Solid-state and dynamic solution behavior of a cationic, two-coordinate gold(I) π-allene complex

Article abstract of DOI:10.1021/om100688t

The cationic gold π-allene complex {[P(t-Bu)₂o-biphenyl] Au[η²-H₂C=C=C(CH₃)₂]} ⁺SbF₆^- was isolated in 98% yield from reaction of 3-methyl-1,2-butadiene with a mixture of [P(t-Bu)₂o-biphenyl]AuCl and AgSbF₆ and was characterized by X-ray crystallography and variable-temperature NMR spectroscopy. These studies revealed preferential binding of gold to the less substituted C=C bond of the allene in both the solid state and solution and also revealed fluxional behavior consistent with π-face exchange of the allene ligand via an η¹-C2 allene intermediate or transition state.

Full text of DOI:10.1021/om100688t

Organometallics 2010, 29, 4207–4209 4207
DOI: 10.1021/om100688t
Solid-State and Dynamic Solution Behavior of a Cationic,
Two-Coordinate Gold(I) π-Allene Complex
Timothy J. Brown, Atsushi Sugie, Marina G. Dickens, and Ross A. Widenhoefer*
French Family Science Center, Duke University, Durham, North Carolina 27708-0346
Received July 13, 2010
Summary: The cationic gold π-allene complex {[P(t-Bu)2o-
gleaned from related transition metal π-allene complexes5-10
and gold π-complexes.11,12 Here we report the synthesis,
X-ray crystal structure, and fluxional behavior of a cationic,
two-coordinate gold(I) π-allene complex.
-
biphenyl]Au[η2-H2CdCdC(CH3)2]}þSbF6 was isolated in
98% yield from reaction of 3-methyl-1,2-butadiene with a
mixture of [P(t-Bu)2o-biphenyl]AuCl and AgSbF6 and was char-
acterized by X-ray crystallography and variable-temperature
NMR spectroscopy. These studies revealed preferential bind-
ing of gold to the less substituted CdC bond of the allene in
both the solid state and solution and also revealed fluxional
behavior consistent with π-face exchange of the allene ligand
via an η1-C2 allene intermediate or transition state.
Reaction of 3-methyl-1,2-butadiene with a mixture of
[P(t-Bu)2o-bipheny]AuCl (1) and AgSbF6 (1:1) employing a
procedure similar to that used for the synthesis of cationic
gold(I) π-alkene complexes12 (excess allene, room tempera-
ture, ∼12 h) led to formation of black suspensions that con-
tained only traces of {[P(t-Bu)2o-biphenyl]Au[η2-H2CdCd
-
1
C(CH3)2]}þSbF6 (2) as determined by H NMR analysis.
Over the past 10 years considerable progress has been
made in the utilization of soluble gold complexes, in partic-
ular cationic gold(I) complexes, as catalysts for the electro-
philic activation of C-C multiple bonds.1 Allenes have
played a central role in the development of this chemistry,
not only as substrates but also as intermediates and prod-
ucts.1 Although it is widely assumed that gold π-allene
complexes are generated as reactive intermediates in these
transformations, no well-defined gold π-allene complex
has been documented.2 As such, information regarding the
structure and behavior of gold π-allene complexes is limited
to that provided by recent computational studies3,4 or
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2010 American Chemical Society
Published on Web 09/02/2010
pubs.acs.org/Organometallics

4208 Organometallics, Vol. 29, No. 19, 2010
Brown et al.
Key to the isolation of pure 2 was lower allene concentration
coupled with shorter reaction time. For example, treatment
of a methylene chloride suspension of 1 and AgSbF6 (1:1)
with 3-methyl-1,2-butadiene (1.5 equiv) at room tempera-
ture for 15 min led to isolation of 2 in 98% yield as an air
and thermally stable white solid that was characterized by
spectroscopy, elemental analysis, and X-ray crystallography
(eq 1).13,14
Figure 2. Partial variable-temperature 1H (left) and 13C (right)
NMR spectra of 2 in CD2Cl2.
the allene unit is bent with a C1-C2-C3 angle of 165°
(Figure 1).
π-Complexation of the 3-methyl-1,2-butadiene group of 2
to gold through the less substituted C1dC2 bond in solu-
1
tion was established by 13C and H NMR spectroscopy at
-60 °C. In particular, the 13C NMR chemical shifts of the
allenyl carbon atoms of 2 [δ 105.6 (CMe2), 197.0 (dCd), 63.1
(CH2)] were shifted significantly from those of free 3-methyl-
1,2-butadiene [δ 94.4, 205, 72.6], and resonances correspond-
ing to diastereotopic allenyl methyl groups were observed in
both the 13C (δ 22.3 and 20.3) and 1H (δ 1.85 and 1.80) NMR
spectra (Figure 2). The presence of a single allenyl proton
resonance in the 1H NMR spectrum of 2 at δ 4.07 indicates
that the allenyl protons adopt either a static or time-averaged
orientation about the pseudo-mirror plane that reflects the
two tert-butyl groups, as was observed in the solid state. As
the temperature was raised, the allenyl methyl resonances
broadened and coalesced, forming single time-averaged
resonances in both the 13C (δ 21.9 at 25 °C) and 1H (δ 1.85
at -30 °C) NMR spectra (Figure 2). The allenyl proton
Figure 1. ORTEP diagram of2 1/2CH2Cl2. Ellipsoidsare shown
at 50% probability. Solvent, counterion, and hydrogen atoms are
˚
omitted for clarity. Key bond lengths (A) and bond angles (deg):
C1-C2 = 1.340, C2-C3 = 1.311, Au-C1 = 2.191, Au-C2 =
2.306, C1-C2-C3=165.0, P-Au-C1dC2(cent)=167.3.
3
Slow diffusion of hexanes into a CH2Cl2 solution of 2 at
4 °C gave colorless crystals of 2 1/2CH2Cl2 suitable for X-ray
3
analysis (Figure 1).14 In the solid state, the 3-methyl-1,2-
butadiene ligand of 2 is bound unsymmetrically to gold
through the less substituted C1dC2 π-bond with a shorter
˚
Au-C1 and a longer Au-C2 interaction (Δd = 0.116 A).
1
Complex 2 adopts a distorted linear conformation with a
P-Au-CdC(cent) angle of 167° and with the uncomplexed
portion of the allene directed away from the protruding
phenyl group of the o-biphenyl moiety. Within the allene
ligand, the coordinated C1dC2 bond is elongated by
resonance in the H NMR spectrum shifted slightly with
increasing temperature, but was otherwise unchanged.
An energy barrier of ΔGq
conversion of the allenyl methyl groups of 2 was deter-
= 11.0 kcal mol-1 for inter-
220K
mined from a peak separation of Δν = 26 Hz and coales-
cence temperature of -53 °C in the 1H NMR spectrum [k =
˚
∼0.03 A relative to the uncomplexed C2dC3 bond, and
√
π(Δv)/ 2].
The rate of methyl group interconversion of 2 was inde-
pendent of concentration (25-50 mM), which argues against
an intermolecular exchange mechanism. Rather, the minimal
intramolecular process that accounts for interconversion of
the allenyl methyl groups of 2 involves migration of gold
between orthogonal allene π-faces via an η1-C2 allene inter-
mediate or transition state in which gold is positioned 45°
relative to the orthogonal π-faces (Scheme 1). Similar π-face
exchange processes have been documented for Fe(0),5 Fe-
(II),6 and Pt(II)7,8 π-allene complexes, and the involvement
of η1-allene complexes in these processes is supported by
both experimental observation6,7,9 and computational ana-
lyses.3 Although an η1-allyl cation intermediate or transition
state would also account for the observed π-face exchange
behavior of 2, analysis of the fluxional behavior of cationic
Fe(II) π-allyl complexes ruled out the involvement of an
(13) Experimental details for 2: 3-Methyl-1,2-butadiene (7.7 mg,
0.11 mmol) was added via syringe to a stirred suspension of 1 (40 mg,
0.075 mmol) and AgSbF6 (26 mg, 0.075 mmol) in CH2Cl2 (2 mL), and
the reaction mixture was stirred at room temperature for 15 min. The
resulting suspension was filtered through a pad of Celite, which was
flushed with additional CH2Cl2. The filtrate was concentrated to ∼2 mL,
diluted with one volume of hexanes, and cooled at 4 °C for 24 h to form 2
(59 mg, 98%) as a white solid. 1H NMR (500 MHz, 25 °C): δ 7.95-7.87
(m, 1H), 7.68-7.59 (m, 2H), 7.54-7.47 (m, 3H), 7.33-7.20 (m, 3H), 4.21
(sept, J = 2.8 Hz, 2H), 1.90 (t, J = 2.8 Hz, 6H), 1.44 (d, J = 16.4 Hz,
18H). 13C{1H} NMR (126 MHz, 25 °C): δ 197.0, 148.8, 143.6 (d, J = 6.9
Hz), 134.4 (d, J = 33.3 Hz), 133.7, 132.2, 130.2, 129.7, 129.5, 128.4, 124.6
(d, J = 43.9 Hz), 105.6, 63.1, 39.3 (d, J = 23.8 Hz), 31.3 (d, J = 6.8 Hz),
21.9. 31P{1H} NMR (202 MHz, 25 °C): δ 66.5. Anal. Calcd (found) for
C25H35PF6AuSb: H, 4.41 (4.44); C, 37.57 (37.72).
˚
(14) X-ray data for 2: triclinic, P1, T = 173 K, a = 7.8865(11) A,
˚
˚
b = 11.4932(16) A, c = 16.466(2) A, R = 98.452(8)°, β = 94.196(8)°,
3
2
γ = 96.769(8)°, V = 1459.8(3) A , Z = 2, R[F > 2σ(F2)] = 0.032,
˚
wR(F2) = 0.077.

Communication
Organometallics, Vol. 29, No. 19, 2010 4209
Scheme 1
η1-allyl cation species in the lowest energy π-face exchange
process,6 a conclusion that is supported by the computa-
tional studies of Malacria and co-workers.3 Also worth
noting is that the computational analysis of Malacria and
co-workers suggests that the allene moiety of Au(I), Au(III),
and Pt(II) η1-allene complexes adopts a bent conformation
(C1-C2-C3 = 129-140°) and that the η1-allene complexes
of (PMe3)Auþ exist as transition states rather than as local
minima.3
In summary, we have synthesized {[P(t-Bu)2o-biphenyl]-
Au[η2-H2CdCdC(CH3)2]}þSbF6- (2), which represents the
first example of a gold π-allene complex. Solution and solid-
state analysis of 2 established preferential binding of gold
to the less substituted CdC bond of the allene. Variable-
temperature NMR analysis revealed fluxional behavior con-
sistent with facile π-face exchange of the allene ligand via an
η1-allene intermediate or transition state. More thorough
studies regarding the structure, fluxional behavior, and reac-
tivity of cationic gold π-allene complexes will be reported in
due course.
Acknowledgment is made 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.
T.J.B. thanks Duke University for a Zeilik fellowship,
and A.S. acknowledges the JSPS for a Young Scientist
Research Fellowship. We thank Alethea N. Duncan for
performing some preliminary experiments.
Supporting Information Available: X-ray crystallographic
data. This material is available free of charge via the Internet
at http://pubs.acs.org.