80.2(6), C15ꢀC20ꢀC21ꢀC26 = 108.8(6). Selected, unique bond
distances (Å) and angles (deg) for 2b: AuꢀC1 = 2.245(4), AuꢀC2 = 2.356(11), PꢀAuꢀC1dC2(cent) = 162.49.
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Figure 4. ORTEP drawings of 4a (left structure) and 4b (right structure). Ellipsoids are shown at the 50% probability level. Counterion and hydrogen
atoms are omitted for clarity. Selected bond distances (Å) and angles (deg) for 4a: AuꢀC1 = 2.215(4), AuꢀC2 = 2.312(3), C1ꢀC2 = 1.376(5), C2ꢀC3 =
1.456(5), C3ꢀC4 = 1.326(7), Auꢀarene(plane) = 2.92, AuꢀC19 = 3.064, PꢀAuꢀC1dC2(cent) = 165.98, C1ꢀC2ꢀC3 = 123.2(5), C2ꢀC3ꢀC4 =
121.9(6), C1ꢀC2ꢀC3ꢀC4 = 167.2(4), C13ꢀC14ꢀC19ꢀC20 = 125.76(17), C13ꢀC14ꢀC19ꢀC24 = 63.5(2). Selected, unique bond distances (Å) and
angles (deg) for 4b: AuꢀC1 = 2.212(7), AuꢀC2 = 2.300(6), C1ꢀC2 = 1.351(11), C2ꢀC3 = 1.474(11), C3ꢀC4 = 1.346(14), PꢀAuꢀC1dC2(cent)
167.82, C1ꢀC2ꢀC3 = 123.3(10), C2ꢀC3ꢀC4 = 120.2(11), C1ꢀC2ꢀC3ꢀC4 = 172.1(8).
=
the C4 carbon atom, reveals that the shift of the C4 carbon atom
to higher frequency relative to free diene increases with increas-
ing substitution at the C4 carbon atom in the order 4 (Δδ = 11.1)
< 1 (Δδ = 14.5) < 2 (Δδ = 17.5). This behavior may be indicative
of increased donation from the uncomplexed to complexed
CdC bond with increasing methyl substitution of the C4 carbon
atom. Conversely, complexes 3 and 5, which possessed a C2
methyl group, displayed more pronounced shift of the C2 diene
carbon atom to higher frequency (Δδ = ꢀ16.3 for 3 and Δδ =
ꢀ11.1 for 5) than did the complexes lacking substitution at the
C2 carbon atom. This behavior may suggest a more even
distribution of partial positive charge between the C2 and C4
carbon atoms of complexes 3 and 5 relative to complexes lacking
substitution at the diene C2 position.
Fluxional Behavior of Complexes 3ꢀ7. As was observed for
1,3-pentadiene complex 1, 2,4-dimethyl-1,3-pentadiene complex
3 displayed fluxional behavior consistent with intramolecular
exchange of the prochiral C1dC2 π-faces. The 13C NMR
spectrum of 3 at ꢀ90 °C displayed a pair of phosphorus-coupled
doublets at δ 36.3 and 36.2 (JCP = 24 Hz) corresponding to the
quaternary carbon atoms of diastereotopic tert-butyl groups
(Figure 6). As the temperature was raised, these resonances
broadened and coalesced, forming a single phosphorus-coupled
doublet at δ 36.8 (JCP = 24 Hz) at ꢀ60 °C; this coalescence
behavior was unaffected by the presence of free 2,4-dimethyl-1,
3-pentadiene (150 mM). These observations are consistent with
racemization of 3 via intramolecular CdC bond exchange
through s-cis and s-trans η4-diene transition states and inter-
mediates, as was proposed for complex 1 (Scheme 2).
Complexes 4ꢀ7, which contain diene ligands composed of
identically substituted CdC groups, displayed variable-tempera-
ture NMR behavior consistent with facile intramolecular ex-
change of the complexed and uncomplexed CdC bonds of the
diene ligand. For example, the 1H NMR spectrum of 1,3-butadiene
complex 4 at ꢀ90 °C displayed six one-proton multiplets at
Figure 5. ORTEP drawing of 7 with ellipsoids shown at the 50%
probability level. Counterion and hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): AuꢀC1 = 2.264(5),
AuꢀC2 = 2.295(5), C1ꢀC2 = 1.352(7), C2ꢀC3 = 1.496(8), C3ꢀC4 =
1.509(9), C4ꢀC5 = 1.314(8), C5ꢀC6 = 1.504(8), Auꢀarene(plane)
=
3.01, AuꢀC21 = 3.060, PꢀAuꢀC1dC2(cent) = 171.69, C1ꢀC2ꢀC3 =
121.4(5), C2ꢀC3ꢀC4 = 112.7(5), C3ꢀC4ꢀC5 = 123.2(5), C4ꢀC5ꢀ
C6 = 123.3(5), C2ꢀC1ꢀC6 = 123.7(5), C14ꢀC19ꢀC20ꢀC21 =
15.7(7), C15ꢀC20ꢀC21ꢀC22 = 91.0.
2ꢀ6 displayed a shift of the C1 diene carbon resonance to lower
frequendy and a shift of the uncomplexed C4 diene carbon to
higher frequency relative to the corresponding resonances of free
diene, as was observed for 1 (Table 1). In the cases of complexes
2 and 3, which possess dissimilarly substituted CdC double
bonds, these features established binding of the diene to gold in
solution through the less substituted C1dC2 bond, as was
observed for 1. Further comparison of the 13C spectra of com-
plexes 1, 2, and 4, which differ in the degree of methyl substitution at
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Figure 6. Temperature dependence of the quaternary tert-butyl 13C
NMR resonances of 3 from ꢀ90 to ꢀ60 °C in CD2Cl2.
Figure 8. Temperature dependence of the vinylic (left column) and
1
diene methyl (right column) H NMR resonances of 5 from ꢀ60 to
20 °C. Resonances at δ 5.0 and 1.88 correspond to free diene, and the
resonance at δ ∼1.65 corresponds to water. Vertical and horizontal
scales are not consistent between the two sets of spectra.
broadened and coalesced, forming a broad four-proton multiplet
at δ ∼4.8 at 20 °C. As was the case with 4, significant broadening
of the vinylic and diene methyl resonances of 5 occurred prior to
detectable broadening of the resonances of residual 2,3-dimethyl-
1,3-butadiene, arguing against an intermolecular pathway for the
exchange. An energy barrier of ΔG‡261K = 11.9 kcal molꢀ1 for the
diene CdC bonds of 5 was determined from line shape analysis
of the diene methyl resonances at the coalescence temperature
(τc = ꢀ12 °C).
1
The H NMR spectrum of 1,3-cyclohexadiene complex 6 at
ꢀ85 °C displayed a 1:1:1:1 ratio of broad one-proton singlets at
δ 6.18, 6.01, 5.59, and 5.13, corresponding to the H4, H3, H2,
and H1 vinyl protons, respectively. As the temperature was
raised, the H2/H3 pair of vinylic resonances at δ 6.01 and 5.59
broadened and coalesced at ca. ꢀ40 °C, forming a broad singlet
at δ 5.89 at 0 °C. Over the same temperature range, the H1/H4
pair of vinylic resonances at δ 6.18 and 5.13 broadened and
coalesced at ca. ꢀ30 °C, forming a broad singlet at ∼5.7 at 0 °C
(Figure 9). An energy barrier of ΔG‡233K = 10.4 kcal molꢀ1 for
exchange of the complexed and uncomplexed CdC bonds of 6
was determined from line shape analysis of the H2/H3 vinylic
resonances near the coalescence point (τc = ꢀ40 °C). Worth
noting is the 1:1 pair of mutiplets at δ 2.46 and 2.32 (Δν = 71
Hz) in the fast-exchange (0 °C) spectrum of 6, corresponding to
the time-averaged C5/C6 protons positioned either cis or trans to
the gold phosphine group (Figure 9). The presence of chemically
inequivalent cis and trans C5/C6 protons establishes that the
interconversion of complexed and uncomplexed CdC bonds of
the 1,3-cyclohexadiene ligand of 6 occurs without concomitant
exchange of the two faces of the 1,3-cyclohexadiene ligand.
Because an intermolecular pathway for exchange of the com-
plexed and uncomplexed CdC bonds of the 1,3-cyclohexadiene
ligand of 6 would also interconvert the cis-C5/C6 and trans-C5/
C6 protons, we can safely rule out an intermolecular pathway for
exchange of the complexed and uncomplexed CdC bonds of the
1,3-cyclohexadiene ligand of 6.
Figure 7. Temperature dependence of the vinylic 1H NMR resonances
of 4 from ꢀ90 to 15 °C in CD2Cl2. The small multiplets at δ 6.3, 5.2, and
5.1 correspond to free 1,3-butadiene.
δ 6.48 and 6.18, corresponding to the C2 and C3 vinylic protons,
and at δ 5.66, 5.56, 4.11, and 3.93 corresponding to the C4
(higher frequency) and C1 (lower frequency) vinylic resonances
(Figure 7). As the temperature was raised, the resonances
corresponding to C2 and C3 protons broadened and coalesced
at ꢀ60 °C to form a time-averaged multiplet at δ 6.40 at 15 °C.
Over the same temperature range, the resonances corresponding
to the C1 and C4 protons broadened and coalesced at ꢀ45 °C to
form a two-proton multiplet at δ ∼5.0 at 15 °C. Also worth
noting was that significant broadening of the vinylic resonances
of 4 occurred prior to detectable broadening of the vinylic
resonances of residual 1,3-butadiene, arguing against an inter-
molecular pathway for the exchange. An energy barrier of ΔG‡
=
214
9.6 kcal molꢀ1 for interconversion of the diene CdC bonds of 4
was obtained from line shape analysis of the C2/C3 diene
resonances near the coalescence temperature (τc = ꢀ59 °C).
1
The H NMR spectrum of 2,3-dimethyl-1,3-butadiene com-
plex 5 at ꢀ60 °C displayed a 1:1 ratio of three-proton singlets at
δ 2.25 and 1.73 corresponding to the diene methyl protons and a
1:1:1:1 ratio of one-proton resonances at δ 5.65, 5.48, 3.99, and
3.64 corresponding to the C4 (higher frequency) and C1 (lower
frequency) vinylic protons (Figure 8). As the temperature was
raised, the diene methyl resonances broadened and coalesced
at ꢀ12 °C, forming a broad singlet (ν1/2 = 19 Hz) at 2.04 at
20 °C. Over the same temperature range, the vinylic resonances
1
The H NMR spectrum of 1,4-cyclohexadiene compound 7
at ꢀ70 °C displayed a 1:1 ratio of resonances at δ 5.57 and 5.71
corresponding to the vinylic protons of the complexed and
uncomplexed CdC bond of the diene, respectively (Figure 10).
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Table 2. Line-Broadening Data and Observed Rate Con-
stants (kobs) for the Exchange of the 2,4-Dimethyl-1,3-pen-
tadiene Ligand of 3 ([3] = 161 mM) with Free 2,4-Dimethyl-
1,3-pentadiene in CD2Cl2 at 25 °C
[2,4-dimethyl-1,
3-pentadiene] (M)
Δν1/2 (Hz)
Δν1/2(excess) (Hz)
kobs (sꢀ1
)
0
6.9
11.0
13.7
15.8
17.9
19.2
0
0
0.09
0.14
0.18
0.22
0.25
4.0
12.7
21.3
27.9
34.3
38.5
6.8
8.9
10.9
12.3
(ΔG‡233 = 9.6 kcal molꢀ1), while the 2,3-dimethyl-1,3-butadiene
‡
complex 5 displayed the highest barrier for exchange (ΔG261
=
11.9 kcal molꢀ1). Although we cannot rule out ground-state
destabilization of 4 relative to 5 as the origin of these differences
(see below), the higher energy barrier for exchange in the case of
5 presumably reflects increased steric destabilization of the
transition state for formation of the three-coordinate, η4-diene
intermediate. In the case of complexes 4 and 5, exchange of the
complexed and uncomplexed CdC bonds of the diene ligand
though either s-cis or s-trans η4-diene intermediates and transi-
tion states fully accounts for the observed fluxional behavior. Of
course in the case of 1,3-cyclohexadiene complex 6, exchange
must occur solely though s-cis η4-diene intermediates and
transition states. In comparison, interconversion of the diaster-
eotopic tert-butyl groups of 3 requires CdC bond exchange
through both s-cis and s-trans η4-diene intermediates and transi-
tion states, as was proposed for complex 1. Although we were
unable to obtain satisfactory simulations of the temperature-
dependent 13C NMR spectra of complexes 1 and 3, the facile
racemization of 1 and 3 suggests that both the s-cis and s-trans η4-
diene intermediates and transition states are easily accessible.
Furthermore, the similar energy barriers observed for CdC bond
exchange of 1,3-cyclohexadiene complex 6 and 1,4-cyclohexa-
diene complex 7 further suggest that the stability of the three-
coordinate intermediates is only modestly sensitive to the
geometry of the two alkene ligands and also that conjugation
plays no significant role in the energetics of CdC bond
exchange.
Figure 9. Temperature dependence of the vinylic (left column) and
1
allylic (right column) H NMR resonances of 6 from ꢀ85 to 0 °C in
CD2Cl2. The resonance indicated with an “ꢁ” corresponds to an
impurity. Vertical and horizontal scales are not consistent between the
two sets of spectra.
1
Figure 10. Temperature dependence of the vinylic H NMR reso-
nances of 7 from ꢀ70 to ꢀ20 °C in CD2Cl2.
As the temperature was raised, the vinylic resonances broadened
and coalesced at ꢀ45 °C, forming a single resonance at δ 5.7
at ꢀ10 °C. An energy barrier of ΔG‡233 = 10.9 kcal molꢀ1 for
exchange of the complexed and uncomplexed CdC bonds of 7
was determined from line shape analysis of the ꢀ45 °C spectrum.
Kinetics of Intermolecular Diene Exchange with 3. All of
our observations regarding the exchange of complexed and
uncomplexed CdC bonds of diene complexes 1 and 3ꢀ7 argue
against intermolecular pathways for CdC bond exchange. To
independently determine the rate of intermolecular diene ex-
change, we studied the kinetics of 2,4-dimethyl-1,3-butadiene
exchange with 3 as a function of [2,4-dimethyl-1,3-butadiene]
in CD2Cl2 at 25 °C employing 1H NMR line-broadening
techniques.37 Addition of 2,4-dimethyl-1,3-butadiene (93 mM)
to a solution of 3 (160 mM) in CD2Cl2 at 25 °C led to excess
line broadening of the C3 vinylic proton resonance at δ 5.74 of
Δν1/2(excess) = 4.0 Hz, which corresponds to a first-order rate
constant for exchange of kobs = π[Δν1/2(excess)] = 12.7 sꢀ1
(Table 2). A plot of kobs versus [2,4-dimethyl-1,3-butadiene]
over the concentration range 0.093ꢀ0.25 M was linear, with a
second-order rate constant for 2,4-dimethyl-1,3-butadiene exchange
1
The H NMR spectrum of 7 at ꢀ70 °C also displayed a four-
proton multiplet at δ 2.85 corresponding to the allylic pro-
tons, which shifted slightly to higher frequency with increasing
temperature, but remained otherwise unchanged over the tem-
perature range ꢀ70 to ꢀ20 °C. As was the case with 1,3-cyclo-
hexadiene complex 6, this observation argues against an inter-
molecular process for CdC bond exchange in 7 that would
equilibrate the cis and trans allylic protons, leading to collapse of
the allylic multiplet.
Several points are worth noting regarding the intramolecular
exchange of the complexed and uncomplexed CdC bonds of
the diene ligands of complexes 3ꢀ7. The energy barriers for
exchange of complexed and uncomplexed CdC bonds for
of kex = 169 ( 1 Mꢀ1 sꢀ1 (ΔG298‡ = 14.40 ( 0.01 kcal molꢀ1
;
complexes 4ꢀ7 fall in the range ΔG‡ = 9.6ꢀ11.9 kcal molꢀ1
.
Figure 11). These data support an associative pathway for inter-
molecular diene exchange through the cationic, three-coordinate
Butadiene complex 4 displayed the lowest barrier for exchange
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Table 3. Equilibrium Constants for Displacement of Isobu-
tylene from [P(t-Bu)2o-biphenyl]Au(η2-H2CdCMe2)]þ-
SbF6ꢀ (9) with Dienes in CD2Cl2 at ꢀ70 °C
diene
Keq
1,3-cyclohexadiene
1.21 ( 0.05
0.93 ( 0.06
0.6 ( 0.2
2,4-dimethyl-1,3-pentadiene
1,4-cyclohexadiene
Figure 11. Plot of kobs versus [2,4-dimethyl-1,3-butadiene] for the
intermolecular exchange of the 2,4-dimethyl-1,3-butadiene ligand of 3
([3] = 160 mM) with free 2,4-dimethyl-1,3-butadiene from 93 to
245 mM at 25 °C in CD2Cl2.
2,3-dimethyl-1,3-butadiene
trans-1,3-pentadiene
0.14 ( 0.02
0.09 ( 0.01
in the order 1,3-butadiene < trans-1,3-pentadiene < 2,4-dimethyl-
1,3-pentadiene. This observation further supports the bonding
model of conjugated dienes to cationic gold(I) involving stabi-
lization of the gold π-interaction through donation of electron
density from the uncomplexed CdC bond to the complexed
CdC bond of the diene ligand, as was also suggested by 13C
NMR analysis of cationic gold η2-diene complexes.
bis(η2-diene) intermediate {[P(t-Bu)2o-biphenyl]Au[η2-H2Cd
C(Me)C(H)dCMe2]2}þSbF6ꢀ. As expected, the energy barrier
for intermolecular diene exchange is considerably higher than
was observed for the intramolecular exchange of complexed and
uncomplexed CdC bonds in complexes 4ꢀ7 (ΔG‡ = 9.6ꢀ11.9
kcal molꢀ1). Also worth noting is that the rate of intermolecular
exchange of 2,4-dimethyl-1,3-butadiene with 3 is only slightly
lower that the rate of isobutylene exꢀchange with [P(t-Bu)2o-
’ CONCLUSION
biphenyl]Au(η2-H2CdCMe2)}þSbF6 (9; ΔG‡ = 15.0 (
We have synthesized seven cationic gold η2-diene complexes
that contain the sterically hindered P(t-Bu)2o-biphenyl ligand,
and we have characterized these complexes by variable-tempera-
ture NMR spectroscopy and, in four cases, by X-ray crystal-
lography. Both in solution and in the solid state, dienes that
contain dissimilarly substituted CdC bonds bind to gold(I)
selectively through the less substituted CdC bond. Analysis of
the 13C NMR chemical shifts of the bound diene ligands and
analysis of diene binding affinities points to stabilization of the
gold π-diene interaction through donation of electron density
from the uncomplexed CdC bond to the complexed CdC bond
of the gold(I) π-diene complex. Variable-temperature NMR
analysis established the fluxional behavior of cationic gold(I)
π-diene complexes involving the facile (ΔG‡ = 9.6ꢀ11.9 kcal
molꢀ1) intramolecular exchange of the complexed and uncom-
plexed CdC bonds of the diene ligand, presumably involving 16-
electron gold η4-diene intermediates and transition states.
298
0.01 kcal molꢀ1).12
Relative Binding Affinities of Dienes to Gold(I). We sought
to determine the relative binding affinities of conjugated die-
nes to the cationic 12-electron gold fragment [P(t-Bu)2o-
biphenyl]Auþ both to determine the effect of diene substitution
on the binding affinity of the dienes to gold(I) and to make direct
comparisons among the binding affinities of conjugated dienes,
aliphatic monoenes, and vinyl arenes. To this end, equilibrium
constants for the displacement of isobutylene from 9 with several
diene ligands were measured in CD2Cl2 at ꢀ70 °C employing 1H
NMR analysis (Table 3). The relative binding affinity of the
dienes was only modestly sensitive to substitution and/or con-
jugation and decreased by a factor of ∼13 in the order 1,
3-cyclohexadiene > 2,4-dimethyl-1,3-pentadiene > 1,4-cyclohex-
adiene > 2,3-dimethyl-1,3-butadiene > trans-1,3-pentadiene
(Table 3). Furthermore, addition of butadiene to a solution of
9 in CD2Cl2 at ꢀ70 °C led to no detectable formation of
butadiene complex 4, which suggests that 1,3-butadiene binds
less tightly to gold than does trans-1,3-pentadiene.
’ EXPERIMENTAL SECTION
We have previously shown that the binding affinities of
p-substituted vinyl arenes to the cationic gold fragment [P(t-
Bu)2o-biphenyl]Auþ increased significantly with the increasing
electron density of the aryl group.12 Furthermore, aliphatic
monoenes displayed significantly higher binding affinities to
gold(I) than did vinyl arenes, presumably due to the electron-
withdrawing nature of the sp2 substituent of the vinyl arenes. For
example, the relative binding affinity of isobutylene exceeded that
of styrene by a factor of ∼60 at ꢀ60 °C (ΔG = 1.73 kcal molꢀ1).
In contrast, comparison of the relative binding affinities of 1,3-
cyclohexadiene (Keq = 1.21 ( 0.05) and 1,4-cyclohexadiene
(Keq = 0.60 ( 0.2) points to conjugation as a nominally
stabilizing feature of gold π-diene complexes. Also worth noting
is that in the case of acyclic dienes that possessed an unsubsti-
tuted ꢀCHdCH2 group the relative binding affinity increased
with increasing substitution at the uncomplexed C4 carbon atom
General Methods. Reactions were performed under a nitrogen
atmosphere employing standard Schlenk and glovebox techniques
unless specified otherwise. NMR spectra were obtained on a Varian
1
spectrometer operating at 500 MHz for H NMR, 125 MHz for 13C
NMR, and 202 MHz for 31P NMR in CD2Cl2. Elemental analysis was
performed by Complete Analysis Laboratories (Parsippany, NJ). Methy-
lene chloride was purified by passage through columns of activated
alumina under nitrogen. CDCl3 and CD2Cl2 were dried over CaH2 prior
to use. {[P(t-Bu)2o-biphenyl]Au[η2-H2CdC(CH3)2]}þSbF6ꢀ (9) was
synthesized using a published procedure.12 [P(t-Bu)2o-biphenyl]AuCl,
AgSbF6, hexanes, 4-methylpentadiene, trans-1,3-pentadiene, 2,4-di-
methyl-1,3-pentadiene, 1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 1,
3-cyclohexadiene, 1,4-cyclohexadiene, and isobutylene were purchased
from major chemical suppliers and used as received. Line shape analysis
was performed using WINDNMR-Pro.38 NMR probe temperatures
were calibrated using a neat methanol thermometer.
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Synthesis of Gold(I) η2-Diene Complexes. {[P(t-Bu)2o-
31P{1H} NMR: δ 63.3. Anal. Calcd (found) for C24H33AuF6PSb: H,
ꢀ
biphenyl]Au[η2-(E)-H2CdC(H)C(H)dC(H)Me]}þSbF6 (1). A mixture
4.24 (4.33); C, 36.71 (36.67).
ꢀ
of [P(t-Bu)2o-biphenyl]AuCl (40 mg, 7.5 ꢁ 10ꢀ2 mmol), AgSbF6 (26
mg, 7.5 ꢁ 10ꢀ2 mmol), and trans-1,3-pentadiene (51 mg, 0.75 mmol)
was dissolved in CH2Cl2 (2 mL), forming a white suspension. The
suspension was stirred in a sealed flask in the dark at room temperature
for 6 h, then filtered through Celite. Volatile material was evaporated
under vacuum to give 1 as a white solid (56 mg, 93%). 1H NMR (30 °C):
δ 7.92ꢀ7.86 (m, 1 H), 7.67ꢀ7.54 (m, 5 H), 7.32ꢀ7.18 (m, 3 H), 6.51
(ddd, J = 9.0, 10.5, 16.0 Hz, 1 H), 6.26 (qd, J = 7.0, 13.5 Hz, 1 H), 5.99
(ddd, J = 1.0, 10.5, 14.5 Hz, 1 H), 4.17 (d, J = 16.5 Hz, 1 H), 4.11 (d, J =
8.5 Hz, 1 H), 1.91 (dd, J = 1.0, 7.0 Hz, 3 H), 1.38 (d, J = 16.0 Hz, 18 H).
13C{1H} NMR (ꢀ80 °C): δ 147.4 (d, J = 13.9 Hz), 144.3, 142.6 (d, J =
7.4 Hz), 137.3, 133.5, 132.6 (d, J = 7.4 Hz), 131.0, 129.0, 129.0, 128.8,
128.3, 127.6 (d, J = 7.4 Hz), 127.3, 123.7 (d, J = 47.3 Hz), 90.6, 37.0 (d,
J = 23.5 Hz), 36.9 (d, J = 23.6 Hz), 29.6 (br s), 18.4. 31P{1H} NMR: δ
65.9 Anal. Calcd (found) for C25H35AuF6PSb: H, 4.32 (4.41); C, 37.46
(37.57).
{[P(t-Bu)2o-biphenyl]Au[η2-H2CdC(Me)C(Me)dCH2]}þSbF6 (5).
Complex 5 was isolated as a white solid (56 mg, 92%) from the reaction
of 2,3-dimethyl-1,3-butadiene with [P(t-Bu)2o-biphenyl]AuCl and
AgSbF6 employing a procedure similar to that used to synthesize 1.
1H NMR (ꢀ50 °C): δ 7.90ꢀ7.85 (m, 1 H), 7.67ꢀ7.58 (m, 5 H),
7.26ꢀ7.20 (m, 3 H), 5.65 (s, 1 H), 5.48 (s, 1 H), 3.99 (br d, J = 3.5 Hz, 1
H), 3.64 (br d, J = 4.0 Hz, 1 H), 2.25 (s, 3 H), 1.74 (s, 3 H), 1.36 (d, J =
16.5 Hz, 9 H), 1.25 (d, J = 17.0 Hz, 9 H). 13C{1H} NMR (ꢀ60 °C): δ
154.7, 147.6 (d, J = 15.0 Hz), 143.1 (d, J = 7.5 Hz), 140.2, 133.7, 132.9
(d, J = 6.3 Hz), 131.3, 129.5, 129.4, 129.3, 128.9, 127.8 (d, J = 7.5 Hz),
127.7, 124.2, 123.9 (d, J = 47.5 Hz), 91.9 (d, J = 15.0 Hz), 36.4 (d, J =
23.7 Hz), 36.2 (d, J = 23.7 Hz), 30.2 (d, J = 5.0 Hz), 29.5 (d, J = 6.2 Hz),
24.0, 20.1. 31P{1H} NMR (ꢀ60 °C): δ 63.5. Anal. Calcd (found) for
C26H37AuF6PSb: H, 4.59 (4.52); C, 38.40 (38.46).
{[P(t-Bu)2o-biphenyl]Au[η2-C(H)dC(H)C(H)dC(H)CH2CH2}þSbF6
ꢀ
(6). Complex 6 was isolated as a white solid (61 mg, 100%) from the
reaction of 1,3-cyclohexadiene with [P(t-Bu)2o-biphenyl]AuCl and
AgSbF6 following the procedure used to synthesize 1. 1H NMR
(ꢀ70 °C): δ 7.86ꢀ7.79 (m, 1 H), 7.61ꢀ7.52 (m, 5 H), 7.26ꢀ7.18 (m,
3 H), 6.18 (br s, 1 H), 6.01 (br s, 1 H), 5.59 (br s, 1 H), 5.13 (br s, 1 H),
2.46ꢀ2.31 (m, 3 H), 2.21ꢀ2.08 (m, 1 H), 1.32 (d, J = 16.5 Hz, 9 H), 1.28
(d, J = 16.5 Hz, 9 H). 13C{1H} NMR (ꢀ70 °C): δ 147.4 (d, J = 13.2 Hz),
142.6 (d, J = 6.5 Hz), 136.4, 133.5, 132.5 (d, J = 7.2 Hz), 131.0, 129.3,
129.1, 128.7, 128.5, 127.5 (d, J = 6.6 Hz), 127.2, 123.4,123.0 (d, J = 5.0
Hz), 121.1, 117.4 (d, J = 6.2 Hz), 114.6 (d, J = 8.3 Hz), 37.4 (d, J = 24.0
Hz), 37.2 (d, J = 23.8 Hz), 29.8 (d, J = 15.2 Hz), 24.4, 21.7. 31P{1H} NMR
(ꢀ70 °C): δ 61.9. Anal. Calcd (found) for C26H35AuF6PSb: H, 4.35
(4.43); C, 38.49 (38.44).
{[P(t-Bu)2o-biphenyl]Au[η2-H2CdC(H)C(H)dCMe2]}þSbF6ꢀ (2). A
mixture of [P(t-Bu)2o-biphenyl]AuCl (40 mg, 7.5 ꢁ 10ꢀ2 mmol),
AgSbF6 (26 mg, 7.5 ꢁ 10ꢀ2 mmol), and 4-methyl-1,3-pentadiene
(12 mg, 0.15 mmol) was dissolved in CH2Cl2 (2 mL), forming a white
suspension. The suspension was stirred in a sealed flask in the dark at
room temperature for 6 h, then filtered through a plug of Celite, eluting
with CH2Cl2. The filtrate was concentrated to ∼2 mL, diluted with
hexanes (2 mL), and cooled at 4 °C overnight to form 1 (50 mg, 82%) as
a white solid. 1H NMR (25 °C): δ 7.95ꢀ7.91 (m, 1 H), 7.70ꢀ7.60 (m, 5
H), 7.35ꢀ7.22 (m, 3 H), 6.81 (ddd, J = 8.5, 11.5, 16.0 Hz, 1 H), 5.85 (d,
J = 11.5 Hz, 1 H), 4.13 (d, J = 16.0 Hz, 1 H), 4.08 (d, J = 8.0 Hz, 1 H),
1.97 (s, 3 H), 1.95 (s, 3 H), 1.40 (d, J = 16.5 Hz, 18 H). 13C{1H} NMR
(25 °C): δ 153.1, 148.8 (d, J = 13.7 Hz), 143.7 (d, J = 6.3 Hz), 134.9,
134.3, 133.7 (d, J = 7.5 Hz), 132.1, 130.0, 129.8, 128.4 (d, J = 6.2 Hz),
128.3, 124.7 (d, J = 46.3 Hz), 123.5, 90.6 (d, J = 15.0 Hz), 38.6 (d, J =
23.7 Hz), 30.8 (d, J = 7.5 Hz), 26.4, 19.5. 31P{1H} NMR: δ 67.3. Anal.
Calcd (found) for C26H37AuF6PSb: H, 4.59 (4.70); C, 38.40 (38.42).
{[P(t-Bu)2o-biphenyl]Au[η2-C(H)dC(H)CH2C(H)dC(H)CH2}þSbF6
ꢀ
(7). Complex 7 was isolated as a white solid (61 mg, 93%) from the
reaction of 1,4-cyclohexadiene with [P(t-Bu)2o-biphenyl]AuCl and
AgSbF6 employing a procedure similar to that used to synthesize 1. 1H
NMR (ꢀ60 °C): δ 7.86ꢀ7.79 (m, 1 H), 7.62ꢀ7.52 (m, 5 H), 7.25ꢀ7.22
(m, 1 H), 7.21ꢀ7.14 (m, 2 H), 5.71 (s, 2 H), 5.58 (s, 2 H), 2.95ꢀ2.76 (m,
1 H), 1.31 (d, J = 17.0 Hz, 18 H). 13C{1H} NMR (ꢀ85 °C): δ 147.4 (d, J
= 13.0 Hz), 142.5 (d, J = 7.0 Hz), 133.4 (d, J = 3.0 Hz), 132.5 (d, J = 7.5
Hz), 131.0, 129.2, 128.5, 127.5 (d, J = 6.4 Hz), 127.2, 123.4, 122.9 (d, J =
47.3 Hz), 120.7 (d, J = 6.9 Hz), 36.9 (d, J = 24.1 Hz), 29.8, 27.0. 31P{1H}
NMR (ꢀ60 °C): δ 62.2. Anal. Calcd (found) for C26H35AuF6PSb: H,
4.32 (4.35); C, 38.40 (38.49).
X-ray Crystal Structure Determinations. Crystals of 1, 2, 4, and
7were obtained from slow diffusion of hexanes into CH2Cl2 solutions of the
respective complexes at 4 °C. The crystals were mounted on a Mitegen
polyimide micromount with a small amount of Paratone N oil. Diffraction
data were obtained with graphite-monochromated Mo KR radiation (λ =
0.71073 Å) on a Bruker-Nonius Kappa Axis X8 Apex2 diffractometer at 110
K. Crystallographic data for complexes 1, 2, 4, and7 are collected in Table 4.
The data collection strategy employed ω and j scans that collected data up
to 2θ. The frame integration was performed using SAINT.39 The resulting
raw data were scaled and absorption corrected using a multiscan averaging
of symmetry equivalent data using SADABS.40
The structures of 1, 4, and 7 were solved by direct methods using the
XS program;41 the structure of complex 2 was solved by direct methods
using the SIR92 program.42 All non-hydrogen atoms were obtained from
the initial solution. The hydrogen atoms were introduced at idealized
positions and were allowed to ride on the parent atom. The structural
models were fit to the data using full matrix least-squares based on F2.
The calculated structure factors included corrections for anomalous
dispersion from the usual tabulation. The structures were refined using
the XL program from SHELXTL;43 graphic plots were produced using
the NRCVAX crystallographic program suite.
ꢀ
{[P(t-Bu)2o-biphenyl]Au[η2-H2CdC(Me)C(H)dCMe2]}þSbF6 (3).
Complex 3 was isolated as a white solid (62 mg, 100%) from the reaction
of 2,4-dimethyl-1,3-pentadiene, [P(t-Bu)2o-biphenyl]AuCl, and AgSbF6
1
employing a procedure similar to that used to synthesize 2. H NMR
(25 °C): δ 7.93ꢀ7.88 (m, 1 H), 7.65ꢀ7.55 (m, 5 H), 7.28ꢀ7.20 (m, 3
H), 5.04 (s, 1 H), 3.89 (d, J = 3.5 Hz, 1 H), 3.81 (d, J = 4.0 Hz, 1 H), 2.27
(s, 3 H), 1.96 (s, 3 H), 1.92 (s, 3 H), 1.37 (d, J = 16.5 Hz, 18 H). 13C{1H}
NMR (25 °C): δ 158.7, 149.4, 148.8 (d, J = 13.8 Hz), 143.7 (d, J = 7.5
Hz), 134.3 (d, J = 2.5 Hz), 133.8 (d, J = 7.5 Hz), 132.0 (d, J = 2.5 Hz),
130.0, 129.8, 128.3 (d, J = 7.5 Hz), 128.3, 125.1 (d, J = 5.0 Hz), 124.8 (d,
J = 47.5 Hz), 91.7 (d, J = 16.3 Hz), 37.8 (d, J = 22.5 Hz), 30.7 (d, J = 6.2
Hz), 28.8, 28.0, 21.3. 31P{1H} NMR: δ 67.5. Anal. Calcd (found) for
C27H39AuF6PSb: H, 4.68 (4.75); C, 39.09 (39.20).
ꢀ
{[P(t-Bu)2o-biphenyl]Au[η2-H2CdC(H)C(H)dCH2]}þSbF6 (4).
Methylene chloride (2 mL) was added to a mixture of [P(t-Bu)2o-
biphenyl]AuCl (40 mg, 7.5 ꢁ 10ꢀ2 mmol) and AgSbF6 (26 mg, 7.5 ꢁ
10ꢀ2 mmol) under 1,3-butadiene atmosphere. 1,3-Butadiene was
bubbled through the resulting suspension with stirring for 6 h at room
temperature in the dark. The resulting suspension was filtered through
Celite, and volatile material was evaporated under vacuum to give 4 (55
mg, 93%) as a white solid. 1H NMR (ꢀ90 °C): δ 7.84ꢀ7.78 (m, 1 H),
7.59ꢀ7.52 (m, 5 H), 7.21ꢀ7.13 (m, 3 H), 6.48 (td, J = 10, 17.8 Hz, 1 H),
6.18 (td, J = 10, 17.0 Hz, 1 H), 5.66 (d, J = 16.8 Hz, 1 H), 5.56 (d, J = 9.8
Hz, 1 H), 4.11 (d, J = 16.0 Hz, 1 H), 3.93 (br dd, J = 4.1, 7.2 Hz, 1 H),
1.26 (d, J = 16.7 Hz, 18 H). 13C{1H} NMR (ꢀ90 °C): δ 147.2 (d, J =
12.5 Hz), 142.5 (d, J = 7.5 Hz), 134.7 (d, J = 2.5 Hz), 133.5 (d, J = 2.5
Hz), 133.3, 132.4 (d, J = 7.5 Hz), 130.9 (d, J = 1.2 Hz), 129.1, 129.1,
128.9, 128.6, 128.5, 127.6 (d, J = 6.9 Hz), 127.3, 123.5 (d, J = 47.2 Hz),
94.9 (d, J = 12.6 Hz), 36.8 (d, J = 23.4 Hz), 36.5 (d, J = 23.6 Hz), 29.5.
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ARTICLE
Table 4. Crystal Data and Structure Refinement for Complexes 1, 2, 4, and 7
1
2
4
7
empirical formula
fw
C
25.50H35AuClF6PSb
C26H37AuF6PSb
813.24
C24H33AuF6PSb
785.19
C26H35AuF6PSb
811.23
840.67
cryst size (mm3)
color and habit
T (K)
0.31 ꢁ 0.20 ꢁ 0.07
colorless prism
110(2)
0.32 ꢁ 0.18 ꢁ 0.06
colorless prism
110(2)
0.37 ꢁ 0.29 ꢁ 0.28
colorless prism
110(2)
0.16 ꢁ 0.14 ꢁ 0.11
colorless plate
110(2)
λ (Å)
0.71073
0.71073
0.71073
0.71073
cryst syst
monoclinic
orthorhombic
Pca21
triclinic
monoclinic
space group
unit cell dimens
P2(1)/n
P1
P21/n
a = 10.488(2) Å
b = 17.861(4) Å
c = 15.443(3) Å
β = 93.948(10)°
a = 12.7007(4) Å
b = 11.9861(4) Å
c = 18.9847(6) Å
a = 9.4763(8 Å
b = 9.8824(8) Å
c = 14.0672(11) Å
R = 83.970(3)°
β = 87.373(4)°
γ = 81.222(4)°
1294.14(18)
2
a = 9.9305(6) Å
b = 24.7210(15) Å
c = 11.5076(7) Å
β = 94.593(3)°
V (Å3)
2886.0(10)
2890.08(16)
2815.9(3)
Z
4
4
4
Dcalc (g/cm3)
1.935
1.869
2.015
1.913
2θmax (deg)
56.66
72.58
86.78
68.88
abs coeff (mmꢀ1
)
6.214
6.112
6.821
6.273
total no. of reflns
91845
98 369
73 102
67 039
no. of unique reflns
no. params refined/restraints
F(000)
6678
12 497
15 686
11 764
6678/311
12 497/360
15 686/341
752
11 764/322
1616
1568
1560
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
max., min. ΔF (e/Å3)
1.188
1.033
1.002
1.029
R1 = 0.0265, wR2 = 0.0595
R1 = 0.0346, wR2 = 0.0619
ꢀ2.044, 1.907
R1 = 0.0364, wR2 = 0.0722
R1 = 0.0624, wR2 = 0.0848
ꢀ0.710, 1.857
R1 = 0.0260, wR2 = 0.0604
R1 = 0.0312, wR2 = 0.0619
ꢀ2.359, 4.902
R1 = 0.0463, wR2 = 0.1033
R1 = 0.0942, wR2 = 0.1172
ꢀ1.264, 2.295
The 4-methyl-1,3-pentadiene ligand of complex 2 exhibited an
orientational disorder. Atoms C2 and C5 occupied single positions,
while atoms C1, C3, C4, and C6 occupied a second set of sites denoted
as C10, C30, C40, and C60, respectively. The occupancies for the
orientations were refined and normalized to 0.574(11) and 0.426(11)
for the unprimed and primed sites respectively. The 1,3-butadiene ligand
of 4 was likewise disordered over two orientations. The refined
occupancy for the major component was 0.643(8).
Kinetics of Intermolecular Diene Exchange with 3. An NMR
tube capped with a rubber septum that contained a solution of 3 (59 mg,
0.072 mmol) and 1,3-dimethoxybenzene (0.018 mmol, 2.4 μL, internal
standard) in CD2Cl2 (0.445 mL) was placed in the probe of an NMR
spectrometer maintained at 298 K. Analysis of the singlet at δ 5.74
corresponding to the C3 proton of the 2,4-dimethyl-1,3-pentadiene
ligand of 3 in the 1H NMR spectrum gave a peak width at half-height of
Δv1/2 = 6.9 Hz (Table 2). The tube was removed from the probe, and
2,4-dimethyl-1,3-pentadiene was added via syringe. The tube was
inverted several times, returned to the probe, and allowed to equilibrate
for 10 min. The concentration of free 2,4-dimethyl-1,3-pentadiene was
determined as 0.09 M by integration of the C3 proton resonance of free
2,4-dimethyl-1,3-pentadiene at δ 5.65 relative to the multiplet at δ 6.48
corresponding to the C4, C5, and C6 aryl protons of 1,3-dimethox-
ybenzene. The peak width at half-height for the singlet at δ 5.74 was
Kinetics of Intramolecular CdC Bond Exchange. An NMR
tube containing a solution of 4 (0.55 mg, 0.070 mmol) in CD2Cl2
(0.5 mL) was placed in the probe of an NMR spectrometer precooled
at ꢀ90 °C, allowed to equilibrate for 10 min, and analyzed by 1H NMR
spectroscopy. The solution was warmed incrementally, equilibrated
at ꢀ80, ꢀ70, ꢀ60, ꢀ59, ꢀ45, ꢀ30, ꢀ15, and 15 °C, and analyzed at
each temperature by 1H NMR spectroscopy. Simulation of the portion
1
Δν1/2 = 11.0 Hz, corresponding to an excess broadening of Δν1/2(excess)
=
of the H NMR spectrum containing the C2/C3 vinylic resonances
4.0 Hz (Table 2). Using this value, the rate of exchange of the diene ligand
with 3 was determined through application of the slow exchange
approximation to be kobs = π(Δv1/2(excess)) = 12.7 sꢀ1. The first-order
rate constant for diene exchange with 3 was determined in a similar
manner at [2,4-dimethyl-1,3-butadiene] = 0.14, 0.18, 0.22, and 0.25 M
(Table 2). The second-order rate constant for the rate of exchange of the
diene ligand of 3 was determined from the slope of a plot of kobs versus
[2,4-dimethyl-1,3-butadiene] where kex = 169 ( 1 Mꢀ1 sꢀ1 (ΔG‡ = 14.40
( 0.01 kcal molꢀ1) (Table 2, Figure 11).
Determination of Diene Binding Constants. 2,3-Dimethyl-
1,3-butadiene (2.4 mg, 0.029 mmol) was added via syringe to a CD2Cl2
solution of {[P(t-Bu)2o-biphenyl]Au(η2-H2CdCMe2)}þSbF6ꢀ (9;20mg,
0.029 mmol) in an NMR tube sealed with a rubber septum. The contents
(δ 6.1ꢀ6.6) at ꢀ59 °C employing the parameters Δω = 144.9 Hz, ωa =
5 Hz, ωb = 4 Hz, JAB = 10.0 Hz, JAtrans = 17.8, JAcis = 8.8 Hz, JBcis = 10.2
Hz, JBtrans = 17.0 Hz gave a best fit with an exchange rate of kex = 701 sꢀ1
(ΔG214 = 9.6 kcal molꢀ1; Figure S7). Peak separations (Δω) were
‡
obtained directly from the ꢀ90 °C 1H NMR spectrum of 4; coupling
constants (J) were obtained from the ꢀ90 °C 1H NMR spectrum of 4
and then varied within (0.2 Hz to best reproduce the slow-exchange
spectrum. Natural peak widths (ω) were estimated by averaging the
peak width at half-height of the outer lines of the respective multiplets.
The energy barriers for interconversion of the diene CdC bonds of
complexes 5ꢀ7 were determined though similar analyses. Comparisons
of experimental and simulated 1H NMR spectra are included as Figures
S7ꢀS10 in the Supporting Information.
3191
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Organometallics
ARTICLE
of the tube were thoroughly mixed, placed in the probe of an NMR
spectrometer precooled at ꢀ70 °C, and maintained at this temperature
for 10 min. The relative concentrations of 9, free isobutylene, 5, and free
2,3-dimethyl-1,3-butadiene were measured by integrating the reso-
nances corresponding to the vinyl protons of bound (δ 3.73) and free
(δ 4.59) isobutylene and the vinyl protons of bound (δ 5.60, 5.44, 3.95,
3.52) and free (δ 5.01, 4.95) 2,3-dimethyl-1,3-butadiene. An equilibrium
constant of Keq = ([5][isobutylene])/([9][2,3-dimethyl-1,3-butadiene]) =
0.13 ( 0.02 was determined as the average of two separate experiments
(Table 3). To ensure that equilibrium was achieved under these
conditions, a similar experiment was performed through addition of
isobutylene (0.017 mmol) to a CD2Cl2 solution of 5 (0.025 mmol). An
equilibrium constant of Keq = ([5][isobutylene])/([9][2,3-dimethyl-
1,3-butadiene]) = 0.12 ( 0.01 was determined, which is not significantly
different from that obtained by treatment of 9 with 2,3-dimethyl-1,
3-butadiene. The relative binding affinities of 1,3-cyclohexadiene, 2,
4-dimethyl-1,3-pentadiene, 1,4-cyclohexadiene, and trans-1,3-penta-
diene relative to isobutylene at ꢀ70 °C were determined employing
procedures analogous to that used to determine the binding affinity of
2,3-dimethyl-1,3-butadiene (Table 3).
(10) Kovꢀacs, G.; Ujaque, G.; Lledoꢀs, A. J. Am. Chem. Soc. 2008,
130, 853.
(11) Formation of a gold(I) π-diene complex from a mixture of
(PPh3)AuCl, AgOTf, 3-methyl-1,3-pentadiene, and benzyl carbamate
was proposed on the basis of in situ 31P and 13C NMR analysis; however,
neither isolation nor full characterization of the purported gold(I)
π-diene complex was reported.2
(12) Brown, T. J.; Dickens, M. G.; Widenhoefer, R. A. Chem.
Commun. 2009, 6451.
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(17) Herrero-Gꢀomez, E.; Nieto-Oberhuber, C.; Loꢀpez, S.; Benet-
Buchholz, J.; Echavarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 5455.
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’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data,
b
assignments of 13C and H diene resonances of complexes
1
1ꢀ6, and plots of experimental and simulated 1H NMR spectra
used to determine energy barriers for interconversion of diene
CdC bonds of complexes 4ꢀ7. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(19) For a recent review of gold π-complexes see: Schmidbaur, H.;
Schier, A. Organometallics 2010, 29, 2.
Corresponding Author
*E-mail: rwidenho@chem.duke.edu.
(20) Pꢀerez-Galꢀan, P.; Delpont, N.; Herrero-Gꢀomez, E.; Maseras, F.;
Echavarren, A. M Chem.ꢀEur. J. 2010, 16, 5324.
(21) (a) Partyka, D. V.; Robilotto, T. J.; Zeller, M.; Hunter, A. D.;
Gray, T. G. Organometallics 2008, 27, 28. (b) Partyka, D. V.; Updegraff,
J. B.; Zeller, M.; Hunter, A. D.; Gray, T. G. Organometallics 2009,
28, 1666.
’ ACKNOWLEDGMENT
We are grateful to the NSF (CHE-0555425) for support of
this research and to the NCBC (2008-IDG-1010) for support of
the Duke University NMR facility. R.E.M.B. thanks Duke Uni-
versity for a Burroughs Welcome fellowship. We thank Dr. Paul
D. Boyle (North Carolina State University) for performing X-ray
crystallographic analyses.
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