Diazo complexes of rhenium with phosphite ligands: Facile synthesis of bis(dinitrogen) [Re(N2)2P4]BPh4 derivatives

Article abstract of DOI:10.1021/ic0104508

The synthesis of bis(dinitrogen) [Re(N₂)₂P₄]BPh₄ [P=P(OEt)₃ or P(OMe)₃] complexes, new diazenido [Re(PhN₂)(PhNHNH₂){P(OMe)₃}₄] (BPh₄)<

Full text of DOI:10.1021/ic0104508

Inorg. Chem. 2001, 40, 5465-5467
5465
Infrared spectra were recorded on a Nicolet Magna 750 FT-IR
spectrophotometer. NMR spectra (1H, 31P) were obtained on a Bruker
AC200 spectrometer at temperatures between +30 and -90 °C, unless
Diazo Complexes of Rhenium with Phosphite
Ligands: Facile Synthesis of Bis(dinitrogen)
[Re(N2)2P4]BPh4 Derivatives
1
otherwise noted. H spectra are referred to internal tetramethylsilane;
31P{1H} chemical shifts are reported with respect to 85% H3PO4, with
downfield shifts considered positive. The SwaN-MR software package5
was used to treat NMR data. The conductivity of 10-3 M solutions of
the complexes in CH3NO2 at 25 °C were measured with a CDM 83
Radiometer.
Gabriele Albertin,* Stefano Antoniutti,
Emilio Bordignon, and Eros Visentin
Dipartimento di Chimica, Universita` Ca' Foscari di Venezia,
Dorsoduro 2137, 30123 Venezia, Italy
Synthesis of Complexes. Compound ReOCl3(AsPh3)2 was prepared
as previously reported.6
ReceiVed April 27, 2001
[Re(N2)2P4]BPh4 (1) [P ) P(OEt)3 (a), P(OMe)3 (b)]. An excess
of the appropriate phosphite (6 mmol) was added to a solution of
ReOCl3(AsPh3)2 (1 mmol, 0.92 g) in 8 mL of tetrahydrofuran (thf),
and the reaction mixture stirred for 1 h. Ethanol or methanol (16 mL),
an excess of NEt3 (20 mmol, 2.8 mL), and then an excess of solid
tBuNHNH2‚HCl (10 mmol, 1.25 g) were sequentially added to the
solution, which was stirred for about 4 h. After filtration, the solvent
was removed under reduced pressure, giving an oil which was treated
with ethanol (10 mL) containing an excess of NaBPh4 (6.5 mmol, 2.2
g). The white solid that separated out from the resulting solution was
filtered off and discarded. A yellow solid appeared after the remaining
solution had been concentrated to 3-4 mL and cooled to -25 °C, which
was filtered and crystallized from CH2Cl2 (2 mL) and ethanol (5 mL);
yield 40-60%. Anal. Calcd for 1a: C, 47.02; H, 6.58; N, 4.57.
Found: C, 47.16; H, 6.44; N, 4.70. ΛM ) 52.9 Ω-1 mol-1 cm2. IR
(KBr): 2108 [s, ν(N2)] cm-1. 1H NMR [(CD3)2CO, 25 °C; δ]: 7.35-
6.70 (m, 20 H, Ph), 4.21 (m, 24 H, CH2), 1.41 (t, 36 H, CH3). 31P{1H}
NMR [(CD3)2CO, 25 °C; δ]: 110.3 s. Anal. Calcd for 1b: C, 40.88;
H, 5.34; N, 5.30. Found: C, 41.05; H, 5.49; N, 5.24. ΛM ) 56.3
Introduction
Previous reports1 from our laboratory dealt with studies of
the reaction of rhenium trichloro complexes, ReCl3[PPh(OEt)2]3,
with hydrazines, which gives new “diazo” derivatives, including
the first bis(dinitrogen) complex for this metal, [Re(N2)2P4]BPh4.
However, the compound was obtained as a byproduct in low
yield. We have now extended these studies to include the
reaction of phosphite P(OR)3 (R ) Me, Et) containing ReCl3P3
derivatives, and we have found a rapid and easy method for
the preparation, in high yield, of bis(dinitrogen) complexes.
In view of current interest2,3 in the chemistry of N2 deriva-
tives, which may be considered the first stage of the N2 fixation
processes,3,4 this paper reports the synthesis and characterization
of new bis(dinitrogen) complexes of rhenium. A study on the
influence of phosphite ligands in the reaction of ReCl3P3 species
with hydrazines, which allows new aryldiazenido and hydrazine
complexes to be prepared, is also reported.
1
Ω-1 mol-1 cm2. IR (KBr): 2109 [s, ν(N2)] cm-1. H NMR [CD2Cl2,
25 °C; δ]: 7.40-6.84 (m, 20 H, Ph), 3.72 (t, 36 H, CH3). 31P{1H}
NMR [CD2Cl2, 25 °C; δ]: 113.8 s.
Experimental Section
[ReCl(PhN2){P(OEt)3}4]BPh4 (2). An excess of triethyl phosphite
(6 mmol, 1 mL) was added to a solution of ReOCl3(AsPh3)2 (1 mmol,
0.92 g) in 10 mL of thf, and the reaction mixture was stirred for 1 h.
Ethanol (10 mL), an excess of NEt3 (10 mmol, 1.4 mL), and then an
excess of phenylhydrazine (10 mmol, 1.1 mL) were sequentially added
to the solution, which was refluxed for 30 min. The solvent was
removed under reduced pressure, giving an oil which was treated with
ethanol (10 mL) containing an excess of NaBPh4 (6 mmol, 2.05 g). A
reddish-brown solid slowly separated out after vigorous stirring of
the resulting solution, which was filtered and crystallized by dissolving
in CH2Cl2 (5 mL) and, after filtration and concentration, adding enough
ethanol (5 mL) to give separation of the solid; yield g40%. Anal.
Calcd: C, 49.49; H, 6.54; N, 2.14; Cl, 2.71. Found: C, 49.38; H, 6.48;
N, 2.20; Cl, 2.88. ΛM ) 55.5 Ω-1 mol-1 cm2. IR (KBr): 1603 [m,
All synthetic work was carried out in an inert atmosphere (Ar or
N2) using standard Schlenk techniques or a vacuum atmosphere drybox.
Once isolated, the complexes were found to be relatively stable in air,
but were nevertheless stored in an inert atmosphere at -25 °C. All
solvents were dried over appropriate drying agents, degassed on a
vacuum line, and distilled into vacuum-tight storage flasks. Metallic
rhenium was a Chempur (Germany) product and used as received.
Phosphites P(OMe)3 and P(OEt)3 were Aldrich products, which were
purified by distillation under nitrogen. Hydrazines CH3NHNH2,
tBuNHNH2‚HCl, PhNHNH2, and NH2NH2‚H2O were also Aldrich
products and used as received. Other reagents were purchased from
commercial sources in the highest available purity and used as received.
1
ν(N2)] cm-1. H NMR [(CD3)2CO, 25 °C; δ]: 7.60-6.80 (m, 25 H,
* To whom correspondence should be addressed.
(1) Albertin, G.; Antoniutti, S.; Bacchi, A.; Bordignon, E.; Miani, F.;
Pelizzi, G. Inorg. Chem. 2000, 39, 3283.
Ph), 4.06 (m, 24 H, CH2), 1.23 (t, 36 H, CH3). 31P{1H} NMR [(CD3)2-
CO, 25 °C; δ]: 95.9 s.
(2) (a) Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C.; Re, N.; Rizzoli,
C.; Chiesi-Villa, A. J. Am. Chem. Soc. 2000, 122, 3652. (b) Clentsmith,
G. K. B.; Bates, V. M. E.; Hitchcock, P. B.; Cloke, F. G. N. J. Am.
Chem. Soc. 1999, 121, 10444. (c) Peters, J. C.; Cherry, J.-P. F.;
Thomas, J. C.; Baraldo, L.; Mindiola, D. J.; Davis, W. M.; Cummins,
C. C. J. Am. Chem. Soc. 1999, 121, 10053. (d) Fryzuk, M. D.; Johnson,
S. A.; Rettig, S. J. J. Am. Chem. Soc. 1998, 120, 11024. (e)
Nishibayashi, Y.; Iwai, S.; Hidai, M. J. Am. Chem. Soc. 1998, 120,
10559. (f) Roussel, P.; Scott, P. J. Am. Chem. Soc. 1998, 120, 1070.
(g) O’Donoghue, M. B.; Zanetti, N. C.; Davis, W. M.; Schrock, R. R.
J. Am. Chem. Soc. 1997, 119, 2753. (h) Hahn, J.; Landis, C. R.;
Nasluzov, V. A.; Neyman, K. M.; Ro¨sch, N. Inorg. Chem. 1997, 36,
3947.
[{Re[P(OEt)3]4}2(µ-NH2NH2)2](BPh4)2 (3). An excess of P(OEt)3
(6 mmol, 1 mL) was added to a solution of ReOCl3(AsPh3)2 (1 mmol,
0.92 g) in 10 mL of thf, and the reaction mixture was stirred for 1 h.
An excess of NH2NH2‚H2O (6 mmol, 0.29 mL) and 10 mL of ethanol
were then added to the solution, which was stirred at room temperature
for 3 h. The solvent was removed under reduced pressure, giving an
oil which was treated with ethanol (10 mL) containing an excess of
NaBPh4 (6 mmol, 2.05 g). The white solid (probably the hydrazinium
salt) that separated out from the resulting solution was filtered off and
discarded. A reddish-brown solid slowly appeared after the remaining
solution had been concentrated to 5 mL and cooled to -25 °C. This
was filtered and crystallized from CH2Cl2 (2 mL) and ethanol (4 mL);
(3) (a) Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95, 1115. (b) Leigh, G.
J. New J. Chem. 1994, 18, 157. (c) Chatt, J.; Dilworth, J. R.; Richards,
R. L. Chem. ReV. 1978, 78, 589.
(4) (a) Shilov, A. E. Metal Complexes in Biomimetic Chemical Reactions;
C.R.C.: Boca Raton, FL, 1997. (b) Sellmann, D.; Sutter, J. Acc. Chem.
Res. 1997, 30, 460. (c) Richards, R. L. Coord. Chem. ReV. 1996, 154,
83. (d) Leigh, G. J. Science 1995, 268, 827.
(5) Balacco, G. J. Chem. Inf. Comput. Sci. 1994, 34, 1235. Upgrades via
the Internet at: http://qobrue.usc.es/jsgroup/swan/index.html.
(6) Garcia-Fonta´n, S.; Marchi, A.; Marvelli, L.; Rossi, R.; Antoniutti, S.;
Albertin, G. J. Chem. Soc., Dalton Trans. 1996, 2779.
10.1021/ic0104508 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/13/2001

5466 Inorganic Chemistry, Vol. 40, No. 21, 2001
Notes
Scheme 1a
obtained1 for the reaction of the related ReCl3P3 complex
containing PPh(OEt)2 ligand, which afforded the neutral mono-
diazoto ReCl(N2)P4 compound in the reaction with t-butylhy-
drazine; but the bis(dinitrogen) complex with PPh(OEt)2 ligand
was obtained as a byproduct in low yield (15%) from the
reaction of ReCl3P3 with methylhydrazine. Instead, with phos-
phites P(OEt)3 and P(OMe)3, bis(dinitrogen) complexes 1 may
easily be prepared in pure form and in good yield.
Dinitrogen complexes 1 were also obtained by operating in
both argon and dinitrogen atmospheres, and therefore, the N2
ligand must come from the t-butylhydrazine, which can liberate
N2 in the reduction reaction of the central metal from Re(III)
to Re(I).
Bis(dinitrogen) complexes 1 are yellow solids that are stable
in air and in solutions of polar organic solvents, in which they
behave as 1:1 electrolytes.8 The analytical and spectroscopic
data support the proposed formulation and suggest a mutually
trans position for the two dinitrogen ligands.
The IR spectra show only one strong νN2 band at 2108 for
1a and 2109 cm-1 for 1b, whereas the 31P{1H} NMR spectra
appear as sharp singlets, in the temperature range +30 to -90
°C, fitting the proposed geometry.
Reactivity studies indicate that 1 are very robust complexes,
in which the N2 ligand cannot be substituted by CO, phosphites,
halogenide, or other donor molecules, and starting complexes
were found unchanged after 48 h of reaction at room temper-
ature. The N2 groups are also unreactive toward both electro-
philic and nucleophilic reagents. Treatment with an excess of
Brønsted acid (CF3SO3H, HBF4‚Et2O, CF3COOH) as well as
with CF3SO3CH3 always gives unreacted dinitrogen compound
1, even after 48 h of reaction. Treatment with an excess of
methyl- or buthyllithium at room temperature or in refluxing
thf also gives unreacted complex 1, and only after long reaction
times was some decomposition observed, thus confirming the
noteworthy stability of bis(dinitrogen) derivatives.
Results obtained with t-buthylhydrazine prompted us to
extend the study of the reactivity of phosphite-containing
ReCl3P3 complexes to other hydrazines to determine the
influence of the phosphite ligands on these reactions. The results
are reported in Scheme 2.
The nature of the phosphite ligand seems to be crucial in
determining the reaction course and stoichiometry of the
resulting diazo complexes. Thus, under reflux, the reaction of
ReCl3P3 with phenylhydrazine afforded the [ReCl(PhN2)P4]BPh4
(2) compound in the case of P(OEt)3; the hydrazine-phenyl-
diazenido [Re(PhN2)(PhNHNH2)P4](BPh4)2 (4) derivative was
separated by operating at room temperature with P(OMe)3.
Various experimental conditions in the reaction between ReCl3P3
and phenylhydrazine, i.e. 25 °C with P(OEt)3 and under reflux
with P(OMe)3, afforded only intractable oils.
a P ) P(OEt)3 (a), P(OMe)3 (b); (i) ) in thf/ROH (R ) Et, Me)
(1:2) solution containing an excess of NEt3 (20:1 ratio).
yield g20%. Anal. Calcd: C, 47.96; H, 7.04; N, 2.33. Found: C, 48.14;
H, 6.95; N, 2.27. ΛM ) 124.9 Ω-1 mol-1 cm2. IR (KBr): 3367, 3346
[m, ν(NH)], 1616 [δ(NH2)] cm-1. 1H NMR [CD2Cl2, 25 °C; δ]: 7.40-
6.84 (m, 40 H, Ph), 4.02, 3.87 (m, 48 H, CH2), 2.30 (br, 8 H, NH2),
1.26, 1.19 (t, 72 H, CH3). 31P{1H} NMR [CD2Cl2, -55 °C; δ]: spin
system A2B2, δA 120.3, δB 115.7, JAB ) 36.5 Hz.
[Re(PhN2)(PhNHNH2){P(OMe)3}4](BPh4)2 (4). To a solution of
ReOCl3(AsPh3)2 (1 mmol, 0.92 g) in 10 mL of thf were added first an
excess of P(OMe)3 (6 mmol, 0.74 mL) and, after 1 h of stirring, then
an excess of NEt3 (10 mmol, 1.4 mL), an excess of PhNHNH2 (10
mmol, 1.1 mL), and 10 mL of methanol. The reaction mixture was
stirred for 3 h, and then the solvent was removed under reduced
pressure, giving an oil which was treated with 7 mL of methanol
containing an excess of NaBPh4 (6 mmol, 2.05 g). An orange solid
separated out from the resulting solution, which was cooled to -25
°C. This was filtered, and the complex extracted with three 5 mL
portions of CH2Cl2. The extracts were evaporated to dryness, giving a
pink solid which was crystallized from ethanol; yield g35%. Anal.
Calcd: C, 56.37; H, 5.85; N, 3.65. Found: C, 56.24; H, 5.93; N, 3.55.
ΛM ) 128.7 Ω-1 mol-1 cm2. IR (KBr): 3356 w, 3325 m, 3263 m
[ν(NH)], 1755 [s, ν(N2)] cm-1. H NMR [CD2Cl2, 25 °C; δ]: 7.50-
1
6.80 (m, 50 H, Ph), 4.65 (br, 2 H, NH2), 3.61 (m, 36 H, CH3). 31P{1H}
NMR [CD2Cl2, 25 °C; δ]: spin system ABC2, δA 108.8, δB 100.3, δC
96.6, JAB ) 59.7, JAC ) 34.9, JBC ) 52.4 Hz.
[ReCl(CH3N2)(CH3NHNH2){P(OMe)3}3]BPh4 (5). To a solution
of ReOCl3(AsPh3)2 (1 mmol, 0.92 g) in 10 mL of thf were added, first
an excess of P(OMe)3 (6 mmol, 0.74 mL) and, after 1 h of stirring, an
excess of NEt3 (10 mmol, 1.4 mL), an excess of methylhydrazine (10
mmol, 0.53 mL), and 10 mL of methanol. The reaction mixture was
stirred at room temperature for 3 h, and then the solvent was removed
under reduced pressure. The oil obtained was treated with methanol (7
mL) containing an excess of NaBPh4 (6 mmol, 2.05 g). A dark green
solid slowly separated out from the resulting solution, which was filtered
and crystallized from CH2Cl2 (2 mL) and ethanol (4 mL); yield
g20%. Anal. Calcd: C, 41.94; H, 5.63; N, 5.59; Cl, 3.54. Found: C,
41.78; H, 5.76; N, 5.45; Cl, 3.72. ΛM ) 54.9 Ω-1 mol-1 cm2. IR
(KBr): 3331 w, 3230 sh, 3217 m [ν(NH)], 1734 [m, ν(N2)] cm-1. H
1
NMR [CD2Cl2, 25 °C; δ]: 7.40-6.87 (m, 20 H, Ph), 5.20 (br, 2 H,
NH2), 3.70 t, 3.62 d (27 H, CH3 phos), 3.21 (s, 3 H, CH3N2), 3.15 (br,
1 H, NH), 2.43 (d, 3 H, CH3NH). 31P{1H} NMR [CD2Cl2, 25 °C; δ]:
spin system AB2, δA 112.3, δB 99.8, JAB ) 35.0 Hz.
Results and Discussion
Treatment of ReCl3[P(OMe)3]3 with methylhydrazine gave
a mixture of bis(dinitrogen) complex 1b and methyldiazenido
[ReCl(CH3N2)(CH3NHNH2)P3]BPh4 (5) derivative, which were
separated by fractional crystallization. In the case of P(OEt)3,
the reaction gave only decomposition products.
Finally, the reaction of ReCl3P3 with the hydrazine NH2NH2‚
H2O afforded binuclear [{ReP4}2(µ-NH2NH2)2](BPh4)2 (3) spe-
cies in the case of P(OEt)3, and an intractable mixture of
products with P(OMe)3.
Bis(dinitrogen) complexes [Re(N2)2P4]BPh4 (1) may be
prepared in 40-60% yield by treating ReOCl3(AsPh3)2 first with
phosphites P(OR)3 and then with an excess of t-butylhydrazine,
as shown in Scheme 1.
The reaction of ReOCl3(AsPh3)2 with phosphites probably
proceeds to give the ReCl3P3 complexes, which cannot be
isolated as solids but only as oily products. The NMR data,
however, show the proton resonances at unusual chemical shifts
(between +20 and +6 ppm), as observed in the related ReCl3-
[PPh(OEt)2]3 compound6 and due to second-order paramagnet-
ism of an octahedral Re(III) ReCl3P3 complex.7 Treatment of
these ReCl3P3 complexes with t-butylhydrazine gave exclusively
bis(dinitrogen) derivatives 1, which were isolated as yellow
solids and characterized. This result contrasts with those
(7) (a) Chatt, J.; Leigh, G. J.; Mingos, D. M. P.; Randall, E. W.; Shaw,
D. J. Chem. Soc., Chem. Commun. 1968, 419. (b) Chatt, J.; Leigh, G.
J.; Mingos, D. M. P. J. Chem. Soc. A 1969, 1674. (c) Randall, E. W.;
Shaw, D. J. Chem. Soc. A 1969, 2867.
(8) Geary, W. J. Coord. Chem. ReV. 1971, 7, 81.

Notes
Inorganic Chemistry, Vol. 40, No. 21, 2001 5467
Scheme 2a
spectroscopic data support the proposed formulation, and a
geometry in solution of the type shown in Scheme 2 may also
be assigned for the diazo derivatives.
The IR spectrum of the compound [ReCl(ArN2)P4]BPh4 (2)
showed only one νN2 band at 1603 cm-1. By comparison with
the νN2 values of other diazenido complexes,1,10,11 this value
suggests the presence of a singly bent ArN2 ligand. In the
temperature range +30 to -80 °C, the 31P{1H} NMR spectra
showed a sharp singlet, which was compatible with the presence
of four equivalent phosphite ligands. On this basis, a trans
geometry was proposed for the aryldiazenido species 2.
The mutually cis position of the aryldiazenido and hydrazine
ligands occurs in the [Re(PhN2)(PhNHNH2)P4]2+ (4) cation
according to the ABC2 multiplet observed in the 31P{1H} NMR
spectra. The IR spectra support the presence of diazenido and
hydrazine ligands, showing a medium-intensity band at 1755
cm-1 attributed to the νNN of ArN2 ligand and three medium-
weak absorptions between 3356 and 3263 cm-1 due to the νNH
of the PhNHNH2 group. The presence of the hydrazine ligand
1
is also confirmed by the H NMR spectrum, which shows a
broad signal at 4.65 ppm due to the NH2 group. The NH signal
is probably masked by the signals of the phenyl protons.
The IR spectra of methylhydrazine complex 5 show the νN2
band at 1734 cm-1 and νNH absorptions of the hydrazine ligand
1
at 3331-3217 cm-1. The H NMR data confirm the presence
of the CH3NHNH2 group, showing the characteristic NH2 and
NH signals at 5.20 and 3.15 ppm and the methyl substituent as
a doublet at 2.43 ppm. These assignments were confirmed by
integration data and homodecoupling experiments. In the
temperature range +30 to -80 °C, the 31P{1H} NMR spectra
appear as an AB2 multiplet, indicating two magnetically
equivalent phosphite ligands which are different from the third.
The spectroscopic data do not allow an unambiguous assignment
of geometry in solution for this compound, but comparison with
the related [ReCl(CH3N2)(CH3NHNH2){PPh(OEt)2}3]BPh4 com-
pound,1 whose structure is known, suggests a cis geometry for
complex 5 (Scheme 2).
a Prepared in situ by reacting ReOCl3(AsPh3)2 with an excess (6:1
ratio) of the appropriate phosphite; (i) in thf/EtOH (1:1) solution
containing an excess of NEt3 (10:1 ratio) and a slight excess of
P(OEt)3 (2:1); (ii) in thf/MeOH (1:1) solution containing an excess of
both NEt3 (10:1 ratio) and P(OMe)3 (3:1).
The formation of various diazo species upon changing the
ancillary phosphite ligand is not surprising but difficult to
rationalize in our case, because the electronic and steric
properties of P(OMe)3 and P(OEt)3 are not sufficiently different9
as to explain the results shown in Scheme 2. However, the yields
of the isolated diazo complexes are quite low, and the final
product often contains only decomposition products, thus
hindering clear-cut comparisons among reactions involving the
two phosphites. Also, comparisons among these results and
previous ones1 on the reactivity of ReCl3P3 containing PPh-
(OEt)2 with hydrazines confirm the important role of the
ancillary ligand in determining the nature of the resulting diazo
species, although no reasonable trend can be deduced. It seems
that every phosphite leads to its own stable complex, according
to a reaction course, and depending on the peculiar properties
of all the ligands.
The IR spectrum of hydrazine complex [{Re[P(OEt)3]4}2(µ-
NH2NH2)2](BPh4)2 (3) shows only two νNH bands at 3367 and
3346 cm-1 of the NH2NH2 ligand. In addition, in the temperature
1
range +30 to -80 °C, the H NMR spectra shows only one
signal for the NH2 protons in any solvent used [CD2Cl2, (CD3)2-
CO, or CDCl3]. Finally, an A2B2 multiplet was observed in the
31P{1H} NMR spectra of 3, which can be simulated with the
parameters reported in Experimental Section. On these bases,
a symmetric geometry of type of Scheme 2, with two bridging
NH2NH2 ligands, is proposed for hydrazine complex 3.
All diazo complexes 2-5 were isolated as diamagnetic air-
stable solids, soluble in polar organic solvents, in which they
behave as 1:1 (2, 5) or 2:1 (3, 4) electrolytes.8 Analytical and
Acknowledgment. The financial support of MURST, Rome
(Programmi di Ricerca Scientifica di Rilevante Interesse Na-
zionale, Cofinanziamento 2000/2001) is gratefully acknowledged.
IC0104508
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