Copper(II) complexes of new potentially hexadentate N3S3- or N6- donor podand ligands based on the tris(pyrazolyl)borate or tris(pyrazolyl)methane core

Article abstract of DOI:10.1039/a809777a

The mononuclear copper(II) complexes of the following potentially hexadentate podand ligands have been prepared and crystallographically characterised: tris[3-{2-(methylsulfanyl)phenyl}pyrazol-1-yl]hydroborate [L¹]^-, phenyltris[3-(2-pyridyl)pyrazol-l-yl]methane (L²), and tris[3- {(6-methyl)-pyrid-2-yl}pyrazol-1-yl]hydroborate [L³]^-. Of these, [L¹]^- [a potentially N₃S₃ donor with three pyrazolyl and three thioether groups, based on a tris(pyrazolyl)borate core] and L² [a potentially N₆ donor with three pyrazolyl and three pyridyl groups, based on a tris(pyrazolyl)methane core] have been prepared for the first time. In [Cu(L¹)][PF₆] the Cu(II) centre is in a five-coordinate N₃S₂ coordination environment which is approximately square pyramidal; the pendant thioether group has a weak, long- range interaction with the sixth coordination site of Cu(II). [Cu(L¹)][PF₆] undergoes a reversible Cu(I)-Cu(II) redox conversion. In [Cu(L²)(MeOH)][PF₆]₂ the ligand is four-coordinate via two bidentate pyridyl/pyrazolyl arms, with the third arm pendant; an axial methanol ligand completes the square-pyramidal coordination. In [Cu(L³)(H₂O)][PF₆], which is five coordinate and approximately trigonal bipyramidal, [L³]^acts as an N₄ donor via all three pyrazolyl groups but only one pyridyl group; the two pendant pyridyl groups are involved in O-H···N hydrogen-bonding interactions with the coordinated water molecule. This 'second-sphere' stabilisation of a coordinated ligand is strongly reminiscent of the cooperative interactions by which substrates are bound to the active sites of metalloproteins. The EPR spectra of this complex are solvent-dependent, showing a change from a d(z)₂ ground state in non-donor solvents to a d(x)_2-(y)₂ ground state in donor solvents.

Full text of DOI:10.1039/a809777a

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Copper(II) complexes of new potentially hexadentate N S - or
3 3
N -donor podand ligands based on the tris(pyrazolyl)borate or
6
tris(pyrazolyl)methane core
Elizabeth R. Humphrey, Karen L. V. Mann, Zoe R. Reeves, Andreas Behrendt, John C. Je†ery,
John P. Maher, Jon A. McCleverty* and Michael D. Ward*
School of Chemistry, University of Bristol, CantockÏs Close, Bristol, UK BS8 1T S.
E-mail: mike.ward=bristol.ac.uk
Received (in Cambridge, UK) 16th December 1998, Accepted 20th January 1999
The mononuclear copper(II) complexes of the following potentially hexadentate podand ligands have been prepared
and crystallographically characterised: tris[3-M2-(methylsulfanyl)phenylNpyrazol-1-yl]hydroborate [L1]~,
phenyltris[3-(2-pyridyl)pyrazol-1-yl]methane (L2), and tris[3-M(6-methyl)-pyrid-2-ylNpyrazol-1-yl]hydroborate
[L3]~. Of these, [L1]~ [a potentially N S donor with three pyrazolyl and three thioether groups, based on a
3 3
tris(pyrazolyl)borate core] and L2 [a potentially N donor with three pyrazolyl and three pyridyl groups, based on
6
a tris(pyrazolyl)methane core] have been prepared for the Ðrst time. In [Cu(L1)][PF ] the Cu(II) centre is in a
Ðve-coordinate N S coordination environment which is approximately square pyramidal; the pendant thioether
6
3 2
group has a weak, long-range interaction with the sixth coordination site of Cu(II). [Cu(L1)][PF ] undergoes a
6
reversible Cu(I)ÈCu(II) redox conversion. In [Cu(L2)(MeOH)][PF ] the ligand is four-coordinate via two bidentate
6 2
pyridyl/pyrazolyl arms, with the third arm pendant; an axial methanol ligand completes the square-pyramidal
coordination. In [Cu(L3)(H O)][PF ], which is Ðve coordinate and approximately trigonal bipyramidal, [L3]~
2
6
acts as an N donor via all three pyrazolyl groups but only one pyridyl group; the two pendant pyridyl groups are
4
involved in OÈHÉ É ÉN hydrogen-bonding interactions with the coordinated water molecule. This “second-sphereÏ
stabilisation of a coordinated ligand is strongly reminiscent of the cooperative interactions by which substrates are
bound to the active sites of metalloproteins. The EPR spectra of this complex are solvent-dependent, showing a
change from a d ground state in non-donor solvents to a d
z2
ground state in donor solvents.
x2~y2
Tripodal ligands (podands) in which three polydentate coordi-
nating arms are attached to a central linking group are
popular ligands for conferring high stability on metal com-
plexes and imposing upon them relatively rigid and well
deÐned donor sets. The vast majority of these are hexadentate
and have all-nitrogen, all-oxygen or mixed N,O-donor sets.1
We have recently started to develop new tripodal ligands of
this type based on the well known tris(pyrazolyl)borate core.2
Our Ðrst example of this was the ligand TpPy, in which attach-
ment of a 2-pyridyl group to the C3 position of the pyrazolyl
rings makes each arm a bidentate chelate.3,4 This completely
changes the character of the ligand compared to the parent Tp
ligand and its simple substituted derivatives, and resulted in
(inter alia) (i) an extensive coordination chemistry with lantha-
nide ions,3 and (ii) multinuclear complexes with Ðrst-row tran-
sition metals displaying unusual multinuclear cage-like
structures.4
group is replaced by a CPh group. Although L2 is structurally
similar to [TpPy] it therefore di†ers in being neutral rather
than anionic, which is likely to make a signiÐcant di†erence to
its coordination behaviour. Tris(pyrazolyl)methanes have
received less attention than their tris(pyrazolyl)borate ana-
logues, although recently a systematic comparison of the
structures of several main-group complexes with tris-
(pyrazolyl)methanes with those of their tris(pyrazolyl)borate
analogues has been undertaken by Reger and coworkers.7 The
syntheses for both new ligands, together with the crystal struc-
tures of their complexes with copper(II), are described in this
paper.
Also included is the structure of the copper(II) complex of
the related podand ligand tris[3-M(6-methyl)-pyrid-2-ylN-
pyrazol-1-yl]hydroborate [L3]~. The synthesis of this ligand,
a derivative of TpPy in which sterically hindering methyl
groups are attached to the C6 positions of the pyridyl rings,
was reported recently together with the structures and lumi-
nescence properties of some lanthanide complexes.8 However
no complexes with Ðrst-row transition metal ions have been
reported until now. The ligand structures are shown in
Scheme 1.
We describe here the preparation of two new potentially
hexadentate podand ligands. The Ðrst is tris[3-M2-(methyl-
sulfanyl)phenylNpyrazol-1-yl]hydroborate [L1]~, a potentially
N S -donor hexadentate podand in which ÈC H SMe substit-
3 3
6 4
uents are attached at the pyrazolyl C3 positions, such that
each arm is now potentially an N,S-bidentate chelate. There
are a very few examples of tripodal ligands with N S donor
sets, the two principal classes being the ÏsarcophaganesÏ of
Sargeson, Hambley and coworkers,5 and the tri-
azacyclononane derivatives of Wieghardt and coworkers
Experimental
3 3
General details
Instrumentation used for routine spectroscopic and electro-
chemical studies has been described in previous papers.3,4,8
3-(2-Pyridyl)pyrazole (Hpypz),3 2-(methylsulfanyl)acetophe-
none (I)9 and KL3 (ref. 8) were prepared according to the
published methods.
which contain aromatic thiols pendant from the N -donor
3
macrocyclic ring.6 The second new podand is phenyltris[3-(2-
pyridyl)pyrazol-1-yl]methane (L2), an analogue of our parent
tris(pyrazolyl)borate TpPy in which the anionic BH head-
New J. Chem., 1999, 23, 417È423
417

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cmv1. 1H NMR (300 MHz, CD COCD ): d 7.78 (3 H, d,
3
3
J \ 2.2, pyrazolyl), 7.59 (3 H, d, J \ 7.3, phenyl H6), 7.23 (6 H,
m, 2 ] phenyl), 7.10 (3 H, m, phenyl), 6.46 (3 H, d, J \ 2.2 Hz,
pyrazolyl), 2.43 (9 H, s, SCH ).
3
Phenyltris[3-(2-pyridyl)pyrazol-1-yl]methane
(L2).
A
mixture of 3-(2-pyridyl)pyrazole (1.00 g, 6.9 mmol) and NaH
(0.248 g, 10 mmol) in toluene (60 cm3) was heated at reÑux
under N , with vigorous stirring, for 2 hours. After cooling to
2
room temperature, a,a,a-trichlorotoluene (0.449 g, 2.3 mmol)
and NBu I (0.085 g, 0.23 mmol) were added to the reaction
4
mixture which was then heated at reÑux under N for 12 h.
2
After cooling, the mixture was Ðltered, placed in a separating
funnel and shaken with water. The organic phase was col-
lected, dried with MgSO and evaporated to dryness in vacuo.
4
The crude product was puriÐed by column chromatography
on alumina, initially using neat CH Cl and then using
2
2
CH Cl containing 1È2% MeOH (v/v) as the eluent. The
2
2
fourth fraction to elute was the desired product L2, and
repeated chromatography was usually necessary to get it
clean. Yield: 0.097 g, 8%. EIMS: m/z 521 (M`). (Found: C,
66.8; H, 4.5; N, 22.7. C
4.8; N, 22.6%). 1H NMR (300 MHz, CDCl ): d 8.62 (3 H, d,
J \ 4.2, pyridyl H6), 7.92 (3 H, d, J \ 7.9, pyridyl H3), 7.66 (3
H
N É 2H O requires C, 66.8; H,
31 23
9
2
Scheme 1 Ligands used in this study.
3
Preparations
H, td, J \ 7.7, 1.8, pyridyl H4), 7.62 (3 H, d, J \ 2.7, pyrazolyl
H4 or H5), 7.50 (1 H, td, J \ 4.6, 1.5, phenyl H4), 7.43 (2 H, m,
phenyl H3/H5), 7.35 (2 H, dd, J \ 7.2, 1.6, phenyl H2/H6), 7.21
(3 H, ddd, J \ 7.3, 4.9, 1.1, pyridyl H5), 7.07 (3 H, d, J \ 2.7
Hz, pyrazolyl H5 or H4).
1- [(2-Methylsulfanyl)phenyl] -3-dimethylamino-2-propen-1-
one (II). A solution of 2-(methylsulfanyl)acetophenone (3.00 g,
0.018 mol) in dimethylformamideÈdimethylacetal (5 cm3, a
large excess) was heated to reÑux for 8 h under N . After this
2
time the excess solvent was removed in vacuo to leave a red
oil. PuriÐcation by column chromatography using
[Cu(L1)] [PF ]. A mixture of KL1 (0.062 g, 0.1 mmol) and
6
CH Cl Èthf (93 : 7, v/v) a†orded II as a yellow oil (2.90 g,
Cu(O Me) É H O (0.020 g, 0.1 mmol) dissolved in 20 cm3
2
2
2
2
2
73%). EIMS: m/z 206 (M` [ Me). 1H NMR (300 MHz,
MeOH was stirred at room temperature to give a clear green-
CDCl ): d 7.60 (1 H, d, J \ 12, vinylic CH), 7.45 (1 H, d,
brown solution. Addition of excess aqueous KPF precipi-
3
6
J \ 7.5, phenyl), 7.30 (2 H, m, 2 ] phenyl), 7.16 (1 H, td,
tated a brown solid which was Ðltered o† and washed with
J \ 7.2, 1.5, phenyl), 5.48 (1 H, d, J \ 12 Hz, vinylic CH), 3.11
water, and dried in vacuo. Recrystallisation by di†usion of
(3 H, s, NCH ), 2.90 (3 H, s, NCH ), 2.44 (3 H, s, SCH ).
Et O vapour into a concentrated MeCN solution of the
3
3
3
2
(Found: C, 63.2; H, 7.3; N, 6.2. C
H
NOS É 0.5H O requires
complex a†orded X-ray quality crystals.
12 15
2
C, 62.6; H, 7.0; N, 6.1%).
[Cu(L2)(MeOH)] [PF ] . A mixture of L2 (0.050g, 0.096
6 2
3-[2-(methylsulfanyl)phenyl]pyrazole (III). A mixture of II
(2.90 g, 0.013 mmol) and hydrazine hydrate (4 cm3, excess) in
EtOH (30 cm3) was heated to reÑux for 30 minutes, after
which it was cooled and water (100 cm3) added. A white solid
started to precipitate; the mixture was refrigerated overnight
to allow precipitation to complete. The solid was Ðltered o†,
washed with water, and dried in vacuo. Recrystallisation from
mmol) and Cu(O Me) É H O (0.019 g, 0.095 mmol) dissolved
2
2
2
in 20 cm3 MeOH was stirred at room temperature to give a
clear green solution. Addition of excess aqueous KPF pre-
cipitated a green solid which was Ðltered o† and washed with
6
water, and dried in vacuo. Recrystallisation by di†usion of
Et O vapour into a concentrated MeCN solution of the
2
complex a†orded X-ray quality crystals.
CHCl Èhexane a†orded III as an o†-white crystalline solid
3
(1.70 g, 68%). EIMS: m/z \ 190 (M`). 1H NMR (300 MHz,
[Cu(L3)(H O)] [PF ]. A mixture of Cu(O Me) É H O (7.6
CDCl ): d 9.00 (1 H, d, J \ 3.0, pyrazolyl), 7.66 (1 H, dd,
2
6
2
2
2
3
mg, 0.038 mmol) and KL3 (0.020 g, 0.038 mmol) in MeOH (50
cm3) was stirred at room temperature for 1 h. Addition of
J \ 7.5, 1.5, phenyl), 7.44 (1 H, t, J \ 7.8, phenyl), 7.30 (2 H,
m, 2 ] phenyl), 6.96 (1 H, d, J \ 3.0, pyrazolyl), 2.46 (3 H, s,
aqueous KPF precipitated a pale green solid, which was
extracted from the aqueous suspension with CH Cl . Concen-
tration of the organic layer followed by di†usion of pentane
SCH ) (Found: C, 63.0; H, 5.5; N, 14.7. C
C, 63.2; H, 5.3; N, 14.7%).
H
N S requires:
6
3
10 10
2
2
2
into the green solution resulted in a crop of X-ray quality
crystals.
Potassium
tris[3-{2-(methylsulfanyl)phenyl}pyrazol-1-yl]-
Characterisation data for the complexes are summarised in
Table 1.
hydroborate (KL1). A mixture of III (1.45 g, 7.6 mmol) and
KBH (0.118 g, 2.2 mmol) was ground together to ensure inti-
4
mate mixing and was then heated gradually to 200 ¡C. The
mixture melted at ca. 90 ¡C, and started to evolve H at ca.
110 ¡C. After 2 h at 200 ¡C the mixture was cooled to leave a
X-Ray crystallography
2
glassy solid which was dissolved in warm toluene (100 cm3)
with the aid of an ultrasound cleaning bath. After addition of
pentane (100 cm3) to the toluene solution a white precipitate
of KL1 formed which was Ðltered o† and dried in vacuo (0.77
Suitable crystals were quickly transferred from the mother
liquor to a stream of cold N on a Siemens SMART di†rac-
2
tometer Ðtted with a CCD-type area detector. In all cases a
full sphere of data was collected at [100 ¡C using graphite-
monochromatised Mo-Ka radiation. A detailed experimental
description of the methods used for data collection and inte-
g, 57%) (Found: C, 58.5; H, 4.8; N, 13.6. C
H
BKN S
6 3
2409
30 28
requires: C, 58.3; H, 4.5; N, 13.6%). IR (KBr disc): l
BhH
418
New J. Chem., 1999, 23, 417È423

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Table 1 Characterisation and spectroscopic data for the new complexes
Mass spectrum Elemental analysisa (%)
UVÈVIS spectrab
/nm (10~3e/dm3 mol~1 cm~1)
EPR spectrac
Complex
m/z
Assignment
C
H
N
j
g
g
A (Cu)/G
A
max
A
M
[Cu(L1)][PF ]
642d
584d
549f
MCu(L1)N`
MCuML2)N`
MCu(L3)N`
45.0
(44.7)
42.1
(42.4)
44.8
(45.5)
3.4
(3.7)
2.8
(3.0)
4.3
(4.7)
10.3
(10.4)
13.7
(13.9)
14.4
(14.5)
290 (sh), 370 (3.1), 630 (sh)e
241 (44), 279 (36), 660 (0.12)e
242 (25), 286 (19), 785 (0.12)e
2.19 2.02 156
2.25 2.06 169
2.33 2.06 153g
6
[Cu(L2)(MeOH)][PF ]
6 2
[Cu(L3)(H O)][PF ]
2
6
a Required values in parentheses. b Recorded in MeCN. c Recorded as frozen CH Cl Èthf glasses at 77 K. All spectra are consistent with a
2 2
ground state. d FAB mass spectrum. e Cu(II)-centred dÈd transition. f ES mass spectrum. g This EPR spectrum is strongly solvent
d
(x2~y2)
dependent; see main text.
gration using the SMART system has been published.3 Table
2 contains a summary of the crystal parameters, data collec-
tion and reÐnement. The absorption corrections were applied
using SADABS.10 In all cases the structures were solved by
conventional direct methods and reÐned by the full-matrix
least-squares method on all F2 data using the SHELXTL 5.03
package on a Silicon Graphics Indy computer.11 Non-
hydrogen atoms were reÐned with anisotropic thermal param-
eters; hydrogen atoms were included in calculated positions
and reÐned with isotropic thermal parameters riding on those
of the parent atom.
In [Cu(L1)][PF ] É H O the asymmetric unit contains one
6
2
complex cation and one complete water molecule, as well as
three hexaÑuorophosphate ions whose P atoms lie on sites
of threefold symmetry such that one-third of each of the
three anions lies within the asymmetric unit. In
[Cu(L2)(MeOH)][PF ] É H O É 2MeOH the F atoms of one
6 2
2
of the hexaÑuorophosphate anions were disordered over two
Table 2 Crystallographic data for the three new complexes
Compound
[Cu(L1)][PF ] É H O
[Cu(L2)(MeOH)][PF ] É H O É 2MeOH
6 2
[Cu(L3)(H O)][PF ] É CH Cl É C H
12
6
2
2
2
6
2
2
5
Formula
M
C
806.1
H
BCuF N OPS
C
989.2
H
CuF N O P
C
870.0
H
BCl CuF N OP
30 30
6
6
3
34 37
12 4 2
9
33 41
2
6 9
System, space group
Cubic, Pa3
27.283(3)
27.283(3)
27.283(3)
90
90
90
20308(4)
24
1.582
Monoclinic, P2 /c
Triclinic, P1
10.020(2)
13.254(2)
15.142(3)
105.878(13)
106.30(2)
92.291(14)
1841.5(6)
2
1.569
0.855
0.26 ] 0.20 ] 0.04
15701, 6449, 0.0583
1
a/Ó
b/Ó
c/Ó
a/¡
19.585(3)
9.9751(9)
20.884(2)
90
92.845(11)
90
4075.0(8)
4
1.612
0.720
0.50 ] 0.40 ] 0.12
25058, 9265, 0.0429
b/¡
c/¡
U/Ó3
Z
D /g cm~3
c
k/mm~1
0.947
0.40 ] 0.36 ] 0.14
81102, 4431, 0.1297
Crystal size/mm
ReÑections collected:
total, independent, R
Data, restraints, parameters
int
4427, 15, 472
9265, 0, 617
0.0577, 0.1692
0.1061, 0
]1.079, [0.813
6426, 74, 516
0.0627, 0.1811
0.087, 1.84
]0.639, [0.899
Final R , wR b, c
0.0601, 0.1279
0.0369, 86.3345
]1.689, [0.443
1
2
Weighting factorsc
Largest residuals/e Ó~3
a Details in common: MoÈKa radiation (0.71073 Ó); temperature for data collection, 173(2) K; 2h limit for data collected, 55¡. b Structure was
reÐned on F2 using all data; the value of R is given for comparison with older reÐnements based on F with a typical threshold of F P 4p(F).
o
1
o
c wR \ [&[w(F2 [ F2)2]/& w(F2)2]1@2 where w~1 \ [p2(F2) ] (aP)2 ] bP] and P \ [max(F2 , 0) ] 2F2]/3.
2
o
c
o
o
o
c
Table 3 Selected bond lengths (Ó) and angles (¡) for the new structures
[Cu(L1)][PF ] É H O [Cu(L2)(MeOH)][PF ] É H O É 2MeOH
[Cu(L3)(H O)][PF ] É CH Cl É C H
6
2
6 2
2
2
6
2
2
5 12
Cu(1)ÈN(12)
Cu(1)ÈN(32)
Cu(1)ÈN(52)
Cu(1)ÈS(27)
Cu(1)ÈS(67)
1.968(5)
2.048(5)
2.141(5)
2.313(2)
2.372(2)
Cu(1)ÈN(21)
Cu(1)ÈN(41)
Cu(1)ÈN(31)
Cu(1)ÈO(3)
Cu(1)ÈN(51)
1.944(3)
1.950(3)
2.051(3)
2.156(3)
2.059(3)
Cu(1)ÈN(52)
Cu(1)ÈO(1)
Cu(1)ÈN(32)
Cu(1)ÈN(12)
Cu(1)ÈN(62)
1.898(4)
1.901(4)
2.171(4)
2.212(4)
2.212(4)
N(12)ÈCu(1)ÈN(32)
N(12)ÈCu(1)ÈN(52)
N(32)ÈCu(1)ÈN(52)
N(12)ÈCu(1)ÈS(27)
N(32)ÈCu(1)ÈS(27)
N(52)ÈCu(1)ÈS(27)
N(12)ÈCu(1)ÈS(67)
N(32)ÈCu(1)ÈS(67)
N(52)ÈCu(1)ÈS(67)
S(27)ÈCu(1)ÈS(67)
88.1(2)
90.4(2)
89.3(2)
N(21)ÈCu(1)ÈN(41)
N(21)ÈCu(1)ÈN(31)
N(41)ÈCu(1)ÈN(31)
N(21)ÈCu(1)ÈN(51)
N(41)ÈCu(1)ÈN(51)
N(31)ÈCu(1)ÈN(51)
N(21)ÈCu(1)ÈO(3)
N(41)ÈCu(1)ÈO(3)
N(31)ÈCu(1)ÈO(3)
N(51)ÈCu(1)ÈO(3)
84.34(11)
79.98(11)
160.34(11)
159.98(11)
79.80(11)
112.47(11)
103.29(11)
105.89(12)
89.30(11)
92.81(11)
N(52)ÈCu(1)ÈO(1)
N(52)ÈCu(1)ÈN(32)
O(1)ÈCu(1)ÈN(32)
N(52)ÈCu(1)ÈN(12)
O(1)ÈCu(1)ÈN(12)
N(32)ÈCu(1)ÈN(12)
N(52)ÈCu(1)ÈN(62)
O(1)ÈCu(1)ÈN(62)
N(32)ÈCu(1)ÈN(62)
N(12)ÈCu(1)ÈN(62)
177.2(2)
88.4(2)
94.4(2)
87.0(2)
93.6(2)
86.1(2)
76.9(2)
100.8(2)
135.2(2)
133.9(2)
89.9(2)
158.48(14)
112.13(13)
167.7(2)
93.06(14)
77.39(13)
93.36(6)
New J. Chem., 1999, 23, 417È423
419

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sites whose site occupancies reÐned to 0.6/0.4; the position of
the P atom was common to both components of the disorder.
In [Cu(L3)(H O)][PF ] É CH Cl É C H the Cl atoms of the
2
6
2
2
5 12
CH Cl were disordered over two sites with fractional
2
2
occupancies of 0.3 and 0.7; restraints were applied to the geo-
metric and thermal parameters of this solvent molecule to
keep the reÐnement stable. In addition there was a collection
of Ðve closely-spaced electron-density peaks which clearly cor-
responds to a disordered alkane; these reÐned successfully as
carbon atoms with unit site occupancy (corresponding to a
disordered molecule of pentane).
Selected bond lengths and angles for all three structures are
collected in Table 3.
CCDC reference number 440/096. See http://www.rsc.org/
suppdata/nj/1999/417/ for crystallographic Ðles in .cif format.
Fig. 1 Crystal
structure
of
the
complex
cation
of
[Cu(L1)][PF ] É H O.
Results and discussion
6
2
the metal atom from the mean basal plane, 0.09 Ó]. According
to the method of Addison et al. for classifying Ðve-coordinate
geometries,12 the parameter q for this complex is 0.15, where 0
denotes a perfect square-based pyramid and 1 denotes a
perfect trigonal bipyramid. The remaining donor atom S(47) is
too far from the metal to be considered as forming a normal
coordinate bond [Cu(1)É É ÉS(47), 3.053(2) Ó] but its weak inter-
action with the second axial coordination site is consistent
with the stereoelectronic requirements of the JahnÈTeller
e†ect. The bidentate arms are not individually coplanar
because of substantial dihedral twists between the pyrazolyl
and phenyl rings [19.8¡ between rings 1 and 2; 42.6¡ between
rings 3 and 4; 31.3¡ between rings 5 and 6, where ring 1 con-
tains N(11) to C(15), ring 2 contains C(21) to C(26), etc.].
Copper(II) complexes with mixed N/S donor sets based on
heterocyclic N donors and thioether S donors have been of
interest as structural and spectroscopic models for various
type I copper proteins which have similar coordination
spheres.13,14 Of particular interest is the Cu(I)ÈCu(II) redox
potential, which is related to both the geometry of the coordi-
nation sphere and the nature of the donor atoms. For
Synthesis of KL1 and the crystal structure of
[Cu(L1)] [PF ] Æ H O
6
2
The new ligand [L1]~ was prepared (as its potassium salt)
according to Scheme 2. The key intermediate in the synthesis
is 2-(methylsulfanyl)acetophenone I, which was prepared
according to the literature method9 from commercially avail-
able 2-sulfanylbenzoic acid by methylation of the thiol with
MeI, followed by conversion of the carboxylic acid function to
an acetyl group with two equivalents of methyl lithium in thf.
Conversion of the acetyl group to a pyrazole in two steps is
facile, following the route we have used before:3,8 (i) reaction
with neat dmfÈdimethylacetal at reÑux for 8 h to give the
dimethylamino-substituted enone II, followed by (ii) ring-
closure to the pyrazole III by reaction with an excess of
hydrazine in EtOH at reÑux for 30 minutes. Finally, the
tris(pyrazolyl)borate [L1]~ was prepared in the usual manner
by melting together a mixture of KBH and III (1 : 3.5 molar
4
ratio) at 200 ¡C for 3 h under N , followed by recrystallisation
2
of the resultant glassy solid from tolueneÈpentane (1 : 1).
Reaction of KL1 with copper(II) acetate hydrate in MeOH
a†orded a green solution from which a precipitate was iso-
[Cu(L1)][PF ] in MeCN, cyclic voltammetry showed a sym-
6
metric, reversible redox process at E \ [0.42 V vs. ferro-
lated after treatment with methanolic KPF . Recrystallisation
1@2
6
ceneÈferrocenium (peakÈpeak separation, 90 mV) which we
from MeCNÈdiethyl ether a†orded X-ray quality crystals of a
assign to the Cu(I)ÈCu(II) couple. This translates to ca. 0.0 V
vs. NHE, and may be compared to the redox potential
range of type 1 copper proteins which runs from ]0.18 V
material whose mass spectrum and elemental analysis sug-
gested the formulation [Cu(L1)][PF ] É H O, and this was
6
2
subsequently conÐrmed crystallographically. The structure of
the complex cation is shown in Fig. 1. The metal ion is in a
(stellacyanin) to ]0.68 V (rusticyanin).14 In [Cu(L1)][PF ]
6
therefore the Cu(II) state is stabilised compared to related bio-
Ðve-coordinate N S coordination environment which may be
3 2
logical systems, which we ascribe to the higher coordination
number; stabilisation of Cu(I) in type 1 copper proteins is
ascribable in part to the fact that the metal centres are basi-
cally four-coordinate and pseudo-tetrahedral.
approximately described as square pyramidal, with N(52)
being the axial ligand and N(12), S(27), S(67) and N(32) com-
prising the basal plane [mean separation of these four atoms
from the best mean plane through them, 0.31 Ó; separation of
Synthesis of L2 and the crystal structure of
[Cu(L2)(MeOH)] [PF ] Æ H O Æ 2MeOH
6 2
2
The standard synthesis of tris(pyrazolyl)methanes involves
reaction of the appropriate pyrazole (three equivalents) with
CHCl under phase transfer conditions using aqueous NaOH
3
as base.15 The reaction requires substitution of all three chlo-
rine atoms of CHCl by the pyrazole anion, and competing
3
side-reactions occur due to the formation of the reactive :CCl
2
diradical by loss of HCl from CHCl in the strongly basic
3
conditions. Nonetheless reasonable yields of simple
tris(pyrazolyl)methanes may be obtained. However all
attempts to prepare the tris(pyrazolyl)methane from 3-(2-
pyridyl)pyrazole (Hpypz) under these conditions failed, giving
crude reaction mixtures containing small amounts of large
numbers of products. To reduce the likelihood of carbene-
based side-reactions we therefore replaced CHCl by PhCCl
Scheme 2 Preparation KL1: (i) dimethylformamideÈdimethylacetal;
(ii) hydrazine hydrate, EtOH, reÑux; (iii) KBH , melt.
4
3
3
420
New J. Chem., 1999, 23, 417È423

View Article Online
in the synthesis. By this method we managed to obtain modest
amounts of L2, although there were still numerous by-
products of which the most abundant was (on the basis of its
mass spectrum) Ph(pypz)C\C(pypz)Ph, showing that
carbene-based side reactions were not entirely eliminated. The
new ligand L2 was satisfactorily characterised on the basis of
its mass and NMR spectra, and elemental analysis.
because of the changes in coordination geometry caused by
the sterically hindering methyl substituents.8 We were there-
fore interested to examine Ðrst-row transition-metal complex-
es with [L3]~ to see how they compared with [TpPy]~, and
the crystal structure of [Cu(L3)(H O)][PF ] is shown in Fig.
2
6
3 (see also Table 3). The metal centre is Ðve coordinate from
the three pyrazolyl donors, one pyridyl donor and one water
molecule. The parameter q is 0.70,12 indicating that the struc-
ture lies nearer the trigonal bipyramidal limit than the square
pyramidal limit; the two axial ligands in this description are
N(52) and O(1), with N(12), N(32) and N(62) forming the
equatorial plane. The two pendant pyridyl donors are
involved in NÉ É ÉHÈO hydrogen-bonding interactions with the
protons of the coordinated water molecule [O(1)É É ÉN(22)
2.642(7) Ó, O(1)É É ÉN(42) 2.685(7) Ó]. The water molecule is
therefore a conventional Ðrst-sphere ligand, and the pendant
groups which coordinate in turn to the water are “second-
sphere ligandsÏ in that they constitute the second coordination
sphere of the metal ion. This is very similar behaviour to that
of the lanthanide complexes of [L3]~;8 the steric interference
between the pyridyl methyl substituents which would result if
the ligand were hexadentate, with the methyl groups all
directed towards one another, is relieved by two of the pyridyl
ligands twisting away from the metal and binding instead to
the water ligand. Consequently these pyridyl/pyrazolyl arms
are not planar, with dihedral twist angles of 33.5¡ between
rings 3 and 4, and 22.4¡ between rings 5 and 6 [where ring 3
contains N(31)ÈC(35), etc.]. This also results in the associated
CuÈN(pyrazolyl) distances being stretched as the pyridyl
groups move away from the metal atom: the distances Cu(1)È
N(32) and Cu(1)ÈN(12) [2.171(4) and 2.212(4) Ó, respectively]
are considerably longer than Cu(1)ÈN(52) [1.898(4) Ó].
Reaction of L2 with copper(II) acetate followed by precipi-
tation of the complex as its hexaÑuorophosphate salt a†orded
a material whose FAB mass spectrum indicated formation of
a 1 : 1 Cu : L2 complex. The crystal structure of [Cu(L2)-
(MeOH)][PF ] É H O É 2MeOH is shown in Fig. 2 (see also
6 2
2
Table 3) and shows that L2 is coordinated as a tetradentate
ligand via two bidentate arms, resulting in a square-pyramidal
Cu(II) centre whose Ðfth (axial) ligand is a molecule of meth-
anol. The four atoms N(31), N(22), N(42) and N(51) form a
near-perfect plane (the mean deviation from the mean plane
through them is 0.028 Ó), and the metal atom is displaced
0.291 Ó out of this mean basal plane towards the axial meth-
anol ligand. The third pyrazolyl-pyridine arm is pendant with
the pyrazolyl and pyridyl rings being trans-coplanar to maxi-
mise the separation between the lone pairs on N(61) and
N(71). The axial methanol ligand is involved in an OÈHÉ É ÉO
hydrogen-bonding interaction with one of the lattice MeOH
molecules [O(3)É É ÉO(7), 2.670(5) Ó], which in turn is
hydrogen-bonded to the other lattice MeOH molecule
[O(7)É É ÉO(5), 2.537(5) Ó]. In addition the pendant pyridyl
residue is involved in an NÉ É ÉHÈO hydrogen-bonding
interaction with the lattice water molecule [N(71)É É ÉO(8),
2.767(5) Ó]. The neutral tris(pyrazolyl)methane L2 therefore
coordinates to Cu(II) in essentially the same way as the
isostructural but negatively charged tris(pyrazolyl)borate
analogue [TpPy]~,16 which forms a square-pyramidal complex
[Cu(TpPy)(H O)][PF ], although in this latter case the pen-
This “second-sphereÏ stabilisation of a coordinated ligand is
strongly reminiscent of the cooperative interactions by which
substrates are bound to the active sites of metalloproteins.14 It
is relatively rare in simple coordination complexes because it
requires a ligand of sufficient complexity and Ñexibility to
bond the metal with some of its donors whilst simultaneously
interacting with a coordinated substrate with the others, but
has recently become the focus of interest in the areas of host-
guest, supramolecular and biomimetic chemistry.17 The struc-
ture of [Cu(L3)(H O)][PF ] is in interesting contrast to that
2
6
dant arm is involved in hydrogen-bonding interactions with
the axial water ligand. This is consistent with the recent
observations of Reger and coworkers that simple tris-
(pyrazolyl)borates and tris(pyrazolyl)methanes can display
essentially identical structures in their metal complexes.7
Synthesis, crystal structure and EPR spectra of
[Cu(L3)(H O)] [PF ] Æ CH Cl Æ C H
2
6
2
6
2
2
5 12
of [Cu(TpPy)(H O)][PF ],16 which is recalled in Fig. 4. In this
2
6
We showed recently that the luminescence properties of
Eu(III) and Tb(III) complexes of [L3]~ were considerably dif-
ferent from those of the parent (unsubstituted) ligand [TpPy]~
latter structure two bidentate arms of TpPy and a water ligand
also form a Ðve-coordinate structure, but in contrast to
[Cu(L3)(H O)][PF ] it is square-pyramidal. Adoption of such
a structure with L3 would result in an unfavourable steric
interaction between the methyl substituents of the two coordi-
nated pyridyl ligands.
2
6
We recorded EPR spectra of all of the complexes described
in this paper as part of their routine characterisation (Table 1),
Fig. 2 Crystal
structure
6 2
of
the
complex
cation
of
Fig. 3 Crystal
structure
of
the
.
complex
cation
of
[Cu(L2)(MeOH)][PF ] É H O É 2MeOH.
[Cu(L3)(H O)][PF ] É CH Cl É C H
2
2
6
2
2
5 12
New J. Chem., 1999, 23, 417È423
421

View Article Online
gests that the structure observed in the solid state changes on
dissolution in this particular solvent combination, either due
to solvation or to rearrangement (e.g. coordination of one of
the pendant pyridyl residues). Secondly, the spectrum appears
to be of rather poor quality with evidence for what we
assumed to be impurities but which appeared consistently
even after recrystallisation, suggesting the presence of a
mixture of species in solution. Both of these observations
prompted us to investigate the solvent-dependence of the EPR
spectrum of this complex.
In a non-donor solvent mixture (1 : 1 dichloromethaneÈ
dichloroethane) the spectrum of Fig. 5(a) was obtained at 77
K, and it is suggestive of a d ground state arising from an
z2
approximately trigonal bipyramidal structure.18,19 There are
three g-values rather than two however, consistent with sig-
niÐcant distortion from ideal trigonal bipyramidal geometry,
and the parameters are g \ 2.26 (A B 70 G), g \ 2.16, and
1
1
2
g \ 2.02 (A B 130 G). The peaks marked A and B, which
3
3
are not reproduced by simulation with the above parameters,
are consistent with a small proportion of a species with a
d
ground state and in fact correspond closely to features
x2~y2
in the spectrum of Fig. 5(c) (see later). Thus it appears that the
crystal structure is largely retained in this non-donor solvent,
with only a small proportion of the complex being converted
Fig. 4 Crystal
structure
of
the
complex
cation
of
[Cu(TpPy)(H O)][PF ] (from ref. 16).
2
6
to some other geometry which has a d
ground state
x2~y2
and noticed two unusual features in the spectrum of
(presumably, either by solvation or by attachment of one of
the pendant pyridyl groups). We emphasise that the presence
of the second set of peaks (A and B) make simulation difficult
and render the paremeters for the main part of the spectrum
somewhat unreliable, although the basic pattern of the spec-
trum is clear.
On thawing the sample, addition of a few drops of thf, and
re-freezing to 77 K, the spectrum changes substantially [Fig.
5(b)]. This shows that most of the sample is converted to a
solvated species (presumably a thf adduct) with a basically
[Cu(L3)(H O)][PF ] recorded as a CH Cl Èthf glass at 77 K
2
6
2 2
[Fig. 5(b)]. Firstly, the spectrum indicates a d
ground
x2~y2
state, which is very common for tetragonally-elongated four-,
Ðve- or six-coordinate Cu(II) complexes but is not consistent
with the basically trigonal bipyramidal crystal structure, for
which a d ground state would be expected.18,19 This sug-
z2
tetragonal geometry giving a d
x2~y2
eters are g \ 2.33 (A \ 153 G); g \ 2.06. The relatively
ground state. The param-
A
A
M
high value of g and the relatively low value of A [cf. the
A
A
spectrum of Fig. 5(c)] are both indicative of a tetrahedral dis-
tortion of the four equatorial ligands.20 We can also see how
the additional features in this spectrum, which we initially
ascribed to impurities, arise from a small proportion of the
unchanged non-solvated species [Fig. 5(a)]. In particular the
inÑexion labelled C and the two weak features to the high-
Ðeld side of the main signal (D and E) correspond almost
exactly to features in the spectrum of Fig. 5(a).
On addition of a drop of pyridine to the dichloromethaneÈ
dichloroethane solution of the complex, the spectrum of Fig.
5(c) is obtained. This is a clean spectrum and consistent with
complete conversion of the complex to a tetragonal structure
(presumably a pyridine adduct) with a d
ground state;
x2~y2
the parameters are g \ 2.27 (A \ 170 G); g \ 2.06. The
A
A
M
fact that addition of pyridine results in a single solvated
species, whereas addition of thf results in a mixture due to
only partial solvation, is in accord with the fact that pyridine
is a much better ligand for Cu(II) than is thf.
Acknowledgements
We thank the EPSRC for Ðnancial support.
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Paper 8/09777A
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423