Mitigating repulsions in close quarters: Copper tells us how!

Article abstract of DOI:10.1016/j.poly.2006.08.004

Four copper(I) complexes of short bite ligands, bis(diphenylphosphino)amine (dppa) and bis(diphenylphosphino)isopropylamine (dppipa) were synthesized from appropriate precursors. All complexes were characterized by single crystal X-ray crystallography and spectroscopic techniques. In each of these complexes, two filled shell cations are forced into close proximity (≈2.7-2.8 ?). With no strong π acid ligands to siphon electron density from the filled d shell, the unavoidable repulsive d¹⁰-d¹⁰ interaction is mitigated when an unsymmetrical coordination environment around the copper atoms exists. The coordinatively saturated copper ion functions as a donor to the coordinatively unsaturated copper. A Cambridge Structural Database (CSD) search reveals the greater propensity of clusters with short contacts to adopt unsymmetrical coordination.

Full text of DOI:10.1016/j.poly.2006.08.004

Polyhedron 26 (2007) 142–148
www.elsevier.com/locate/poly
Mitigating repulsions in close quarters: Copper tells us how!
*
Ritu Ahuja, M. Nethaji, A.G. Samuelson
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
Received 28 September 2005; accepted 8 August 2006
Available online 11 August 2006
Abstract
Four copper(I) complexes of short bite ligands, bis(diphenylphosphino)amine (dppa) and bis(diphenylphosphino)isopropylamine
(dppipa) were synthesized from appropriate precursors. All complexes were characterized by single crystal X-ray crystallography and
˚
spectroscopic techniques. In each of these complexes, two filled shell cations are forced into close proximity (ꢀ2.7–2.8 A). With no strong
p acid ligands to siphon electron density from the filled d shell, the unavoidable repulsive d10–d10 interaction is mitigated when an unsym-
metrical coordination environment around the copper atoms exists. The coordinatively saturated copper ion functions as a donor to the
coordinatively unsaturated copper. A Cambridge Structural Database (CSD) search reveals the greater propensity of clusters with short
contacts to adopt unsymmetrical coordination.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Copper; Cambridge Structural Database (CSD) analysis; Diphosphinoamine; Metal–metal interactions; Short bite ligand; Unsymmetrical
coordination
1. Introduction
valence shell. Whereas in the lighter congeners, the exis-
tence of analogous cuprophilicity and argentophilicity
[1,4,5,9] have not been substantiated by many ligand-
unsupported M(I)–M(I) (M = Cu, Ag) contacts in coordi-
nation compounds. Very short distances, for example
Closed shell interactions in inorganic chemistry have
been a challenge to experimental and theoretical chemists
alike [1,2]. Group 11 monocations have been the subject
of innumerable studies for they form many polynuclear
complexes and extended lattices with short contacts in
the solid state despite the formally closed shell (d10) elec-
tronic configuration of the metals involved. The nature of
this closed shell interaction has been an area of consider-
able debate. In the case of gold, the attractive nature of this
interaction has been particularly well documented and
accepted. The term ‘‘aurophilicity’’ is used to collectively
describe Au–Au interactions [3–6]. Some of these interac-
tions are unsupported by exogenous ligands. This attrac-
tive interaction has found its theoretical justification in
the ligand-induced [7] and relativistically-supported [8] sta-
bilization, which leads to favorable mixing of the empty 6s
with 5d states to effectively reduce the population of the 5d
˚
2.42 A between two copper centers has been observed by
White et al. [10] in a complex with a short bite ligand
2-((6-methyl)pyridyl)trimethylsilylamine. Another example
˚
of an unusually short Cu(I)–Cu(I) distance (2.35 A) [11] is
found in [Cu(tolyl-NNNNN-tolyl)]3. This complex has been
analyzed at different levels of theory and the Cu–Cu inter-
action has been shown to be attractive as a result of
s þ pz þ dz2 mixing [12]. In the few unsupported short con-
tacts, strong electrostatic interactions between the sub-
units cloud the analysis. This has resulted in a controversy
about the degree of stabilization, if any, offered by metal–
metal contacts in ligand supported cases.
We have examined in this study, four complexes of
a diphosphinoamines of the type Ph2PN(R)PPh2 (dppa;
R = H, dppipa; R = iPr). These ligands are well known for
their short bite [13], which force short metal–metal contacts.
There would be considerable difficulty in accommodating
*
Corresponding author. Fax: +91 80 2360 1552.
E-mail address: ashoka@ipc.iisc.ernet.in (A.G. Samuelson).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.08.004

R. Ahuja et al. / Polyhedron 26 (2007) 142–148
143
Table 1
the short-bite of the ligand with the closed shell repulsion of
Cu(I). Yet copper(I) adopts another route for relieving the
repulsive interactions in these examples. Unsymmetrical
coordination makes one copper electron rich relative to the
other by the presence of an extra ligand. This makes one
copper a donor and the other an acceptor and masks the
otherwise repulsive interaction. The generality of this phe-
nomenon in complexes with a short contact was brought
out by analysis of data readily available in the Cambridge
Structural Database (CSD) [14].
˚
Selected bond distances (A) and bond angles (ꢁ) for complex 1
Bond distances
Cu1–Cu2
Cu1–P1
Cu1–P3
Cu1–N3
Cu2–P2
Cu2–P4
2.7483(9)
2.235(1)
2.2213(12)
1.965(4)
2.2645(12)
2.2632(13)
Cu2–Ow1
Cu2–O5a
P1–N1
P2–N1
P3–N2a
P4–N2a
2.189(5)
2.172(19)
1.676(3)
1.676(3)
1.673(3)
1.688(3)
Bond angles
P1–Cu1–P3
P1–Cu1–N3
P3–Cu1–N3
P1–Cu1–Cu2
P3–Cu1–Cu2
N3–Cu1–Cu2
P2–Cu2–O5a
P2–Cu2–Cu1
P4–Cu2–O5a
131.95(5)
107.56(13)
116.87(13)
95.87(4)
98.69(4)
93.49(14)
111.1(5)
81.65(3)
109.4(5)
P2–Cu2–P4
139.46(5)
101.50(15)
98.80(14)
58.4(5)
108.4(5)
166.31(17)
122.2(2)
121.8(2)
P2–Cu2–Ow1
P4–Cu2–Ow1
O5a–Cu2–Ow1
O5a–Cu2–Cu1
Ow1–Cu2–Cu1
P1–N1–P2
2. Results and discussion
2.1. X-ray crystal structure determinations
P3–N2a–P4
Complex 1 consists of a cationic dppa bridged dimer
with an unsymmetrical coordination. The perspective view
is represented in Fig. 1. Although Cu1 and Cu2 have 3 and
4 coordinating atoms around them respectively, including
the distant copper in the coordination sphere makes the
description easy to follow. Cu1 has a distorted tetrahedral
geometry with P1, P3, N3 and Cu2 around it. Cu1 is merely
˚
2.9766(1) A from the P1 P3 N3 plane. The two trigonal
planes P1, P3, N3 and P2, P4, O5a, have a dihedral angle
of 14.82(6) A. The eight membered ring formed by the
two PNP bridges adopts a twist boat conformation. Nitro-
gen atoms of the ligand are almost planar with the angles
not very different from those of the free ligand. There is
no significant change in the metrical parameters of the
ligand on complexation (P–N bond length in the free ligand
˚
˚
0.2336(1) A from the plane formed by two P atoms (P1 and
P3) and the N3 of acetonitrile. The sum of the three angles
in this trigonal plane is approximately equal to 360ꢁ (see
Table 1). The tetrahedral coordination is provided by the
second copper atom (Cu2). Cu2 is found to adopt a
severely distorted trigonal bipyramidal geometry. It has
two P atoms P2, P4 and an oxygen atom of the perchlorate
anion in the equatorial plane. Cu2 deviates from this plane
˚
is 1.692 A with a PNꢁP angle of 118.9ꢁ).
The second ClO4 is foundꢁ outside the coordination
sphere. The counter anion ClO4 shows weak intermolecu-
lar hydrogen bonding interactions with the aromatic pro-
tons, H of the acetonitrile and the NH protons. The
bound water is found to have a short contact with a phe-
nylic proton as well. Some of the short contacts are listed
in the Supplementary information (Table S1).
˚
by 0.227(1) A. The axial ligands around Cu2 are the oxy-
gen atom of the water (Ow1) and Cu1 subtending an angle
of 166.3(2)ꢁ along this axis. Cu1 and Cu2 deviate by
˚
˚
ꢁ2.6182(1) A and ꢁ0.010(1) A from the P2, P4, and O5a
The ORTEP plot of the dication of complex 2 is given in
Fig. 2 and shows that complexes 1 and 2 are similar. Cu1 is
plane respectively, while Cu2 has
a deviation of
Fig. 1. Cationic fragment of complex 1 [Cu2(dppa)2(CH3CN)(H2O)-
(OClO3)]ClO4 with a Cu–Cu distance of 2.7483(9) A (phenyl rings on
Fig. 2. Cationic fragment of complex 2 [Cu2(dppa)2(CH3CN)3](BF4)2 with
a Cu–Cu distance of 2.8215(8) A (phenyl rings on phosphorus and
˚
˚
phosphorus and hydrogen atoms removed for clarity).
hydrogen atoms removed for clarity).

144
R. Ahuja et al. / Polyhedron 26 (2007) 142–148
found in a severely distorted tetrahedral geometry. Cu1
˚
deviates by ꢁ0.2237(5) A from the P1, P3, N3 plane and
Cu2 forms the fourth coordination site of the tetrahedron.
The second metal atom Cu2 adopts an irregular trigonal
˚
bipyramidal geometry with Cu2 being ꢁ0.352(1) A away
from the P2, P4, N5 plane. The axial sites are occupied
by Cu1 and N4 (CH3CN) subtending an angle of
166.92(9)ꢁ at Cu2 (Table 2). The two counter anions lie
far from the metal center.
˚
˚
Cu1 and Cu2 deviate by 2.4525(5) A and ꢁ0.352(1) A
˚
from the P2, P4, N5 plane respectively, ꢁ0.2237(5) A and
˚
ꢁ3.043(1) A from the P1, P3, N3 plane respectively. The
two planes P1, P3, N3 and P2, P4, N5 have a dihedral angle
ꢁ
˚
of 6.92(7) A. The short contacts formed by BF4 with phe-
nylic protons and NH protons do not appear to affect the
structure significantly and are listed in the Supplementary
information (Table S2).
Fig. 3. Complex
3 [Cu2(dppipa)2(Cl)2] with a Cu–Cu distance of
˚
2.7664(14) A (phenyl rings on phosphorus and hydrogen atoms removed
for clarity).
The movement of the two Cu atoms in complex 1 towards
one other is quite striking. Complex 1 is also remarkable for
it is the first example of a Cu(I) complex having a water mol-
ecule and a phosphine in the coordination sphere. Surpris-
ingly, in spite of very similar synthetic conditions, they
have different ancillary donors and Cu–Cu distances.
ꢁ
Recently, an analogous complex with PF6 was character-
˚
ized in both symmetrical (Cu–Cu distance = 3.341(2) A)
and unsymmetrical environments (Cu–Cu distance =
˚
2.869(4) A) [15]. Significantly, the shorter distance is associ-
ated with the unsymmetrical structure. While the unsym-
metrical structure has donors similar to 2, an entirely
different orientation of the ligand leads to shorter Cu–Cu
˚
distance (2.8215(8) A) in 2. In 1 and 2, the complex is coord-
inatively unsaturated in one of the metal centers although
excess acetonitrile is present during the synthesis.
Fig. 4. Complex
4 [Cu2(dppipa)2(Br)2] with a Cu–Cu distance of
˚
In a related study with the ligand dppipa, we isolated
complexes 3 and 4. The ORTEP plot of the dppipa com-
plexes 3 and 4 are shown in Figs. 3 and 4 respectively.
The two halide (chloride and bromide) dimers have highly
unsymmetrical structures. There are two dppipa units, out
of which one is found in a chelating and the other one in a
2.7675(8) A (phenyl rings on phosphorus and hydrogen atoms removed
for clarity).
bridging mode. Important bond lengths and bond angles
for complex 3 and 4 are given in Tables 3 and 4 respec-
tively. Both complexes have Cu–Cu distances within the
sum of the Van der Waal’s radii. The chloride complex 3
Table 2
˚
has two copper centers at a distance of 2.7664(14) A and
˚
Selected bond distances (A) and bond angles (ꢁ) for complex 2
the bromide complex 4 has a Cu–Cu distance of
Bond distances
˚
2.7675(8) A.
Cu1–Cu2
Cu1–P1
Cu1–P3
Cu1–N3
Cu2–P2
Cu2–P4
2.8215(8)
2.2725(10)
2.2316(11)
2.023(3)
2.273(1)
2.2723(11)
Cu2–N4
Cu2–N5
P1–N1
P2–N1
P3–N2a
P4–N2a
2.073(3)
2.046(3)
1.687(3)
1.677(3)
1.680(3)
1.693(3)
In complex 3, Cu1 has three of its coordination sites
occupied by P1, P4 and Cl1 (bridged). The fourth coordina-
tion site is held by P3 at a comparatively longer distance of
˚
2.263(1) A. The other copper, Cu2 is trigonal planar with
Cl1, Cl2 and P2 in its coordination sphere with sum of
the three angles close to 360ꢁ. The two trigonal planes P2,
Cl1, Cl2 and P1, P4, Cl1 are hinged by a chloride bridge,
Cl1 and the dihedral angle is 69.59(2)ꢁ. The deviations of
Bond angles
P1–Cu1–P3
P1–Cu1–N3
P3–Cu1–N3
P1–Cu1–Cu2
P3–Cu1–Cu2
N3–Cu1–Cu2
P2–Cu2–P4
P2–Cu2–N5
129.87(4)
96.07(9)
130.71(8)
97.93(3)
93.66(3)
96.87(8)
130.68(4)
118.21(9)
P4–Cu2–N5
P2–Cu2–N4
P4–Cu2–N4
N4–Cu2–N5
N4–Cu2–Cu1
P1–N1–P2
103.33(9)
95.71(9)
105.55(9)
96.15(12)
166.92(9)
123.31(15)
124.92(16)
˚
Cu1 and Cu2 from P1, P4, Cl1 planes are ꢁ0.358(1) A
˚
and 2.115(1) A respectively. On the other hand, Cu1 and
˚
˚
Cu2 deviate by 2.161(1) A and 0.1883(6) A from P2, Cl1,
Cl2 plane respectively. This indicates that Cu2 moves
towards Cu1 in a cuprophilic fashion.
P3–N2a–P4

R. Ahuja et al. / Polyhedron 26 (2007) 142–148
145
Table 3
p-donation of bridging chloride ligands leads to larger
Cu–Cu distances.
˚
Selected bond distances (A) and bond angles (ꢁ) for complex 3
Bond distances
Cu1–Cu2
Cu1–P1
Cu1–P3
Cu1–P4
Cu1–Cl1
Cu2–P2
2.7664(14)
2.2604(12)a
2.339(1)b
2.301(1)b
2.320(1)c
2.186(1)a
Cu2–Cl1
Cu2–Cl2
P1–N1
P2–N1
P3–N2
P4–N2
2.363(1)c
2.1824(12)d
1.708(3)
1.720(3)
1.709(3)
1.721(3)
2.2. Solution behavior
One expects that complex 1 would exhibit conductivity
corresponding to a 1:1 electrolyte and complex 2 would
show conductivity corresponding to that of a 1:2 electrolyte
[17]. Indeed complex 2 showed a conductivity value of
220.64 S cmꢁ1 molꢁ1 (10ꢁ3 M in acetone), which shows
that it is a 1:2 electrolyte. On the other hand complex 1
gave a value of 203.64 S cmꢁ1 molꢁ1 (10ꢁ3 M in acetone),
which was considerably higher than that of a 1:1 electrolyte
suggesting significant dissociation in solution. This obser-
vation clearly indicates that the perchlorate anion, which
is g1-bound to the metal center, dissociates in solution
and a 1:2 electrolyte is formed. The dissociation of the per-
chlorate in solution suggests that the structure in solution
could be symmetrical even though the solid state structure
is unsymmetrical.
Bond angles
P1–Cu1–P3
P1–Cu1–P4
P3–Cu1–P4
P1–Cu1–Cl1
P3–Cu1–Cl1
P4–Cu1–Cl1
a
111.60(4)
126.51(4)
71.95(4)
108.72(4)
115.10(4)
117.47(4)
P2–Cu2–Cl1
P2–Cu2–Cl2
Cl1–Cu2–Cl2
P1–N1–P2
108.49(4)
134.33(4)
114.97(4)
115.10(13)
105.27(14)
P3–N2–P4
Bridging dppipa.
b
c
Chelating dppipa.
Bridging Clꢁ.
Terminal Clꢁ.
d
Table 4
˚
Selected bond distances (A) and bond angles (ꢁ) for complex 4
2.3. Cambridge Structural Database analysis
Bond distances
Cu1–Cu2
Cu1–P1
Cu1–P2
Cu1–P3
Cu1–Br1
Cu2–P4
2.7675(8)
Cu2–Br1
Cu2–Br2
P1–N1
P2–N1
P3–N2
2.4437(8)c
2.3094(8)d
1.706(4)
1.722(4)
1.703(3)
1.715(3)
To probe the frequency of unsymmetrical coordination,
we explored the CSD and found 1227 instances where
Cu–Cu distances were below 2.8 A. Out of these, 829 frag-
2.3404(12)a
2.2956(13)a
2.263(1)b
˚
2.4496(7)c
2.1853(13)b
ments were obtained when the search was limited to ‘‘sym-
metrical’’ dimers with coordination numbers 2,2; 3,3 and
4,4, etc. Among these symmetrically coordinated frag-
ments, 530 of them have coordination numbers up to 4,4.
The remaining have higher coordination numbers. Frag-
ments with coordination number of four each (221) were
analyzed in detail. Although the CSD lists them as having
the same coordination number, and it was found that
about 76% (163) have an unsymmetrical coordination by
virtue of different coordinating atoms or very different
bond distances around the metal centers. In addition, these
apparently ‘‘symmetrically bridged’’ compounds several
(143 out of 221) have single atom bridges with acute angles
in structures that lead to 3c–2eꢁ bonds and so promote
attractive Cu–Cu interactions.
P4–N2
Bond angles
P1–Cu1–P3
P1–Cu1–P2
P2–Cu1–P3
P1–Cu1–Br1
P3–Cu1–Br1
P2–Cu1–Br1
a
112.11(4)
72.23(4)
126.21(5)
114.18(4)
108.49(4)
118.10(4)
P4–Cu2–Br1
P4–Cu2–Br2
Br1–Cu2–Br2
P1–N1–P2
109.24(4)
132.91(4)
114.64(3)
105.7(2)
P3–N2–P4
115.47(19)
Bridging dppipa.
b
c
Chelating dppipa.
Terminal Brꢁ.
Bridging Brꢁ.
d
Similarly in case of complex 4, there are two trigonal
planes Br1, Br2, P4 and P2, P3, Br1 (common vertex at
Br1) at a dihedral angle of 67.66(2)ꢁ. Cu1 is four coordinate
with three coordination sites, Br1, Br2 and P3 almost in a
On the contrary, unsymmetrical coordination occurred
in 398 cases, which accounts for about one-third of the
total number. Of these, 305 fragments have coordination
number less than or equal to 4. There are 142 hits for coor-
dination number 3 and 4, 119 hits with coordination num-
bers 2 and 3 and the remaining 44 have 2 and 4 coordinate
CuI ions. Fig. 5 represents the distribution of M–M dis-
tances in complexes with coordination numbers 3/4 and
2/3 respectively. The histograms clearly reflect a well-
defined interaction between the metal ions around 2.6–
˚
plane with a weak interaction at a distance of 2.3404(12) A
˚
with P1. Cu1 and Cu2 deivate by 0.3613(4) A and
˚
ꢁ2.1246(6) A from P2, P3, Br1 plane respectively. The
˚
three coordinate Cu2 deviates by 0.2346(6) A from Br1,
˚
Br2, P4 plane with Cu1 deviating by 2.2484(4) A from
the same plane. In this case also, one can find Cu2 moving
towards Cu1.
˚
One would expect the Cu–Cu distance in the case of the
bromide bridged dimer to be longer than in the case of
analogous chloride complex due to larger size of the bridg-
ing bromide ion. Surprisingly the two Cu–Cu distances are
not very different. This behavior is reminiscent of the
behavior of copper in trimeric complexes [16]. The greater
2.7 A. This is below the sum of the van der Waal’s radii
I
˚
for two Cu ions (2.8 A) suggesting the presence of an
attractive interaction between the two copper(I) centers.
A similar analysis of structures having Cu–Cu distances
˚
above 2.8 A shows that the probability of encountering
unsymmetrical coordination is less (1/4th).

146
R. Ahuja et al. / Polyhedron 26 (2007) 142–148
Fig. 5. Histograms of Cu–Cu distances in (a) CuL3 and CuL4 and
(b) CuL2 and CuL3 complexes.
Fig. 6. Two unsymmetrical copper(I) dimers in wireframe style with
refcodes: (a) CAGBIW and (b) ODODAM.
Among the complexes short listed from the CSD search,
two complexes closely related to 3 and 4 were encountered
with CCDC refcodes CAGBIW [18] and ODODAM [19]
(Fig. 6(a) and (b)). These two structures have very similar
coordination environments to those observed in the present
study. The former has one copper having a coordination
number of four with one bromide, two nitrogen atoms
and one phosphorus bound to it. The three coordinate cop-
per has two bromides (one terminal and one bridging) and
one nitrogen in its coordination sphere. Similarly the latter
structure has the three coordinate copper with two chlo-
rides, one nitrogen and the four coordinate copper with
one chloride, two nitrogen atoms and one phosphorus.
3. Conclusions
As expected for the short bite dppa as well as dppipa
ligands, dinuclear copper(I) complexes formed have two
copper(I) centers close to each other. In the absence of sig-
nificant p-acceptor character in the ligand, such a short
Cu–Cu distance is clearly repulsive and is forced by the
ligand. Due to the plasticity of the coordination sphere in
monovalent group 11 complexes, a change in the coordina-
tion number is energetically accessible. Hence, the coordi-
nation environment around only one of the copper(I)
ions is altered to give a 4 coordinate–3 coordinate system.
The four coordinate copper(I) can act as an electron donor
and pump some electron density into a coordinatively
unsaturated three coordinate copper. Computational work
to justify this statement is underway. A CSD analysis
reveals more frequent occurrence of unsymmetrical coordi-
nation in complexes with short contacts. Four complexes
investigated in the present study along with two of the clo-
sely related copper(I) dimers with ((dimethylamino-
methyl)phenyl)phosphine-N,N0,N00,P ligand and analogous
˚
The Cu–Cu distances are 2.658 and 2.665 A respectively.
These two examples again augment the fact that complexes
that have Cu–Cu distances within the sum of the Van der
Waal’s radii of two copper centers tend to be unsymmetri-
cally coordinated. A survey of short metal–metal contacts
in systems with incomplete d shells, invariably reflected les-
ser preference for forming unsymmetrical dimers in com-
parison with the formation of symmetrical structures.

R. Ahuja et al. / Polyhedron 26 (2007) 142–148
147
hexafluorophosphate copper(I) dppa dimers reinforce the
importance of unsymmetrical coordination in relieving
the repulsion between two closed shells. In spite of closed
shell repulsions, a close contact results because the
resourceful copper(I) adopts unsymmetrical coordination!
7.12–7.45 (m, 40H, Ph). 31P{1H} NMR (CDCl3): d = 47.8
(s). IR (KBr, cmꢁ1): 2303 (w), 2264 (w) (CH3CN); 1102,
1052 (vs, br) (BF4ꢁ). HRESMS: [Mꢁ(CH3CN)3]2+ obsd.
m/z 448.0487, calc. for C48H42N2P4 448.0445.
Complex 3: To 15 ml of CH2Cl2 was added dppipa
(0.22 g, 0.50 mmol) followed by the addition of CuCl
(0.025 g, 0.25 mmol). The reaction was stirred at room tem-
perature for half an hour. Subsequently the obtained yellow
colored solution was concentrated to 2 ml and about 7 ml of
petroleum ether was allowed to diffuse through the dichlo-
romethane solution. Bright yellow colored shaped crystal
were obtained in about 10–12 h (yield: 91%). 1H NMR
4. Experimental
4.1. General remarks
Dichloromethane, diethyl ether, acetonitrile, petroleum
ether (b.p. 60–80 ꢁC) and benzene (Caution: benzene is car-
cinogenic and should be handled with extreme caution!)
were purified and dried under nitrogen atmosphere by con-
ventional methods [20]. Bis(diphenylphosphino)amine [21]
and bis(diphenylphosphino)isopropylamine [22] were syn-
thesized using existing procedures and were further purified
using column chromatography. [Cu(CH3CN)4]ClO4 [23],
[Cu(CH3CN)4]BF4 [24], CuCl [25] and CuBr [26] were pre-
pared by reported methods. All manipulations were carried
out under an atmosphere of purified N2 using a standard
double manifold and Schlenk ware.
i
(CDCl3, 25 ꢁC): d = 0.06, 0.50 (d, 6H, Me of Pr) (1:1),
i
3.57 (sep., 2H, br, –CH of Pr), 6.98–7.49 (m, 40H, Ph).
31P{1H} NMR: d = 77.1 (w), 55.9 (s). Elemental Anal. Calc.
for Cu2C54H54N2P4Cl2 Æ 0.5CH2Cl20.5H2O: C, 59.26; H,
5.07; N, 2.54. Found: C, 59.28; H, 5.06; N, 2.53%.
Complex 4: A similar procedure was adopted except
that CuBr (0.036 g, 0.25 mmol) was used for the reaction
that gave bright yellow single crystals (yield: 91%). 1H
i
NMR (CDCl3, 25 ꢁC): d = 0.05, 0.44 (d, 6H, Me of Pr)
i
(1:1), 3.54 (sep., 2H, br, –CH of Pr), 6.97–7.51 (m, 40H,
Ph). 31P{1H} NMR: d = 77.1 (w), 55.9 (s). Elemental Anal.
Calc. for Cu2C54H54N2P4Br2 Æ CH2Cl2: C, 53.84; H, 4.57;
N, 2.28. Found: C, 53.78; H, 4.69; N, 2.20%.
4.2. Physical measurements
1H NMR spectra were recorded either on a Bruker ACF
200 MHz or AMX 400 MHz spectrometer with tetra-
methylsilane (TMS) as the internal reference. 31P{1H}
NMR spectra were recorded either on a Bruker AMX
400 MHz spectrometer operating at 162.2 MHz or a Bruker
ACF 200 MHz operating at 81.1 MHz with H3PO4 (85%) as
the external reference. IR spectra were recorded in the solid
state as KBr pellets on an Equinox 55 Bruker spectrometer.
Elemental analyses were recorded with a Carlo Erba Ele-
mental Analyzer model 1106 and HRESMS on Micromass
Q-Tof micro. The conductivity measurements were carried
out with a Control Dynamics Conductivity Meter with
KCl (10ꢁ3 M) used as a standard solution.
4.4. X-ray crystallographic study
Single crystals of complex 1, 3 and 4 were mounted in a
Lindemann capillary with paraffin oil and crystal of com-
plex 2 was glued to the tip of a glass fiber along the largest
dimension and coated with paraffin oil. Data were collected
on a Bruker AXS single crystal diffractometer equipped
with SMART APEX CCD detector and a sealed Mo Ka
source working at 1.75 kW. Intensity data were collected
at room temperature. Crystallographic computations were
performed using the WINGX package [27]. The data were
corrected for Lorentz and polarization effects. Absorption
correction was applied using Psi-Scan [28] for complex 1,
3 and 4 and SADABS [29] for complex 2. The positions of
heavy atoms were determined by SHELXS86 [30]. The remain-
ing atoms were located from the difference Fourier map
using SHELXL-97 [31]. Hydrogen atoms were geometrically
fixed for all four complexes.
4.3. Synthesis of complexes
Complex 1: dppa (0.19 g, 0.5 mmol) was added to 15 ml
CH2Cl2 containing [Cu(CH3CN)4]ClO4 (0.16 g, 0.5 mmol)
and stirred for 1 h at room temperature. Subsequently the
solution was concentrated to about 7 and 15 ml of diethyl
ether was allowed to diffuse through the solution to obtain
Single crystal structure of complex 1 (C51H49Cl4Cu2-
N3O9P4): (0.42 · 0.40 · 0.38 mm3); triclinic; P1; a =
ꢀ
˚
1
˚
˚
colorless crystals (yield: 86%). H NMR (CDCl3, 25 ꢁC):
11.0536(16) A, b = 12.3546(18) A, c = 22.106(3) A, a =
78.332(3)ꢁ, b = 84.031(2)ꢁ, c = 68.189(2)ꢁ; V = 2743.5(7)
d = 1.98 (s, 3H, CH3CN), 4.13 (s, 2H, NH), 7.12–7.43 (m,
40H, Ph); 31P{1H} NMR (CDCl3): d = 46.3 (s); IR (KBr,
cmꢁ1): 2303 (w), 2270 (w) (CH3CN); 1099 (vs, br) 1064
(br) (ClO4ꢁ). Elemental Anal. Calc. for Cu2C50H45N3P4-
Cl2O8 Æ H2O Æ CH2Cl2: C, 49.15; H, 3.74; N, 3.16. Found:
C, 49.3; H, 3.95; N, 3.38%.
3
˚
A ; Z = 2;
q
calc. = 1.502 Mg/m3;
l ;
calc. = 1.143 mmꢁ1
R = 0.0594 for 10753 independent reflections with
(I > 2r(I)).
Complex 2: C54H51B2Cu2F8N5P4: (0.36 · 0.32 ·
3
˚
˚
ꢀ
0.20 mm ); triclinic; P1; a = 11.169(3) A, b = 12.349(4) A,
˚
Complex 2: A similar procedure was adopted except that
[Cu(CH3CN)4]BF4 (0.15 g, 0.5 mmol) was used for the reac-
tion which afforded colorless crystals (yield: 88%). 1H NMR
(CDCl3, 25 ꢁC): d = 1.98 (s, 9H, CH3CN), 4.63 (s, 2H, NH),
c = 22.083(7) A, a = 94.374(5)ꢁ, b = 95.555(5)ꢁ, c =
106.850(4)ꢁ; Z = 2;
q lcalc. = 0.914
calc. = 1.376 Mg/m3;
mmꢁ1; R = 0.0519 for 13298 independent reflections
(I > 2r(I)).

148
R. Ahuja et al. / Polyhedron 26 (2007) 142–148
[3] H. Schmidbaur, Gold Bull. 23 (1990) 11.
Complex 3: C55H56Cl4Cu2N2P4: (0.45 · 0.4 · 0.38 mm3);
[4] P. Pyykko¨, Y.-F. Zhao, Angew. Chem., Int. Ed. Engl. 30 (1991)
604.
˚
˚
monoclinic; P21/c; a = 21.042(10) A, b = 13.733(7) A, c =
˚
20.649(10) A, a = 90ꢁ, b = 114.365(8)ꢁ, c = 90ꢁ; V =
[5] P. Pyykko¨, J. Li, Chem. Phys. Lett. 197 (1992) 586.
[6] P. Pyykko¨, J. Li, Inorg. Chem. 32 (1993) 2630.
[7] O.D. Ha¨berlen, H. Schmidbaur, N. Rosch, J. Am. Chem. Soc. 116
(1994) 8241.
3
q lcalc. = 1.135
calc. = 1.390 Mg/m3;
˚
5435(5) A ; Z = 4;
mmꢁ1; R = 0.0502 for 12748 independent reflections with
(I > 2r(I)).
[8] P. Pyykko¨, Chem. Rev. 88 (1988) 563.
[9] M.A. Omary, R.R. Webb, Z. Assefa, G.E. Shankle, H.H. Patterson,
Inorg. Chem. 37 (1998) 1380.
Complex 4: C55H56Br2Cl2Cu2N2P4: (0.24 · 0.22 ·
0.21 mm3); monoclinic; P21/c; a = 21.287(3) A, b =
˚
˚
˚
13.678(2) A, c = 20.660(3) A, a = 90ꢁ, b = 114.071 (2)ꢁ,
[10] L.M. Engelhardt, G.E. Jacobsen, W.C. Patalinghug, B.W. Skelton,
C.L. Raston, A.H. White, J. Chem. Soc., Dalton Trans. (1991)
2859.
[11] J. Beck, J. Stra¨hle, Angew. Chem., Int. Ed. Engl. 24 (1985)
409.
c = 90ꢁ; V = 5492.5(15) A ; Z = 4; qcalc. = 1.483 Mg/m3;
3
˚
l
calc. = 2.481 mmꢁ1; R = 0.0545 for 13015 independent
reflections with (I > 2r(I)).
[12] K.M. Merz Jr., R. Hoffmann, Inorg. Chem. 27 (1988) 2120.
[13] J.T. Mague, J. Cluster Sci. 6 (1995) 217.
[14] F.H. Allen, O. Kennard, Chem. Des. Autom. News. 8 (1993)
31.
Acknowledgements
We acknowledge the financial support of CSIR, New
Delhi, DST for CCD facility, Bioinformatics, IISc for the
CSD and Prof. Dr. Frenking and Dr. Sivasankar for useful
discussions.
´
[15] H. Liu, M.J. Calhorda, M.G.B. Drew, V. Felix, J. Novosad, L.F.
Verios, F.F. de Biani, P. Zanello, J. Chem. Soc., Dalton Trans. (2002)
4365.
[16] J.K. Bera, M. Nethaji, A.G. Samuelson, Inorg. Chem. 38 (1999) 218.
[17] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81.
[18] H. Lang, M. Leschke, H.A. Mayer, M. Melter, C. Weber, G.
Rheinwald, O. Walter, G. Huttner, Inorg. Chim. Acta 324 (2001)
266.
Appendix A. Supplementary material
CCDC-200619, CCDC-200620, CCDC-191476, CCDC-
247449 contain the supplementary crystallographic data
for 1, 2, 3 and 4. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Center, 12 Union Road,
Cambridge CB2 1EZ, UK; fax (+44) 1223 336 033; or
e-mail: deposit@ccdc.cam.ac.uk).
[19] M. Leschke, H. Lang, M. Melter, G. Rheinwald, C. Weber, H.A.
Mayer, H. Pritzkow, L. Zsolnai, A. Driess, G. Huttner, Z. Anorg.
Allg. Chem. 628 (2002) 349.
[20] D.D. Perin, W.L.F. Armarego, Purification of Laboratory Reagents,
3rd ed., Pergamon Press, London, 1988.
[21] V.H. No¨th, L. Meinel, Z. Anorg. Allg. Chem. 5–6 (1967)
225.
[22] M.S. Balakrishna, T.K. Prakasha, S.S. Krishnamurthy, U. Siriwar-
dane, S.N.J. Hosmane, Organomet. Chem. 390 (1990) 203.
[23] B.J. Hathaway, D.G. Holah, J.D. Postlethwaite, J. Chem. Soc. (1961)
3215.
[24] G.J. Kubas, Inorg. Synth. 19 (1979) 90, and references therein.
[25] R.N. Keller, H.D. Wycoff, Inorg. Synth. 50 (1946) 1.
[26] R.N. Keller, H.D. Wycoff, Inorg. Synth. 50 (1946) 3.
[27] L.J. Farrugia, G.X. Win, J. Appl. Crystallogr. 32 (1999) 837.
[28] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr. Sect. A
24 (1968) 351.
ꢁ
Weak hydrogen bonding interactions of ClO4ꢁ and BF4
ions in complex 1 and 2, list of ‘‘refcodes’’ having Cu–Cu dis-
˚
tances (<2.8 A) with unsymmetrical coordination, histo-
gram of short contacts in CuL2 and CuL4, and references
dealing with apparent ‘‘cuprophilic’’ interactions. Supple-
mentary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2006.08.004.
[29] R.H. Blessing, Acta Crystallogr. Sect. A 51 (1995) 33.
[30] G.M. Sheldrick, SHELXS-86, Program for Crystal Structure Determina-
tion, University of Cambridge, Cambridge, England, 1986.
[31] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement,
University of Go¨ttingen, Germany, 1997.
References
[1] P. Pyykko¨, Chem. Rev. 97 (1997) 597.
[2] M. Jansen, Angew. Chem. 99 (1987) 1136.