Syntheses and structures of thermally stable diketiminato complexes of gold and copper

Article abstract of DOI:10.1039/c0dt01422b

While most metallic elements across the Periodic Table form stable chelating β-diketiminato complexes, examples of Au(i) are conspicuous by their absence. We report here the reaction of K[HC(F₃CCNR) ₂] with AuCl(PPh₃) which provides a rare example of a thermally stable gold(i) diketiminato complex, (Ph₃P)Au[RNC(CF ₃)CH(CF₃)CNR] [R = 3,5-C₆H₃(CF ₃)₂]. The complex is highly fluxional in solution but in the solid state adopts a U-conformation. By contrast, the analogous reaction of K[HC(F₃CCNR)₂] with CuBr(PPh₃)₃ gives the rigid 18-electron chelate complex (Ph₃P) ₂Cu[κ²-HC{(CF₃)CNR}₂].

Full text of DOI:10.1039/c0dt01422b

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 1016
www.rsc.org/dalton
COMMUNICATION
Syntheses and structures of thermally stable diketiminato complexes of gold
and copper†
Nora Carrera, Nicky Savjani, Jason Simpson, David L. Hughes and Manfred Bochmann*
Received 19th October 2010, Accepted 18th November 2010
DOI: 10.1039/c0dt01422b
While most metallic elements across the Periodic Table form
stable chelating b-diketiminato complexes, examples of Au(I)
are conspicuous by their absence. We report here the reaction
of K[HC(F3CC NR)2] with AuCl(PPh3) which provides
a rare example of a thermally stable gold(I) diketiminato
complex, (Ph3P)Au[RN C(CF3)CH(CF3)C NR] [R = 3,5-
C6H3(CF3)2]. The complex is highly fluxional in solution but
in the solid state adopts a U-conformation. By contrast, the
analogous reaction of K[HC(F3CC NR)2] with CuBr(PPh3)3
Preliminary screening reactions of AuCl or AuCl(tht) (tht =
tetrahydrothiophene) with either Li[HC(MeC NC6H3Pri2)2] or
Na[HC(MeC NC6H4But-4)2] in THF or diethyl ether at 0 ◦C led
in all cases to the rapid precipitation of metallic gold; evidently,
these electron-rich ligands lead only to reduction. By contrast,
the potassium salt of the less electron-donating trifluoromethyl-
substituted ligand 16 reacted cleanly with AuCl(PPh3) in diethyl
ether at 0–20 ◦C to give the corresponding gold(I) diketiminato
complex 2 as orange crystals in essentially quantitative yield
(Scheme 1; for experimental details and the crystal structure of
1-H see the ESI†). In the solid state and in solution this complex
is remarkably temperature-stable.
gives the rigid 18-electron chelate complex (Ph3P)2Cu[j2-
HC{(CF3)C NR}2].
Diketiminato anions are widely used as versatile ligands with
easily tuneable steric and electronic properties. Derivatives with
sterically hindered N-aryl substituents, notably mesityl and 2,6-
diisopropylphenyl, have proved particularly useful. Most metallic
elements across the Periodic Table are now known to form b-
diketiminato complexes of the well-known N,N-bonded chelate
type (structure A).1 One element that is conspicuous by its absence
in this important class of compounds is gold(I). Although AuCl
mixed with the diketiminate Li[HC(MeC NC6H3Pri2)2] has been
reported as an oxidation catalyst, the assumption being that under
such conditions Au(I) diketiminato chelate complexes form in situ
as catalytically active species,2,3 evidence for its formation was
scant. To our knowledge the only examples of isolated and fully
characterised b-diketiminato gold(I) complexes are the binuclear
species [Au(RN CHCH CH-NR)]2 (R = 2,6-Pri2C6H3 and
2,4,6-Br3C6H2), in which Au(I) is linearly coordinated to two
ligands, each of which adopts a W conformation (structure B).4 By
contrast, b-diketiminato complexes of gold(III) show the expected
chelate structure.5 We report here the synthesis and structure of
the first examples of monomeric gold(I) diketiminato complexes
supported by phosphine ligands.
The crystal structure of 2 (Fig. 1) shows that the diketiminato
ligands adopts a U-conformation, a with linear N–Au–P moiety.
There is no coordination to the second nitrogen atom. In the solid
state both C–N bonds adopt E-conformations. The C6H3(CF3)2
rings are approximately parallel.
Complex 2 is fluxional. In toluene-d8 at 22 ◦C, two separate but
broadened 19F signals are observed for the two CF3 substituents
in 2- and 4-positions of the diazapentadiene, at d -71.2 and
-65.5, respectively (Fig. 2). These signals coalesce at 67 ◦C and
◦
at 105 C have merged into a singlet at d -66.5. This behaviour
is consistent with the exchange of the metal centre between the
two nitrogen donors of the diketiminato ligand. However, as the
chemical shift of the coalescence singlet differs from the arithmetic
mean of the two low-temperature resonances, it is possible
that a third component participates in the equilibrium under
these conditions, conceivably a ligand rearrangement product of
the type [Au(PR3)2]+[Au(N–N)2]- that is not observed at lower
temperatures but that contributes to the time-averaged chemical
shift (Scheme 2).
Another form of fluxionality is the inversion at the non-
coordinating nitrogen atom.6 At room temperature and above the
19F chemical shift at -65.5 is apparently due to a mixture of E and
Z isomers of the C(CF3) NAr moiety. Cooling to -94 ◦C shifts
Wolfson Materials and Catalysis Centre, School of Chemistry, University
of East Anglia, Norwich, UK NR4 7TJ. E-mail: m.bochmann@uea.ac.uk;
Fax: +44 01603 592044
† Electronic supplementary information (ESI) available. CCDC reference
numbers 800528–800530. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt01422b
1016 | Dalton Trans., 2011, 40, 1016–1019
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Scheme 1
Fig. 1 Two views of the molecular structure of 2, indicating the atom numbering scheme. Hydrogen atoms have been omitted for clarity. Only the
atoms of the ma◦jor components of the disordered groups are shown. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths
˚
(A) and angles ( ): Au–N(21) 2.075(4), Au–P(1) 2.2360(12), N(21)–C(22) 1.368(6), C(24)–N(25) 1.263(7); C(22)–C(23) 1.342(7), C(23)–C(24) 1.467(7);
N(21)–Au–P(1) 175.59(11).
this equilibrium, most probably towards the E isomer present in
the crystal structure (see ESI†).
In summary, while there was no evidence for the formation
of thermally stable diketiminato gold(I) complexes with the
anions [HC(MeC NAr)2]- (Ar = 2,6-C6H3Pri2 or C6H4But-4), the
introduction of electron withdrawing CF3 substituents leads to
the first examples of mononuclear gold(I) diketiminato comple◦xes
with phosphine ligands which are stable in solution to >100 C.
In the ground state these compounds show 2-coordinate gold
but also intricate fluxional behaviour. By contrast, copper(I)
forms rigid tetrahedral 18-electron N–N chelates with this
ligand.
An analogous copper(I) complex was prepared for comparison.
Treating 1-K with CuBr(PPh3)3 gives the bis(phosphine) complex
3 as red crystals (Scheme 3). Unlike 2, the copper(I) diketiminate
adopts the expected chelate structure (Fig. 3), analogous to a
number of previously reported Cu(I) benzene and CO com-
plexes with the 1- ligand.7 Silver also forms a chelate with this
diketiminate but gives a trigonal complex with only one PPh3
ligand.8 A number of related mono-phosphine copper complexes
are known.9–13 Tetrahedral copper diketiminato complexes with
two phosphine ligands are comparatively rare but are favoured
by sterically less hindered chelate ligands.14 In the case of 3 the
formation of an 18-electron bis(phosphine) complex is seen as a
reflection of the electron withdrawing properties of the fluorinated
ligand.
Acknowledgements
We gratefully acknowledge support of this work by Johnson
Matthey plc and the Spanish Ministry of Science and Innovation.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 1016–1019 | 1017
©

View Article Online
Scheme 2
Scheme 3
Fig. 2 19F NMR spectra of 2 at 22–105 ◦C (toluene-d8). At 22 ◦C the CF3 substituents in 2- and 4-positions of the diazapentadiene are found at d -71.2
and -65.5, respectively, while peaks around d -63 are due to aryl-CF3.
1018 | Dalton Trans., 2011, 40, 1016–1019
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Fig. 3 Molecular structure of 3, indicating the atom numbering scheme. Hydrogen atoms and the phenyl groups have been omitted for clarity. Two of
◦
˚
the phenyl-CF3 substituents are disordered. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (A) and angles ( ): Cu–N(4)
2.0929(15), Cu–P(1) 2.2953(5), N(4)–C(5) 1.316(2), C(5)–C(51) 1.398(2); N(4a)–Cu–N(4) 94.65(8), N(4)–Cu–P(1a) 102.67(4), N(4)–Cu–P(1) 113.31(4),
P(1a)–Cu–P(1) 126.03(3).
7 D. S. Laitar, C. J. N. Mathison, W. M. Davis and J. P. Sadighi, Inorg.
References
Chem., 2003, 42, 7354.
1 L. Bourget-Merle, M. F. Lappert and J. R. Severn, Chem. Rev., 2002,
102, 3031.
2 B. Guan, D. Xing, G. Cai, X. Wan, N. Yu, Z. Fang, L. Yang and Z.
Shi, J. Am. Chem. Soc., 2005, 127, 18004.
3 A. S. K. Hashmi and G. J. Hutchings, Angew. Chem., Int. Ed., 2006,
45, 7896.
4 H. V. Rasika Dias and J. A. Flores, Inorg. Chem., 2007, 46, 5841.
5 A. Venugopal, M. K. Ghosh, H. Ju¨rgens, K. W. To¨rnroos, O. Swang,
M. Tilset and R. H. Heyn, Organometallics, 2010, 29, 2248.
6 (a) H. O. Kalinowski and H. Kessler, Top. Stereochem., 1973, 7, 295;
(b) G. E. Hall, W. J. Middleton and J. D. Roberts, J. Am. Chem. Soc.,
1971, 93, 4778.
8 H. A. Chiong and O. Daugulis, Organometallics, 2006, 25, 4054.
9 P. O. Oguadinma and F. Schaper, Organometallics, 2009, 28,
4089.
10 P. O. Oguadinma and F. Schaper, Organometallics, 2009, 28, 6721.
11 Y. M. Badiei, A. Krishnaswamy, M. M. Melzer and T. H. Warren,
J. Am. Chem. Soc., 2006, 128, 15056.
12 Y. M. Badiei and A. T. H. Warren, J. Organomet. Chem., 2005, 690,
5989.
13 J. T. York, V. G. Young and W. B. Tolman, Inorg. Chem., 2006, 45,
4191.
14 X. Li, J. Ding, W. Jin and Y. Cheng, Inorg. Chim. Acta, 2009, 362,
233.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 1016–1019 | 1019
©