Contrasting connectivity of amidine and phosphaamidine (1,3-P,N) Cu(I) complexes

Article abstract of DOI:10.1016/j.ica.2016.02.024

Aprotic acetamidine {Me₂N-C(Me)=N(R)(CuBr)}₂ {R = ⁱPr (2a), Cy (2b), Ph (2c)} and neutral 1,3-P,N phosphaamidine copper bromide complexes {(CuBr)(Ph₂PC(Ph)=NPh)}₄ (6a), {(CuBr)₄(Ph₂PC(Ph)=NⁱPr)₂}·{2CH₂Cl₂} (6b), (CuBr)₄(ⁱPr₂PC(Ph)=NPh)₂} (7a) and {(CuBr)₄(ⁱPr₂PC(Ph)=NⁱPr)₂} (7b) are prepared by direct combination of corresponding amidine and phosphaamidine with CuBr(DMS). X-ray crystallographic analysis of the acetamidine complexes reveal monodentate N_imine coordination to copper with significant degree of delocalization about the N-C=N framework and a relatively short Cu-Cu interaction of 2.5758(8) ? in 2c compared to (2.9801)(10) ? 2a. The phosphaamidine ligands are never η¹-N_imine bound; instead they are η¹-P bound in the cubane complex 6a or η²-bound in the step cluster complexes 6b, 7a and 7b.

Full text of DOI:10.1016/j.ica.2016.02.024

Accepted Manuscript
Contrasting Connectivity of Amidine and Phosphaamidine (1,3-P,N) Cu(I)
complexes
Ramjee Kandel, Keith Huynh, Lauren Dalgliesh, Ruiyao Wang, Philip G. Jessop
PII:
S0020-1693(16)30040-8
http://dx.doi.org/10.1016/j.ica.2016.02.024
ICA 16894
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
10 January 2016
9 February 2016
13 February 2016
Please cite this article as: R. Kandel, K. Huynh, L. Dalgliesh, R. Wang, P.G. Jessop, Contrasting Connectivity of
Amidine and Phosphaamidine (1,3-P,N) Cu(I) complexes, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/
10.1016/j.ica.2016.02.024
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Contrasting Connectivity of Amidine and Phosphaamidine (1,3-P,N) Cu(I) complexes
Ramjee Kandel, Keith Huynh, Lauren Dalgliesh, Ruiyao Wang, and Philip G. Jessop*
Department of Chemistry, Queen's University, Kingston, ON, Canada, K7L 3N6
* Corresponding Author E-mail for P.G. Jessop: jessop@chem.queensu.ca
A B S T R A C T
i
Aprotic acetamidine {Me2N‒C(Me)=N(R)(CuBr)}2 {R = Pr (2a), Cy (2b), Ph (2c)} and neutral 1,3-P,N phosphaamidine copper
bromide complexes {(CuBr)(Ph2PC(Ph)=NPh)}4 (6a), {(CuBr)4(Ph2PC(Ph)=NiPr)2}{2CH2Cl2} (6b), (CuBr)4 (iPr2PC(Ph)=NPh)2}
(7a) and {(CuBr)4(iPr2PC(Ph)=NiPr)2}(7b) are prepared by direct combination of corresponding amidine and phosphaamidine with
CuBr(DMS). X‒ray crystallographic analysis of the acetamidine complexes reveal monodentate Nimine coordination to copper with
significant degree of delocalization about the N‒C=N framework and a relatively short Cu‒Cu interaction of 2.5758(8) Å in 2c
compared to (2.9801)(10) Å 2a. The phosphaamidine ligands are never η1‒Nimine bound; instead they are η1‒P bound in the cubane
complex 6a or η2‒bound in the step cluster complexes 6b, 7a and 7b.
Keywords: Amidines, Phosphaamidines, Hemilabile ligands, Copper(I) complexes
1. Introduction
Because there might be significant advantages to
Nitrogen-based ligands have played an integral role
in the development of coordination chemistry, with a historical
focus on pyridine-derived ligands [1]. Pyridine-based ligands
are attractive for coordination chemistry as a result of their
structural diversity and substituent tenability [2]. Amidines (A
in Scheme 1) are another class of nitrogen‒based ligands, with
a similar degree of electronic and steric tunability, but they
have received far less attention [3]. The most widespread
application of amidine-derived ligands in transition metal [4]
and main group chemistry [5] are the anionic amidinates (B)
rather than the neutral amidines themselves.
having the Nimine atom available for ligand‒based reactivity,
one might prefer that atom to be either weakly bound to the
metal (e.g. in an η2‒binding geometry) or unbound (e.g. in an
η1‒Namine), we chose to also explore the chemistry of acyclic
phosphaamidines, in which a phosphorus atom replaces the
Namine (C, Scheme 1). Phosphaamidines have been made by the
reaction of acid chloride (N-phenyl imide chloride of benzoic
acid) with MePR'2 (R' = C6H5) to give RC(NR)-PR'2 [19]. A
few examples of such 1,3-P,N ligands have been prepared by
reaction of imidoyl chloride or N‒aryl formimidates with alka-
li metal phosphide [20] or silylphosphanes [21]. Recently,
Lammertsma and his groups prepared phosphaamidines using
nitrilium ions as an imine synthon [22], while different routes
for preparation of phosphaamidines are being explored, the
coordination chemistry of such ligands with transition metals
have received little attention [22a,22c,23].
Research in our laboratory has exploited the basicity
of amidines in reactions with CO2, in aqueous or alcoholic
media, for the generation of amidinium bicarbonate/carbonate
salts [6] for applications as ionic liquids [7] and surfactants
[8]. We have also explored the use of amidines as promoters
of CO2 fixation to other products [9] and thus a natural exten-
sion of our research program leads to the preparation of
amidine-metal complexes.
The substitution of Namine of amidine with phospho-
rus should encourage transition metal binding to the P atom,
leaving the Nimine atom either unbound or weakly coordinated.
The anticipated advantages of a free or hemilabile Nimine atom
include its Brønsted basicity for proton transfer and its
nucleophilicity for CO2 binding. While those aspects are cur-
rently being explored, the subject of this paper is a comparison
of the binding geometries of aprotic acyclic amidines and
phosphaamidines, to determine whether a soft metal such as
Cu(I) would bind exclusively to the phosphorus atom, leaving
the basic Nimine atom free, or whether the Nimine would also be
bound. In this study, we restrict the discussion to acyclic and
aprotic amidines and phosphaamidines and their copper(I)
complexes.
Scheme 1. Amidines, amidinate anion and phosphaamidine
A modest number of examples of cyclic amidine-
metal complexes, such as η1‒Nimine complexes of imidazole
[10], DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) [11], and
DBN (1,5-diazabicyclo[4.3.0]non-5-ene) [12], exist in the
literature. In addition, examples involving bidentate chelating
amidines as ligands, either as a bisamidine [13], or pyridyl
substituted amidine [14] have been reported. However, acyclic
amidine-metal complexes are comparatively scarce. Complex-
es of formamidine [15,16], benzamidine [15c,17], and protic
acetamidines [18] with η1‒Nimine have been observed but no
examples of aprotic acetamidines complexes (aprotic in the
sense of lacking an N‒H bond) have been reported so far to
the best of our knowledge.
2. Results and discussion
2.1. Synthesis of amidine Cu(I) Complexes
The coordination chemistry of a protic acyclic amidine with
Cu(I) has been studied before. Coles and co-workers reported
the preparation of a N,N'‒diphenylbenzamidine copper com-
plex [CuCl(PhC{NPh}{NHPh})2]2 via direct combination of
the benzamidine ligand with CuCl in THF solution [17]. Each
Cu(I) is coordinated with the Nimine atoms of two benzamidine
ligands with the ligand coordination supported by the intermo-

lecular hydrogen bonding of the Namine‒H proton to bridging
chloride atoms.
We studied amidine Cu(I) complexes but using
amidines that contain no N‒H bonds. The binding geometry is
affected by this change because intermolecular hydrogen
bonding is prevented. The direct combination of a CH2Cl2
solution of acetamidine 1a–b with an equimolar suspension of
CuBr·dimethylsulfide (DMS) in CH2Cl2 at ambient tempera-
ture (Scheme 2) results in the gradual formation of a clear
orange solution within 30 min. Removal of all volatiles from
the reaction solution yields an orange oil (an orange foam (R =
1
iPr (2a), Cy (2b)). H NMR spectroscopic characterization, in
CDCl3, of the crude material reveals downfield shifts for the
diagnostic N(CH3)2 and N–C(CH3)=N protons, in addition to
downfield shifts for the R substituent protons compared to the
free ligand.[Supporting information (SI)] Crystals of 2a were
grown via Et2O vapour diffusion into a saturated CH2Cl2 solu-
tion at 15 °C and its molecular structure was determined by a
single‒crystal X‒ray diffraction study (Figure 1, crystallo-
graphic data in the SI).
Figure 1. Molecular structure of 2a (top) and 2c (below)
showing displacement ellipsoids at the 50% probability level.
The hydrogen atoms have been omitted for clarity. Symmetry
transformations used to generate equivalent atoms for 2a: *: 1
- x, -y, -z and 2c: *: 1 - x, 1 - y, 1 - z.
Scheme 2. Synthesis of acetamidine Cu(I) Complexes
Direct combination of acetamidine 1c with
CuBr(DMS) in CH2Cl2 did not result in an observable reaction
as was observed for 1a-b, even at elevated temperatures (40
C) because of the low basicity of 1c (pKaH 8.3) compared to
1a–b (pKaH 12.3–12.5) [24]. Product 2c was prepared under
neat conditions at an elevated temperature with addition of
twelve equivalents of 1c to CuBr(DMS), which gradually
formed a yellow solution after 2 h at 55 °C. 1H NMR spectros-
copy of the crude material suggested coordination of 1c to
copper from the diagnostic downfield shifts compared to the
free ligand, although the magnitude of the shifts were smaller
than those observed for 2a–b. 2c crystals were grown and
subjected to a single-crystal X‒ray diffraction study (Figure 1,
crystallographic information in SI).
amine (~1.46 Å) and a C=N double bond of an imine (~1.28
Å). A value of CN = 0 Å indicates a fully delocalized system
whereas a value of ~0.18 Å indicates a fully localized system
[21]. For 2a the Nimine– C(4) and Namine–C(4) bond lengths are
similar at 1.296(4) Å and 1.352(5) Å respectively with a CN
of only 0.056 Å, which indicates a significant degree of delo-
calization. The sum of bond angles around the amine nitrogen
is 360, i.e. a planar nitrogen, and thus further supports the
description of a delocalized system in the N–C=N framework.
The sum of bond angles around the imine N (N1) is also 360˚,
showing that the Cu is coplanar with the amidine, consistent
with a sigma coordination.
The molecular structure of 2c is analogous to that of
The molecular structure of 2a (Figure 1) exists as a
2a. The Cu–Cu distance is shorter at 2.5758(8) Å, with a
Nimine–Cu bond length of 1.971(3) Å. The Nimine– C(7) and
Namine–C(7) bond lengths are again similar at 1.301(4) Å and
1.353(4) Å respectively with a CN of only 0.052 Å. With a
sum of bond angles of 359, the amine nitrogen again adopts a
planar geometry, thus indicating a significant degree of delo-
calization. Again, the Cu is coplanar with the amidine. While
the molecular structures of 2a and 2c are similar, it is interest-
ing to note that they are appreciably different from the analo-
,-dibromobridged dimer with a Nimine–Cu bond length of
1.968(3) Å. No close contact is present between the amine
nitrogen and Cu. The Cu–Cu distance is 2.9801(10) Å, which
is greater than the sum of the covalent radii of two Cu atoms
(2.34 Å) [25], indicating the absence of a Cu–Cu bond. The
extent of delocalization within the N–C=N framework can
approximately be described by CN, where CN equals the
bond length difference between a C–N single bond of an
2

gous benzamidine copper chloride [17], where binding appears
to be supported by intramolecular hydrogen bonding between
the amidine proton and the bridging halide atoms; an interac-
tion which is not possible with our aprotic amidines.
2.2. Synthesis of phosphaamidines
The phosphaamidines were prepared from imidoyl
chloride (3a, 3b) which was itself prepared from benzoyl chlo-
ride [26]. A lithiated secondary phosphine (Ph2PLi or Pr2PLi)
Scheme 4. Cubane (left) and step (right)
i
[27], was reacted with imidoyl chloride [PhC(Cl)NR'] (R' = iPr
or Ph) in situ or separately to give phosphaamidines (Scheme
3). All the phosphaamidines were obtained in pure form with
little purification required, in 35 to 88% yield.
Phosphaamidine, N‒[(diphenylphosphino)phenylmethylene]
benzenamine (Ph2PC(Ph)=NPh) 4a which has been reported
before [19], was prepared by using lithiated diphenyl phos-
phine instead of the published reagent methyl diphenyl phos-
Figure 2. Molecular structure of 6a showing displacement
phine.
isopropylamine
4a,
N‒[(diphenylphosphino)phenylmethylene]
(Ph2PC(Ph)=NiPr)
(4b), and
N‒[(diisopropylphosphino)phenylmethylene]benzenamine
(iPr2PC(Ph)=NPh) (5a) were obtained in solid form while
N‒[(diisopropylphosphino)phenylmethylene]isopropylamine
(iPr2PC(Ph)=NiPr) (5b) was obtained as a viscous liquid.
Compound 4b has been reported recently by the reaction of
diphenylphosphane
with
either
pyridine
or
dimethylaminopyridine (DMAP) adduct and is obtained in the
form/mixture of E and Z-conformers [22b]. 31P{1H} NMR
spec trum at 6.95 ppm and 8.68 ppm shows two types of
conformer. The identity of each product was confirmed by 1H,
13C, 31P{1H} NMR, EI‒MS and EI‒HRMS (SI)
ellipsoids at the 30% probability level. The hydrogen atoms
have been omitted for clarity. Symmetry transformations used
to generate equivalent atoms: *: ¼ - y, ¼ + x, ¼ -z; ‘: -x, ½ -y,
-z; #: -1/4 + y, ¼ - x, ¼ -z.
The coordination geometry of complex 6a is signifi-
cantly different from that of acetamidine complexes 2a and 2c;
the amidine complexes bind η1 by the basic Nimine atom, while
the phosphaamidine in 6a binds η1 by the P‒phosphine, leav-
ing the basic Nimine atom dangling free. A Au(I) phosphanyl
trimer with core has been reported with a phosphaamidine
coordinated leaving the Nimine free [22], the core of 6a is a
Cu4Br4 cubane. Other phosphorus ligands like PPh3 when re-
acted with copper halide are known to form cubane like [28]
or step like [29] cores (Scheme 4), with the type of core de-
pending on the conditions, the metal to phosphine ratio, or the
steric properties of the ligand used [30]. The obtained Cu4Br4
core is distorted with Br‒Cu‒Br angles of 100.981(14)° and
101.24(1)° and Cu‒Br‒Cu angles of 77.81(1)° and 77.98(1)°.
The bond lengths and angles are shown in the SI. The failure
of the Nimine atom to bind to the metal may be due either to
steric factors (which would not be surprising considering the
large number of phenyl groups around the core) or to the poor
basicity of the phenyl-substituted Nimine atom. To explore this
Scheme 3. Synthesis of phosphaamidine ligands
2.3. Phosphaamidine Cu(I) complexes
CuBr(DMS) was reacted with phosphaamidines to
compare the coordination modes of the phosphaamidines to
those of the amidines. Compound 6a was prepared by the re-
action of a suspension of CuBr(DMS) in CH2Cl2 with a yellow
solution of 4a in CH2Cl2. A yellow crystalline compound was
obtained at 15 °C by slow evaporation of ether into
CH2Cl2/toluene. The crystal is tetragonal with space group
I41/a. The molecular structure consists of a Cu4Br4 cubane
core (Scheme 4) with only the phosphaamidine phosphorus
coordinated to the Cu(I) leaving the Nimine uncoordinated (Fig.
2). Crystallographic information is given in the SI.
3

further, we made related complexes with other phosphaamidi-
nes.
On reaction of 4b, 5a and 5b with CuBr(DMS) in
CH2Cl2 (Scheme 5), isostructural crystals were obtained with
Cu4Br4 step core structures, rather than cubane cores. Com-
pound 6b cocrystallized with two molecules of CH2Cl2 in the
unit cell.
6b
Scheme 5. Synthesis of copper(I) complexes with 4b, 5a
and 5b
The molecular structures of 6b, 7a and 7b are shown in Fig. 3
and the crystallographic information is given in SI. Such step
type structures are common for CuCl or CuBr complexes with
bidentate ligands [31] but have not previously been reported
for phosphaamidine complexes. The core can be denoted as
Cu4(µ‒Br)2(µ3‒Br)2, where P and three Br atoms coordinate
the inner Cu atoms in a distorted tetrahedral geometry and N
and two Br atoms coordinate to the outer Cu atoms in a dis-
torted trigonal structure. The four Cu atoms lie in a plane
while Br atoms above and below the plane. The Cu‒N bond
lengths range from 1.954 to 1.970 Å, similar to amidine
complexs 2a (1.968 Å ) and 2c (1.971 Å). However, the C=N
bond is slightly longer in η1‒amidines complexes (1.296 Å in
2a and 1.301 Å in 2c) than in the η2‒bound phosphaamidine
complexes (1.281‒1.293 Å) and especially the η1-
phosphamidine complex 6a (1.268(3) Å), where Nimine is free
and sp2 hybridized.
7a
7b
Figure 3. Molecular structure of 6b,7a and 7b showing dis-
placement ellipsoids at the 50% probability level. Solvent
molecules in 6b are removed for clarity. The hydrogen atoms
have been omitted for clarity. Symmetry transformations used
to generate equivalent atoms 6b: *: 1 - x, -y, 1 – z, 7a: *: 1 - x,
1 - y, 1 – z, and 7b: *: 2 - x, 1 - y, 1 - z.
The basicity of the Nimine atom in acetamidines played a sig-
nificant role in the binding of Nimine with Cu(I) with 1c. How-
ever, the failure of the basic Nimine to bind to the metal in the
η1‒P phosphaamidine complex 6a appears to be the result of
the steric bulk of the phenyl rings in 4a rather than or in addi-
tion to the weak basicity of the Nimine. If weak basicity alone
were the cause then η2‒coordination would not have occurred
with the similarly weak basic Nimine in complex 7a.
3. Conclusions
A series of aprotic and acyclic amidine and
phosphaamidine Cu(I) bromide complexes have been pre-
pared. Acetamidine ligands 1a and 1c coordinate to Cu
through solely the Nimine. The ease of complex formation is
dependent on the basicity of the acetamidine ligand under
consideration. X‒ray diffraction studies reveal a significant
degree of delocalization about the N–C=N framework. The
lack of an N–H proton in these aprotic ligands prevents them
from coordinating to copper with the same binding geometry
4

as a protic amidine ligand in an analogous copper complex
reported previously [17]. Structurally similar, neutral
phosphaamidine ligands coordinate η1‒P with Cu(I)Br to form
cubane Cu4Br4 complex or η2‒P,N to form step Cu4Br4 com-
plexes. Phosphaamidine 4a coordinated by the P atom alone,
leaving Nimine free, because of the bulky phenyl group. This
steric effect is reduced in other phosphaamidines so that they
coordinate through both sites to form step structures. The co-
ordinating ability of these ligands to other metals ions and
catalytic properties of such complexes are currently under
exploration.
amount of acetamidine (1a and 2b) dissolved in CH2Cl2. Typi-
cal reaction scale was approximately 10 mmol with an overall
concentration of approximately 0.01 M. After the mixture was
stirred for 2 h, all volatiles were removed from the resulting
orange solution. The residue was isolated and purified by pre-
cipitation induced by addition of a 5 mL CH2Cl2 solution into
rapidly stirred hexanes to obtain compounds 2a and 2b.
Me2NC(Me)=NiPr·CuBr (2a). Yellow powder. Yield = 1.84 g,
3
68%. 1H NMR (CDCl3): δ = 1.35 (6H, d, JHH = 6.3 Hz,
C(CH3)2), 2.11 (3H, s, CCH3), 3.31 (6H, s, N(CH3)2), and 3.66
ppm (1H, sept, 3JHH = 6.3 Hz, NCH). 13C{1H} NMR (CDCl3):
δ = 15.4, 26.4, 40.6, 51.6, and 163.3 ppm. IR (nujol): 1640
4. Experimental
All reactions were carried out with the exclusion of
air and moisture under an atmosphere of pre-purified nitrogen
or argon (Praxair), unless otherwise noted, using standard
Schlenk techniques or an inert atmosphere glovebox (Vacuum
Atmospheres Company). Acetamidines were prepared accord-
ing to the literature method [24,25]. N-phenylbenzimidoyl
chloride and N-isopropylbenzimidoyl chloride were prepared
according to the literature method [26]. CuBr(DMS) was pur-
chased from Sigma-Aldrich and used as received. CH2Cl2 and
CDCl3 were dried by refluxing over CaH2 for 4 h, followed by
distillation prior to use. Et2O and hexanes were dried by re-
fluxing over sodium ketyl radical for 4 h followed by distilla-
cm-1 (C=N).
EI-MS (70 eV, m/z, %): 267.27 ([M+]-
C7H16BrCuN2, 10%), 128.13 (C7H16N2, 100%).
Me2NC(Me)=NCy·CuBr (2b). Yellow powder. Yield = 1.90 g,
61%. 1H NMR (CDCl3): δ = 1.28 (4H, m, CH2CH2CH), 1.65
(4H, m, CH2CH2CH2), 1.81 (2H, m, CH2(CH2)2), 2.20 (3H, s,
CCH3); 3.20 (1H, m, CH(CH2)2), and 3.31 ppm (6H, s,
N(CH3)2). 13C{1H} NMR (CDCl3): δ = 15.6, 25.0, 25.4, 37.4,
40.7, and 60.4 ppm. IR (nujol): 1638 cm-1 (C=N). EI-MS (70
eV, m/z, %): 551.67 ([M+]-C6H11, 5%), 169.16 (C10H20N2,
100%).
1
tion prior to use. H and 13C{1H} NMR spectra were recorded
on a Bruker Avance 300 spectrometer (300 and 75 MHz re-
spectively) and referenced to protic impurities (CHCl3) in the
solvent (1H) or externally to Si(CH3)4 (13C{1H}). The amidine
complexes and phosphaamidine complexes are unstable if
exposed to air.
4.3. Me2NC(Me)=NPhCuBr (2c). Copper bromide dimethyl
sulfide (0.50 g, 2.43 mmol) and N-phenyl-N',N'-
dimethylacetamidine 1c (5.0 g, 0.031 mol) were combined in a
glass pressure tube and heated for 2 h at 55 C. The resulting
yellow solution was added dropwise to hexanes yielding a
beige precipitate, which was then subsequently filtered. The
crude material was washed with 3 x 10 mL of hexanes and
then dried in vacuo. Yield = 0.50 g (67.3%). Crystals of 2c
were grown by vapour diffusion of Et2O into a saturated
4.1 X-ray diffraction Studies: A crystal of each compound
suitable for single–crystal X-ray diffraction was mounted on a
glass fiber with grease and cooled to 93 °C in a stream of
nitrogen gas controlled with Cryostream Controller 700. Data
collection was performed on a Bruker SMART APEX II X-ray
diffractometer with graphite-monochromated Mo Kα radiation
( = 0.71073 Å), operating at 50 kV and 30 mA over 2θ rang-
es of 3.68 ~ 52.00º. No significant decay was observed during
the data collection. Data were processed on a PC using the
Bruker AXS Crystal Structure Analysis Package [32]. Data
collection: APEX2 [32a]; cell refinement: SAINT [32b]; data
reduction: SAINT [32b]; absorption correction: SADABS
[32c]; structure solution: XPREP [32d]; and SHELXTL [32e];
structure refinement: SHELXTL; molecular graphics:
SHELXTL; publication materials: SHELXTL. Neutral atom
scattering factors were taken from Cromer and Waber [33].
The structure was solved by direct methods. Full-matrix least-
1
CH2Cl2 solution. H NMR (CDCl3): δ = 1.90 (3H, s, CCH3),
3.23 (6H, s, N(CH3)2), 6.84 (2H, d, ArH), 7.05 (1H, t, ArH),
and 7.28 ppm (2H, t, ArH). 13C{1H} NMR (CDCl3): δ = 16.1,
39.3, 123.3, 123.6, 129.2, 153.0, and 163.7 ppm. IR (nujol):
1576 cm-1 (C=N). EI-MS (70 eV, m/z, %): 304.58 ([M+]-
C10H14BrCuN2, 5%), 162.11 (C10H14N2, 75%).
4.4. Preparation of phosphaamidines 4a and 4b: A Et2O solu-
tion of Ph2PLi was prepared by the addition of a 1.6 M hex-
n
anes solution of BuLi (4.3 mL, 6.88 mmol) to a clear and
colourless Et2O (80 mL) solution of Ph2PH (1.26 g, 6.77
mmol) at 0 C. The resulting yellow solution was stirred for 1
h at 0 C, then warmed to ambient temperature and stirred for
an additional 1 h. To this yellow solution was slowly added an
Et2O (20 mL) solution of N-phenylbenzimidoyl chloride (1.46
g, 6.77 mmol) or N-isopropylbenzimidoyl chloride (1.23 g,
6.77 mmol) at 78 C separately to obtain compound 4a or 4b
respectively. The yellow mixture was allowed to slowly warm
to ambient temperature then stirred for an additional 18 h. The
resulting pale yellow suspension was reduced to dryness in
vacuo and the residue was extracted with refluxing hexanes
(200 mL) for 2 h. After slowly cooling to ambient tempera-
ture, the mixture was filtered and the yellow filtrate was con-
centrated to incipient crystallization. Pale yellow crystalline
2
square refinements minimizing the function w (Fo – Fc2)2
were applied to the compound. All non-hydrogen atoms were
refined anisotropically. All H atoms bound to carbon atoms
were placed in geometrically idealized position using the ap-
propriate HFIX instructions in SHELXL. The isotropic ther-
mal parameters of these hydrogen atoms were fixed at 1.2
(phenyl-CH, ipso-CH) and 1.5 (CH3 groups) that of the pre-
ceding carbon.
4.2. Preparation of 2a and 2b. To a suspension of copper bro-
mide dimethyl sulfide in CH2Cl2 was added an equimolar
5

compounds of 4a (44%) and 4b (35%) were grown from the
concentrated hexanes solution at -30 C.
Hz), 22.2 (d, J = 14.0 Hz), 23.3, 24.0, 53.4, 127.0, 128.1,
129.8, 140.0, and 172.6 ppm (d, J = 17.8 Hz). EI-MS (70 eV,
m/z, %): 263.18 ([M+], 4%), 104.09 (C7H6N, 100%). EI-
HRMS (C16H26NP): 263.1803 (calcd.), 263.1795 (found).
Ph2PC(Ph)=NPh (4a). Yellow solid. Yield = 1.09 g, 44%. 31
P
1
NMR (CDCl3): δ = 9.0 ppm. H NMR (CDCl3): δ = 6.72 (m,
2H), 6.80 (m, 1H), 6.93 (m, 1H), 7.11-7.14 (m, 6H), 7.24 (m,
1H), 7.34 (m, 6H), and 7.60 ppm (m, 3H). 13C NMR (CDCl3):
δ = 119.3, 120.7, 123.3, 123.6, 127.7, 128.4, 128.8, 129.1,
134.2, 135.0 (d, J = 19.2 Hz), 151.6, and 178.9 ppm. EI-MS
(70 eV, m/z, %): 365.14 ([M+], 100%). EI-HRMS (C25H20NP):
365.1333 (calcd.), 365.1325 (found).
4.6. General preparation of 6a and 6b. To a suspension of
CuBr(DMS) (0.256 g, 1.24 mmol) in CH2Cl2 (5 mL) was add-
ed dropwise a CH2Cl2 (5 mL) solution of either 4a (0.455 g,
1.25 mmol) or 4b (0.209 g, 0.63 mmol). The solids in the ini-
tially beige suspension slowly dissolved into solution after 1 h
yielding a burnt orange solution. All volatiles from the reac-
tion solution were removed in vacuo to afford orange foam.
The remaining solids were washed with toluene (2 x 2 mL)
and dried under vacuum to obtain 6a or 6b in 72% or 83%
yield, respectively. Yellow crystals were grown from Et2O
vapour diffusion into a saturated 1:1 (vol.) CH2Cl2/toluene
solution at 15 C.
Ph2PC(Ph)=NiPr (4b). Yellow solid. Yield = 0.78 g, 35%. 31P
1
NMR (CDCl3): δ = 6.95, 8.68 ppm. H NMR (CDCl3): δ =
1.22 (d, J = 6.1 Hz, 6H), 3.61 (sept, J = 6.1 Hz, 1H), 6.81 (m,
2H), 7.03 (m, 2H), 7.14 (m, 3H), 7.30 (m, 9H), and 7.34 ppm
(m, 4H). 13C NMR (CDCl3): δ = 23.0, 23.7, 55.6, 126.3, 127.4,
127.7 (d, J = 7.1 Hz), 128.0 (d, J = 6.9 Hz), 128.4 (d, J = 6.8
Hz), 128.6 (d, J = 4.5 Hz), 133.5 (d, J = 19.2 Hz), 134.4 (d, J
= 18.9 Hz), and 174.7 ppm. EI-MS (70 eV, m/z, %): 331.15
([M+], 1%), 146.09 ([M+]-PPh2, 100%). EI-HRMS
(C22H22NP): 331.1490 (calcd.), 331.1502 (found).
[Ph2PC(Ph)=NPh·CuBr]4 (6a). Orange foam. Yield = 0.45 g,
72%. 31P NMR (CDCl3): δ = 10.8 ppm. 1H NMR (CDCl3): δ =
6.70 (m, 2H), 6.91-6.96 (m, 5H), 7.07-7.09 (m, 2H), 7.28-7.41
(m, 6H), and 7.61-7.72 ppm (m, 5H). 13C NMR (CDCl3): δ =
122.2, 125.8, 123.3, 127.6, 128.1, 128.5, 128.8, 128.9, 129.9,
130.9, 134.4 (d, J = 14.8 Hz), 149.1, and 180.9 ppm.
4.5. Preparation of phosphaamidines 5a and 5b: A Et2O solu-
tion of iPr2PLi was prepared from the addition of a 1.6 M hex-
[(Ph2PC(Ph)=NiPr·(CuBr)2]2 (6b). Orange foam. Yield = 0.36
g, 83%. 31P NMR (CDCl3): δ = 8.7 ppm. 1H NMR (CDCl3): δ
= 1.39 (d, J = 5.6 Hz, 6H), 3.85 (sept, J = 6.1 Hz, 1 H), 6.76
(br, 2H), 7.09 (m, 2H), 7.16 (m, 4H), 7.30 (m, 2H), and 7.47
ppm (m, 5H). 13C NMR (CDCl3): δ = 23.9, 57.4, 126.2, 128.1,
128.4, 129.2, 130.1, 132.1, 134.4 (d, J = 49.6 Hz), and 172.7
ppm. ESI-HRMS (C44H44Br3Cu4N2P2): 1156.7665 (calcd.),
1156.7542 (found).
n
anes solution of BuLi (4.3 mL, 6.88 mmol) to a clear and
i
colourless Et2O (80 mL) solution of Pr2PH (10 mL, 6.77
mmol) (10 wt% in hexane) at 0 C. The resulting yellow solu-
tion was stirred for 1 h at 0 C, then warmed to ambient tem-
perature and stirred for an additional 1 h. To this yellow solu-
tion was slowly added an Et2O (20 mL) solution of N-
phenylbenzimidoyl chloride (1.46 g, 6.77 mmol) or N-
isopropylbenzimidoyl chloride (1.23 g, 6.77 mmol) at 78 C.
The yellow mixture was allowed to slowly warm to ambient
temperature then stirred for an additional 18 h. The resulting
solution was reduced to dryness in vacuo and the residue was
dissolved in minimum CH2Cl2 and then filtered. The filtrate
was dried to obtain white solid compound 5a and red oily
compound 5b in good yield.
4.7. General preparation of 7a and 7b. To a suspension of
CuBr(DMS) (0.655 g, 3.18 mmol) in CH2Cl2 (40 mL) was
added a solution of either 5a (0.472 g, 1.58 mmol) or 5b
(0.415 g, 1.58 mmol) separately. The resulting orange mixture
was stirred for 1 h then filtered through Celite, and the filtrate
was then reduced to dryness to yield an orange residue. The
residue was taken up in CH2Cl2 (10 mL) then precipitated into
hexanes (50 mL). Orange solid was obtained following filtra-
tion to obtain 7a or 7b at 63% or 43%, respectively. Single
crystals of 7a were grown from Et2O vapour diffusion into a
saturated CH2Cl2 solution of 7a at 18 C, while crystals of 7b
were obtained by slow evaporation of a CH2Cl2 solution at
room temperature
iPr2PC(Ph)=NPh (5a). White solid. Yield = 1.44 g, 72%. 31P
1
NMR (CDCl3): δ = 27.7 ppm. H NMR (CDCl3): δ = 1.04 (m,
6H), 1.23 (m, 6H), 2.22 (d of sept, J = 3.0 and 7.1 Hz, 2H),
6.66 (d, J = 7.5 Hz, 2H), 6.92 (t, J = 7.1 Hz, 1H), 7.15 (t, J =
7.6 Hz, 1H), 7.22 (br, 5H), and 7.49 ppm (m, 1H). 13C NMR
(CDCl3): δ = 19.0 (d, J = 10.8 Hz), 20.3 (d, J = 11.9 Hz), 22.7
(d, J = 12.6 Hz), 120.4, 122.8, 127.9, 128.4, 128.7, 128.8,
129.4, 139.3, 139.7, 152.2 (d, J = 3.8 Hz), and 178.8 ppm (d, J
= 21.3 Hz). EI-MS (70 eV, m/z, %): 297.17 ([M+], 6%),
180.11 ([M+]-PiPr2, 100%). EI-HRMS (C19H24NP): 297.1646
(calcd.), 297.39 (found).
[(iPr2PC(Ph)=NPh·(CuBr)2]2 (7a). Orange solid. Yield = 0.58
1
g, 63%. 31P NMR (CDCl3): δ = 35.5 ppm. H NMR (CDCl3):
δ = 1.34 (d, J = 7.0 Hz, 6H), 1.39 (d, J = 6.9 Hz, 6H), 2.38 (m,
2H), 6.84 (d, J = 7.5 Hz, 2H), 6.96 (t, J = 7.4 Hz, 1H), 7.13
(m, 4H), and 7.15 ppm (m, 3H). 13C NMR (CDCl3): δ = 19.4
(d, J = 8.3 Hz), 20.1 (d, J = 5.1 Hz), 25.1 (d, J = 13.9 Hz),
121.8, 125.2, 127.6, 128.6, 128.8, 129.2, 136.5 (d, J = 11.3
Hz), 149.5 (d, J = 16.2 Hz), and 179.1 ppm. ESI-HRMS
(C38H48Br3Cu4N2P2): 1088.7975 (calcd.), 1088.7880 (found).
iPr2PC(Ph)=NiPr (5b). Red oil. Yield = 1.56 g, 88%. 31P NMR
(CDCl3): δ = 26.0 ppm. 1H NMR (CDCl3): δ = 0.95 (m, 6H),
1.10 (m, 12H), 2.04 (d of sept, J = 3.4 and 7.1 Hz, 2H), 3.65
(sept, J = 6.2 Hz, 1H), 7.18 (m, 2H), and 7.33 ppm (m, 3H).
13C NMR (CDCl3): δ = 18.8 (d, J = 9.9 Hz), 18.9 (d, J = 11.4
6

[(iPr2PC(Ph)=NiPr·(CuBr)2]2 (7b). Orange solid. Yield = 0.37
g, 43%. 31P NMR (CDCl3): δ = 35.2 ppm. 1H NMR (CDCl3):
δ = 1.32 (d, J = 6.9 Hz, 3H), 1.34 (d, J = 6.9 Hz, 3H), 1.37 (d,
J = 6.9 Hz, 3H), 1.40 (d, J = 6.9 Hz, 3H), 1.44 (d, J = 6.2 Hz,
6H), 2.17 (m, 2H), 3.89 (sept, J = 6.2 Hz, 1H), 7.13 (m, 2H),
and 7.50 ppm (m, 3H). 13C NMR (CDCl3): δ = 19.9 (d, J = 8.7
Hz), 20.0 (d, J = 5.3 Hz), 24.8 (d, J = 11.8 Hz), 58.4 (d, J =
13.4 Hz), 124.8, 125.2, 128.8, 129.4, 136.2 (d, J = 7.4 Hz),
Cunningham, Macromolecules 44 (2011) 2501-2509; c) M.
Mihara, M. Cunningham, P. Jessop, Macromolecules 44 (2011)
3688-3693; d) C. Liang, J.R. Harjani, T. Robert, E. Rogel, D.
Kuehne, C. Ovalles, V. Sampath, P.G. Jessop, Energy Fuels 26
(2012) 488-494; e) X. Su, P.G. Jessop, M.F. Cunningham, Mac-
romolecules 45 (2012) 666-670; f) C.I. Fowler, P.G. Jessop, M.F.
Cunningham, Macromolecules 45 (2012) 2955-2962.
[9] a) P. Munshi, A.D. Main, J. Linehan, C.-C. Tai, P.G. Jessop, J.
Am. Chem. Soc. 124 (2002) 7963-7971; b) P. Munshi, D.J.
Heldebrant, E.P. McKoon, P.A. Kelly, C.-C. Tai, P.G. Jessop,
Tetrahedron Lett. 44 (2003) 2725-2727; c) B.J. Flowers, R.
Gautreau-Service, P.G. Jessop, Adv. Synth. Catal. 350 (2008)
2947-2958.
and 178.0 ppm (d,
(C32H52Br3Cu4N2P2): 1020.8287 (calcd.), 1020.7412 (found).
J
=
27.3 Hz). ESI-HRMS
Supporting Information
Text, figures, tables, and CIF and xyz files giving spectroscopic
and crystallographic details. This material is available free of
charge via the Internet at http://pubs.acs.org. CCDC .. also contain
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[10] a) J.C. Wang, J.E. Bauman Jr. Inorg. Chem. 4 (1965) 1613-
1615; b) D.M.L. Goodgame, A.A.S.C. Machado, J. Chem. Soc.,
Chem. Commun. (1969) 1420-1421; c) A.B.P. Lever, B.S.
Ramaswamy, S.H. Simonsen, L.K. Thompson, Can. J. Chem. 48
(1970), 3076-3088; d) P.B. Hitchcock, M.F. Lappert, P.L. Pye, J.
Chem. Soc., Dalton Trans. (1977) 2160-2172; e) B. Çetinkaya, E.
Çetinkaya, P.B. Hitchcock, M.F. Lappert, I. Özdemir, J. Chem.
Soc., Dalton Trans. (1997) 1359-1362; f) F. Zamora, V.M. Gon-
zález, J.M. Pérez, J.R. Masaguer, C. Alonso, C. Navarro-
Ranninger, Appl. Organomet. Chem. 11 (1997) 659-666; g) S.
Haneda, C. Ueba, K. Eda, M. Hayashi, Adv. Synth. Cat. 349
(2007) 833-835; h) V.de la Fuente, R. Marcos, X.C. Cambeiro, S.
Castillón, C. Claver, M.A. Pericàs, Adv. Synth. Cat. 353 (2011)
3255-3261.
Acknowledgment
The authors gratefully acknowledge the Natural Sciences and
Engineering Research Council (NSERC) of Canada for financial
support (grant number RGPIN/288288-2009) and Dr. G. Schatte
for assistance in verifying X-ray structures. PGJ also thanks the
Canada Research Chairs Program.
Rererences
[1] P. Tomasik, Z. Ratajewicz, Pyridine Metal Complexes (Eds.:
G.R. Newkome, L. Strekowski), Part 6A, Volume 14, John Wiley
& Sons, New York, 1985.
[11] a) H. Hoberg, Y. Peres, C. Krüger, Y.-H. Tsay, Angew.
Chem. Int. Ed. Engl. 26 (1987) 771-773; b) R.D Sole, A.De Luca,
G. Mele, G. Vasapollo, J. Porphyrins Pthalocyanines 9 (2005)
519-527; c) V.de la Fuente, C. Godard, E. Zangrando, C. Claver,
S. Castillón, Chem. Commun. 48 (2012) 1695-1697.
[2] B.K. Breedlove, R. Kandel, A.Md. Habib, M. Yamashita,
Dalton Trans. 43 (2014) 7683–7686.
[12] U. Flörke, U. Ortmann, H.-J. Haupt, Acta Crys. C C48
(1992) 1663-1665.
[3] M.P. Coles, Dalton Trans. (2006) 985-1001.
[13] a) L. Böttcher, D. Walther, H. Görls, Z. Anorg. Allg. Chem.
629 (2003) 1208-1216; b) L. Böttcher, A. Scholz, D. Walther, N.
Weisbach, H. Görls, Z. Anorg. Allg. Chem. 629 (2003) 2103-
2112; c) C. Valdebenito, M.T. Garland, R. Quijada, R. Rojas, J.
Organomet. Chem. 694 (2009) 717-725; d) J. Li, D.W. Widlicka,
K. Fichter, D.P. Reed, G.R. Weisman, E.H. Wong, A. DiPasquale,
K.J. Heroux, J.A. Golen, A.L. Rheingold, Inorg. Chim. Acta 364
(2010) 185-194.
[4] For representative reviews, see: a) S. Collins, Coordin. Chem.
Rev. 255 (2011) 118-138; b) F.T. Edelmann, Chem. Soc. Rev. 38
(2009) 2253-2268; c) F.T. Edelmann, Adv. Organomet. Chem. 57
(2008) 183-352.
[5] For representative reviews, see: a) P.C. Junk, M.L. Cole,
Chem. Commun. (2007) 1579-1590; b) D.A. Kissounko, M.V.
Zabalov, G.P. Brusova, D.A. Lemenovskii, Russ. Chem. Rev. 75
(2006) 351-374.
[14] a) C.-W. Lee, Z.-K. Chan, T.-R. Chen, J.-D. Chen, Inorg.
Chim. Acta 348 (2003) 135-142; b) H.-C. Liang, Y.-Y. Wu, F.-C.
Chang, P.-Y. Yang, J.-D. Chen, J.-C. Wang, J. Organomet. Chem.
669 (2003) 182-188; c) F.A. Cotton, P. Lei, C.A. Murillo, L.-S.
Wang, Inorg. Chim. Acta 349 (2003) 165-172.
[6] P.G. Jessop, S.M. Mercer, D.J. Heldebrant, Energy Envron.
Sci. 5 (2012) 7240-7253.
[7] a) P.G. Jessop, D.J. Heldebrant, X. Li, C.A. Eckert, C.L.
Liotta, Nature 436 (2005) 1102-1102; b) D.J. Heldebrant, C.R.
Yonker, P.G. Jessop, L. Phan, Chem. Eur. J. 15 (2009) 7619-
7627; c) D.J. Heldebrant, P.K. Koech, M.T.C. Ang, C. Liang,
J.E. Rainbolt, C.R. Yonker, P.G. Jessop, Green Chem. 12 (2010)
713-721; d) S.M. Mercer, P.G. Jessop, ChemSusChem 3 (2010)
467-470; e) P.G. Jessop, L. Phan, A. Carrier, S. Robinson, C.J.
Dürr, J.R. Harjani, Green Chem. 12 (2010) 809-814.
[15] a) F.A. Cotton, L.M. Daniels, D.J. Maloney, J.H. Matonic,
C.A. Murillo, Polyhedron 13 (1994) 815-823; b) F.A. Cotton,
L.M. Daniels, C.A. Murillo, Inorg. Chim. Acta 224 (1994) 5-9; c)
D.I. Arnold, F.A. Cotton, D.J. Maloney, J.H. Matonic, C.A. Mu-
rillo, Polyhedron 16 (1997) 133-141; d) D.I. Arnold, F.A. Cotton,
J.H. Matonic, C.A. Murillo, Polyhedron 16 (1997) 1837-1841.
[16] F. Nicolò, G. Bruno, S.L. Schiavo, M.S. Sinicropi, P. Piraino,
Inorg. Chim. Acta 223 (1994) 145-149.
[8] a) Y. Liu, P.G. Jessop, M. Cunningham, C.A. Eckert, C.L.
Liotta, Science 313 (2006) 958-960; b) C.I. Fowler, C.M.
Muchemu, R.E. Miller, L. Phan, C. O’Neill, P.G. Jessop, M.F.
7

[17] S.H. Oakley, D.B. Soria, M.P. Coles, P.B. Hitchcock, Dalton
Trans. (2004) 537-546.
[33] D.T. Cromer, J.T. Waber, International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, UK, 1974, 4 Table
2.2 A.
[18] J. Barker, N.D. Cameron, M. Kilner, M.M. Mahmoud, S.C.
Wallwork, J. Chem. Soc., Dalton Trans. (1991) 3435-3445.
[19] V.K. Issleib, O. Lӧw, Z. Anorg. Allg. Chem. 346 (1966) 241-
254.
[20] a) J. Heinicke, A. Tzschach, Z. Chem. 18 (1978) 452-453; b)
X. Li, H. Song, C. Cui, Dalton Trans. (2009) 9728-9730; c) M.
Song, B. Donnadieu, M. Soleilhavoup, G. Bertrand, Chem. Asian
J. 2 (2007) 904-908.
[21] a) K. Issleib, H. Schmidt, H. Meyer, J. Organomet. Chem.
160 (1978) 47-57; b) A. Schmidpeter, A. Willhalm, Angew.
Chem. 96 (1984) 901-902; Angew. Chem. Int. Ed. Engl. 23
(1984) 903-904.
[22] a) T. van Dijk, S. Burck, M.K. Rong, A.J. Rosenthal, M.
Nieger, J.C. Slootweg, K. Lammertsma, Angew. Chem. Int. Ed.
53 (2014) 9068-9071; b) T. van Dijk, S. Martijn, M.S. Bakker, F.
Holtrop, M. Nieger, J.C. Slootweg, K. Lammertsma, Org. Lett. 17
(2015) 1461−1464; c) T. van Dijk, S. Burck, A.J. Rosenthal, M.
Nieger, A.W. Ehlers, J.C. Slootweg, K. Lammertsma, Chem. Eur.
J. 21 (2015) 9328-9331
[23] Shenai-Khatkhate et al US patent application US
2008/0026577 Al
24 J. Oszczapowicz, E. Raczyńska, J. Chem. Soc., Perkin Trans. 2,
(1984) 1643-1646.
[25]. B. Cordero, V. Gómez, A.E. Platero-Prats, M. Revés, J.
Echeverría, E. Cremades, F. Barragán, S. Alvarez, Dalton Trans.
(2008) 2832-2838.
[26] H. Takahashi, T. Fukami, H. Kojima, T. Yamakawa,
H. Takahashi, T. Sakamoto, T. Nishimura, M. Nakamura,
T. Yosizumi, K. Niiyama, N. Ohtake, T. Hayama, Tetrahedron 61
(2005) 3473-3481.
[27] M.D. Fryzuk, A. Carter, A. Westerhaus, Inorg. Chem. 24
(1985) 642-648.
28] a) M.R. Churchill, K.L. Kalra, Inorg. Chem. 13 (1974) 1065-
1071; b) M.R. Churchill, K.L. Kalra, Inorg. Chem. 13 (1974)
1899-1904; c) M.R. Churchill, B.G. Deboer, S.J. Mendak, Inorg.
Chem. 14 (1975) 2041-2047; d) M.R. Churchill, F.J. Rotella,
Inorg. Chem. 16 (1977) 3267-3273.
[29] J.C. Dyason, P.C. Healy, L.M. Engelhardt, C. Pakawatchai,
V.A. Patrick, C.L. Raston, A.H. White, Dalton Trans. (1985) 831-
838.
[30] M.R. Churchill, B.G. Deboer, D.J. Donovan, Inorg. Chem.
14 (1975) 617-623.
[31] S. Naik, J.T. Mague, M.S. Balakrishna, Inorg. Chem. 53
(2014) 3864-3873.
[32] APEX2 Version a) 2010.3-0; b) 2009.7.68A; c) 2008.1; d)
2008.2; e) 2000.6.14; Bruker-AXS: Madison, Winconsin, USA,
2010, 2009, 2008, 2000
8

Graphical abstract synopsis
Coordination geometries of uncharged and apro-
tic amidines and 1,3-(P,N) phosphaamidines on
Cu(I) centres.
9

Highlights
• Aprotic acetamidine ligands coordinate to Cu
through solely the Nimine
.
• A bulky aprotic phosphaamidine ligand coordi-
nates 1•P with Cu(I)Br to form cubane Cu4Br4
complex, leaving the Nimine basic site dangling.
• Less bulky aprotic phosphaamidine ligands co-
2
ordinate •P,N to form step Cu4Br4 complexes.
10