Copper(II) acetate complexes, [CuLm(OAc)2]n (L = HNPPh3), stable in the solid state either as a dimer (m = 1, n = 2) or a monomer (m = 2, n = 1)

Article abstract of DOI:10.1021/ic0155636

The synthesis and X-ray structures of two novel copper (II) phosphinimine acetate complexes, monomeric Cu(HNPPh₃)₂(OAc)₂ and dimeric [Cu(HNPPH₃)(OAc)₂]₂, are described. Intramolecular hydrogen bonding between acetate ligands and the imino hydrogen stabilizes Cu(HNPPH₃)₂(OAc)₂, whereas the copper (II) tetraacetato core of [Cu(HNPPh₃)(OAc)₂]₂ possesses neither intra- nor intermolecular hydrogen bonding.

Full text of DOI:10.1021/ic0155636

5064
Inorg. Chem. 2001, 40, 5064-5065
Copper(II) Acetate Complexes, [CuLm(OAc)2]n (L ) HNPPh3), Stable in the Solid State Either as a Dimer
(m ) 1, n ) 2) or a Monomer (m ) 2, n ) 1)
Jeremy C. Stephens, Masood A. Khan, and Robert P. Houser*
Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman, Oklahoma 73019
ReceiVed July 10, 2001
The familiar “paddlewheel” structure of copper(II) carboxylates
has been known for many years.1 There are far fewer examples
of crystal structures of copper carboxylate monomers, most of
which are stabilized by bulky alkyl or aryl groups on the
carboxylate anion.2 Although there have been reports of equilibria
in solution between the copper acetate monomer and dimer,3 to
our knowledge, there are no examples of copper acetate complexes
where the monomer and dimer have been structurally character-
ized in the solid state. Herein we report the first example of copper
acetate complexes, [CuLm(OAc)2]n, that have been structurally
characterized as a monomer (m ) 2, n ) 1) and a dimer (m ) 1,
n ) 2).
Scheme 1
Our interest in these compounds arose from ongoing research
on copper clusters that mimic the CuZ catalytic site in nitrous
oxide reductase.4 Our copper clusters contain copper cations
bridged by phosphoraneiminato ligands. The phosphoraneiminate
anion, NPR3-, has been used as a ligand with many transition
metals,5 as well as with main group elements.6 The coordination
When a CH2Cl2 slurry of Cu(OAc)2‚H2O is treated with a
CH2Cl2 solution of Me3SiNPPh3, the Si-N bond is hydrolyzed
to produce triphenylphosphinimine, Ph3PNH, which coordinates
to the copper ions via the imine nitrogen atom to produce deep
blue Cu(HNPPh3)2(OAc)2 (1, Scheme 1).9 The solid-state structure
of 1, determined from a single-crystal X-ray diffraction study,
reveals a square planar geometry around the copper(II) ion with
the ligands in a trans conformation (Figure 1).10 The copper atom
lies on the inversion center, and the acetate anions are coordinated
in a syn-monodentate fashion. The same arrangement of ligand
atoms was observed in the closely related copper phosphinimine
complex where R ) Et, Cu(HNPEt3)2(OAc)2,9a and the Cu-N
and Cu-O bond distances in 1 are nearly identical to those
reported for Cu(HNPEt3)2(OAc)2.
-
mode of the NPR3 ligand is varied, ranging from µ1 to µ3,
depending predominantly on the oxidation state of the metal.
Complexes of divalent metal ions with phosphoraneiminato
ligands have the general formula [MX(NPR3)]4 (X ) halide; R
) Me, Et, Ph), possess a heterocubane structure, and are well
represented for most of the mid- to late-transition metals, with
solid-state structures reported for Mn2+, Fe2+, Co2+, Ni2+, Zn2+
,
and Cd2+.5 Interestingly, there are no examples of the heterocu-
bane cluster where M ) Cu2+. A recent computational study of
[MX(NPR3)]4 heterocubane clusters suggests that there have been
no reported examples of [CuX(NPR3)]4 due to destabilization from
Jahn-Teller distortions.7 Very few copper complexes of any type
with phosphoraneiminato ligands have been reported.8
The Cu-N-P bond angle of 138.01(11)° for 1 is slightly
more obtuse than the analogous angle of 133.3° reported for
Cu(HNPEt3)2(OAc)2, presumably due to increased steric interac-
tions between the phenyl rings and the acetate ligands. Indeed,
the acetate methyl group is in a position that minimizes interaction
with the nearby atoms, lying between the two proximal phenyl
rings of the phosphinimine ligand. The N1-H1‚‚‚O2A angle and
the short H1‚‚‚O2A distance (see Figure 1 caption) indicate that
hydrogen bonding occurs, perhaps giving rise to the stabilization
* To whom correspondence should be addressed.
(1) Harcourt, R. D.; Skrezenek, F. L.; Maclagan, R. G. A. R. J. Am. Chem.
Soc. 1986, 108, 5403-5408, and references therein.
(2) (a) Chavez, F. A.; Que, L.; Tolman, W. B. Chem. Commun. 2001, 111-
112. (b) Lah, N.; Giester, G.; Lah, J.; Segedin, P.; Leban, I. New J.
Chem. 2001, 25, 753-759. (c) Psomas, G.; Raptopoulou, C. P.;
Iordanidis, L.; Dendrinou-Samara, C.; Tangoulis, V.; Kessissoglou, D.
P. Inorg. Chem. 2000, 39, 3042-3048. (d) Muhonen, H. Acta Crystal-
logr., Sect. C 1983, 39, 536-540.
(3) Szpakowska, M.; Uruska, I. Pol. J. Chem. 1980, 54, 1661-1670.
(4) (a) Brown, K.; Tegoni, M.; Prudeˆncio, M.; Pereira, A. S.; Besson, S.;
Moura, J.; Moura, I.; Cambillau, C. Nat. Struct. Biol. 2000, 7, 191-
195. (b) Rasmussen, T.; Berks, B. C.; Sanders-Loehr, J.; Dooley, D.
M.; Zumft, W. G.; Thomson, A. J. Biochemistry 2000, 39, 12753-12756.
(c) Brown, K.; Djinovic-Carugo, K.; Haltia, T.; Cabrito, I.; Saraste,
M.; Moura, J. J. G.; Moura, I.; Tegoni, M.; Cambillau, C. J. Biol. Chem.
2000, 275, 41133-41136.
(5) Dehnicke, K.; Krieger, M.; Massa, W. Coord. Chem. ReV. 1999, 182,
19-65.
(6) Dehnicke, K.; Weller, F. Coord. Chem. ReV. 1997, 158, 103-169.
(7) Sundermann, A.; Schoeller, W. W. J. Am. Chem. Soc. 2000, 122, 4729-
4734.
(9) Transition metal complexes of the neutral phosphinimine species, HNPR3,
have been reported. (a) Ackermann, H.; Geiseler, G.; Harms, K.; Leo,
R.; Massa, W.; Weller, F.; Dehnicke, K. Z. Anorg. Allg. Chem. 1999,
625, 1500-1506. (b) Stahl, M. M.; Faza, N.; Massa, W.; Dehnicke, K.
Z. Anorg. Allg. Chem. 1997, 623, 1855-1856.
(10) Crystal data for 1: C40H38CuN2O4P2, MW ) 736.20, blue, block-shaped
crystal (0.54 × 0.52 × 0.48 mm), triclinic, space group P-1 with a )
8.7983(10), b ) 9.4952(10), c ) 12.5588(13) Å, R ) 109.609(7), â )
103.008(8), γ ) 99.475(8), V ) 929.42(17) Å3, Z ) 1, Fcalcd ) 1.315 g
cm-3, Mo KR radiation (λ ) 0.71073 Å), 198(2) K. Data were collected
using a Siemens P4 single-crystal diffractometer, and the structure was
solved via direct methods. Full-matrix least-squares refinement on F2
using SHELXTL V5.1 converged with final R1 ) 0.0350 and wR2 )
0.0930 for 3273 independent reflections, with I > 2σ(I) and 229
parameters.
(8) (a) Riese, U.; Faza, N.; Massa, W.; Dehnicke, K. Angew. Chem., Int.
Ed. Engl. 1999, 38, 528-531. (b) Meyer zu Ko¨cker, R.; Dehnicke, K.;
Fenske, D. Z. Naturforsch. 1994, 49b, 987-990. (c) Meyer zu Ko¨cker,
R.; Behrendt, A.; Dehnicke, K.; Fenske, D. Z. Naturforsch. 1993, 49b,
301-308. (d) Meyer zu Ko¨cker, R.; Pebler, J.; Friebel, C.; Dehnicke,
K. Z. Anorg. Allg. Chem. 1995, 621, 1311-1317.
10.1021/ic0155636 CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/28/2001

Communications
Inorganic Chemistry, Vol. 40, No. 20, 2001 5065
shown in Figure 2.12 The coordination geometry around the copper
atoms is square pyramidal, with the phosphinimine ligands
occupying the apical positions and the oxygen atoms of the acetato
bridging ligands making up the basal plane. The Cu‚‚‚Cu distance
of 2.664(3) Å is typical for binuclear copper(II) acetates that
possess N-donating apical ligands, and the Cu-O and Cu-N bond
lengths are all within expected ranges.13 The copper(II) ions lie
0.217(1) Å above the basal planes created by the oxygen atoms
of the bridging acetates. The Cu-N-P bond angle in 2 is identical
to the Cu-N-P bond angle in 1 as a result of the similar
disposition of the phenyl rings with respect to the acetate groups.
In contrast to 1, there is no intramolecular hydrogen bonding
in 2, as evinced by the sharp N-H stretch at 3352 cm-1 in the
FTIR spectrum (Figure S1). Moreover, the H1‚‚‚O3A distance
in 2 is over 2.9 Å and the N1-H1‚‚‚O3A angle is 91°, precluding
intramolecular hydrogen bonding. Inspection of the packing
diagram for 2 (Figure S2) reveals the absence of any potential
intermolecular hydrogen bonding contacts.
Although dimeric 2 is stable in the solid state, upon dissolution
it dissociates into a monomeric species. When green 2 is dissolved
in either CH2Cl2 or CH3CN, the color quickly changes to blue,
and the UV-visible spectrum of 2 in solution is identical to the
spectrum of 1. Furthermore, electrospray MS of CH2Cl2 solutions
of 1 and 2 are nearly identical, with both spectra dominated by
a parent ion at m/z ) 676.1 ([M - OAc]+). The ESI MS of 2
also contains a small fraction (<3% relative intensity) of higher
molecular weight ions that are presumably multinuclear in nature.
Finally, EPR spectra of frozen solutions of 1 and 2 both display
axial signals typical for tetragonal copper complexes.14
Figure 1. Representation of the X-ray crystal structure of 1 as 50%
thermal ellipsoids. H atoms have been omitted for clarity. Selected
bond distances (Å) and angles (deg): Cu1-N1, 1.9309(15); Cu1-O1,
1.9621(13); P1-N1, 1.5811(15); N1-O2A, 2.7651(15); N1-H1, 0.81(2);
O2A‚‚‚H1, 2.01(2); N1-Cu1-N1A, 180.00(11); O1A-Cu1-O1,
180.00(9); N1-Cu1-O1A, 91.23(6); N1-Cu1-O1, 88.77(6); P1-N1-
Cu1, 138.01(11) N1-H1‚‚‚O2A, 154(2).
In summary, the structures of copper acetate monomer 1 and
dimer 2 have been determined. Hydrogen bonding plays a role
in the stabilization of the monomer, while no hydrogen bonding
is apparent in the dimer. Although both 1 and 2 are stable
indefinitely in the solid state, only the monomer is present in
solution.
Figure 2. Representation of the X-ray crystal structure of 2 as 50%
thermal ellipsoids. H atoms have been omitted for clarity. Selected bond
distances (Å) and angles (deg): Cu1-Cu1A, 2.664(3); Cu1-N1, 2.128(3);
Cu1-O1A, 1.966(3); Cu1-O2, 1.966(3); Cu1-O3A, 1.963(3); Cu1-
O4, 1.968(3); P1-N1, 1.511(3); O3A-Cu1-O1A, 87.98(13); O1A-
Cu1-O4, 90.15(13); O2-Cu1-O4, 89.98(12); O3A-Cu1-O2, 89.09(12);
O1A-Cu1-O2, 167.26(8); O3A-Cu1-O4, 167.26(8); O3A-Cu1-N1,
98.69(14); O1A-Cu1-N1, 94.97(12); O2-Cu1-N1, 97.73(12); O4-
Cu1-N1, 94.02(13); P1-N1-Cu1, 138.05(13).
Acknowledgment. We thank the donors of the Petroleum
Research Fund, administered by the American Chemical Society,
the National Science Foundation (NSF CAREER Award CHE-
0094079), and the University of Oklahoma for financial support
for this research. We also acknowledge Mr. Scott Collins for the
preliminary synthesis of 1, and Dr. Larry Russon for collecting
ESI MS data.
of the monomeric form of 1.11 Further proof of hydrogen bonding
is evident in the FTIR spectrum of 1, which displays a broadened
N-H stretch centered at approximately 3400 cm-1 (Figure S1).
This type of hydrogen bonding in copper(II) carboxylate mono-
mers has been observed in the related complex Cu(HNPEt3)2-
(OAc)2,9a and in a complex coordinated by a sterically bulky
carboxylate and methanol.2a
Supporting Information Available: Experimental details and spec-
troscopic characterization for 1 and 2; FTIR of 1 and 2 (Figure S1);
packing diagram of 2 (Figure S2) (PDF); X-ray structural information
on 1 and 2 (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
IC0155636
Adjusting the stoichiometry of the reaction in Scheme 1 by
using excess Cu(OAc)2‚H2O results in a mixture of 1 and a green
crystalline product characterized as dimeric [Cu(HNPPh3)(OAc)2]2
(2). Furthermore, monomer 1 can be converted to dimer 2. When
a CH2Cl2 solution of analytically pure 1 was treated with excess
Cu(OAc)2‚H2O, 2 was isolated as the sole product upon recrys-
tallization. The identity of 2 was verified by X-ray crystallography
to be a copper(II) dimer with the aforementioned familiar
paddlewheel structure containing the four acetate bridging units,
(12) Crystal data for 2: C44H44Cu2N2O8P2, MW ) 917.83, green, prism-shaped
crystal (0.58 × 0.54 × 0.32 mm), triclinic, space group P-1 with a )
8.866(9), b ) 9.685(10), c ) 13.041(13) Å, R ) 90.06(9), â )
104.59(11), γ ) 104.45(10)°, V ) 1047.0(18) Å3, Z ) 1, Fcalcd ) 1.456
g cm-3, Mo KR radiation (λ ) 0.71073 Å), 173(2) K. Data were collected
using a Siemens P4 single-crystal diffractometer, and the structure was
solved via direct methods. Full-matrix least-squares refinement on F2
using SHELXTL V5.1 converged with final R1 ) 0.0504 and wR2 )
0.1379 for 4779 independent reflections, with I > 2σ(I) and 264
parameters.
(13) Melnik, M. Coord. Chem. ReV. 1982, 42, 259-293.
(14) EPR of 1 (50:50 CH2Cl2/toluene, 9.420 GHz, 77 K) g ) 2.056, g| )
2.271, A|Cu ) 165 G.
(11) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford Univer-
sity: New York, 1997; Chapters 4 and 11.