Amine-amide equilibrium in gold(III) complexes and a gold(III)-gold(I) aurophilic bond

Article abstract of DOI:10.1021/ic061911y

The ligands HN(CH₂-2-C₅H₄N)₂, BPMA, and PhCH₂N(CH₂-2-C₅H₄N) ₂, BBPMA, react with Na[AuCl₄] to give the cationic complexes [AuCl(BPMA-H)]⁺ and [AuCl(BBPMA)]²⁺, respectively. The amido complex [AuCl(BPMA-H)]⁺ undergoes easy inversion at the amido nitrogen atom and can be reversibly protonated by triflic acid to give [AuCl(BPMA)]²⁺. The complex [AuCl(BBPMA)]²⁺ is easily decomposed in aqueous solution by cleavage of a carbon-nitrogen bond or, in dilute HCl solution, by protonation of the ligand to give [BBPMAH ₂]Cl[AuCl₄] The complexes [BBPMAH₂]Cl[AuCl ₄] and [BBPMAH₂]Cl[AuCl₂] can be formed by direct reaction of BBPMA with H[AuCl₄]. Unusual forms of gold(III)...gold(III) and gold(III)...gold(I) aurophilic bonding are observed in the salts [AuCl(BPMA-H)][PF₆] and [AuCl(BPMA-H)] [AuCl₂], respectively. The first comparison of the structures of gold(III) amine and amido complexes, in the cations [AuCl(BPMA-H)]⁺ and [AuCl(BPMA)]²⁺, indicates that there is little p _π-d_π bonding in the amido-gold bond and that the amide exerts a stronger trans influence than the amine group.

Full text of DOI:10.1021/ic061911y

Inorg. Chem. 2007, 46, 1361−1368
Amine−Amide Equilibrium in Gold(III) Complexes and a Gold(III)−Gold(I)
Aurophilic Bond
Lingyun Cao, Michael C. Jennings, and Richard J. Puddephatt*
Department of Chemistry, UniVersity of Western Ontario, London, Canada N6A 5B7
Received October 6, 2006
The ligands HN(CH2
cationic complexes [AuCl(BPMA
undergoes easy inversion at the amido nitrogen atom and can be reversibly protonated by triflic acid to give
[AuCl(BPMA)]2+. The complex [AuCl(BBPMA)]2+ is easily decomposed in aqueous solution by cleavage of a carbon
−
2-C5H4N)2, BPMA, and PhCH2N(CH2
−
2-C5H4N)2, BBPMA, react with Na[AuCl4] to give the
−
H)]+ and [AuCl(BBPMA)]2+, respectively. The amido complex [AuCl(BPMA
−
H)]+
−
nitrogen bond or, in dilute HCl solution, by protonation of the ligand to give [BBPMAH2]Cl[AuCl4] The complexes
[BBPMAH2]Cl[AuCl4] and [BBPMAH2]Cl[AuCl2] can be formed by direct reaction of BBPMA with H[AuCl4]. Unusual
forms of gold(III)‚‚‚gold(III) and gold(III)‚‚‚gold(I) aurophilic bonding are observed in the salts [AuCl(BPMA
[PF6] and [AuCl(BPMA H)][AuCl2], respectively. The first comparison of the structures of gold(III) amine and amido
complexes, in the cations [AuCl(BPMA dπ bonding in the
H)]+ and [AuCl(BPMA)]2+, indicates that there is little pπ
amido gold bond and that the amide exerts a stronger trans influence than the amine group.
−H)]-
−
−
−
−
Introduction
(III), in which case decomposition to metallic gold tends to
occur.2 One potentially useful feature of primary and
secondary amine complexes of gold(III) is that the high
charge on gold(III) polarizes the NH group of the coordinated
amine and leads to easy deprotonation to form amido
complexes.2-7 These amido complexes have been known for
many years,3 and the effects of the deprotonation on
spectroscopic properties, on the rates and mechanisms of
ligand-substitution reactions, and on bioinorganic chemistry
have been well established.2-8 Judging from known amine-
The realization that gold(I) and gold(III) complexes can
have high activity in either homogeneous or heterogeneous
catalysis has created a critical need for a better understanding
of the role of supporting ligands for these metal ions.1 In
particular, ligands are needed that can prevent easy decom-
position to gold metal while maintaining high catalytic
activity and that can facilitate mechanistic studies of the
catalytic reactions. For gold(III) catalysts, these supporting
ligands should also stabilize the higher oxidation state, thus
ruling out soft donor ligands such as tertiary phosphines that
tend to favor gold(I).2 On the other hand, many simple
nitrogen-donor ligands, while effective in stabilizing gold-
(III) with respect to gold(I), are easily displaced from gold-
(3) Kharasch, M. S.; Isbell, H. S. J. Am. Chem. Soc. 1931, 53, 3059.
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Inorg. Chem. 1963, 2, 921. (b) Weick, C. F.; Basolo, F. Inorg. Chem.
1964, 3I, 576. (c) Baddley, W. H.; Basolo, F. J. Am. Chem. Soc. 1966,
88, 2944.
* Corresponding author. E-mail: pudd@uwo.ca.
(5) (a) Cinellu, M. A.; Minghetti, G.; Pinna, M. V.; Stoccoro, S.; Zucca,
A.; Manassero, M. Eur. J. Inorg. Chem. 2003, 2304. (b) Oyaizu, K.;
Ohtani, Y.; Shiozawa, A.; Sugawara, K.; Saito, T.; Yuasa, M. Inorg.
Chem. 2005, 44, 6915. (c) Cheung, T.-C.; Lai, T.-F.; Che, C.-M.
Polyhedron 1994, 13, 2073. (d) Simonov, Y.; Bologa, O.; Bourosh,
P.; Gerbeleu, N.; Lipkowski, J.; Gdaniec, M. Inorg. Chim. Acta 2006,
359, 2006.
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Inorg. Chim. Acta 2003, 349, 91. (b) Best, S. L.; Chattopadhyay, T.
K.; Djuran, M. I.; Palmer, R. A.; Sadler, P. J.; Sovago, I.; Varnagy,
K. J. Chem. Soc., Dalton Trans. 1997, 2587. (c) Koch, D.; Sunkel,
K.; Beck, W. Z. Naturforsch. B 1999, 54, 96. (d) Horvath, U. E. I.;
Cronje, S.; McKenzie, J. M.; Barbour, L. J.; Raubenheimer, H. G. Z.
Naturforsch. B 2004, 59, 1605.
(1) (a) Hashmi, A. S. K. Gold Bull. 2004, 37, 51. (b) Ivanova, S.; Petit,
C.; Pitchon, V. Gold Bull. 2006, 39, 3. (c) Fan, D.; Melendez, E.;
Ranford, J. D.; Lee, P. F.; Vittal, J. J. J. Organomet. Chem. 2004,
689, 2969. (d) Fierro-Gonzalez, J. C.; Gates, B. C. J. Phys. Chem. B
2004, 108, 16999. (e) Jones, C. J.; Taube, D.; Ziatdinov, V. R.; Periana,
R. A.; Nielsen, R. J.; Oxgaard, J.; Goddard, W. A. Angew. Chem.,
Int. Ed. 2004, 43, 4626. (f) Gonzalez-Arellano, C.; Corma, A.; Iglesias,
M.; Sanchez, F. Chem. Commun. 2005, 3431. (g) Lo, V. K.-Y.; Liu,
Y.; Wong, M.-K.; Che, C.-M. Org. Lett. 2006, 8, 1529. (h) Comas-
Vives, A.; Gonzalez-Arellano, C.; Corma, A.; Iglesias, M.; Sanchez,
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K.; Bianco, M. C.; Fischer, D.; Bats, J.W. Eur. J. Org. Chem. 2006,
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(2) (a) Schmidbaur, H. Gold: Progress in Chemistry, Biochemistry and
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Chemistry of Gold; Elsevier: Amsterdam, 1978.
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Laguna, A. Eur. J. Inorg. Chem. 2003, 2170.
10.1021/ic061911y CCC: $37.00
Published on Web 01/17/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 4, 2007 1361

Cao et al.
Chart 1
amide ligand properties, gold chemistry has potential in
reactions such as outer-sphere hydrogenation of unsaturated
groups (ketones, imines, etc.) or C-H bond activation.1,9
However, to date, no structural comparisons of amine and
amido complexes of gold(III) have been conducted that might
provide insight into the bond-activation processes.
Figure 1. View of the structure of complex 2b. Selected bond param-
eters: AuCl(1), 2.258(1); AuCl(2), 2.255(1) Å; Cl(1)AuCl(2), 179.03(5)°.
Hydrogen bonding: N(13)Cl(3), 3.067(4); N(33)Cl(3), 3.061(4) Å. The
structure of the tetrachloroaurate salt 2a is similar (Figure S1).
Scheme 1
This article reports the chemistry of bis(2-pyridylmethyl)-
amine (BPMA, 1a) and its N-benzyl derivative, BBPMA
(1b), with gold(III) (Chart 1). Clearly, BPMA is capable of
undergoing deprotonation to form the amide, but BBPMA
is not. The reactions of BPMA with square-planar palladium-
(II),10 platinum(II),11 rhodium(I),12 and iridium(I)12 have been
reported, but we are not aware of any similar chemistry with
gold(III). The first comparison of the structures of amine
and amido derivatives of gold(III) is reported herein.
Results and Discussion
Reactions of Ligand BBPMA, 1b. Initial attempts to
prepare gold(III) complexes by reaction of BPMA or
BBPMA with H[AuCl4] were unsuccessful and resulted only
in protonation of the ligand. For example, reaction of
BBPMA, 1b, with H[AuCl4] under mild conditions gave the
mixed chloride/tetrachloroaurate(III) salt [BBPMAH2]Cl-
[AuCl4], 2a, and under more forcing conditions, the reaction
gave the corresponding chloride/dichloroaurate(I) salt
[BBPMAH2]Cl[AuCl2], 2b (Scheme 1). In both salts, the
chloride ion is strongly hydrogen-bonded to the [BBP-
MAH2]2+ ion, as illustrated in Figure 1 for complex 2b. There
appears to be a slow reduction of [AuCl4]- to [AuCl2]- in
this reaction medium, with release of HCl. The released
chloride is strongly bound through hydrogen bonding to the
doubly protonated BBPMA ligand, and the cation [BBPMA-
H2‚‚‚Cl]+ can crystallize with either the [AuCl4]- or [AuCl2]-
ion.
Scheme 2
was characterized by its spectroscopic properties. Most
diagnostic in the 1H NMR spectrum was the observation that
the methylene protons of the 2-pyridylmethyl groups of
complex 3 are diastereotopic and appear as an “AB” multiplet
[δ(Ha) ) 5.19, δ(Hb) ) 5.87, J(HaHb) ) 14 Hz], whereas
the methylene protons of the benzyl group are equivalent
and appear as a singlet resonance [δ(H) ) 4.85]. (See
Figure2.)
2
In neutral solution, the ligand BBPMA (1b) reacted with
Na[AuCl4] to give [AuCl(BBPMA]Cl2, 3 (Scheme 2), which
(8) Fernandez, E. J.; Gil, M.; Olmos, M. E.; Crespo, O.; Laguna, A.; Jones,
P. G. Inorg. Chem. 2001, 40, 2170.
(9) (a) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV.
2004, 248, 2201. (b) Fulton, J. R.; Sklenak, S.; Bouwkamp, M. W.;
Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 4722.
Complex 3 had limited thermal stability in aqueous
solution. After complete decomposition, metallic gold was
(10) (a) Bugargic, Z. D.; Liehr, G.; van Eldik, R. J. Chem. Soc., Dalton
Trans. 2002, 951. (b) Nagy, Z.; Fabian, I.; Benyei, A.; Sovago, I. J.
Inorg. Biochem. 2003, 94, 291.
(11) (a) Pitteri, B.; Annibale, G.; Marangoni, G.; Bertolasi, V.; Ferretti, V.
Polyhedron 2002, 21, 2283. (b) Pitteri, B.; Marangoni, G.; Cattalini,
L.; Visentin, F.; Bertolasi, V.; Gilli, P. Polyhedron 2001, 20, 869. (c)
Jenkins, H. A.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1997,
16, 1946.
(12) (a) Thewissen, S.; Reijnders, M. D. M.; Smits, J. M. M.; de Bruin, B.
Organometallics 2005, 24, 5964. (b) de Bruin, B.; Verhagen, J. A.
W.; Schouten, C. H. J.; Gal, A. W.; Feichtinger, D.; Plattner, D. A.
Chem. Eur. J. 2001, 7, 416.
1
Figure 2. Methylene region of the H NMR spectrum (400 MHz, CD3-
NO2 solution) of complex 3, showing the diastereotopic nature of the
CHaHb-2-py protons.
1362 Inorganic Chemistry, Vol. 46, No. 4, 2007

Amine-Amide Equilibrium in Gold(III) Complexes
Scheme 3
formed, and the compound [PhCH2NH2CH2-2-C5H4NH]Cl2,
4, was isolated from the solution and characterized crystal-
lographically (Figure S2). The formation of compound 4
involves the cleavage of a 2-C5H4NCH2-N bond of the
ligand 1b to give PhCH2NHCH2-2-C5H4N, which is then
doubly protonated by HCl formed during the decomposition.
The cleavage of the C-N bond is probably aided by
polarization of the ligand BBPMA when bound to gold(III),
and the N-CH2-2-pyridyl group is activated more than the
N-CH2Ph group toward nucleophilic attack at carbon by
coordination of both the amine and pyridyl units to gold-
(III) in complex 3. The fate of the cleaved 2-pyridylmethyl
group has not been determined. The ligand BBPMA is stable
in boiling water or in dilute hydrochloric acid solution, so
coordination to gold(III) is clearly involved in the ligand-
cleavage reaction to give 4.
Reactions of the Ligand BPMA, 1a. The gold(III)
complexes [AuCl(BPMA-H)]+X-, 5a-5c, were prepared
by reaction in a neutral solution of the ligand 1a with Na-
[AuCl4], as shown in Scheme 3. The reactions were usually
carried out in the presence of tetrafluoroborate or hexafluo-
rophosphate salts to give the corresponding salts 5a (X )
BF4) or 5b (X ) PF6). If no large anion was present, the
cation crystallized as the dichloroaurate(I) salt 5c. The
complexes were isolated as red, light-sensitive solids.
Complexes 5a and 5b were soluble in acetone or acetonitrile,
but 5c was sparingly soluble in most organic solvents,
although it dissolved in dimethyl sulfoxide. The reactions
of Scheme 3 occur with spontaneous deprotonation of the
BPMA ligand to give the gold(III) amido complexes 5a-
5c.
Figure 3. 1H NMR spectra (400 MHz) of complex 5a in acetone-d6 at (a)
20 and (b) -60 °C.
Figure 4. View of the structure of complex 5a.
gold(III) to the amido nitrogen is slightly shorter than those
to the pyridyl nitrogen atoms (Table 1). The main distortion
from square-planar coordination is associated with the
formation of the five-membered chelate rings, with angle
N(1)AuN(15) ) 165.4(2)°. The amido nitrogen atom has
trigonal-pyramidal stereochemistry [sum of angles at N(8)
) 330(1)°; Table 1], and the two methylene carbon atoms
are displaced to the same side of the square plane [displace-
ments of C(7) and C(9) ) 0.60 and 0.55 Å; Table 2], such
that the two pyridyl groups are close to coplanarity (Table
2). The structural data indicate that there is limited π-bonding
between gold(III) and the amido nitrogen atom and that there
is a stereochemically active lone pair of electrons on the
nitrogen atom (Table 1 and theoretical studies below). The
molecular structures of 5a-5c are similar, as can be seen
by comparison of the structural and conformational param-
eters in Tables 1 and 2.
The cations in complex 5b pack in columns as shown in
Figure 5. The closest Au‚‚‚Au separations are Au‚‚‚AuB )
3.54 Å and Au‚‚‚AuA ) 3.73 Å. The shorter of these
distances is in the range expected for attractions between
d8-d8 complexes.13,14 Such interactions are rare for gold-
(III), but they have been observed previously in a few bridged
digold(III) compounds.13,14 The antiparallel orientation of the
square-planar gold(III) complexes (Au and AuB in Figure
Complexes 5 were characterized by their 1H NMR spectra
and by X-ray structure determinations. In complexes 5, the
NCHaHb protons are expected to be nonequivalent as a result
of the trigonal-pyramidal stereochemistry at the amido
nitrogen, but only a single resonance was observed for these
1
protons in the H NMR spectrum of each complex. For
example, complex 5a gave a resonance at δ(NCH2) ) 4.42
1
in the H NMR spectrum, and this peak broadened but did
not split at low temperature (Figure 3).
The structure of complex 5a is shown in Figure 4, with
selected structural parameters listed in Tables 1 and 2. There
are no strong interactions between the cation and the
tetrafluoroborate anion, which is not shown.
In complex 5a, the gold(III) atom has distorted square-
planar coordination and is bonded to a chloride ligand, two
nitrogen atoms from pyridyl groups, and one amido nitrogen
atom from the BPMA-H ligand (Figure 4). The bond from
(13) Vicente, J.; Chicote, M.-T.; Saura-Llamas, I.; Jones, P. G.; Meyer-
Base, K.; Erdbrugger, C. F. Organometallics 1988, 7, 997.
(14) (a) Mendizabal, F.; Pyykko, P. Phys. Chem. Chem. Phys. 2004, 6,
900. (b) Canales, S.; Crespo, O.; Gimeno, M. C.; Jones, P. G.; Laguna,
A.; Mendizabal, F. Organometallics 2001, 20, 4812. (c) Pyykko, P.
Angew. Chem., Int. Ed. 2004, 43, 4412. (d) Mendizabal, F.; Zapata-
Torres, G.; Olea-Azar, C. Chem. Phys. Lett. 2003, 382, 92. (e) Bennett,
M. A.; Hockless, D. C. R.; Rae, A. D.; Welling, L. L.; Willis, A. C.
Organometallics 2001, 20, 79.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1363

Cao et al.
Table 1. Selected Bond Distances and Angles for Complexes 5a-5c and 6 and DFT-Calculated Values for [AuCl(BPMA-H)]+ (5calc) and
[AuCl(BPMA)]2+ (6calc
)
5a
5b
5c
6
5calc
2.05
2.03
2.05
2.39
6calc
Au-N(1)
Au-N(8)
Au-N(15)
Au-Cl
2.014(6)
1.994(5)
2.023(6)
2.341(2)
2.03(1)
1.96(1)
2.00(1)
2.335(3)
2.025(8)
1.974(9)
2.008(9)
2.342(3)
2.006(8)
2.008(7)
2.006(8)
2.265(3)
2.05
2.09
2.05
2.33
N(1)-Au-N(15)
N(8)-Au-Cl
165.4(2)
164.8(4)
172.8(3)
113(1)
110(1)
108(1)
331
164.5(4)
166.5(3)
163
165
179
119
106
106
331
176.8(2)
113.4(6)
107.7(4)
108.6(4)
330
175.9(2)
113.3(8)
107.4(6)
110.2(6)
331
179.1(3)
122.3(9)
109.5(6)
109.8(6)
342
174
115
108
108
331
C(7)-N(8)-C(9)
C(7)-N(8)-Au(1)
C(9)-N(8)-Au(1)
∑N(8)a
a ∑N(8) ) sum of bond angles at the amido (5a-5c) or amine (6) nitrogen atom N(8).
Table 2. Conformational Parameters for Complexes 5a-5c and 6
5a
36
32
26
5b
32
32
22
24
0.47
0.45
9
5c
40
6
AuN(8)C(7)C(6)
AuN(8)C(9)C(10)
N(8)C(7)C(6)N(1)
N(8)C(9)C(10)N(15)
δC(7)a
27
29
30
22
0.62
0.47
13
8
30
21
24
0.45
0.50
9
23
0.60
0.55
13
11
2
δC(9)a
ωAuN(1)b
ωAuN(15)b
7
2
10
6
ωN(1)N(15)c
7
a δC ) displacement (Å) of the carbon atom from the AuN3Cl plane.
b ωAuN ) angle between the plane of the pyridyl group (containing N1 or
N15) and the plane of the AuN3Cl atoms. c ωN(1)N(15) ) angle between
the planes of the two pyridyl groups.
Figure 6. Structure of complex 5c. Intermolecular distances: Au(1)‚‚‚
Au(2) ) 3.339 Å, Cl(22)‚‚‚Au(1A) ) 3.49 Å. Symmetry operators: A, 1/2
1
1
1
+ x, /2 - y, -1/2 + z; B, -1/2 + x, /2 - y, /2 + z.
Au(I)‚‚‚Au(I) aurophilic bonds.14,17 The dichloroaurate ion
is oriented roughly parallel to the (py)N-Au-N(py) axis
of the [AuCl(BPMA-H)]+ ion in complex 5c. This is the
orientation that is expected on the basis of electrostatic
factors. In the alternative orientation with the dichloroaurate
ion oriented roughly parallel to the (amido)N-Au-Cl axis
of the [AuCl(BPMA-H)]+ ion, there would be unfavorable
electrostatic repulsions between the negatively charged
ligands on gold(I) and gold(III).14 The aurophilic bonds
themselves arise from nondirectional dispersion forces and,
therefore, do not control the preferred conformation.14 We
note that, if electrostatic forces alone were present, the
orientation with the chloride ligands of the [AuCl2]- ion
interacting with the cationic gold(III) center on both sides
would be favored.
Figure 5. Packing of the [AuCl(BPMA-H)]+ ions in complex 5b.
Symmetry operators: A, -x + 1, -y + 2, -z + 2; B, -x + 2, -y + 2,
-z + 2.
5) is expected to minimize electrostatic repulsions between
the chloride ligands and also allows π-π stacking interac-
tions between the pyridyl groups.14
The structure of complex 5c is shown in Figure 6. The
complex packs in columns with alternating gold(III) complex
cations and gold(I) complex anions. There is a short
aurophilic bond with Au(1)‚‚‚Au(2) ) 3.34 Å. However, on
the other side of the gold(III) center, the shortest contact is
to a chloride ligand, with Au(1)‚‚‚Cl(22B) ) 3.49 Å (Figure
6). There are few examples of Au(I)‚‚‚Au(III) aurophilic
bonds14 or of gold(I) bound in an analogous way to other
square-planar d8 metal ions.14-16 Theoretical studies have
indicated bonding energies for Au(I)‚‚‚Au(III) secondary
bonds that are close to those for the well-established
Amido-Amine Equilibrium. The amido complex 5a
reacted with excess triflic acid by protonation of the amido
nitrogen atom to give the corresponding amine complex,
isolated as the triflate salt [AuCl(BPMA)](OTf)2 (6; Scheme
4). The product has limited thermal stability in acid solution,
and decomposition is accompanied by a color change from
(16) (a) Singh, A.; Sharp, P. R. J. Chem. Soc., Dalton Trans. 2005, 2080.
(b) Crespo, O.; Laguna, A.; Fernandez, E. J.; Lopez-de-Luzuriaga, J.
M.; Jones, P. G.; Teichert, M.; Monge, M.; Pyykko, P.; Runeberg,
N.; Schutz, M.; Werner, H.-J. Inorg. Chem. 2000, 39, 4786.
(17) (a) Schneider, D.; Schuster, O.; Schmidbaur, H. Organometallics 2005,
24, 3547. (b) Fackler, J. P., Jr. Inorg. Chem. 2002, 41, 6959.
(15) For a different type of gold(I)-gold(III) bond, see: Uson, R.; Laguna,
A.; Laguna, M; Tarton, M. T.; Jones, P. G. J. Chem. Soc., Chem.
Commun. 1988, 740.
1364 Inorganic Chemistry, Vol. 46, No. 4, 2007

Amine-Amide Equilibrium in Gold(III) Complexes
Figure 7. 1H NMR spectra (400 MHz, acetone-d6) of complex 6 in the
region of the NCH2 and NH resonances at temperatures of (a) 20, (b) -20,
(c) -60, and (d) -90 °C. The low intensity of the NH resonance occurs as
a result of H-D exchange with solvent.
Scheme 4
Figure 8. View of the structure of the complex [AuCl(BMPA)](OTf)2, 6.
Two triflate ions involved in NH‚‚‚O-S-O‚‚‚Au bridging are shown.
Scheme 5
pale yellow to black. The pure product 6 decomposes quickly
in acetone, but is more stable in methanol solution. The pKa
value for 6, obtained as described for diethylenetriamine
derivatives,4 is estimated at 3.5. For comparison, the pKa
values of [AuCl(dien)]2+ and [AuCl(Et4dien)]2+ are estimated
to be 4.0 and 2.2, respectively, and the spectroscopic changes
accompanying protonation or deprotonation are similar in
each case.4
1
In the H NMR spectrum in acetone-d6 solution at room
temperature, the NCH2 protons of complex 6 appeared as a
single resonance at δ ) 5.66, considerably shifted from the
value for 5a of δ ) 4.44, consistent with transfer of some
positive charge to the CH2 protons upon protonation of 5a.
There were smaller increases in the chemical shifts of the
pyridyl protons in 6 compared to that of 5a (see the
Experimental Section). At lower temperatures, the NCH2
resonance gradually became broader and finally split into
two peaks (Figure 7). No resonance for the NH proton was
resolved except at very low temperature (Figure 7), and the
intensity was low as a result of H-D exchange with the
(5) Å], whereas the Au-Cl bond in 6 [2.265(3) Å] is shorter
than in 5a [2.341(2) Å]. Hence, protonation of the amido
nitrogen atom weakens the Au-N bond because the amine
nitrogen is a weaker σ-donor than the amido nitrogen.
Because the amido nitrogen in 5a is a stronger σ-donor than
the amine nitrogen donor in 6, it has a stronger trans
influence than the amine nitrogen donor, and so, the Au-
Cl bond is longer in 5a than in 6. It is interesting that
protonation of the nitrogen atom leads to a less pyramidal
stereochemistry at N(8) in 6 than in 5a. Thus, the sum of
the bond angles at N(8) (Table 1) is greater in 6 (342°) than
in 5a (330°), as expected if there were little Au-N(8)
π-bonding in the amido complexes.
In complex 6, both triflate ions are weakly coordinated to
gold(III) with Au ‚‚O(36) ) 2.79 Å and Au‚‚‚O(28) ) 2.84
Å. In addition, there is a hydrogen bond between the NH
group of 6 and one of the oxygen atoms of a triflate anion
of a neighboring formula unit [N(8) ‚‚O(26A) ) 2.98(1) Å].
The bridging of the triflate ion N(8)H‚‚‚O(26A)-S-O(28A)
‚‚Au(A) leads to formation of a supramolecular polymer.
1
solvent. The changes in the H NMR spectra as a function
of temperature are interpreted in terms of overall inversion
at the amine nitrogen atom, which leads to effective
equivalence of the CHaHb protons of each methylene group.
The mechanism is likely to involve reversible deprotonation
of the amine group, with inversion at the nitrogen of the
amido derivative, as shown in Scheme 5. The inversion at
nitrogen is slowed by the addition of free triflic acid. The
protonation of 5a in acetone-d6 with H[PF6] gives NMR
spectra very similar to those reported for 6, but it has not
been possible to isolate this product in pure form. It is likely
that the hydrogen bonding of the NH proton of 6 with triflate
is less important in solution than in the solid state (see
below).18
The structure of complex 6 is shown in Figure 8, and its
structural parameters are included in Tables 1 and 2. The
overall structure is similar to those of 5a-5c (Tables 1 and
2), but there are subtle differences. For example, the Au-
N(8) bond in 6 [2.008(7) Å] is longer than that in 5a [1.994-
(18) (a) Romeo, R.; Minniti, D.; Alibrandi, G.; de Cola, L.; Tobe, M. L.
Inorg. Chem. 1986, 25, 1944. (b) Romeo, R.; Nastasi, N.; Scolaru,
M.; Plutino, M. R.; Albinati, A.; Macchioni, A. Inorg. Chem. 1998,
37, 5460.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1365

Cao et al.
amide donor in 5 compared to the amine donor in 6 leads to
weakening and polarization of the trans Au-Cl bond in 5.
Conclusions
The neutral ligands BBPMA and BPMA can coordinate
to gold(III) to give the cationic complexes [AuCl(BBP-
MA)]2+ (3; Scheme 2) and [AuCl(BPMA)]2+ (6; Scheme
4). The gold(III) center is strongly polarizing, and the effect
is exhibited by the activation of a C-N bond of complex 3
(Scheme 2) or by the easy deprotonation of complex 6 to
give the corresponding amido complex [AuCl(BPMA-H)]+
(5; Schemes 3 and 4). The much greater polarizing power
of gold(III) compared to, for example, platinum(II) is seen
in the differences in pKa values for [Pt(DMSO)(dien)]2+
(11.9),18 [AuCl(dien)]2+ (4.0),4a [Pt(OH)(BPMA)]+ (11.5),11a
and [AuCl(BPMA)]2+ (3.5). This difference is not purely
an oxidation-state phenomenon, as BPMA complexes of
molybdenum(V) are not spontaneously deprotonated.20 The
difference between gold(III) and platinum(II) is important
in coordination chemistry, and especially in aqueous bioi-
norganic chemistry, because under normal conditions, the
BPMA complexes of platinum(II) and gold(III) exist in the
amine (BPMA) and amido (BPMA-H) forms, respec-
tively.10-13,18,21 The ability of gold(III) to act as a strong
σ-electrophile, with intermediate hard-soft character, is
considered to be a key property in the bond-activation
reactions involved in catalytic chemistry.1
The stronger trans influence of the amido group in 5
compared to the amine group in 6 on the trans Au-Cl bond
is evident from the structure determinations and also from
the DFT calculations (Table 1). Previous work has shown
that the trans effect, measured by the rate of ligand
substitution, tends to be higher for the amido ligand with
neutral nucleophiles and higher for the amine group with
anionic nucleophiles.4,18 The present work suggests that the
actual ligand-substitution step should be faster for the trans
amide but that the more favorable pre-equilibrium of anionic
reagents X- with the dicationic amine complex {[AuCl-
(BPMA)]2+‚‚‚X- in the case of 6} compared to the mono-
cationic amido complex {{[AuCl(BPMA-H)]+‚‚‚X- in the
case of 5} can mask this effect.
Figure 9. (a,d) Calculated structures, (b,e) HOMOs, and (c,f) LUMO for
complexes (a-c) 5 and (d-f) 6.
Theoretical Studies. To gain further insight into the
structural features of complexes 5 and 6, calculations were
carried out on the cations [AuCl(BPMA-H)]+ and [AuCl-
(BPMA)]2+ using density functional theory (DFT), with a
relativistic potential for gold.19 Data are reported in Table 1
and in Figure 9. Table 1 shows that the DFT calculations
give reasonable values for the bond distances and also
accurately predict the trigonal-pyramidal stereochemistry of
the amido nitrogen atom in 5 (calculated sum of bond angles
at N ) 331°). The calculation also predicts the longer Au-
Cl and shorter Au-N(amide) distances in 5 compared to 6
(Table 1).
In complex 5, the HOMO (Figure 9b) has mostly the
character of the amido nitrogen lone pair, with smaller
2
2
2
components from gold (5dx -y , 5dxy, 5dz ) and chlorine (3pz),
2
2
while the LUMO (Figure 9c) is the σ*(Au 5dx -y ) molecular
orbital. The gold 6pz orbital is too high in energy to play a
significant part in the bonding, and the lack of π-bonding
with this gold 6pz orbital accounts for the pyramidal
stereochemistry of the amido nitrogen atom in 5. Protonation
of the amido nitrogen lone pair in 5 leads to stabilization of
the σ(NH) orbital in 6, and the HOMO in 6 (Figure 9e) has
mostly chlorine 3pz character. The character of the LUMO
in 6 (Figure 9f) is similar to that in 5 (Figure 9c).
The nearly planar structure of [AuCl(BPMA-H)]+ allows
easy axial approach to the gold(III) center, and there is a
competition between electrostatic attraction to negatively
charged anions or dipoles and the formation of aurophilic
bonds. Unusual examples of Au(III)‚‚‚Au(III) and
Au(III)‚‚‚Au(I) aurophilic bonds are identified, in which the
orientation of the two gold units is mostly controlled by
electrostatic interactions between the supporting ligands on
each gold center.14
For the trans N-Au-Cl atoms, the Hirschfeld analysis
of the atomic charges is useful.19 These charges are N,
-0.16e; Au, 0.44e; and Cl, -0.16e in 5 and N, -0.03e; Au,
0.50e; and Cl, -0.06e in 6. The charge on the NH proton in
6 is calculated to be 0.48e. Thus, upon protonation of 5, a
charge of 0.52e is transferred from 5 to the proton, and this
is contributed by the atoms of the trans N-Au-Cl unit
(0.29e, consisting of 0.13e from N, 0.06e from Au, and 0.10e
from Cl) and by the atoms of the pyridylmethyl groups
(0.23e). Of course, this transfer of charge is fully consistent
with the concept of a trans influence, in which the stronger
Experimental Section
NMR spectra were recorded using a Varian Mercury or Inova
400 NMR spectrometer. The ligands BPMA and BBPMA were
prepared according to the literature methods.22
(19) (a) Baerends, E. J. et al. ADF2006.01; Vrije Universiteit: Amsterdam,
The Netherlands, 2006; available at http://www.scm.com. (b) Becke,
A. D. J. Chem. Phys. 1993, 98, 5648. (c) Hirschfeld, F. L. Theor.
Chim. Acta 1977, 44, 129.
(20) van Staveren, D. R.; Bothe, E.; Weyhermuller, T.; Metzler-Nolte, N.
Eur. J. Inorg. Chem. 2002, 1518.
(21) Gibson, D. H.; Wu, J. G.; Mashuta, M. S. Inorg. Chim. Acta 2006,
359, 309.
1366 Inorganic Chemistry, Vol. 46, No. 4, 2007

Amine-Amide Equilibrium in Gold(III) Complexes
Table 3. Crystal Data for Compounds 2a, 2b, and 4
H, 3.56; N, 7.07%. Found: C, 38.01; H, 3.34; N, 6.96%. The NMR
data were very similar to the data for 2a. Crystals were grown from
acetone/ether.
2a
2b
4
formula
fw
C19H21AuCl5N3 C19H21AuCl3N3 C13H16Cl2N2
[AuCl(BBPMA)]Cl2, 3. To a stirred solution of NaAuCl4 (0.338
g, 1.00 mmol) in H2O (20 mL) was added a solution of BBPMA
(0.267 g, 1.00 mmol). The mixture was stirred for 4 h to give the
product as a brown solid precipitate, which was separated by
filtration; washed with H2O, ethanol, Et2O, and pentane; and dried
under vacuum. Yield: 57%. Mp: 138-141 °C (decomp). Anal.
Calcd for C19H19AuCl3N3: C, 38.50; H, 3.23; N, 7.09%. Found:
C, 38.28; H, 3.03; N, 6.98%. NMR in nitromethane-d3: δ(1H) )
8.73 (d, 2H, py-H6), 8.21 (m, 2H, py-H4), 7.72 (d, 2H, py-H3),
7.60 (m, 2H-py-H5), 7.48 (d, 2H, Ph-H2H6), 7.10-7.15 (m, 3H‚
‚‚Ph-H3H4H5), 5.75 (d, 2H, N-CHaHb-py), 5.19 (d, 2H,
N-CHaHb-py), 4.86 (s, 2H, N-CH2-Ph).
[PhCH2NH2CH2-2-C5H4NH]Cl2, 4. To a solution of NaAuCl4
(0.415 g, 1.18 mmol) in distilled water (30 mL) was added BBPMA
(0.342 g, 1.18 mmol). The mixture was stirred for 12 h at 80 °C,
cooled to room temperature, and filtered to remove insoluble
material, and the solvent from the filtrate was evaporated under
vacuum. The solid product was recrystallized from MeOH/Et2O.
Yield: 35%. Mp: 125-129 °C (decomp). Anal. Calcd for C13H16-
Cl2N2: C, 57.58; H, 5.95; N, 10.33%. Found: C, 57.51; H, 5.83;
N, 9.97%. NMR in CD3OD: δ(1H) ) 8.87 (d, 1H, py-H6), 8.44
(m, 1H, py-H4), 8.06 (d, 1H, py-H3), 7.92 (m, 1H, py-H5), 7.60
(m, 2H, Ph-H2H6), 7.45-7.50 (m, 3H, Ph-H3H4H5), 4.65 (s, 2H,
N-CH2-py), 4.43 (s, 2H, N-CH2-Ph).
[AuCl(BPMA-H)]PF6, 5b. To a solution of NaAuCl4 (0.873
g, 2.22 mmol) in H2O (30 mL) was added NaHCO3 (0.306 g, 2.22
mmol) and KPF6 (0.815 g, 4.43 mmol). To this solution was added
dropwise a solution of BPMA (0.455 g, 2.22 mmol) in MeOH (5
mL). The product precipitated as a red solid, which was separated
by filtration; washed with H2O (2 × 30 mL), EtOH (30 mL), and
ether (3 × 30 mL); and recrystallized from acetone/ether. Yield:
1.19 g, 90%. Mp: 112 °C (decomp). Anal. Calcd for C12H12-
AuClF6N3P: C, 25.04; H, 2.10; N, 7.30%. Found: C, 24.69; H,
2.02; N, 7.18%. NMR in acetone-d6: δ(1H) ) 9.15 (d, 2H, py-
H6), 8.46 (m, 2H, py-H4), 8.12 (d, 2H, py-H3), 7.91 (t, 2H, py-H5),
4.44 (s, 4H, CH2).
665.6
594.7
271.2
T (K)
150(2)
150(2)
299(2)
0.71073
triclinic
P1h
λ (Å)
0.71073
orthorhombic
Pnma
0.71073
orthorhombic
Pbca
cryst syst
space group
cell dimens
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
13.5338(3)
7.7333(2)
21.7974(6)
90
90
90
2281.3(1)
4
1.938
7.045
1280
11.7116(3)
14.5205(4)
23.9645(6)
90
90
90
4075.4(2)
8
1.939
7.621
2288
5.0844(4)
11.8297(7)
12.038(1)
93.929(5)
98.807(3)
94.546(4)
710.9(1)
2
Z
d (calcd) (Mg m-3
)
1.267
0.437
284
µ (mm-1
F(000)
)
abs corr
integration
integration
4665/0/236
1.021
0.036
0.096
integration
2470/0/172
1.054
data/constr/params 3566/0/196
GOF
R1 [I > 2σ(I)]
wR2[I > 2σ(I)]
1.076
0.042
0.091
0.062
0.168
[(BBPMA)H2]Cl[AuCl4], 2a. To a solution of HAuCl4·3H2O
(0.394 g, 1.00 mmol) in acetone (20 mL) was added a solution of
BBPMA (0.290 g, 1.00 mmol) in acetone (5 mL). The mixture
was stirred for 5 h at room temperature; then, the solution was
filtered, the volume of the filtrate was reduced to 5 mL under
vacuum, and pentane (20 mL) was added to precipitate the product
as a pale brown solid. The product was filtered, washed with ether
and pentane, and recrystallized from acetone/pentane. Yield: 85%.
Mp: 117 °C (decomp). Anal. Calcd for C19H21AuCl5N3: C, 34.28;
H, 3.18; N, 6.31%. Found: C, 33.97; H, 3.06; N, 6.22%. NMR in
acetone-d6: δ(1H) ) 8.93 (d, 2H, py-H6), 8.59 (m, 2H, py-H4),
8.06-8.11 (m, 4H, py-H3,H5), 7.43 (d, 2H, ph-H2H6), 7.08-7.18
(m, 3H, Ph-H3H4H5), 4.33 (s, 4H, N-CH2-py), 3.99 (s, 2H,
N-CH2-Ph).
[(BBPMA)H2]Cl[AuCl2], 2b. This compound was prepared
similarly to 2a, but the reaction solution was heated under reflux
for 3 h. Yield: 53%. Anal. Calcd for C19H21AuCl3N3: C, 38.37;
[AuCl(BPMA-H)]BF4, 5a. This compound was prepared in a
similar way except that NaBF4 was used in place of KPF6. Yield:
Table 4. Crystal Data for Compounds 5a-5c and 6‚Me2CO
5a
5b
5c
6‚Me2CO
formula
fw
C12H12AuBClF4N3
517.47
C12H12AuClF6N3P
575.63
C12H12Au2Cl3N3
698.53
C17H19AuClF6N3O7S2
787.89
T (K)
150(2)
150(2)
296(2)
150(2)
λ (Å)
0.71073
monoclinic
P2/c
0.71073
triclinic
P1h
0.71073
monoclinic
P21/n
0.71073
monoclinic
P21/n
cryst syst
space group
cell dimens
a (Å)
14.3491(8)
7.6628(3)
14.2562(8)
90
112.434(2)
90
1448.9(1)
4
2.372
7.1133(8)
9.2734(1)
13.218(2)
108.123(5)
97.509(7)
102.762(7)
789.3(2)
2
7.4839(3)
19.8209(7)
10.7631(4)
90
95.441(2)
90
710.9(1)
4
2.919
8.4086(4)
13.8090(7)
22.193(1)
90
97.336(3)
90
2555.8(2)
4
2.048
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
d (calcd) (Mg m-3
)
2.422
9.655
540
µ (mm-1
F(000)
)
10.379
968
18.935
1256
6.111
1520
abs corr
data/constr/params
GOF
integration
2557/4/200
0.983
integration
2780/0/217
1.044
integration
2793/0/182
0.964
integration
4501/0/335
1.048
R1 [I > 2σ(I)]
wR2[I > 2σ(I)]
0.035
0.056
0.051
0.105
0.038
0.087
0.054
0.091
Inorganic Chemistry, Vol. 46, No. 4, 2007 1367

Cao et al.
88%. Mp: 102-103 °C. Anal. Calcd for C12H12AuBClF4N3: C,
27.85; H, 2.34; N, 8.12%. Found: C, 27.56; H, 2.09; N, 7.86%.
NMR in acetone-d6 similar to 5b.
DFT Calculations. Calculations were carried out using the
Amsterdam density functional theory package ADF2006.01.19 The
hybrid functional BLYP was used, with a relativistic potential for
gold.19 Atomic charges were estimated using the Hirschfeld
procedure.19
X-ray Structure Determinations. A crystal was mounted on a
glass fiber. Data were collected using a Nonius Kappa-CCD
diffractometer with COLLECT software. Unit cell parameters were
calculated and refined from the full data set, and the absorption
correction was made using the HKL2000 DENZO-SMN program.
The refinement was carried out using the SHELXTL/PC 6.14
programs. Non-hydrogen atoms were refined anisotropically, and
hydrogen-atom positions were included as riding on the respective
carbon or nitrogen atom. Details are reported in Tables 3 and 4.
[AuCl(BPMA-H)][AuCl2], 5c. This compound was prepared
similarly to 5b but with no KPF6 added and with a reaction time
of 4 h. A red powder was obtained and was recrystallized from
acetone/pentane. Yield: 91% (based on gold). Mp: 114 °C
(decomp). Anal. Calcd for C12H12Au2Cl3N3: C, 20.63; H, 1.73; N,
6.02%. Found: C, 21.02; H, 1.95; N, 6.21%. NMR in DMSO-d6:
δ(1H) ) 8.97 (d, 2H, py-H6), 8.35 (t, 2H, py-H4), 8.00 (d, 2H,
py-H3), 7.78 (t, 2H, py-H5), 4.13 (s, 4H, CH2).
[AuCl(BPMA)](OTf)2, 6. To a red suspension of complex 5a
(0.183 g, 0.319 mmol) in acetone (10 mL) was added HOTf (0.056
mL, 0.64 mmol). The red color changed to pale yellow. The volume
was reduced to ∼3 mL under vacuum, and the precipitate was
washed with ether (2 × 10 mL) and pentane (10 mL) and
recrystallized from methanol/ether. Yield: 0.152 g, 72%. Mp: 163
°C (decomp). Anal. Calcd for C14H13AuClF6N3O6S2: C, 23.04; H,
1.80; N, 5.76%. Found: C, 23.23; H, 2.06; N, 5.90%. NMR in
acetone-d6: δ(1H) ) 9.14 (m, 2H, py-H6), 8.64 (m, 2H, py-H4),
8.20 (m, 2H, py-H3), 8.05 (m, 2H, py-H5), 5.56 (s, 4H, CH2).
Acknowledgment. We thank the NSERC (Canada) for
financial support. R.J.P. thanks the Government of Canada
for a Canada Research Chair.
Supporting Information Available: X-ray data as electronic
CIF files. Structure of the tetrachloroaurate salt 2a (Figure S1) and
crystallographic characterization of [PhCH2NH2CH2-2-C5H4NH]-
Cl2, 4 (Figure S2). This material is available free of charge via the
Internet at http://pubs.acs.org.
(22) (a) Gruenwedel, D. W. Inorg. Chem. 1968, 7, 495. (b) Dick, S.; Weiss,
A. Z. Naturforsch. B 1997, 52, 188.
IC061911Y
1368 Inorganic Chemistry, Vol. 46, No. 4, 2007