Cycloaurated triphenylphosphine-sulfide and -selenide

Article abstract of DOI:10.1039/b916755b

The first examples of cycloaurated phosphine sulfides and triphenylphosphine selenide have been synthesised; these complexes are fairly rare examples of gold(iii) complexes with potentially reducing sulfur- and selenium-donor ligands. The cycloaurated complex (AuCl₂(2-C ₆H₄P(S)Ph₂) was synthesised in good yield by transmetallation of the organomercury precursor Hg(2-C₆H ₄P(S)Ph₂)₂ with Me₄N[AuCl ₄]. A route to the chloro-mercury analogue ClHg(2-C₆H ₄P(S)Ph₂) was developed by reaction of the cyclomanganated triphenylphosphine sulfide (CO)₄Mn(2-C₆H ₄P(S)Ph₂) with HgCl₂; this mercury substrate was also used in the synthesis of AuCl₂(2-C₆H ₄P(S)Ph₂). The cycloaurated triphenylphosphine selenide complex AuCl₂(2-C₆H₄P(Se)Ph₂) was synthesised by an analogous methodology using the new phosphine selenide Hg(2-C₆H₄P(Se)Ph₂)₂ [prepared from Hg(2-C₆H₄PPh₂)₂ and elemental Se under sonication]. The phosphonamidic analogue AuCl₂(2-C ₆H₄P(S)(NEt₂)₂) has also been synthesised from PhP(S)(NEt₂)₂via lithiation and mercuration. X-Ray crystal structures of several compounds are reported, and show the presence of puckered ring systems. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b916755b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Cycloaurated triphenylphosphine-sulfide and -selenide†
Kelly J. Kilpin, William Henderson* and Brian K. Nicholson*
Received 13th August 2009, Accepted 5th November 2009
First published as an Advance Article on the web 22nd December 2009
DOI: 10.1039/b916755b
The first examples of cycloaurated phosphine sulfides and triphenylphosphine selenide have been
synthesised; these complexes are fairly rare examples of gold(III) complexes with potentially reducing
sulfur- and selenium-donor ligands. The cycloaurated complex (AuCl2(2-C6H4P(S)Ph2) was synthesised
in good yield by transmetallation of the organomercury precursor Hg(2-C6H4P(S)Ph2)2 with
Me4N[AuCl4]. A route to the chloro-mercury analogue ClHg(2-C6H4P(S)Ph2) was developed by
reaction of the cyclomanganated triphenylphosphine sulfide (CO)4Mn(2-C6H4P(S)Ph2) with HgCl2; this
mercury substrate was also used in the synthesis of AuCl2(2-C6H4P(S)Ph2). The cycloaurated
triphenylphosphine selenide complex AuCl2(2-C6H4P(Se)Ph2) was synthesised by an analogous
methodology using the new phosphine selenide Hg(2-C6H4P(Se)Ph2)2 [prepared from Hg(2-C6H4PPh2)2
and elemental Se under sonication]. The phosphonamidic analogue AuCl2(2-C6H4P(S)(NEt2)2) has also
been synthesised from PhP(S)(NEt2)2 via lithiation and mercuration. X-Ray crystal structures of several
compounds are reported, and show the presence of puckered ring systems.
yields of the sulfide complex 3c were excellent (90%) but both the
selenide 3c (22%) and the oxide 3a (41%) were only obtained in
Introduction
Coordination compounds of tertiary phosphine chalcogenides are
well known and a number of reviews are dedicated to complexes
with either the oxygen, sulfur or selenium atom of the ligand
acting as a neutral h donor to the metal centre.1,2 Ph3P O
=
moderate yields because of either the instability of the P Se bond
or the unfavourable bond angle that is formed at the oxygen in
the cyclometallated complex. The tin complex 4 was synthesised
by reaction of the free ligand with Ph3SnCl via an ortho-lithiated
intermediate. The resulting complex had weak O–Sn and S–Sn
interactions both in the solid state and in solution. However, the
1
3,4,5
=
3-5,6
=
(a hard donor ligand) and Ph3P S
(a soft donor ligand) both
form complexes with Au(III) (a soft metal centre) via the electron-
rich chalcogen atom. Very recently, the cycloaurated phosphine
oxide derivative 1 was prepared by transmetallation of the
corresponding methyltin derivatives 2.7 Cycloaurated complexes
have been attracting considerable interest, primarily as a result
of the redox-stabilisation of the Au(III) centre imparted by the
metallacyclic structure.8 However, the majority of cycloaurated
complexes involve C,N chelates, and we are unaware of any
examples of cycloaurated phosphine-sulfides (C,S donors) or–
selenides (C,Se donors). Complex 1 appears to be the sole example
to date of a C,O cycloaurated system.
+
-
cationic complex 5, formed by the reaction of 4 with CPh3 PF6 ,
had much stronger O–Sn and S–Sn interactions;10 stannated
Ph3P O 6 was also synthesised indirectly.11 With the exception of
=
the Pt(II) and Pd(II) compounds (7) containing tridentate S,C,S’
pincer ligands,12 a selection of indirectly synthesised Pt(IV)13 (8
and 9) and Rh14 (10) compounds containing cyclometallated
15,16
=
=
=
Ph3P O moieties, and ortho-mercurated Ph3P O and Ph3P S,
cyclometallated complexes of the heavier transition metal elements
have thus far been largely neglected.
Results and discussion
Synthesis and characterisation of organomercury precursors
Cycloaurated complexes are typically synthesised either by direct
C–H bond activation of the ligand by e.g. H[AuCl4], or by
transmetallation, often from the corresponding ortho-mercurated
compound.8 For phosphine chalcogenide ligands direct cyclomet-
allation with gold was not possible, because of reduction of the
gold(III), so the transmetallation route from the ortho-mercurated
complexes was employed. Synthesis of ortho-mercurated com-
plexes is commonly achieved by either direct mercuration of the
substrate or transmetallation reactions from ortho-lithiated or
Grignard intermediates, with the choice of method often limited
by the functional groups present on the ligand.17
Complexes containing ortho-metallated phosphine chalco-
genide ligands are themselves surprisingly rare in the literature.
=
The series of ortho-manganated complexes of Ph3P O (3a),
9
=
=
Ph3P S (3b) and Ph3P Se (3c) has previously been reported;
Department of Chemistry, University of Waikato, Private Bag 3105,
Hamilton, New Zealand. E-mail: w.henderson@waikato.ac.nz; Fax: +64-
7-838-4219
† CCDC reference numbers (743531) 15, (743530) 17b, (743528) 17c and
(743529) 18. For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b916755b
Bennett et al. have shown that the ortho-mercurated bis-
phosphine complex 11 reacts with either hydrogen peroxide
or sulfur to yield the bisphosphine-dioxide 12a or -disulfide
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12b, respectively.15,16 Identifying these compounds as suitable
cycloauration substrates, we extended this series of compounds to
the diselenide analogue 12c by reaction of 11 with (grey) selenium
powder under ultrasonic conditions. The use of ultrasound allows
the use of shorter reaction times and lower reaction temperatures
than with the more traditional thermal route, e.g. using selenium
in refluxing toluene.18 The selenide 12c is a white, air-stable solid
ionising) phosphine and phosphine chalcogenide compounds for
ESI MS analysis.19,20,21
1
and its 31P{ H} NMR spectrum shows a singlet at 41.4 ppm
with the expected coupling to both 199Hg (284 Hz) and 77Se
(684 Hz). Electrospray ionisation mass spectrometry (ESI-MS)
confirmed that both phosphine groups had been converted to the
corresponding selenide, however ions due to mixed oxide/selenide
species were also present. As only one compound was observed
1
in the 31P{ H} NMR spectrum it is possible that loss of selenium
and subsequent oxidation is occurring inside the spectrometer.
Alternatively, if very small traces of the mixed oxide/selenide
species were present in the sample then they would be likely to
have a higher proton affinity and therefore ionise more efficiently
than 12c. Addition of AgNO3 solution to 12c prior to ESI MS
analysis resulted in silver adducts in the spectrum (e.g. [12 + Ag]+),
however the mixed oxide/selenide species was still present. We
have previously used silver ions to successfully derivatise (poorly-
Organo-manganese reagents provide an alternative pathway to
ortho-mercurated precursors not accessible by traditional routes.22
For example, reaction of ortho-manganated acetophenone 13 with
HgCl2 yielded the ortho-mercurated derivative 14, a compound
that cannot be formed by the classical methods of mercuration.23
The use of lithium or Grignard reagents is excluded because of
the ketone functionality and direct mercuration occurs at the
1856 | Dalton Trans., 2010, 39, 1855–1864
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Table 1 Selected structural parameters for the complex ClHg(2-
C6H4P(S)Ph2) 15
Angles/◦
˚
Atoms
Lengths/A
Atoms
P(1)–S(1)
1.9745(11)
1.824(3)
1.400(4)
2.068(3)
2.3273(8)
2.9585(8)
S(1)–P(1)–C(11)
114.51(11)
121.3(2)
121.7(2)
173.34(9)
106.25(14)
105.86(14)
P(1)–C(11)
C(11)–C(12)
C(12)–Hg(1)
Hg(1)–Cl(1)
S(1)–Hg(1)
P(1)–C(11)–C(12)
C(11)–C(12)–Hg(1)
C(12)–Hg(1)–Cl(1)
C(11)–P(1)–C(31)
C(21)–P(1)–C(11)
methyl carbon (as a result of keto–enol tautomerisation). Ortho-
=
manganated Ph3P S 3b can be synthesised in high yields (ca.
9
=
90%) by reaction of Ph3P S with PhCH2Mn(CO)5 so we inves-
tigated whether transmetallation from 3b would produce ortho-
mercurated triphenylphosphine sulfide 15. Indeed, the desired
compound is formed cleanly and in high yield upon refluxing 3b
with a slight excess of HgCl2 in methanol (Scheme 1). The 31P{ H}
1
NMR spectrum of 15 (in CDCl3) has a single peak at 48.2 ppm,
which is close to the literature value of the bis-phosphine sulfide
12b (47.7 ppm, acquired in CD2Cl2).16
Scheme 1
Fig. 1 Structure of the complex ClHg(2-C6H4P(S)Ph2) 15 showing the
atom numbering scheme. Thermal ellipsoids are shown at the 50%
probability level.
ESI MS of 15 shows ions [M + H]+, [M + Na]+ and [2M -
Cl]+; it also indicated that the diarylmercury complex 12b had
also been formed (by an ion at m/z 789.093 corresponding to
[12b + H]+, 100%), so an X-ray crystal structure was performed in
order to unequivocally characterise the compound. The molecular
structure and the atom labelling scheme are shown in Fig. 1
and selected bond lengths and angles in Table 1. The mercury
atom is bonded to the ortho carbon of one of the phenyl
rings, confirming transmetallation from manganese. In other
structurally characterised complexes where tertiary phosphine
sulfides are coordinated to Hg(II) the S–Hg bond lengths range
observations indicate that the interaction between the mercury and
˚
sulfur [Hg(1) ◊ ◊ ◊ S(1) 2.9585(8) A] is substantial enough to hold the
compound in a pseudo-cyclic structure.
The ortho-mercurated thiophosphonamidic compound 16 was
also prepared from PhP(S)(NEt2)2 and investigated as a cycloau-
ration substrate. Craig et al.27 have previously shown that the
methyl analogue PhP(S)(NMe2)2 undergoes ortho-lithiation on the
phenyl ring; the commercially available ethyl analogue behaves in
an identical manner and the resulting ortho-lithiated compound
can be further reacted with HgCl2 to give 16 in good yield. Like 15,
there is a peak in the mass spectrum (m/z 769.266) corresponding
to the diaryl mercury complex [Hg(2-C6H4P(S)(NEt2)2)2 + H]+.
˚
˚
from 2.500 A to 2.761 A, and the P–S–Hg angles are in the range
of 99.31–110.99◦.24,25 The P–S–Hg angle in 15 (86.58◦) is extremely
acute; most P–S–M angles in coordinated phosphine sulfides fall
within the range of 102–116◦ (see Table 3). The P(1)–S(1) bond
However, the 31P{ H} NMR spectrum shows a 31P-199Hg coupling
1
26
˚
˚
=
length in 15 [1.9745(11) A] is longer than in Ph3P S (1.950 A),
and the coordination around the mercury atom deviates slightly
from linear geometry [C(12)–Hg(1)–Cl(1) angle 173.34(9)◦]. These
of 414 Hz, very similar to the coupling seen in 15. Microelemental
analysis also supports the structure being the mercury chloride
species 16.
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Table 2 Selected structural parameters for the complexes 17b and 17c
Synthesis of cycloaurated phosphine chalcogenides
=
=
The synthetic reactions are summarised in Scheme 2. The cy-
cloaurated triphenylphosphine sulfide complex 17b was obtained
by reaction of the ortho-mercurated phosphine sulfide complexes
12b and 15 with [Me4N][AuCl4] and [Me4N]Cl in acetonitrile.
The cycloaurated triphenylphosphine selenide complex 17c was
likewise formed from the organomercury reagent 12c. When the
donor atom was selenium, the transmetallation reaction proceeded
in three hours compared to the 24 h required with the sulfur
donor. Both 17b and 17c are yellow microcrystalline compounds,
however 17c appears to be moisture sensitive and when isolated
in the solid form it decomposes over time (even in darkness) to
give a black powder. Complex 16 also readily transmetallates
in an analogous manner; when reacted with [Me4N][AuCl4] the
cycloaurated complex 18 was isolated as a fluffy pale-yellow solid.
All of the transmetallation reactions gave adequate to good yields
of cycloaurated product.
17b (E S)
17c (E Se)
˚
Bond lengths/A
Au(1)–Cl(1)
Au(1)–Cl(2)
C(12)–Au(1)
E(1)–Au(1)
P(1)–E(1)
P(1)–C(11)
C(11)–C(12)
2.3635(11)
2.3094(12)
2.045(5)
2.3073(11)
2.0324(16)
1.794(4)
2.3628(6)
2.3222(6)
2.049(2)
2.4091(3)
2.1839(6)
1.783(2)
1.406(3)
1.401(6)
Bond angles/◦
Cl(1)–Au(1)–Cl(2)
Cl(1)–Au(1)–E(1)
E(1)–Au(1)–C(12)
C(12)–Au(1)–Cl(2)
Au(1)–E(1)–P(1)
E(1)–P(1)–C(11)
C(11)–C(12)–Au(1)
90.64(4)
86.25(4)
90.02(13)
93.23(13)
95.24(5)
104.57(15)
120.2(3)
90.52(2)
84.876(18)
91.16(7)
93.60(7)
91.759(18)
104.17(8)
120.99(18)
Scheme 2
=
Attempts at synthesising cycloaurated Ph3P O via transmetal-
lation from 12a were unsuccessful, the same synthetic methodol-
ogy as for 12b and 12c giving a variety of unidentified compounds
Fig. 2 Molecular diagram of AuCl2(2-C6H4P(Se)Ph2) 17c indicating the
atom numbering scheme, with thermal ellipsoids at the 50% probability
level. The cycloaurated phosphine sulfide 17b is isostructural, and has a
comparable numbering scheme.
observed by 31P{ H} NMR spectroscopy of the reaction mixture.
1
Longer reaction times and changing the solvent from acetonitrile
to DMSO resulted in complete loss of 12a and evidence of
the cycloaurated product in ESI mass spectrometry, but, despite
numerous attempts, we were unable to isolate the product, which
would be analogous to the recently reported derivative 1.
metal. The greatest deviations from the gold coordination planes
˚
˚
are 0.0681(14) A (C12) for 17b and 0.0643(8) A (C12) for 17c.
Because of the greater trans influence of the carbon-bonded
phenyl ring, the Au(1)–Cl(1) bonds in 17b and 17c [2.3635(11) and
˚
2.3628(6) A, respectively] are longer than the Au(1)–Cl(2) bonds
X-Ray crystal structures of AuCl2(2-C6H4P(S)Ph2) 17b,
AuCl2(2-C6H4P(Se)Ph2) 17c and AuCl2(2-C6H4P(S)(NEt2)2) 18
˚
[2.3094(12) and 2.3222(6) A] trans to the chalcogen. The Au–Cl
˚
bond trans to the selenium atom [2.3222(6) A] is longer than the
˚
X-Ray crystal structure analyses of 17b and 17c showed that the
compounds are isostructural and isomorphous. The molecular
structure and atom numbering scheme of the selenide are shown
in Fig. 2, with important bond parameters for both structures
presented in Table 2; the numbering scheme for the sulfide
corresponds to that of the selenide. In each case, a new five-
membered metallacyclic ring has formed with the gold bonded in
the ortho position of one of the phenyl rings and either a sulfur or a
selenium atom as the neutral donor. Two chloride ligands complete
the essentially square planar coordination geometry around the
Au–Cl bond trans to the sulfur atom [2.3094(12) A] indicating a
=
greater trans influence of the P Se group.
=
Coordination to the metal lengthens the P E bond, from
26
˚
=
˚
˚
˚
=
1.950(3) A in Ph3P S to 2.0324(16) A in 17b, and from 2.112(1) A
in Ph3P Se to 2.1839(6) in 17c. The Au–S bond [2.3073(11) A]
28
is comparable with other Au(III)-S bond lengths where the sulfur
originates from a tertiary phosphine sulfide group.29,30 Likewise,
the Au–Se bond in 17c is comparable with similar complexes31 and
in conjunction with the P–Se bond length suggests a strong gold–
selenium interaction. The bite angle of the ligand is close to 90◦ for
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Table 3 Results of a search of the Cambridge Crystallographic databasea
Table 4 Selected structural parameters for the complex AuCl2(2-
C6H4P(S)(NEt2)2) 18
1
=
for h -coordinated Ph3P E ligands
=
=
S
=
Ph3P
O
Ph3P
Ph3P Se
Molecule A
Molecule B
˚
Sample number
357
636
158.2
19
24
109.1
3.0
11
13
104.0
4.5
Bond lengths/A
Independent XPPh3 moieties
Mean P–E–M angle/◦
Au(1)–Cl(1)
Au(1)–Cl(2)
Au(1)–S(1)
Au(1)–C(2)
P(1)–S(1)
2.3700(13)
2.3178(13)
2.3108(14)
2.059(5)
2.0403(19)
1.781(5)
2.3715(12)
2.3205(13)
2.3064(13)
2.051(5)
2.0451(18)
1.776(5)
Standard deviation in P–E–M 5.7
angle/◦
Range P–E–M angle/◦
P–E–Au angle/◦
125–180
102–116
95.24 (17b)
97–111
91.759 (17c)
b
P(1)–C(1)
C(1)–C(2)
P(1)–N(1)
P(1)–N(2)
a Version 5.30, May 2009 release. Only those hits with R1 ≤ 0.075 were
1.398(7)
1.628(5)
1.623(4)
1.414(7)
1.638(5)
1.619(4)
included. b P–O–Au angle 117.0(2)◦ in complex 1.
Bond angles/◦
both 17b and 17c. The metallacyclic rings of both complexes are
significantly puckered, which contrasts to the ortho-manganated
analogues 3b and 3c, where the metallacyclic rings are planar.
The greatest deviations from the mean metallacyclic plane are
Cl(1)–Au(1)–Cl(2)
Cl(2)–Au(1)–C(2)
Cl(1)–Au(1)–S(1)
C(2)–Au(1)–S(1)
Au(1)–S(1)–P(1)
S(1)–P(1)–C(1)
91.49(5)
93.21(15)
85.88(5)
89.42(14)
94.58(6)
102.91(17)
116.2(4)
119.3(4)
90.28(5)
92.90(15)
86.43(5)
90.37(15)
95.30(6)
102.68(17)
117.1(4)
118.6(4)
˚
˚
P(1) [+0.3002(14) A] and S(1) [-0.3016(11) A] for 17b and P(1)
˚
[+0.3114(8)] and Se(1) [-0.2976(6) A] for 17c.
Table 3 summarises a review of the Cambridge Crystallographic
P(1)–C(1)–C(2)
C(1)–C(2)–Au(1)
Database for structures of triphenylphosphine chalcogenide lig-
1
ands where the chalcogenide atom acts as an h donor.32 In
these complexes the chalcogenide atom is not constrained so the
intrinsically preferred geometry around the chalcogenide atom
can be ascertained. Comparison shows that cycloauration imposes
much more acute angles at the chalcogenide atoms (P–E–Au) in
the gold complexes. In complexes 17b and 17c the P–E–Au angles
are 95.24(5) and 91.759(18)◦ compared with unconstrained angles
of 102–116◦ and 97–111◦ for Ph3PS and Ph3PSe complexes, re-
spectively. The P–O–Au bond angle of the cycloaurated phosphine
oxide complex 1 is 117.0(2)◦.7 In earlier work on ortho-manganated
triphenylphosphine chalcogenides Depree et al. suggested that
=
incorporation of Ph3P O into a five-membered ring results in
an unfavourable P–O–M bond angle at the oxygen.9
Compound 18 crystallised in the space group Pna21, with
two unique molecules in the asymmetric unit; one of these
molecules is depicted in Fig. 3, clearly confirming the presence
of the cycloaurated thiophosphorylic compound. Selected bond
parameters are listed in Table 4. In both independent molecules
the coordination around the gold atom is essentially square planar,
with the largest deviations from the coordination plane of the
˚
˚
Fig. 3 Molecular structure of AuCl2(2-C6H4P(S)(NEt2)2) 18 showing one
of the unique molecules in the asymmetric unit. Hydrogen atoms have been
excluded for clarity and thermal ellipsoids are shown at the 50% probability
level.
metal being 0.0183(17) A [C(2), molecule A] and 0.0181(11) A
[Au(1), molecule B]. As with 17b and 17c, the bite angle of the
ligand is approximately 90◦. The P–S–Au angles are 94.58(6)◦
(molecule 1) and 95.30(6)◦ (molecule 2), which are again severely
acute and lead to the metallacyclic ring being extremely puckered.
In both molecules it is the sulfur and phosphorus atoms that
show the greatest deviation from the metallacyclic ring. The Au–
Spectroscopic characterisation of cycloaurated phosphine
chalcogenides
˚
˚
S bond lengths are 2.3108(14) A (molecule A) and 2.3064(13) A
(molecule B), which are comparable to the Au–S bond length in
The presence of phosphorus in these compounds introduces a
powerful (31P) NMR spectroscopic probe which aids in both
characterisation of the final products and in monitoring reac-
tions. In isoelectronic iminophosphorane complexes there are
small changes in chemical shift of the compounds upon ortho-
mercuration followed by a larger predictable downfield shift of
approximately 40 ppm after transmetallation to the gold,33,34
attributed to the phosphorus entering a cyclic environment.35
The trend is not as obvious in the present chalcogenide se-
ries, with only small unpredictable changes between the ligand,
˚
17b (2.3073(11) A). As expected, the Au–Cl bonds trans to the
˚
carbon [2.3700(13), 2.3715(12) A] are longer than those trans to
˚
the sulfur [2.3178(13), 2.3205(13) A]. Interestingly, substituting
two phenyl groups (17b) for diethylamino groups (18) results in
an increase in Au–Cl bond lengths. The Au–Cl bond trans to the
sulfur in 17b is 2.3094(12) A—this increases to 2.3178(13) and
2.3205(13) A in 18 indicating that the amine-substituted ligand
˚
˚
has a greater trans influence than the phenyl substituted ligand.
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1
Table 5 31P{ H} NMR chemical shifts (ppm) of the metallated phosphine chalcogenide ligands used in this study (CDCl3)
Ligand
Ortho-mercurated complex
Cycloaurated complex
48.2 (3JP–Hg = 385 Hz)
41.4 (1JP–Se = 684 Hz)
(3JP–Hg = 284 Hz)
56.4
=
Ph3P
Ph3P Se
S
44.0
36.0 (1JP–Se = 731 Hz)
38.8 (1JP–Se = 425 Hz)
=
PhP(S)(NEt2)2
78.4
83.1 (3JP–Hg = 414 Hz)
79.4
ortho-mercurated and cycloaurated complexes hence other effects
must be contributing to the 31P chemical shift. Table 5 presents the
1
31P{ H} NMR data for the complexes studied.
As discussed by Glidewell et al.,36 the coordination of phosphine
selenides to metal centres results in significant deshielding of the
selenium nucleus and a decrease in the 1JPSe coupling constant—
=
this is observed in the series of Ph3P Se complexes. There is
only a small change (47 Hz) upon mercuration, which indicates
little interaction between the selenium and the mercury atom.
1
Transmetallation to gold sees a decrease in the value of JPSe by
259 Hz, which indicates a strong selenium–gold interaction. In
addition, the value of 1JPSe is related to P–Se bond lengths in the
solid state—the longer the P–Se bond the smaller 1JPSe is. This is
in accordance with X-ray crystallographic data: the P–Se bond
Conclusions
Three new ortho-metallated gold(III) complexes containing phos-
phine chalcogenide ligands have been synthesised by transmet-
allation from organomercury substrates and fully characterised.
These are rare examples of cycloaurated complexes, which contain
a neutral donor ligand other than nitrogen. The complexes with the
neutral sulfur donors appear to show similar qualitative stability to
cycloaurated C,N complexes, which is interesting as they contain
an oxidising gold(III) centre and a potentially reducing phosphine
chalcogenide ligand. Reactivity also appears to be similar to the
C,N auracycles: the cis chloride ligands can be replaced by the
chelating dianionic thiosalicylate ligand to give a bi-metallacyclic
complex, which shows better biological activity than the parent
complex against the P388 murine leukemia cell line.
1
˚
=
length in Ph3P Se is 2.112(1) A and JPSe is 731 Hz and in the
˚
cycloaurated complex the P–Se bond length is 2.1839(6) A with
1JPSe = 425 Hz.
-1
=
=
=
The P E vibrations in Ph3P S and Ph3P Se occur at 638 cm
and 561 cm-1, respectively. Upon coordination to the metal this
decreases to 603 cm-1 for 15 and further still to 597 cm-1 for 17b.
In the case of the selenide derivatives, the mercury compound
=
12c shows no change in the P Se vibration upon mercuration,
however coordination to the gold (17c) gives a shift to 545 cm-1.
This further supports the inference from the NMR data that there
is little interaction between the selenium and mercury. Likewise,
-1
=
PhP(S)(NEt2)2 has a P S vibration at 604 cm , which moves
to lower energy for the mercury complex 16 (597 cm-1) and the
cycloaurated species 18 (586 cm-1). These decreases mirror those
seen by King and McQuillan, and are attributed to a weakening
of the P–E p bonds upon coordination to the metal.37
Experimental
General
General experimental procedures were as described previously.47
Compounds 11, 12a, 12b and 3b were prepared by literature
procedures.9,16 Me4N[AuCl4] was prepared by addition of excess
Me4NCl to an aqueous solution of H[AuCl4]. The reagents HgCl2
(BDH), PhP(NEt2)2 (Aldrich), selenium powder (BDH), thios-
alicylic acid (Sigma), tetramethyammonium chloride (Aldrich)
and n-butyllithium (2.0 M in cyclohexane, Aldrich) were used as
received.
Reaction of AuCl2(2-C6H4P(S)(NEt2)2), 18 with thiosalicylic acid
Because gold(III) thiosalicylate derivatives often show a greater
biological activity than the parent dichloride complex,38,39,40,41
compound 18 was reacted with a molar equivalent of thiosalicylic
acid and excess Me3N in methanol to give the thiosalicylate
complex 19 in good yield. In previous examples of gold(III)
thiosalicylate complexes the ligand is coordinated so that the two
soft s donor ligands are mutually cis due to the anti-symbiotic
effect.42,43 Therefore, it is assumed that 19 is also the isomer where
the carbon and the thiosalicylate sulfur are cis to each other.
Given the interest in biological activity of gold(III) complexes,
with promising results,44,45,46 some of the phosphine chalcogenide
complexes reported herein have been subject to preliminary
screening. Complexes 18 (IC50 3474 ng mL-1) and 19 (IC50
1522 ng mL-1) were active against the P388 murine leukemia
cell line, though the activity is poor compared to other gold(III)
thiosalicylate complexes.38–41 Complex 17b shows poor activity
(IC50 >12 500 ng mL-1).
Synthesis of Hg(2-C6H4P(Se)Ph2)2 12c. Hg(2-C6H4PPh2)2 11
(0.300 g, 0.42 mmol) and grey selenium powder (0.066 g,
0.84 mmol) were added to dry degassed toluene (50 mL) and
sonicated in an ultrasonic bath at 50 ◦C until the majority of
the Se had disappeared (~16 h). The toluene was removed under
vacuum and the white solid extracted into dichloromethane (3 ¥
10 mL). Petroleum spirit (30 mL) was added to the filtrate and
storage at -20 ◦C gave Hg(2-C6H4P(Se)Ph2)2 12c as a white solid
(0.225 g, 61%). Found: C 49.1, H 3.3, C36H28P2Se2Hg requires C
49.1, H 3.2%. NMR: 1H: d 7.05 (m, 4 H, H-5 and H-6), 7.40 (m,
2 H, H-4), 7.42 (m, 8 H, H-9), 7.47 (m, 4 H, H-10), 7.68 (m, 8 H,
1860 | Dalton Trans., 2010, 39, 1855–1864
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1
H-8), 7.78 (m, 2 H, H-3). 13C{ H}: d 125.6 (d, 3JPC 12.6 Hz, C-5),
in degassed acetonitrile (15 mL) in a foil-covered flask. The
acetonitrile was removed under vacuum and the residue extracted
into dichloromethane (2 ¥ 10 mL) and filtered. Slow addition
of diethyl ether to the filtrate produced yellow crystals of
AuCl2(2-C6H4P(S)Ph2) 17b (0.060 g, 42%). Found: C 38.4, H 2.6;
C18H14PSCl2Au requires C 38.5, H 2.5%. NMR: H: d 6.97 (m,
1 H, H-6), 7.30 (m, 1 H, H-5), 7.45 (m, 1 H, H-4), 7.64 (m, 4 H,
H-9), 7.72 (m, 2 H, H-10), 7.76 (m, 4 H, H-8), 8.46 (m, 1 H, H-3).
3
4
128.6 (d, JPC 12.6 Hz, C-9), 130.3 (d, JPC 3.6 Hz, C-4), 131.6
(d, 4JPC 2.4 Hz, C-10), 132.3 (d, 2JPC 15.8 Hz, C-6), 132.4 (d, 1JPC
75.2 Hz, C-7), 133.2 (d, 2JPC 9.7 Hz, C-8), 140.2 (d, 1JPC 84.9 Hz,
3
2
C-1), 140.8 (d, JPC 19.4 Hz, C-3), 175.4 (d, JPC 25.1 Hz, C-2).
1
31P{ H}: d 41.4 (3JHgP 284 Hz, 1JSeP 684 Hz) ppm. ESI-MS: m/z:
1
904.964 (100%, [M + Na]+, calcd 904.960), 841.040 (44%, [M -
Se + O + Na]+, calcd 841.038), 819.064 (20%, [M - Se + O + H],
calcd 819.056), 882.981 (15%, [M + H]+, calcd 882.978); AgNO3
added: 988.880 (100%, [M + Ag]+, calcd 988.876), 926.957 (25%,
1
1
3
13C{ H}: d 125.0 (d, JPC 83.4 Hz, C-7), 127.9 (d, JPC 12.6 Hz,
C-5), 130.2 (d, 3JPC 13.7, C-9), 131.9 (d, 2JPC 14.3 Hz, C-6), 133.2 (d,
2JPC 11.5 Hz, C-8), 134.9 (d, 4JPC 3.3 Hz, C-4), 135.0 (d, 4JPC 2.7 Hz,
C-10), 137.2 (d, 3JPC 14.8 Hz, C-3), 140.5 (d, 1JPC 100.0 Hz, C-1),
+
-1
=
[M - Se + O + Ag] , calcd 926.954). IR: u(P Se) 562 (m) cm .
152.0 (d, 2JPC 16.1 Hz, C-2). 31P{ H}: d 56.4 ppm. Scheme 3 gives
1
the NMR numbering system. ESI-MS: m/z: 578.999 (100%, [M -
Cl + OMe + Na]+, calcd 578.998), 1081.002 (15%, [2M - 2Cl +
OMe]+, calcd 1080.999), 1135.010 (14%, [2M - 2Cl + 2OMe +
+
-1
=
Na] , calcd 1135.008). IR: u(P S) 597 (m) cm .
Alternative synthesis
ClHg(2-C6H4P(S)Ph2) 15 (0.250 g, 0.47 mmol), [Me4N][AuCl4]
(0.195 g, 0.47 mmol) and [Me4N]Cl (0.052 g, 0.47 mmol) were
stirred in degassed acetonitrile (20 mL) for 24 h in a foil-covered
flask. The acetonitrile was removed under vacuum and the residue
extracted into dichloromethane (2 ¥ 10 mL) and filtered. Slow
addition of diethyl ether to the filtrate produced yellow crystals
of AuCl2(2-C6H4P(S)Ph2) 17b (0.121 g, 46%), identified by NMR
and ESI mass spectrometry.
NMR numbering scheme for Hg(2-C6H4P(Se)Ph2)2 12c.
Synthesis of ClHg(2-C6H4P(S)Ph2) 15. (CO)4Mn(2-
C6H4P(S)Ph2) 3b (0.406 g, 0.88 mmol) and HgCl2 (0.477 g,
1.77 mmol) were refluxed in methanol (15 mL) for 3 h. The
white solid that formed was removed by filtration and the filtrate
reduced in volume to give a white solid. This was subsequently
re-dissolved in the minimum dichloromethane and passed
through a column of Celite. Removal of the dichloromethane
gave ClHg(2-C6H4P(S)Ph2) 15 as a white solid (0.339 g, 73%).
Found: C 40.8, H 2.7; C18H14PSClHg requires C 40.8, H 2.7%.
NMR: 1H: d 7.30 (m, 1 H, H-5), 7.39 (m, 1 H, H-6), 7.47 (m, 4 H,
H-9), 7.55 (m, 3 H, H-4 and H-10), 7.63 (m, 4 H, H-8). 7.70 (m,
Synthesis of AuCl2(2-C6H4P(Se)Ph2) 17c. Hg(2-C6H4P-
(Se)Ph2)2 12c (0.100 g, 0.11 mmol), [Me4N][AuCl4] (0.094 g,
0.22 mmol) and [Me4N]Cl (0.012 g, 0.11 mmol) were stirred in
degassed acetonitrile (10 mL) for 3 h in a foil covered flask. The
solvent was removed under reduced pressure and the resulting
yellow solid extracted into dichloromethane (2 ¥ 10 mL) and
filtered. The resulting filtrate was treated with diethyl ether
1
1 H, H-3). 31C{ H}: d 127.5 (d, 3JPC 11.8 Hz, C-5), 129.0 (d, 3JPC
12.7 Hz, C-9), 131.6 (d, 4JPC 3.5 Hz, C-4), 132.1 (d, 1JPC 86.3 Hz,
◦
(20 mL) and storage at -20 C gave yellow crystals of AuCl2(2-
4
2
C-7), 132.3 (d, JPC 3 Hz, C-10), 132.6 (d, JPC 10.6 Hz, C-8),
C6H4P(Se)Ph2) 17c (0.082 g, 61%). NMR: 1H: d 6.87 (m, 1 H, H-6),
133.5 (d, 2JPC 13.8 Hz, C-6), 136.8 (d, 1JPC 89.0 Hz, C-1), 138.9 (d,
7.29 (m, 1 H, H-5), 7.45 (m, 1 H, H-4), 7.64 (m, 4 H, H-9), 7.71
3JPC 17.1 Hz, C-3), 155.8 (d, JPC 19.8 Hz, C-2). 31P{ H}: d 48.2
(3JHgP 385 Hz) ppm. Scheme 3 gives the NMR numbering system.
ESI-MS: m/z: 789.093 (100%, [Hg(2-C6H4P(S)Ph2)2 + H]+, calcd
789.089), 1023.016 (20%, [2M - Cl]+, calcd 1023.018), 552.980
2
1
1
(m, 2 H, H-10), 7.75 (m, 4 H, H-8), 8.51 (m, 1 H, H-3). 13C{ H}:
d 124.5 (d, 1JPC 75.4 Hz, C-7), 127.5 (d, 3JPC 13.0 Hz, C-5), 130.2
(d, 3JPC 13.4 Hz, C-9), 132.6 (d, 2JPC 13.9 Hz, C-6), 133.5 (d, 2JPC
4
4
11.9 Hz, C-8), 134.8 (d, JPC 3.0 Hz, C-4), 134.9 (d, JPC 3.0 Hz,
+
-1
=
(9%, [M + Na] , calcd 552.983). IR: u(P S) 603 (m) cm .
3
1
C-10), 138.5 (d, JPC 14.8 Hz, C-3), 140.9 (d, JPC 96.0 Hz, C-1),
150.2 (d, JPC 19.4 Hz, C-2). 31P{ H} d 38.8 (1JSeP 425 Hz) ppm.
Scheme 3 gives the NMR labelling scheme. ESI-MS: m/z: 630.890
(100%, [M + Na]+, calcd 630.893), 1238.793 (28%, [2M + Na]+,
2
1
calcd 1238.797), 1180.838 (9%, [2M - Cl]+, calcd 1180.838). IR:
-1
=
u(P Se) 562 (m) cm .
Synthesis of PhP(S)(NEt2)2
PhP(NEt2)2 (2 mL, 7.9 mmol) and sulfur (0.275 g, 8.6 mmol) were
stirred under nitrogen overnight in dry degassed hexanes (50 mL).
The solvent was removed under reduced pressure to give a pale
yellow oil that failed to crystallise (1.40 g, 62%). Found: C 59.3, H
9.1, N 9.9; C14H25N2PS requires C 59.1, H 8.9, N 9.9%. NMR: 1H:
d 1.04 (t, 12 H, H-8), 3.13 (m, 8 H, H-7), 7.42 (m, 3 H, H-3, H-4
Scheme 3 NMR numbering scheme for 15 (M = HgCl, E = S), 17b (M =
AuCl2, E = S) and 17c (M = AuCl2, E = Se).
Synthesis of AuCl2(2-C6H4P(S)Ph2) 17b. Hg(2-C6H4P(S)Ph2)2
12b (0.100 g, 0.13 mmol), [Me4N][AuCl4] (0.105 g, 0.26 mmol)
and [Me4N]Cl (0.014 g, 0.13 mmol) were stirred overnight
1
3
and H-5), 7.93 (m, 2 H, H-2 and H-6). 13C{ H}: d 13.5 (d, JPC
4 Hz, C-8), 39.4 (d,2JPC 4 Hz, C-7), 128.2 (d, 3JPC 14 Hz, C-3 and
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C-5), 130.9 (d, 4JPC 3 Hz, C-4), 131.7 (d, 2JPC 10 Hz, C-2 and C-6),
1067.116), 573.033 (95%, [M + Na]+, calcd 573.033), 1061.166
1
135.7 (d, 1JPC 123 Hz, C-1). 31P{ H}: d 78.4 ppm. Scheme 4 gives
(73%, [2M - 2Cl + OMe]+, 1061.168), 1125.072 (58%, [2M +
the NMR numbering scheme. ESI-MS: m/z: 307.138 (100%, [M +
Na]+, calcd 1125.075), 515.074 (38%, [M - Cl]+, calcd 515.075).
Na]+, calcd 307.137), 591.287 (5%, [2M + Na]+, calcd 591.284).
IR: u(P S) 586 (m) cm .
-1
=
-1
=
IR: u(P S) 604 cm .
Reaction of AuCl2(2-C6H4P(S)(NEt2)2) 18 with thiosalicylic acid
AuCl2(2-C6H4P(S)(NEt2)2) 18 (0.051 g, 0.09 mmol) and thiosali-
cylicacid(0.014g, 0.09mmol)werestirredinmethanol(10mL). To
the dark orange solution Me3N (1 mL, excess) was added resulting
in the solution becoming immediately lighter. The solution was
stirred for 90 min in a foil covered flask before water (50 mL) was
added to produce a fine precipitate. Stirring was continued for a
further 12 h, the solution was filtered and the yellow solid washed
with water (2 ¥ 10 mL) and diethyl ether (1 ¥ 10 mL). Drying
under vacuum gave 0.043 g (76%) of the thiosalicylate derivative
19. Found: C 39.9, H 4.5, N 4.5, C21H28N2O2PS2Au requires C
39.9, H 4.5, N 4.4%. NMR: 1H: d 1.14 (t, 12 H, H-8), 3.25 (m, 8
H, H-7), 7.12 (m, 1 H, H-6), 7.18 (m, 2 H, H-11 and H-12), 7.27
(m, 1 H, H-5), 7.39 (m, 2 H, H-4 and H-10), 7.93 (m, 1 H, H-3),
Scheme 4 NMR numbering scheme for (Et2N)2P(S)Ph (M = H), 16 (M =
HgCl) and 18 (M = AuCl2).
Synthesis of ClHg(2-C6H4P(S)(NEt2)2) 16
PhP(S)(NEt2)2 (0.500 g, 1.76 mmol) was dissolved in dry, degassed
diethyl ether (20 mL). n-Butyllithium (0.9 mL, 1.76 mmol) was
added dropwise to give a yellow solution that was stirred for 22 h.
HgCl2 (0.478 g, 1.76 mmol) in THF (5 mL) was added dropwise
and the resulting grey solution stirred for a further 4 h. The grey
solid that had formed was filtered and the filtrate evaporated under
reduced pressure. The residue was redissolved in dichloromethane
(20 mL) and re-filtered. The solvent was reduced in volume (to
~5 mL) and hexanes were added until the solution went cloudy.
Storage at -20 ◦C gave white crystals of ClHg(2-C6H4P(S)(NEt2)2)
16 (0.503 g, 55%). Found: C 32.6, H 4.6, N 5.3; C14H24N2PSClHg
requires C 32.4, H 4.7, N 5.4%. NMR: 1H: d 1.05 (t, 12 H, H-8),
3.12 (m, 8 H, H-7), 7.32 (m, 1 H, H-5), 7.47 (m, 1 H, H-4),
1
8.16 (m, 1 H, H-13). 13C{ H}: d 13.7 (3JPC 2.8 Hz, C-8), 40.4 (2JPC
4.5 Hz, C-7), 125.4 (C-12), 127.0 (3JPC 13.0 Hz, C-5), 129.9 (C-10
and C-11), 130.4 (2JPC 10.8 Hz, C-6), 133.2 (C-13), 134.0 (4JPC
3.3 Hz, C-4), 134.1 (C-14), 134.2 (3JPC 16.1 Hz, C-3), 134.7 (2JPC
21.5 Hz, C-2), 136.8 (C-9), 141.6 (1JPC 138.0 Hz, C-1), 168.6 (C-
1
15). 31P{ H}: d 75.0 ppm. ESI-MS: m/z: 633.109 (100%, [M + H]+,
calcd 633.107), 655.092 (64%, [M + Na]+, calcd 655.089), 763.254
(27%, [2-Au(C6H4P(S)(NEt2)2)2]+, calcd 763.247), 1287.192 (21%,
[2M + Na]+, calcd 1287.188), 1265.210 (6%, [2M + H]+, calcd
-1
=
=
1265.206). IR: u(P S) 587 (m); u(C O) 1624 (vs) cm .
1
7.57 (m, 1 H, H-3), 7.84 (m, 1 H, H-6). 13C{ H}: d 13.6 (d,
2
3
3JPC 3.1 Hz, C-8), 39.9 (d, JPC 3.6 Hz, C-7), 127.1 (d, JPC
11.4 Hz, C-5), 130.8 (d, 2JPC 9.3 Hz, C-6), 131.0 (d, 4JPC 3.4 Hz, C-
3
1
4), 138.7 (d, JPC 20.5 Hz, C-3), 139.7 (d, JPC 132.1 Hz, C-1),
156.9 (d, JPC 22.8 Hz, C-2). 31P{ H}: d 83.1 (3JHgP 414 Hz)
ppm. Scheme 4 gives the NMR labelling system. ESI-MS: m/z:
1003.188 (100%, [2M - Cl]+, calcd 1003.186), 769.266 (55%, [Hg(2-
2
1
C6H4P(S)(NEt2)2)2+H]+, calcd 769.258), 543.071 (24%, [M + Na]+,
NMR numbering scheme for 19.
-1
=
calcd 543.068). IR: u(P S) 597 (s) cm .
Antitumour assays
Synthesis of AuCl2(2-C6H4P(S)(NEt2)2) 18
Assays to determine the in vitro antitumour activity against the
P388 Murine Leukemia cell line were carried out by Gill Ellis of
the Marine Chemistry Group at the University of Canterbury.
The sample was dissolved in the solvent and incubated with
P388 murine leukemia cells for 72 h. IC50 values were determined
using absorbance values obtained when the dye MMT tetrazolium
(yellow) was reduced to MMT formazan (purple).
ClHg(2-C6H4P(S)(NEt2)2) 16 (0.150 g, 0.29 mmol), [Me4N][AuCl4]
(0.119 g, 0.29 mmol) and [Me4N]Cl (0.032 g, 0.29 mmol) were
stirred overnight in a foil-covered flask of degassed acetonitrile
(15 mL), under a nitrogen atmosphere. The acetonitrile was
removed under reduced pressure and the residue extracted into
dichloromethane (2 ¥ 10 mL). Hexanes (20 mL) were added and
the solution stored at -20 ◦C to give fluffy yellow crystals of
AuCl2(2-C6H4P(S)(NEt2)2) 18 (0.106 g, 66%). Found: C 30.0, H
4.5, N 4.9; C14H24N2PSCl2Au requires 30.5, H 4.4, N 5.1%. NMR:
1H: d 1.20 (t, 12 H, H-8), 3.25 (m, 8 H, H-7), 7.11 (m, 1 H, H-
6), 7.30 (m, 1 H, H-5), 7.40 (m, 1 H, H-4), 8.39 (m, 1 H, H-3).
X-Ray crystal structure determinations of 15, 17b, 17c and 18
In all cases single crystals suitable for X-ray diffraction were
grown by vapour diffusion of diethyl ether into a dichloromethane
solution of the compound at room temperature. Unit cell and
intensity data were collected at the University of Auckland
on a Bruker Smart CCD Diffractometer, operating at 89 K;
crystallographic details are presented in Table 6. Absorption
corrections were applied by semi-empirical methods (SADABS48).
1
13C{ H}: d 13.9 (3JPC 2.5 Hz, C-8), 40.7 (2JPC 4.7 Hz, C-7), 127.4
(3JPC 13.1 Hz, C-5), 129.9 (2JPC 11.5 Hz, C-6), 134.7 (4JPC 2.9 Hz,
C-4), 135.8 (1JPC 139.0 Hz, C-1), 136.2 (3JPC 17.3 Hz, C-3), 144.0
1
(2JPC 24.6 Hz, C-2). 31P{ H}: d 79.4 ppm. Scheme 4 gives the NMR
labelling system. ESI-MS: m/z: 1067.114 (100%, [2M - Cl]+, calcd
1862 | Dalton Trans., 2010, 39, 1855–1864
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Table 6 Crystal and refinement data for the complexes 15, 17b, 17c and 18
Complex
15
17b
17c
18
Formula
Molecular mass
T/K
C18H14PSClHg
529.36
89
Monoclinic
P21/c
8.8845(1)
17.2034(1)
11.2712(2)
90
101.8490(1)
90
1686.02(4)
4
2.085
0.2524, 0.1026
3430
3258
0.0200
0.0525
1.060
C18H14PSCl2Au
561.19
89
Triclinic
¯
P1
C18H14PCl2SeAu
608.09
89
Triclinic
¯
P1
C14H24N2PSCl2Au
551.25
89
Orthorhombic
Pna21
27.3998(9)
7.9520(3)
17.1925(6)
90
90
90
3746.0(2)
8
1.955
0.2397, 0.2038
7379
6979
Crystal system
Space group
˚
a/A
8.1875(1)
9.1443(1)
12.4011(1)
98.705(1)
102.795(1)
90.484(1)
894.124(16)
2
2.084
0.2742, 0.2285
3657
3562
0.0274
8.1506(1)
9.2374(1)
12.4369(2)
80.142(1)
76.934(1)
89.994(1)
897.94(2)
2
2.249
0.4170, 0.1553
4284
4147
0.0152
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
U/A
Z
D
calcd/g cm-3
Tmax,min
Number of unique reflections
Number of observed reflections [I > 2s(I)]
R[I > 2s(I)]
wR2 (all data)
Goodness of Fit
0.0216
0.0457
0.0717
1.106
—
0.0360
1.073
—
1.130
Flack x parameter
—
0.2364(4)a
a Racemic twin (occupancy 0.76/0.24).
The structures of 15 and 18 were solved by the direct methods
option of SHELXS-97, 17b and 17c by the Patterson methods
option.49 In all cases the heavy metal was initially located and
all other non-hydrogen atoms by a series of difference maps.
Hydrogen atoms were placed in calculated positions. Refinement
was with SHELXL-97.50
An initial inspection of 18 indicated that the two independent
molecules in Pna21 were possibly related by an inversion centre.
A closer examination showed that the heavy atoms (Au, P, N, S
and Cl) were related by approximate symmetry but the lighter
atoms were not. Attempts to solve and refine the structure in
the centrosymmetric space group Pnma were not successful,
confirming Pna21. Refinement suggested racemic twinning with
0.76 : 0.24 occupancy so this was included in the final model.
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Acknowledgements
We thank the University of Waikato for financial support of
this work, including technical support from Wendy Jackson and
Pat Gread, and Prof. Alistair Wilkins for assistance with NMR
spectroscopy. KJK thanks The Tertiary Education Commission
(Top Achievers Doctoral Scholarship) and the New Zealand
Federation of Graduate Women (Merit Award for Doctoral
Study). Dr Tania Groutso (University of Auckland) is thanked
for collection of the X-ray data sets, and Gill Ellis (University of
Canterbury) for biological assay data.
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