Synthesis and NMR Investigation of Dirhodium(II) Formamidinate Complexes Containing Bidentate Phosphorus Donors

Full text of DOI:10.1021/ic9502032

Inorg. Chem. 1996, 35, 1377-1381
1377
Notes
transfer of a proton from a phenyl group to an acetate or
formamidinate fragment.
Synthesis and NMR Investigation of
Dirhodium(II) Formamidinate Complexes
Containing Bidentate Phosphorus Donors. X-ray
Crystal Structure of the Ortho-Metalated
Complex Rh2(form)(µ-O2CCF3)[(C6H4)(C6H5)P-
(CH2)2P(C6H5)2](σ-O2CCF3)(p-toluidine)
In order to assess the generality of such reactions and to
evaluate the influence of the chain length on the reaction
products, we now report the results of the 1:1 reactions between
Rh2(form)2(O2CCF3)2(H2O)2 and the diphosphine ligands
Ph2P(CH2)nPPh2 (n ) 1-4). We report also the X-ray analysis
of the ortho-metalated complex Rh2(form)(µ-O2CCF3)[(C6H4)-
(C6H5)P(CH2)2P(C6H5)2](σ-O2CCF3)(p-toluidine).
Giuseppe Tresoldi,† Giovanni De Munno,‡
Francesco Nicolo`,† Sandra Lo Schiavo,† and
Pasquale Piraino*,†
Experimental Section
Dipartimento di Chimica Inorganica, Analitica e Struttura
Molecolare, Universita` di Messina, Messina, Italy, and
Dipartimento di Chimica, Universita` della Calabria,
Arcavacata, Cosenza, Italy
Rh2(form)2(O2CCF3)2(H2O)2 was prepared according to literature
procedure.7 1,2-bis(diphenylphosphino)methane (dppm), 1,2-bis(di-
phenylphosphino)ethane (dppe), 1,2-bis(diphenylphosphino)propane
(dppp), and 1,2-bis(diphenylphosphino)butane (dppb) were all obtained
from Strem and were used without further purification. Other reagents
and solvents were used as received. Infrared spectra were recorded
on a Perkin-Elmer FT 43 instrument. 31P NMR spectra were obtained
on a Perkin-Elmer AMX 300 instrument.
ReceiVed February 24, 1995
Introduction
4+
Synthesis of Rh2(form)2(µ-O2CCF3)(dppm)(σ-O2CCF3) (2). To
a diethyl ether solution (20 mL) of Rh2(form)2(O2CCF3)2(H2O)2 (0.2
g, 0.21 mmol) was added 0.017 g (0.21 mmol) of solid 1,2-bis-
(diphenylphosphino)methane (dppm). The resulting green solution was
stirred at room temperature for 3 h. The filtrate was evaporated to
dryness and the green powder obtained was crystallized from CHC13/
n-heptane giving green needles of 2. Yield: 113 mg. Anal. Calcd
for C59H52N4P2O4F6Rh2: C, 56.11; H, 4.15; N, 4.43; P, 4.90; F, 9.02.
Found: C, 55.89; H, 4.31; N, 4.39; P, 5.11; F, 8.94. Infrared spectrum
(Nujol mull, cm-l): νasym(CO2) 1645 (s), 1687 (s); (CH3CN) νasym(CO2)
1645 (s), 1695 (s). Molar conductivity (Ω-l cm2 M-l): 180 (CH3CN,
5 × 10-4 M). 31P NMR (CDCl3, J (Hz)): δ 7.37 ddd (1JRh-P ) 123;
2JRh-P ) 3.5; JP-P ) 52.6), 18.81 ddd (1JRh-P ) 137; 2JRh-P ) 4); (CD3-
CN, AA′XX′ spin system): δ 15.5
Synthesis of Rh2(form)(µ-O2CCF3)[(C6H4)(C6H5)P(CH2)2P(C6H5)2]-
(σ-O2CCF3)(p-toluidine) (3). This complex was prepared by reacting
a diethyl ether solution of Rh2(form)2(O2CCF3)2(H2O)2 (0.091 g, 0.1
mmol) with solid dppe (0.04 g, 0.1 mmol) at room temperature for 4
h. After this time stirring was stopped and the resulting dichroic green-
red solution was left standing for an additional 16 h, leading to the
formation of 3 as red crystals. Yield: 59%. Anal. Calcd for C60H54-
N4P2O4F6Rh2: C, 56.44; H, 4.26; N, 4.38; P, 4.85. Found: C, 56.04;
In a previous work we reported that the Rh2 complex
Rh2(form)2(O2CCF3)2(H2O)2 (form ) N,N′-di-p-tolylformamidi-
nate anion) easily reacts with 1,2-bis(diphenylphosphino)ethane
(dppe), molar ratio 1:2, giving the species [Rh2(form)(O2CCF3)-
{(C6H4)(C6H5)P(CH2)2P(C6H5)2}dppe]CF3COO (1) containing
one diphosphine ortho-metalated across the dirhodium core.1
This species represents the first example of a diphosphine ortho-
metalated across a metal-metal bond as well as the first report
on a dirhodium(II) complex supported by three different bridging
ligands. It also represents a further example of the propensity
of P-donor ligands to undergo ortho-metalation across the Rh-
Rh bond. This observation is supported by work carried out
by Cotton and Lahuerta who showed that P-donors such as
P(C6H5)3,2 P(m-CH3C6H4)3,3 P(C6H5)2(C6F4Br),4 and P(o-
5
ClC6H4)(C6H5)2 undergo, under more or less vigorous condi-
tions, ortho-metalation across the Rh-Rh bond of the complexes
Rh2(O2CCH3)4 and Rh2(O2CCF3)4. A search of the literature
for reactions of this type reveals that the aforementioned
reactions are rare for other metal-metal-bonded complexes.6
The mechanism invoked for these reactions2c contains three main
features: (i) axial coordination of the phosphorus ligand, (ii)
axial-equatorial isomerization (very likely this is the crucial
step of these reactions since it allows the close approach of one
phenyl ring at the adjacent rhodium atom), and (iii) formal
H, 4.16; N, 4.28; P, 5.00. Infrared spectrum (Nujol mull, cm-1): νasym
-
(CO2) 1716 (s), 1647 (s). Molar conductivity (Ω- cm2 M-1): 140
(CH3CN, 5 × 10-4 M). 31P NMR (CDCl3, J (Hz)): δ 37.56 ddd (1JRh-P
) 145.8; 2JRh-P ) 3.6; JP-P ) 25.2); 52.4 ddd (1JRh-P ) 131.9; 2JRh-P
) 3.8).
1
† Universita` di Messina.
Synthesis of Rh2(form)2(O2CCF3)2(dppb) (4). Crude dppb (0.047
g, 0.1 mmol) was added to a stirred solution of Rh2(form)2(O2-
CCF3)2(H2O)2 (0.1 g, 0.1 mmol) in CH2C12 (30 mL). The solution
changed rapidly to a dark green and then slowly to a yellow-red color.
Stirring of the mixture was continued for 3 h. After the solvent was
removed, the residue was crystallized from benzene/heptane. Yield:
63%. Anal. Calcd for C62H58N4P2O4F6Rh2: C, 57.68; H, 4.52; N, 3.25;
P, 4.79. Found: C, 57.40; H, 4.52; N, 3.21; P, 4.79. Infrared spectrum
(Nujol mull, cm -1): νasym(CO2) 1632 (vs), 1657 (s). 31P NMR (CDCl3,
‡ Universita` della Calabria.
(1) Bruno, G.; De Munno, G.; Tresoldi, G.; Lo Schiavo, S.; Piraino, P.
Inorg. Chem. 1991, 31, 1538.
(2) (a) Chakravarty, A. R.; Cotton, F. A.; Tocher, D. A.; J. Chem. Soc.
Chem. Commun. 1984, 501. (b) Chakravarty, A. R.; Cotton, F. A.;
Tocher, D. A.; Tocher, J. H. Organometallics 1985, 4, 8. (c) Lauherta,
P.; Paya, J.; Pellinghelli, M. A.; Tiripicchio, A. Inorg. Chem. 1992,
31, 1224.
(3) Lahuerta, P.; Martinez-Manez, R.; Paya, J.; Peris, E.; Diaz, V. Inorg.
Chim. Acta 1989, 28, 139.
(4) (a) Barcelo, F.; Cotton, F. A.; Lauherta, P.; Llusar, R.; Sanau, M.;
Schwotzer, W.; Ubeda, M. A. Organometallics 1986, 5, 808. (b)
Barcelo, F.; Cotton, F. A.; Lauherta, P.; Sanau, M.; Schwotzer, W.;
Ubeda, M. A. Organometallics 1987, 6, 1105.
2
J (Hz)): δ 30.67 dd (1JRh-P ) 135; JRh-P ) 5.8).
X-ray Data Collection and Structure Refinement. Suitable
crystals of complex 3 were obtained by slow evaporation of solvent
from a CHCl3-heptane solution. A summary of the crystallographic
data and the structure refinement is listed in Table 1. The reflection
(5) (a) Lauherta, P.; Paya, J.; Solans, X.; Ubeda, M. A. Inorg. Chem.
1992, 31, 385. (b) Gonzales, G.; Lahuerta, P.; Martinez, M.; Peris,
E.; Sanau, M. J. Chem. Soc., Dalton Trans. 1994, 545 and references
therein.
(6) Barder, T. S.; Tetrick, J. M.; Walton, R. A.; Cotton, F. A.; Powell, G.
L. J. Am. Chem. Soc. 1983, 105, 4090.
(7) Piraino, P.; Bruno, G.; Lo Schiavo, S.; Zanello, P. Inorg. Chem. 1987,
26, 91.
0020-1669/96/1335-1377$12.00/0 © 1996 American Chemical Society

1378 Inorganic Chemistry, Vol. 35, No. 5, 1996
Notes
Table 2. Selected Interatomic Distances (Å) and Angles (deg) for
Rh2(form)(µ-O2CCF3)[(C6H5)2P(CH2)2P(C6H5)(C6H4)](σ-CF3COO)-
(p-toluidine)
Table 1. Crystallographic Data for Rh2(form)(µ-O2CCF3)[(C6H5)2P-
(CH2)2P(C6H5)(C6H4)](σ-CF3COO)(p-toluidine)
formula
fw
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C52H47F6N3O4P2Rh2
1159.7
triclinic
P1h
11.118(2)
12.324(2)
19.753(2)
76.87(2)
80.33(2)
72.11(2)
2494.2(7)
2
Rh(1)-Rh(2)
Rh(1)-P(2)
Rh(1)-N(2)
Rh(2)-O(3)
Rh(2)-N(3)
P(1)-C(33)
P(1)-C(41)
P(2)-C(26)
O(1)-C(1)
O(3)-C(3)
N(1)-C(46)
N(2)-C(12)
N(3)-C(13)
2.606(1)
2.249(1)
2.064(4)
2.125(3)
2.039(4)
1.841(5)
1.807(4)
1.818(4)
1.234(5)
1.229(5)
1.440(5)
1.310(6)
1.441(6)
Rh(1)-P(1)
Rh(1)-O(2)
Rh(2)-O(1)
Rh(2)-N(1)
Rh(2)-C(40)
P(1)-C(34)
P(2)-C(20)
P(2)-C(32)
O(2)-C(1)
O(4)-C(3)
N(2)-C(5)
N(3)-C(12)
2.189(1)
2.169(3)
2.204(3)
2.148(3)
1.992(4)
1.818(4)
1.831(4)
1.821(4)
1.250(5)
1.244(6)
1.426(4)
1.321(4)
Z
F(000)
1172
1.54
0.796
0.813-0.841
0.710 73 (Mo KR)
23
F
calcd, g/cm3
µ, mm-1
Rh(2)-Rh(1)-P(1)
P(1)-Rh(1)-P(2)
P(1)-Rh(1)-O(2)
Rh(2)-Rh(1)-N(2)
P(2)-Rh(1)-N(2)
Rh(1)-Rh(2)-O(1)
O(1)-Rh(2)-O(3)
O(1)-Rh(2)-N(1)
Rh(1)-Rh(2)-N(3)
O(3)-Rh(2)-N(3)
Rh(1)-Rh(2)-C(40)
O(3)-Rh(2)-C(40)
N(3)-Rh(2)-C(40)
Rh(1)-P(1)-C(34)
Rh(1)-P(2)-C(20)
Rh(1)-P(2)-C(32)
Rh(1)-O(2)-C(1)
Rh(2)-N(1)-C(46)
Rh(1)-N(2)-C(12)
86.9(1) Rh(2)-Rh(1)-P(2)
84.8(1) Rh(2)-Rh(1)-O(2)
176.1(1) P(2)-Rh(1)-O(2)
85.1(1) P(1)-Rh(1)-N(2)
169.0(1) O(2)-Rh(1)-N(2)
81.1(1) Rh(1)-Rh(2)-O(3)
105.3(1)
89.3(1)
97.0(1)
92.4(1)
86.5(1)
91.4(1)
rel transm
λ(graphite monochromated), Å
T, °C
2θ range, deg
no. of data colld (2θ-ω)
no. of data refined
no. of variables
Ra
3-54
11734
7632 [F g 6 σ(F)]
628
0.035
0.045
90.8(1) Rh(1)-Rh(2)-N(1) 169.1(1)
88.0(1) O(3)-Rh(2)-N(1)
86.6(1) O(1)-Rh(2)-N(3)
177.2(1) N(1)-Rh(2)-N(3)
87.7(1)
87.0(1)
93.9(1)
b
Rw
GOFc
1.33
98.1(1) O(1)-Rh(2)-C(40) 178.3(1)
a R ) [∑|Fo| - |Fc|]/∑|Fo|. b Rw[∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
c GOF
2
90.8(1) N(1)-Rh(2)-C(40)
91.5(1) Rh(1)-P(1)-C(33)
115.2(1) Rh(1)-P(1)-C(41)
102.4(2) Rh(1)-P(2)-C(26)
109.7(2) Rh(2)-O(1)-C(1)
115.7(2) Rh(2)-O(3)-C(3)
122.3(2) Rh(1)-N(2)-C(5)
122.5(2)
92.8(1)
111.5(1)
114.7(1)
128.0(2)
123.5(2)
126.7(3)
117.9(3)
) [∑w(|Fo| - |Fc|)2/(Nobservns - Nvar)]1/2
.
intensities were evaluated by the learnt-profile procedure8 and then
corrected for Lorentz-polarization and absorption effects.9
The structure was solved by standard methods with the SHELXTL-
PLUS system.10 Neutral-atom scattering factors and anomalous disper-
sion corrections come from ref 11. The final geometrical calculations
and drawings were carried out with the PARST program12 and the XP
utility of the Siemens package,10 respectively. Selected bond distances
and bond angles are listed in Table 2. An Ortep view of the molecular
structure of 3 with the corresponding atom-labeling scheme is shown
in Figure 2.
Results and Discussion
The reactions of the dirhodium(II) complex Rh2(form)2-
(O2CCF3)2(H2O)2 with the diphosphines Ph2P(CH2)nPPh2 (n )
1-4) show an evolution which is critically dependent on the
aliphatic chain length of the ligand.
Figure 1. Proposed solid-state structure for Rh2(form)2(µ-O2-
CCF3)(dppm)(σ-O2CCF3).
Addition of 1 equiv of dppm to a solution of Rh2(form)2(O2-
CCF3)2(H2O)2 results in the immediate formation of a green
solution from which the complex Rh2(form)2(µ-O2CCF3)(dppm)-
(σ-O2CCF3) (2) was easily isolated. The solid IR spectrum of
2 shows two strong absorptions at 1645 and 1687 cm-l
evidencing the presence of nonequivalent trifluoroacetate groups.
The high energy band is shifted to 1695 cm-1 in CH3CN, where
2 behaves as 1:1 electrolyte, while it is little affected in CH2-
Cl2, where 2 behaves as a nonelectrolyte. Comparison of the
properties of 2 with those of the complex Rh2(form)2(O2CCF3)2-
(PPh2Py)13 shows a number of similar features and suggests
that 2 adopts in the solid state a similar geometry (Figure 1)
The complex, which in CH2Cl2 or benzene retains the same
geometry, in CH3CN, according to conductivity and solution
IR data, exhibits a different structure. This can be interpreted
in terms of dissociation of the monoligated trifluoroacetate group
so that the species existing in acetonitrile solution is [Rh2(form)2-
(µ-O2CCF3)(µ-dppm)(L)2](CF3COO) (L ) CH3CN).
In agreement with this suggestion the 31P NMR spectra in
CDCl3 and CH3CN are different. The 31P NMR spectrum in
CDCl3 is temperature invariant from -25 to +25 °C and is
consistent with the structure shown in Figure 1. In particular,
the resonance of the phosphorus atoms appears as two well
formed doublets of doublets of doublets centered at 7.37 and
18.81 ppm with one large (123-137 Hz) and one small (3.5-4
Hz) 1JRh-P and 2JRh-P coupling constant. As already reported14
these values provide conclusive evidence for the equatorial
coordination of both the phosphorus atoms. When the 31P NMR
spectrum is measured in CD3CN a different spin system appears.
At room temperature the spectrum reveals the expected ten lines
of an AA′XX′ spin system (A,A′ ) 103Rh; X,X′ ) 31P).
Analysis of the XX′ portion of the AA′XX′ pattern yielded the
2
following parameters: δ 15.5 (1JRh-P ) 123.1; JRh-P ) 3.85;
(8) Diamond, R. Acta Crystallogr., Sect. A 1969, 25, 43.
(9) Kopfmann, G.; Huber, R. Acta Crystallogr., Sect. A 1968, 24, 348.
(10) SHELXTL-PLUS, Version 4.2. Siemens Analytical X-ray Instruments
Inc., Madison, WI, 1991.
(11) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, England, 1974 Vol. IV.
JP-P ) 53.8; JRh-Rh ) -11.8 Hz).
In contrast to Rh2(form)2(O2CCF3)2(PPh2Py), complex 2 does
not react further with a second equivalent of dppm. The
synthesis of the complexes Rh2(O2CCH3)2(dppm)2Cl2 and Rh2-
(12) Nardelli, M. Comput. Chem. 1983, 7, 95 (version locally modified).
(13) Rotondo, E.; Bruno, G.; Nicolo`, F.; Lo Schiavo, S.; Piraino, P. Inorg.
Chem. 1991, 30, 1195.
(14) Rotondo, E.; Mann, B. E.; Tresoldi, G.; Piraino, P. Inorg. Chem. 1989,
28, 3070.

Notes
Inorganic Chemistry, Vol. 35, No. 5, 1996 1379
ppm with coupling constants to 103Rh of 145.8 and 3.6 Hz and
131.9 and 3.8 Hz, respectively. Spectroscopic and conductivity
data were not helpful in assigning the structure of 3, which was
elucidated only by an X-ray investigation.
Molecular Structure of Rh2(form)(µ-O2CCF3)[(C6H5)2P-
(CH2)2P(C6H5)(C6H4)](σ-O2CCF3)(p-toluidine). In the crystal
structure of the title complex, illustrated in Figure 2, the
asymmetric unit comprises one discrete Rh2(form)(µ-O2CCF3)-
[(C6H5)2P(CH2)2P(C6H5)(C6H4)](σ-O2CCF3)(p-toluidine) mol-
ecule. The dinuclear complex is mainly characterized by a
central unsymmetrical dirhodium unit bridged by three different
ligands: a trifluoroacetate, a formamidinate, and an ortho-
metalated 1,2-bis(diphenylphosphino)ethane which is also che-
lated to Rh(1) through the two phosphorus atoms. A second
trifluoroacetate group, monocoordinated at one equatorial posi-
tion, and a p-toluidine, located at one axial position, complete
the coordination around the Rh(2) atom which adopts a
pseudooctahedral arrangement for its six bonds. The geometry
of the 5-coordinated Rh(1) atom might be described as a
distorted square-pyramid with the equatorial plane orthogonal
to the dirhodium axis. The Rh-Rh separation of 2.606(1) Å,
which is considerably longer than the distances found in most
of the dirhodium(II) complexes,16 is significantly shorter than
the value [2.7331(8) Å] found in the complex [Rh2(form)(µ-
O2CCF3){(C6H5)2P(CH2)2P(C6H5)(C6H4)}(dppe)]CF3COO (1),
which exhibits essentially the same set of bridging ligands.
The Rh(1)-P(2) and Rh(1)-P(1) bond lengths are 2.189(1)
and 2.249(1) Å, respectively, with the longer Rh-P distance
reflecting the influence of the trans nitrogen. The Rh(2)-Rh-
(1)-P(2) bond angle is significantly larger than Rh(2)-Rh(1)-
P(1) [105.3(1) vs 86.9(1)°]. This difference may be explained
in terms of rigidity of the bridging fragment C(40)-C(41)-
P(1) and steric interactions between one phenyl rings ligated to
P(2) atom and the fluorine atoms of the adjacent trifluoroacetate
ligand. However the Rh(2)-Rh(1)-P(2) bond angle is sig-
nificantly smaller than the corresponding value found in 1
[121.68(6)°] where the steric requirement of the ligands force
the 5-coordinated rhodium to assume a different geometry,
namely distorted trigonal-bipyramidal instead of square-
pyramidal.
The formamidinate fragment is bonded to the dirhodium unit
in the usual way, namely σ,σ-N,N′ with delocalized double
bonds. As expected the Rh(2)-N(p-toluidine) bond length
[2.148(3) Å] is significantly longer than both Rh-N(form) bond
lengths. In addition the hydrogen atoms of the p-toluidine
fragment are involved in intramolecular interactions: the most
significant hydrogen bond acts on the uncoordinated oxygen
of the adjacent mono-coordinated trifluoroacetate [H(1A)‚‚‚
O(4) ) 1.93(4) Å; N(1)‚‚‚O(4) ) 2.834(6) Å; N(1)-H(1A)‚‚‚
O(4) ) 156(3)°].
The formation of the p-toluidine fragment is the likely result
of the fragmentation of the neutral p-tolylformamidine produced
during the ortho-metalation. Although fragmentation of neutral
formamidines is a known process in organic chemistry, no
corresponding reactions are found in the chemistry of metal
coordination complexes. The only similar example, reported
by Cotton, concerns the fragmentation of a p-tolylformamidine
with formation of a p-toluido fragment.17
Figure 2. View of the asymmetric unit represented by one discrete
molecule Rh2(form)(µ-O2CCF3)[(C6H5)2P(CH2)2P(C6H5)(C6H4)](σ-O2-
CCF3)(p-toluidine) with numbering scheme. Thermal ellipsoids are
drawn at the 50% probability level while the hydrogen atom size is
arbitrary.
[(C6H4)(C6H5)2P]2(dppm)2Cl2, recently reported by Cotton et
al.,15 shows that cis-coordination of two dppm ligands across
4+
the Rh2 core is possible. The reactions reported by Cotton
are promoted by (CH3)3SiCl whose function is to remove two
acetate groups from the equatorial position of the complexes
Rh2(O2CCH3)4 and Rh2(O2CCH3)2[(C6H5)2P(C6H4)]2, respec-
tively. This synthetic route leaves unoccupied four equatorial
sites where two dppm molecules are easily accommodated.
Although we do not have spectroscopic evidences on the
mechanism operating during the formation of complex 2, on
the basis of the literature data,13,14 we suggest that the reaction
leading to 2 proceeds through the following steps: (i) prelimi-
nary coordination of the phosphorus donor at one axial position
of the parent complex; (ii) axial-equatorial isomerization
followed by ring closure of the uncoordinated phosphorus atom
at the second rhodium atom. The reaction of 2 with a second
equivalent of dppm ought to proceed in a similar way. But
space-filling models show that the first step is hindered. The
presence of two phenyl and two p-tolyl groups does not leave
the metal centers open to axial attack by the phosphorus atom
of a second dppm molecule.
The reaction of Rh2(form)2(O2CCF3)2(H2O)2 with dppe in a
1:1 mole ratio proceeds in a different way, namely via ortho-
4+
metalation of the diphosphine across the Rh2 core. Mi-
croanalysis of the red crystals, obtained by minimal manipulation
in workup, shows that these crystals contain only one forma-
midinate group per dimeric unit and analyze as Rh2(form)(O2-
CCF3)2[(C6H4)(C6H5)P(CH2)2P(C6H5)2](C7H9N) (3). Once
formed, complex 3 reacts with a further equivalent of dppe
giving an ionic species which analyzes as [Rh2(form)(O2CCF3)-
{(C6H4)(C6H5)P(CH2)2P(C6H5)2}dppe]CF3COO (3D).
The solid state IR spectrum of 3 shows, in addition to the
bands associated with the formamidinate fragment, two strong
ν(O-C-O) absorptions at 1716 and 1647 cm-l. These absorp-
tions are also present in dichloromethane where 3 is not
conducting. In acetonitrile, where 3 behaves as 1:1 electrolyte,
NMR Studies. Once the solid-state structure of 3 had been
established, we monitored its formation by 31P NMR spectros-
.
a very strong band appears at 1695 cm-1 The 31P NMR
spectrum of 3, taken in CD3CN at room temperature, exhibits
two doublets of doublets of doublets centered at 37.56 and 52.4
(16) (a) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms,
J Claredon Press: Oxford, England, 1993. (b) Felthouse, T. R. Prog.
Inorg. Chem. 1982, 29, 73. (c) Boyar, E. B.; Robinson, S. D. Coord.
Chem. ReV. 1983, 50, 109.
(15) Cotton, F. A.; Dunbar, K. R.; Verbruggen, M. G. J. Am. Chem. Soc.
1987, 109, 5498.
(17) Cotton, F. A.; Poli, P. Inorg. Chim. Acta 1986, 122, 243.

1380 Inorganic Chemistry, Vol. 35, No. 5, 1996
Notes
Figure 4. 31P NMR spectrum at 245 K of the reaction mixture used
to form 3 in CDCl3. Insets show expansions of the region around +30
and -50 ppm.
Andersen description)18 with a linear Rh-Rh-P fragment. The
positive region of the 31P NMR spectrum at 240 K consists of
two closely spaced groups of resonances: a doublet of triplets
falling at 33.2 ppm (1JRh-P ) 124.1, 2J ) 4.4 Hz) and a broad
doublet at 31.72 ppm (J ) 122.1 Hz). These two sets of
resonances display similar intensity. All of these parameters
are consistent with the formation of equatorial adducts with a
bent arrangement of the Rh-Rh-P fragment (class II).
Figure 3. 31P NMR spectrum at 240 K of the reaction mixture used
to form 3 in CD3CN. Insets show the expansions of the region around
+30, -30, and -50 ppm.
Scheme 1
According to Scheme 1, we suggest that the reaction pattern
could involve sequential axial coordination of dppe through one
phosphorus atom with formation of the axial monoadduct 3A
followed by chelation of the dangling phosphorus at the same
rhodium atom with partial displacement of one trifluoroacetate
group from the equatorial position. In CD3CN at 240 K the
labile monocoordinated CF3COO group is partially dissociated
leading to the formation of two species: one neutral (3B) which
resonates at -45.74 and 33.2 ppm, and another one, mono-
cationic (3C), with resonances at -27.4 and 31.72 ppm. To
confirm this suggestion the same experiment was performed in
a noncoordinating solvent such as chloroform. The spectrum
in this solvent (Figure 4) is invariant from 230 to 270 K and
shows only two signals of similar intensity arising from 3B:
the low-field one, centered at 30.3 ppm, appears as a doublet
2
of triplets (1JRh-P ) 124.8; JRh-P ) 5.1 Hz) while the high-
field shows a doublet of doublets of doublets centered at -50.37
ppm (1JRh-P ) 74; 2JRh-P ) 68, JP-P ) 6 Hz). When the sample
was warmed to 273 K, the spectra became more complicated,
showing the presence of several interconverting species. No
attempts were made to assign these signals which were not
helpful in clarifying the reaction sequence. However the
chemical shift of these signals, which fall in the range 30-55
ppm, clearly indicates that all the phosphorus atoms are
equatorially coordinated. At 295 K the spectrum in CD3CN
reveals only the presence of the signals centered at 37.56 and
52.4 ppm due to the formation of 3. Very likely 3B and 3C
undergo the well established axial-equatorial isomerization with
formation of a bis-equatorial adduct which was not detected in
the spectra. These species undergo ortho-metalation followed
by formal transfer of an ortho-proton of one phenyl group,
closely approaching the adjacent rhodium atom, to a forma-
midinate group with consequent elimination of formamidine
from the “lantern” structure. After one day, these signals also
disappear and are replaced by a pair of doublets centered at
48.16 ppm indicating that in CD3CN complex 3 evolves toward
unidentified species still containing equatorially bonded phos-
copy in order to provide information about the reaction sequence
leading to complex 3. These experiments were performed in a
31P NMR tube by adding at room temperature, Via syringe, an
acetonitrile solution of dppe to a solution of Rh2(form)2(O2-
CCF3)2(H2O)2 dissolved in the same solvent, molar ratio 1:1,
immediately after the reaction mixture was cooled to 240 K.
The spectrum recorded at this temperature clearly shows the
presence of two axial-equatorial adducts. No signals due to
uncoordinated phosphorus atoms were detected. The negative
region of the spectrum, shown in Figure 3, shows two sets of
resonances of similar intensity falling at -45.74 (1JRh-P ) 80,
2JRh-P ) 63.2, JP-P ) 6.3 Hz) and -27.4 ppm (1JRh-P ) 77.8,
2JRh-P ) 50.5, JP-P ) 4.2 Hz). Both the chemical shifts and
coupling constants are consistent with the presence of axially
coordinated phosphorus atoms (class I adduct according to
(18) Girolami, G. S.; Mainz, V. V.; Andersen, R. A. Inorg. Chem. 1980,
18, 805.

Notes
Inorganic Chemistry, Vol. 35, No. 5, 1996 1381
phorus atoms. The latter species is not formed in CDCl3 even
after 6 days.
toward a compound of unknown composition with a resonance
centered at 8.93 ppm as a doublet of doublets.
Once formed, complex 3 easily reacts with a further
equivalent of dppe giving the ionic complex [Rh2(form)(O2-
CCF3){(C6H4)(C6H5)P(CH2)2P(C6H5)2}dppe]CF3COO (3D) whose
analytical, spectroscopic and conductivity data resemble those
of 1. The reaction very likely involves initial displacement of
the labile, equatorially coordinated CF3COO- group by one
phosphorus atom of a second dppe molecule followed by ring
closure by the second phosphorus at the axial site. Ortho-
metalation of a second dppe molecule is prevented. At first
sight it would appear that the synthesis of 3 allows us to
elucidate the overall reaction leading to complex 1. Careful
comparison of the structure of 1 with that of 3D shows that 1
and 3D do not display the same arrangement of ligands around
The reaction between Rh2(form)2(O2CCF3)2(H2O)2 and 1,2-
bis(diphenylphosphino)butane (dppb), molar ratio 1:1, proceeds
in a different way. With this long and fexible ligand the reaction
occurs without ortho-metalation and leads to the formation of
the complex Rh2(form)2(O2CCF3)2(dppb) (4). The 31P NMR
spectrum of 4 exhibits only one resonance, as a double doublet,
centered at 30.67 ppm with coupling constants to 103Rh of 135
and 5.8 Hz. The values of the chemical shift and coupling
constants strongly suggest that in complex 4 both phosphorus
atoms are equivalent and that dppb is chelated in equatorial
positions to one rhodium atom. Complex 4, which is moderately
stable in solution and insoluble in polar solvents, does not
undergo ortho-metalation. This failure may be due to the
flexible organic backbone of dppb which orientates the phenyl
groups away from the rhodium atoms thereby disfavoring the
ortho-metalation. The formation of equatorial adducts prevents
the formation of polymeric, zigzag, or linear chain compounds.
4+
the Rh2 core.
When the parent complex is allowed to react with 1,2-bis-
(diphenylphosphino)propane (dppp), molar ratio 1:1, a reaction
similar to that of dppe was observed. But, although analytical
1
data and the intensity of the IR and H NMR absorptions of
the formamidinate fragment indicate that this reaction also
proceeds by elimination of a formamidine ligand, we were not
able to obtain the compound in an acceptable pure form. By
following the reaction by 31P NMR spectroscopy from 250 to
308 K, we found a possible explanation. The sequence of the
31P resonances is essentially the same as that observed for the
dppe derivative and a species with resonances similar to those
exhibited by 3 was detected. By this time, this species evolves
Acknowledgment. Financial support from MURST and
Italian CNR is gratefully acknowledged.
Supporting Information Available: Tables S1-S6, giving crystal
data, anisotropic thermal parameters, hydrogen atom coordinates, atomic
coordinates, and full listing of bond lengths and bond angles for 3 (11
pages). Ordering information is given on any current masthead page.
IC9502032