Structural variation in gold(I)-chelate systems: Synthesis of an asymmetrically bridged β-diketiminato complex of gold

Article abstract of DOI:10.1016/j.poly.2012.02.026

The reaction of the potassium diketiminate K[RNC(CF₃)CH(CF ₃)CNR)], where R = 3,5-C₆H₃Me₂, with PPh₃AuCl afford the complex [RNC(CF₃)CH(CF ₃)CNR)AuPPh₃]. Unlike gold(I) diketiminates without backbone-CF₃ substituents, the complex is thermally stable in the solid state and in solution. The crystal structure confirms that, unlike previous examples of Au(I) ketiminates, this complex possesses a three-coordinate metal centre with a distorted Y-shaped coordination geometry.

Full text of DOI:10.1016/j.poly.2012.02.026

Polyhedron 38 (2012) 137–140
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Structural variation in gold(I)-chelate systems: Synthesis of an
asymmetrically bridged b-diketiminato complex of gold
⇑
Nicky Savjani, Mark Schormann, Manfred Bochmann
Wolfson Materials and Catalysis Centre, School of Chemistry, University of East Anglia, Norwich NR4 7TJ, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of the potassium diketiminate K[RN@C(CF3)CH@(CF3)CNR)], where R = 3,5-C6H3Me2, with
PPh3AuCl afford the complex [RN@C(CF3)CH@(CF3)CNR)AuPPh3]. Unlike gold(I) diketiminates without
backbone-CF3 substituents, the complex is thermally stable in the solid state and in solution. The crystal
structure confirms that, unlike previous examples of Au(I) ketiminates, this complex possesses a three-
coordinate metal centre with a distorted Y-shaped coordination geometry.
Received 6 February 2012
Accepted 23 February 2012
Available online 8 March 2012
Keywords:
Gold
Ó 2012 Elsevier Ltd. All rights reserved.
Chelate complex
Diketiminate
Coordination compound
Phosphine complex
1. Introduction
b-Diketiminato ligands are attractive as versatile ligands with
easily tuneable steric and electronic properties. They form com-
plexes with most metallic elements across the Periodic table, and
in the majority of cases the complexes are of the well-known
N,N-bonded chelate type (structure A) [1]. The exception from this
important class is gold(I): complexes with methyl-substituted
ligands such as [HC(MeC@NC6H3Pri2)2]À are thermally unstable,
and 1,5-diazapentadienyls form dimers with W-conformation
(structure B; R = 2,6-Pri2C6H3 and 2,4,6-Br3C6H2), in which Au(I)
is linearly coordinated [2]. This lack of chelation of gold(I) towards
F3C
F3C
NH
N
F3C
F3C
N
N
F3C
F3C
N
ClAu(PPh3)
Et2O
KH
Et2O
K
PPh3
Au
N
1-H
1-K
2
Scheme 1. Synthesis of complex 2.
N^N is also seen with imidato and a-diimine complexes [3].
mation (C) [4]. By contrast, b-diketiminato complexes of gold(III)
show the expected N,N-chelate structure [5].
We report here the synthesis of a closely related complex with
non-fluorinated aryl substituents, [RN@C(CF3)CH@(CF3)CNR)AuPPh3]
where R = 3,5-C6H3(CH3)2. It turns out that the comparatively subtle
electronic change of replacing the meta-CF3 substituents of the aryl
ring by methyl groups has unexpected structural consequences.
We have recently reported the synthesis, structure and
fluxional behaviour of a bis(trifluoromethyl)phenyl-substituted
2. Results and discussion
derivative, [(j
1-RN@C(CF3)CH@(CF3)CNR)AuPPh3], where R = 3,5-
C6H3(CF3)2. Here the metal is coordinated to only one N donor,
and in the solid state the C3N2 backbone is twisted into a U-confor-
The potassium salt of the b-diketimide 1 reacted cleanly with
AuCl(PPh3) in diethyl ether at 0–20 °C to give the corresponding
diketiminato complex 2 as orange crystals in essentially quantita-
tive yield (Scheme 1). Like complex C, this compound is tempera-
ture stable in the solid state and in solution.
⇑
Corresponding author. Fax: +44 01603592044.
E-mail address: m.bochmann@uea.ac.uk (M. Bochmann).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.02.026

138
N. Savjani et al. / Polyhedron 38 (2012) 137–140
The crystal structure of 2 (Fig. 1) shows that both N-atoms of
the N-substituents: Replacement of 3,5-C6H3(CF3)2 by 3,5-
C6H3(CH3)2 enforces a trend towards N,N-chelate formation, with-
out however reaching the symmetric chelate structure of typical
transition metal NCCCN chelates [1,3,7,8]. The chelate is stabilised
by C–HÁ Á ÁF hydrogen bonding interactions between the C(3)–H
moiety and the two CF3 substituents. To our knowledge compound
2 represents the first example of a gold(I) complex coordinating to
this class of ligand in chelate fashion. The presence of two CF3 sub-
stituents on the backbone makes the ligand system sufficiently
electron accepting to render this Au(I) diketiminate thermally sta-
ble in solution to P100 °C.
the diketiminato ligand are coordinated to the gold cation, albeit
in an asymmetric manner [Au(1)–N(1) 2.137(4), Au(1)–N(2)
2.384(4) Å]. In line with this, the [NCCCN] backbone shows bond
alternation, i.e. only partial charge delocalisation. Unlike C, the
N(1)–Au–P moiety is distorted significantly from linear towards a
Y-shaped, three-coordinate geometry of the metal centre [N(1)–
Au(1)–P(1) 151.91(12)°]. The backbone of the diketiminato ligand
and the gold-phosphine cation exist within a symmetry plane, with
only two phenyl groups from the phosphine and two from the N-
aryl moieties protruding from this plane. The Au–Au distances
are in excess of 5 Å, which rules out aurophilic interactions.
Within the backbone, there is evidence for strong intramolecu-
lar H–F interactions between the one fluoride of both CF3 groups
and the C–H unit of the b-diketiminate (F(3)–H(2) 2.20(5), F(4)–
H(2) 2.20(6) Å) [6]. An investigation of the X-ray data of related
Cu and Ru complexes bearing CF3-substituted diketiminato ligands
suggest the existence of similar CHÁ Á ÁF bonding patterns, although
these reports make no specific comments [7–10]. The fluorine–
hydrogen-bonding interactions of these compounds fall within
the range of 1.9–2.2 Å, significantly shorter than the sum of the
van der Waals radii (ca. 2.55 Å).
As was seen with the derivative C [4], complex 2 is fluxional. In
toluene-d8 at 22° C, two separate, but broadened 19F signals are ob-
served for the two CF3 substituents in 2- and 4-positions of the
diazapentadienyl ligand, at dÀ70.1 and À64.3, respectively
(Fig. 2). Heating solutions of 2 up to the boiling point of toluene-
d8 results in broadening of the CF3 signals into the baseline. The
spectra closely resemble those of C in the temperature range up
to 73° C; however, in the case of 2 coalescence of the two CF3
groups is not reached below the boiling point of the solvent. The
original spectrum was restored on cooling; without apparent
decomposition.
3. Experimental
All manipulations were performed under an atmosphere of dry
nitrogen using standard Schlenk techniques. Solvents were puri-
fied by distillation under nitrogen was from sodium–potassium al-
loy (light petroleum, bp 40–60 °C) or sodium-benzophenone
(diethyl ether, THF). Deuterated toluene was degassed by several
freeze–thaw cycles and dried over activated 4 Å molecular sieves.
NMR spectra (1H, 13C) were recorded on a Bruker Avance DPX-
300 spectrometer. 1H NMR spectra were referenced to residual sol-
vent protons. The ligand precursor 1-H was prepared following the
method by Sadighi and co-workers [11].
3.1. Synthesis of K[RN@C(CF3)CH@(CF3)CNR)] (R = 3,5-C6H3Me2)
(1-K)
A suspension of potassium hydride (32 mg, 0.81 mmol) in
diethyl ether (15 mL) was added to a solution of 1-H (315 mg,
0.76 mmol) in diethyl ether (10 mL). The solution turned immedi-
ately from yellow to orange. The mixture was stirred for 5 h at
room temperature and filtered. The solvent was removed under
vacuum to leave 1-K an orange solid (320 mg, 0.71 mmol, 93%).
1H NMR (300 MHz, CDCl3): d 6.80 (2H, s, p-Ar), 6.63 (4H, s, o-Ar),
In conclusion, the coordination geometry of b-diketiminato gold
complexes provides a nice example of the surprising sensitivity of
these complexes to minor changes in electronic characteristics of
Fig. 1. Molecular structure of 2. Ellipsoids are drawn at 50% probability. Selected bond lengths (Å) and angles (°): Au(1)–N(1) 2.137(4), Au(1)–N(2) 2.384(4); N(1)–C(1)
1.339(7), N(2)–C(3) 1.308(7); C(1)–C(2) 1.382(7), C(2)–C(3) 1.416(7); N(1)–Au–P(1) 151.91(12); N(2)–Au–P(1) 123.68(10); N(1)–Au–N(2) 84.41(15); F(3)Á Á ÁH(2) 2.20(5);
F(4)Á Á ÁH(2) 2.20(6); F(3)Á Á ÁH(2)Á Á ÁF(4) 143(3).

N. Savjani et al. / Polyhedron 38 (2012) 137–140
139
Fig. 2. 19F NMR spectra of 2 over the temperature range 25–105 °C (toluene-d8).
5.81 (1H, s, CH), 2.27 (12H, s, CH3-Ar). 19F NMR (282.4 MHz, CDCl3):
d À62.76 (6F, s, CF3). Anal. Calc. for C21H19F6KN2: C, 55.74; H, 4.23;
N, 6.19. Found: C, 55.67; H, 4.35; N, 6.11%.
SHELXL [13]. All non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were included in idealised
positions and their Uiso values were set to ride on the Ueq values
of the parent carbon atoms. Except for the methyl groups C(17),
C(18), C(27) and C(28) that were allowed to rotate about the C–C
bond and the coordinates of H(2) that were refined freely.
3.2. Synthesis of [RN@C(CF3)CH@(CF3)CNR)AuPPh3]
(R = 3,5-C6H3Me2) (2)
An orange-red solution of 1-K (226 mg, 0.50 mmol) in dry THF
3.3.1. Crystal data
ꢀ
(15 mL) was added by syringe to
a colourless solution of
C39H34AuF6N2P, M = 872.62, triclinic, space group P1 (No. 2),
AuCl(PPh3) (247 mg, 0.50 mmol) in dry THF (20 mL) cooled on an
ice bath. After 1 h at 0 °C and 1 h at room temperature the THF
was removed to leave an orange-red solid, which was recrystal-
lized from light petroleum at À28 °C. Orange crystals of 2 were ob-
tained (371 mg, 0.43 mmol, 86%). 1H NMR (300 MHz, CDCl3): d 7.47
(15H, m, PPh3), 6.53 (4H, s, o-Ar), 6.43 (2H, s, p-Ar), 5.51 (s, 1H, CH),
2.07 (12H, s, CH3-Ar). 13C NMR (75.5 MHz, CDCl3): d 142.6 (s), 148.6
(s) 138.4 (d, J = 13.6 Hz), 134.2 (s), 131.8 (s), 129.2 (br. s), 127.2 (s),
125.0 (br. s), 121.2 (br. s), 120.2 (br. s), 119.0 (br. s), 114.3 (br. s),
21.2 (s). 19F NMR (300 MHz, CDCl3, 20 °C): d À66.17 (br, 3F, 4-
CF3), À71.14 (br, 3F, 2-CF3). 31P NMR (CDCl3): d 31.02 (s). Anal.Calc.
for C39H34AuF6N2P: C, 53.68; H, 3.93; N, 3.21. Found: C, 53.52; H,
3.78; N, 3.33%.
a = 12.2054(4), b = 12.7181(4), c = 13.6002(4) Å,
b = 111.625(2),
= 108.416(2)°, V = 1737.12(9) Å3. Z = 2, Dcalc
1.668 g cmÀ3, F(000) = 860, T = 120(1) K, k(Mo K
) = 0.71069 Å.
wR2 = 0.0904 and R1 = 0.0654 for all 7939 reflections – for the ‘ob-
served’ data only, R1 = 0.0450.
a = 101.607(2),
c
=
a
Acknowledgements
NS thanks the University of East Anglia for a studentship. This
work was supported by Johnson Matthey plc and the Engineering
and Physical Sciences Research Council. We thank the National
Crystallography Service for the data collection of 2.
3.3. X-ray crystallography
Appendix A. Supplementary data
Crystal data were collected at 120(1) K on a Bruker-Nonius
Roper CCD diffractometer at the EPSRC National Crystallography
Service. Data were processed using DENZO/SCALEPACK [12] programs.
CCDC 865393 contains the supplementary crystallographic data
for complex 2. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax:+44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
The structure was solved by direct methods in the SHELXS program
[13] and refined by full-matrix least-squares methods, on F2’s, in

140
N. Savjani et al. / Polyhedron 38 (2012) 137–140
[8] H.V.R. Dias, J.A. Flores, J. Wu, P. Kroll, J. Am. Chem. Soc. 131 (2009) 11249.
References
[9] A.D. Phillips, O. Zava, R. Scopilitti, A.A. Nazarov, P.J. Dyson, Organometallics 29
(2010) 417.
[10] S. Hong, L.M.R. Hill, A.K. Gupta, B.D. Naab, J.B. Gilroy, R.G. Hicks, C.J. Cramer,
W.B. Tolman, Inorg. Chem. 48 (2009) 4514.
[11] D.S. Laitar, C.J.N. Mathison, W.M. Davis, J.P. Sadighi, Inorg. Chem. 42 (2003)
7354.
[12] Z. Otwinowski, W. Minor, in: C.W. Carter, R.M. Sweet (Eds.), Methods in
Enzymology, Macromolecular Crystallography Part A, vol. 276, Academic
Press, 1997, p. 307.
[1] L. Bourget-Merle, M.F. Lappert, J.R. Severn, Chem. Rev. 102 (2002) 3031.
[2] H.V.R. Dias, J.A. Flores, Inorg. Chem. 46 (2007) 5841.
[3] A.A. Mohamed, Coord. Chem. Rev. 254 (2010) 1918.
[4] N. Carrera, N. Savjani, J. Simpson, D.L. Hughes, M. Bochmann, Dalton Trans. 40
(2011) 1016.
[5] A. Venugopal, M.K. Ghosh, H. Jürgens, K.W. Törnroos, O. Swang, M. Tilset, R.H.
Heyn, Organometallics 29 (2010) 2248.
[6] For a discussion of CH-F bonding criteria see for example: A.J. Mountford, D.L.
Hughes, S.J. Lancaster, Chem. Commun. (2003) 2148.
[13] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.
[7] J.A. Flores, H.V.R. Dias, Inorg. Chem. 47 (2008) 4448.