A hardwon dirhodium paddlewheel with guanidinate type (hpp) bridging ligands

Article abstract of DOI:10.1039/b513406d

A dirhodium paddlewheel compound which has a Rh₂⁵⁺ core enveloped by four guaninate ligands was prepared. The ligand itself was incorporated into paddlewheel compounds of eleven different metal atoms. Considering the instability the tetraacetate precursor, compound 1 is remarkably stable in crystalline form and remains unchanges after exposure to air for several days. In THF solution the brownish color changes rapidly to purple because of axial coordination by solvent molecules.

Full text of DOI:10.1039/b513406d

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C O M M U N I C A T I O N
A hardwon dirhodium paddlewheel with guanidinate type (hpp)
bridging ligands†
John F. Berry, F. Albert Cotton,* Penglin Huang, Carlos A. Murillo* and Xiaoping Wang
Department of Chemistry and Laboratory for Molecular Structure and Bonding,
P.O. Box 30012, Texas A&M University, College Station, Texas, 77842-3255, USA.
E-mail: cotton@tamu.edu, murillo@tamu.edu; Fax: 979-845-9351; Tel: 979-845-4432
Received 22nd September 2005, Accepted 7th October 2005
First published as an Advance Article on the web 21st October 2005
We report a dirhodium paddlewheel compound which has
an Rh25+ core enveloped by four guanidinate ligands and an
axially coordinated bromine atom; Rh2(hpp)4Br crystallizes
as a 1-D chain with a Rh–Rh distance of 2.430(3) e` and a
Rh–Rh bond order of 1.5 (hpp = the anion of 1,3,4,6,7,8-
hexahydro-2H-pyrimido[1,2a]pyrimidine).
becomes unreactive and will remain unchanged after mixing
with bromine.
The preparation of dirhodium compounds with guanidine-type
ligands is of interest because of the known effectiveness of
dirhodium compounds in catalysis1 and their use as antitumor
agents which interact with DNA.2 Also guanidine type ligands,
which are of interest in biological systems,3 have been found to
form unusually strong bonds to a variety of metal atoms, e.g.,
gallium,4 but in general studies of the coordination chemistry
of these ligands has lagged behind that of other N,N-type
ligands,5 such as amidinates. One guanidinate ligand that we
have been particularly interested in is hpp (hpp = the anion
of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine, I). This
ligand is known to stabilize dimetal units in high oxidation
states.6 In one case, W2(hpp)4, it produces a molecule that has an
ionization potential lower than any other known stable closed
shell molecule.7
Considering the instability of the tetraacetate precursor,
compound 1 is remarkably stable in crystalline form and remains
unchanged after exposure to air for several days. However, in
THF solution the brownish color changes rapidly to purple
because of axial coordination by solvent molecules. The CV in
CH2Cl2 shows a wave at −0.34 V vs. Ag/AgCl. The structure§
is that of a typical paddlewheel complex as shown in Fig. 1. The
5+
˚
Rh–Rh distance in this Rh2 species (2.430(3) A) is long when
12
compared to those in [Rh2(O2CCH3)4(H2O)2]ClO4 (2.315(2)
4+
Since there are about 1500 compounds containing the Rh2
core, including one with triphenylguanidate ligands,8 and a few
with the Rh2 core,9 most of which have a set of four bridging
5+
ligands of general type II arranged in paddlewheel fashion about
the central dirhodium core, it might have been supposed that
making one more dirhodium compound with the hpp ligand
would be easy.
n+
The ligand itself has already been incorporated into M2
paddlewheel compounds of eleven different metal atoms (V, Nb,
Cr, Mo, W, Re, Ru, Os, Ir, Pd and Pt).9 However, finding a route
n+
to a Rh2(hpp)4 compound proved to be very challenging. We
report here the only successful effort to synthesize a homoleptic
dirhodium complex with hpp and also describe a few of the
numerous efforts to obtain Rh2(hpp)4 by reaction of Hhpp (or
Lihpp) and either Rh2(O2CCH3)4 or [Rh2(CH3CN)10](BF4)2.
The brown paddlewheel compound 1, Rh2(hpp)4Br, was
prepared‡ in greater than 50% yield by reaction of Khpp
and Rh2(O2CCH3)4Br10 in THF at 50–60 ◦C for 4 h. Because
Rh2(O2CCH3)4Br decomposes readily upon storage (even in
a dry-box) at ambient temperature it was freshly made from
the reaction of unsolvated Rh2(O2CCH3)4 and an excess of
freshly distilled bromine; the excess bromine was removed
under vacuum, and finally the purple solid that remained was
used without further purification. The olive-green, anhydrous
11
Rh2(O2CCH3)4 was made from blue Rh2(O2CCH3)4(CH3OH)2
under a dynamic vacuum at 40–50 ◦C. Care should be exer-
cised to avoid extended heating periods (more than 24 h) or
temperatures above 90 ◦C because the resulting green solid
† Electronic supplementary information (ESI) available: Experimen-
tal and characterization data for 2·H2O and 3·2CH2Cl2. Displace-
ment ellipsoid drawings for 3·2CH2Cl2 and 4·3CH2Cl2. See DOI:
10.1039/b513406d
Fig.
1
One of the orientations of the disordered structure of
Rh2(hpp)4Br (1) drawn with displacement ellipsoids at the 30% proba-
bility level and hydrogen atoms omitted for clarity: (a) a single molecule;
(b) the 1-D chain of alternating Rh2 and Br units.
5+
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 3 7 1 3 – 3 7 1 5
3 7 1 3
©

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˚
and 2.318(2) A) where the axial ligands are r-donors and short
i
13
˚
whencompared to 2.510(1) A in[Rh2(O2CC2H5)4(PPr 3)2]SbF6,
where the axial ligands are p-acceptors and can interact with
the filled antibonding orbitals of the dirhodium unit. This
Rh–Rh distance is about the same as that in formamidinate
14
˚
derivatives, e.g., 2.452(2) A in [Rh2(DTolF)4(H2O)]O2CCF3,
where DTolF = N,Nꢀ-di-p-tolylformamidinate. The molecules
share the one Br atom by interacting with each other to form a
1-D chain that has slightly different Rh–Br distances (2.803(3)
˚
and 2.905(3) A). They reside on crystallographic C4v special
positions with the Rh–Rh bonds aligned on the four-fold axes.
Because the C4v site symmetry is higher than the C4 symmetry of
the molecule, the puckered hpp ligands are disordered as shown
in Fig. 2. Because there are 13 metal based electrons in this
paddlewheel compound, the Rh–Rh bond order is 1.5.
4+
Fig. 3 The structure of the paddlewheel 2 in 2·H2O showing the Rh2
species with one hpp and three acetate paddles, and also the axially
coordinated acetate groups which are hydrogen bonded to an H2hpp
moiety.
Acknowledgements
Financial support from the National Science Foundation, the
Robert A. Welch Foundation and Texas A&M University is
gratefully acknowledged. J. F. B. thanks the National Sci-
ence Foundation for a predoctoral fellowship. We also thank
Johnson-Matthey for a generous loan of RhCl3.
Notes and references
‡ Synthesis of Rh2(hpp)4Br (1): All procedures were carried out under
N2 using standard Schlenk techniques. In one flask, freshly made
Rh2(O2CCH3)4Br was prepared by reaction of finely-ground unsolvated
Rh2(O2CCH3)4 (101 mg, 0.200 mmol) and an excess of Br2, in a
procedure in which addition and removal under vacuum of Br2 was
repeated three times to assure complete conversion of Rh2(O2CCH3)4
to Rh2(O2CCH3)4Br. After the final elimination of Br2 under vacuum,
a purple solid remained. In another flask, Hhpp (139 mg, 1.00 mmol)
and an excess of KH (1.0 g, in mineral oil, 35% by weight, 8.7 mmol)
were mixed and then washed with hexanes (2 × 30 mL) to remove
the mineral oil. To the mixture of solids THF (20 mL) was added and
bubbles were observed. After 10 min a yellowish solution and a colorless
solid were observed. The mixture was passed through a fine fritted filter
stick packed with 3 cm of Celite and the solution was collected into the
flask containing the solid Rh2(O2CCH3)4Br. The mixture was stirred at
50–60 ◦C for 4 h. A yellowish brown supernatant solution was observed
over a colorless solid. The mixture was filtered while still warm. After
slow cooling of the solution overnight a brown microcrystalline solid was
obtained. Typical yields were over 50%. Anal. Calc. for Rh2C28H48N12Br
(Rh2(hpp)4Br): C, 40.10; H, 5.77; N, 20.04%. Found: C, 39.95, H, 5.52;
N, 19.87%.
Fig. 2 Near axial view of 1 showing how the hpp ligands are disordered
over two orientations.
5+
It is interesting to note that the general preparation of Rh2
compounds of the type Rh2L4X,9 where L = a mononegative
bridging ligand, typically involves oxidation of the correspond-
ing Rh2L4 precursor, which is often made from reaction of the lig-
and L and Rh2(O2CCH3)4. Efforts to follow a similar route were
repeatedly unsuccessful in our attempts to synthesize Rh2(hpp)4,
or to prepare an oxidized form of it, using either Rh2(O2CCH3)4
with or without axial ligands, or [Rh2(CH3CN)10](BF4)415 as the
source of rhodium and Lihpp or Hhpp as the source of ligand.
Several solvents such as THF, diethyl ether, acetonitrile and
o-dichlorobenzene were used in the reactions, as well as several
temperatures. In some reactions, oxidizing agents such as I2 were
also added.
§ Crystal data for 1: C28H48BrN12Rh2, M = 838.51, tetragonal, space
3
˚
˚
group P4/nmm (no. 129), a = 13.976(7), c = 8.139(9) A, V = 1590(2) A ,
Z = 2, T = 213(2) K, l(Mo-Ka) = 2.336 mm−1. Full-matrix refinement
on F2, R1 = 0.0478, wR2 = 0.1094, GOF = 1.113 for 1042 unique
reflections and 100 parameters. R1 = 0.0370 and wR2 = 0.0995 for 857
reflections with I > 2r(I).
Crystal data for 2·H2O: C31H57N9O11Rh2, M = 937.68, triclinic, space
¯
˚
group P1 (no. 2), a = 11.272(3), b = 11.400(3), c = 15.856(4) A, a =
◦
3
˚
80.768(5), b = 78.128(5), c = 87.688(5) , V = 1968.3(9) A , Z = 2, T =
There was one species that was obtained several times, namely
[Rh2(hpp)(O2CCH3)3]{(O2CCH3)(H2hpp)}2,§ 2. The structure
of 2·H2O is shown in Fig. 3. Three bridging ligands are acetate
groups while the fourth is an hpp anion and the molecule has
a C2 idealized symmetry. Each molecule has two axial acetate
213(2) K, l(Mo-Ka) = 0.905 mm−1. Full-matrix refinement on F2, R1 =
0.0647, wR2 = 0.1055, GOF = 1.034 for 6911 unique reflections and 501
parameters. R1 = 0.0409 and wR2 = 0.0936 for 5002 reflections with
I > 2r(I).
Crystal data for 3·2CH2Cl2: C29H51Cl4N9O6Rh2, M = 969.41, mono-
ligands that are hydrogen bonded to H2hpp+ cations. Compound
clinic, space group C2/c (no. 15), a = 20.952(3), b = 13.217(2), c =
◦
3
˚
˚
4+
14.875(2) A, b = 108.844(2) , V = 3898(1) A , Z = 4, T = 213(2)
K, l(Mo-Ka) = 1.173 mm−1. Full-matrix refinement on F2, R1 =
0.0353, wR2 = 0.0747, GOF = 1.065 for 4420 unique reflections and
249 parameters. R1 = 0.0281 and wR2 = 0.0692 for 3754 reflections
with I > 2r(I).
˚
2 has an Rh2 core and the Rh–Rh distance is 2.3927(7) A.
When sky-blue CH2Cl2 solutions of 2 were briefly exposed to air
and some of the solvent allowed to evaporate, purple crystals
of [Rh2(hpp)(O2CCH3)3(Hhpp)2]·2CH2Cl2,§ 3·2CH2Cl2, were
formed which has a similar core structure to 2, but contains
neutral Hhpp molecules in the axial positions. The Rh–Rh dis-
Crystal data for 4·3CH2Cl2: C29H52Cl6N6O12Rh2, M = 1095.29, mon-
oclinic, space group C2/c (no. 15), a = 17.368(8), b = 14.227(6), c =
3
˚
˚
20.933(9) A, b = 107.792(7), V = 4925(4) A , Z = 4, T = 213(2) K, l(Mo-
Ka) = 1.049 mm−1. Full-matrix refinement on F2, R1 = 0.0807, wR2 =
0.2021, GOF = 1.079 for 3215 unique reflections and 300 parameters.
R1 = 0.0669 and wR2 = 0.1901 for 2588 reflections with I > 2r(I).
CCDC reference numbers 273861 for 1, 273862 for 2·H2O, 273864 for
3·2CH2Cl2 and 273863 for 4·3CH2Cl2. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b513406d
˚
tance in 3 (2.4262(5) A) is slightly longer than that in 2. Another
compound that has been isolated is the rhodium acetate adduct
[Rh2(O2CCH3)4]{(O2CCH3)(H2hpp)}2·3CH2Cl2,§ 4·3CH2Cl2.
Further studies of the properties of 1 will be undertaken in
an effort to understand the reason for its stability relative to the
still unknown Rh2(hpp)4.
3 7 1 4
D a l t o n T r a n s . , 2 0 0 5 , 3 7 1 3 – 3 7 1 5

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D a l t o n T r a n s . , 2 0 0 5 , 3 7 1 3 – 3 7 1 5
3 7 1 5