Preparation of Rhenium(I) and Rhenium(II) Amine Dinitrogen Complexes and the Characterization of an Elongated Dihydrogen Species

Article abstract of DOI:10.1021/ic9700866

A series of rhenium(I) dinitrogen complexes were prepared containing predominantly amine ligands. On the basis of infrared and electrochemical data, a system was selected that was anticipated to be a suitable precursor for an elongated dihydrogen complex. Upon oxidation by AgOTf, the dinitrogen ligand of fac-[Re(PPh₃)(PF₃)(dien)(N₂)⁺ (3) is replaced with triflate to generate 13, fac-[Re(PPh₃)(PF₃)(dien)(OTf)]OTf, a convenient precursor to rhenium(II) and rhenium(I) amine complexes. Reduction of the rhenium(II) triflate 13 under 1 atm of hydrogen gas generates the complex fac-[Re(dien)(PPh₃)(PF₃)(dien)(H₂)]⁺ (fac-14). T₁ measurements indicate a dihydrogen species with a H-H distance of 1.38 ± 0.03 A?. The HD analog displays a J_HD of 6.7 Hz, corresponding to a H-H distance of 1.31 ± 0.03 A?, a value in good agreement with that determined from T₁ data and among the largest ever measured for an elongated dihydrogen system.

Full text of DOI:10.1021/ic9700866

Inorg. Chem. 1997, 36, 3553-3558
3553
Preparation of Rhenium(I) and Rhenium(II) Amine Dinitrogen Complexes and the
Characterization of an Elongated Dihydrogen Species
R. Martin Chin,† Joseph Barrera,† Raymond H. Dubois,† Lisa E. Helberg,† Michal Sabat,†
Tanya Y. Bartucz,‡ Alan J. Lough,‡ Robert H. Morris,*,‡ and W. Dean Harman*,†
Departments of Chemistry, University of Virginia, Charlottesville, Virginia 22901, and University of
Toronto, 80 Saint George Street, Toronto, Ontario, M5S 3H6 Canada
ReceiVed January 24, 1997X
A series of rhenium(I) dinitrogen complexes were prepared containing predominantly amine ligands. On the
basis of infrared and electrochemical data, a system was selected that was anticipated to be a suitable precursor
for an elongated dihydrogen complex. Upon oxidation by AgOTf, the dinitrogen ligand of fac-[Re(PPh3)(PF3)-
(dien)(N2)]+ (3) is replaced with triflate to generate 13, fac-[Re(PPh3)(PF3)(dien)(OTf)]OTf, a convenient precursor
to rhenium(II) and rhenium(I) amine complexes. Reduction of the rhenium(II) triflate 13 under 1 atm of hydrogen
gas generates the complex fac-[Re(dien)(PPh3)(PF3)(dien)(H2)]+ (fac-14). T1 measurements indicate a dihydrogen
species with a H-H distance of 1.38 ( 0.03 Å. The HD analog displays a JHD of 6.7 Hz, corresponding to a
H-H distance of 1.31 ( 0.03 Å, a value in good agreement with that determined from T1 data and among the
largest ever measured for an elongated dihydrogen system.
Introduction
D, where the H‚‚H notation refers to an elongated dihydrogen
ligand.8
While the vast majority of reported rhenium(I) compounds
fall in the realm of organometallic chemistry,1 a notable
exception is a class of rhenium(I) dinitrogen complexes initially
discovered by Chatt, Dilworth, and Leigh.2 The synthetic
approach to these materials from a general ReV precursor offers
an unusually high flexibility in the preparation of an inorganic
ligand set for rhenium (Vide infra), yet this versatile methodol-
ogy has not been widely utilized. Our intention was to prepare
for rhenium a series of low-valent dinitrogen complexes with
amine-based ligand sets, a class of materials well represented
for group VIII transition metals, but practically unknown for
group VII. Parallel to the synthetic approach used for osmium
ammines,3 we hoped to replace the dinitrogen ligand with triflate
upon oxidation to ReII, thus providing suitable precursors to a
novel class of ReII and ReI coordination complexes.
One interesting application of this methodology would be
toward the study of dihydrogen complexes.4 The few dihy-
drogen complexes with nitrogen donor sets that have been
prepared are currently limited to neutral ruthenium and osmium
complexes containing porphyrin ligands,5 cationic osmium
complexes containing ammine or ethylenediamine ligands,6 and
neutral pyrazolylborate complexes of rhodium7a and ruthenium.7b
We report herein the characterization in solution of the new
triamine complexes fac-[Re(H‚‚X)(dien)(PF3)(PPh3)]+, X ) H,
Results and Discussion
Preparation and Properties of the Dinitrogen Complexes.
The preparation of a number of organodiazenido complexes of
the form ReCl2(PPh3)2(NNC(O)Ph)L has been described by
Chatt et al.,2 and these compounds, as well as their ReV precursor
(1; Figure 1), are versatile synthons to rhenium(I) dinitrogen
complexes containing phosphine ligands. We have found that
this general methodology may be readily applied to amine-based
complexes as the following examples illustrate.
Treatment of the chelate 1 with PF3 results in the formation
of ReCl2(PPh3)2(NNC(O)Ph)(PF3) (2) as originally described
by Chatt et al.2 When 2 is combined with the tridentate ligand
dien, along with NaOTf to facilitate removal of the halide, the
cationic dinitrogen complex fac-[Re(PPh3)(PF3)(dien)(N2)](OTf)
(3) is recovered in 46% yield as a single stereoisomer.
Significant features of 3 include a dinitrogen stretching fre-
quency of 2033 cm-1 and a reversible ReII/ReI reduction
potential of 0.86 V (NHE). Crystals of 3‚2(acetone) were grown
from an acetone solution, and the structure of the triflate salt
was determined by X-ray diffraction (Table 1). An ORTEP
representation of the cation (Figure 2) indicates a facial dien
geometry with the central nitrogen trans to the triphenylphos-
phine. The Re-N2 distance (1.927(9) Å) is predictably shorter
than the corresponding bonds of the dien ligand, and the N-N
† University of Virginia.
‡ University of Toronto.
X Abstract published in AdVance ACS Abstracts, July 1, 1997.
(1) O’Conner, J. M. In ComprehensiVe Organometallic Chemistry II;
Casey, P. C., Ed.; Pergamon Press, Elsevier Science Inc.: New York,
1995; Vol. 6, Chapter 9.
(6) (a) Li, Z. W.; Taube, H. J. Am. Chem. Soc. 1994, 116, 9506-9513.
(b) Hasegawa, T.; Li, Z.; Parkin, S.; Hope, H.; McMullan, R. K.;
Koetzle, T. F.; Taube, H. J. Am. Chem. Soc. 1994, 116, 4352-4356.
(c) Harman, W. D.; Taube, H. J. Am. Chem. Soc. 1990, 112, 2261-
2263.
(7) (a) Eckert, J.; Albinati, A.; Bucher, U. E.; Venanzi, L. M. Inorg. Chem.
1996, 35, 1292-1294. (b) Moreno, B.; Sabo-Etienne, S.; Chaudret,
B.; Rodriguez, A.; Jalon, F.; Trofimenko, S. J. Am. Chem. Soc. 1995,
117, 7441-7451.
(8) (a) Klooster, W. T.; Koetzle, T. F.; Jia, G.; Fong, T. P.; Morris, R.
H.; Albinati, A. J. Am. Chem. Soc. 1994, 116, 7677-7681. (b)
Maseras, F.; Lledos, A.; Costas, M.; Poblet, J. M. Organometallics
1996, 15, 2947. In this account, we use the term “elongated” to
represent the gray region between classical dihydride (dH-H > 1.7 Å;
JHD < 5 Hz) and dihydrogen (dH-H < 1.0 Å; JHD > 25 Hz) complexes.
(2) (a) Chatt, J.; Dilworth, J. R.; Leigh, G. J. J. Chem. Soc., Dalton Trans
1973, 612. (b) Chatt, J.; Hussain, W.; Leigh, G. J.; Ali, H. M.; Pickett,
C. J.; Rankin, D. A. J. Chem. Soc., Dalton Trans. 1985, 1131. (c)
Pombeiro, A. J. L.; Hitchcock, P. B.; Richards, R. L. J. Chem. Soc.,
Dalton Trans. 1987, 319.
(3) Lay, P. A.; Magnuson, R. H.; Taube, H. Inorg. Synth. 1986, 24, 269.
(4) (a) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120. (b) Jessop, P. G.;
Morris, R. H. Coord. Chem. ReV. 1992, 121, 155-284. (c) Heinekey,
D. M.; Oldham, W. J. J. Chem. ReV. 1993, 93, 913.
(5) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E.; Lewis, N. S.;
Lopez, M. A.; Guilard, R.; L’Her, M.; Bothner-By, A. A.; Mishra, P.
K. J. Am. Chem. Soc. 1992, 114, 5654.
S0020-1669(97)00086-4 CCC: $14.00 © 1997 American Chemical Society

3554 Inorganic Chemistry, Vol. 36, No. 16, 1997
Chin et al.
Figure 1. Preparation of assorted rhenium(I) dinitrogen amine complexes.
Table 1. Crystallographic Data for Compounds 3 and 15
3‚2(acetone)
15
empirical formula
formula wt
a (Å)
b (Å)
c (Å)
C29H40N5O5F6P2SRe
932.87
11.363(4)
9.760(3)
33.428(6)
C27H34N3O7F9P2S2Re
995.83
10.123(2)
10.8123(11)
17.577(3)
107.068(12)
101.26(2)
94.479(10)
1784.9(5)
P1h (No. 2)
2
1.85
0.710 73
-100(2)
37.05
R (deg)
â (deg)
90.16(2)
γ (deg)
vol (Å3)
space group
Z
3707(3)
P21/c (No. 14)
4
F
calcd (g cm-3
)
1.67
λ (Å)
0.710 69
-120(2)
35.31
0.039 (I > 3σ(I))
0.052 (I > 3σ(I))
T (°C)
µ (cm-1
R(F)a
)
0.051 (I > 2σ(I))
0.106 (I > 2σ(I))
Rw(F)a
2
a R ) ∑(|Fo| - |Fc|)/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
where w ) 1/σ2(F).
distance has increased to 1.13(1) Å. Both of these observations
are a direct result of the rhenium-dinitrogen π-back-bonding
interaction.
In a similar manner, a phosphite analog (4) of the organo-
diazenido complex 2 may be prepared from 4-ethyl-2,6,7-trioxa-
1-phosphabicyclo[2.2.2]octane (etpb). This material reacts with
dien to give the dinitrogen complex 5, which is isolated in a
∼5:4 ratio of two isomers. Chromatographic separation on
alumina provides pure samples of both isomers, and the major
component (5A) has been determined by X-ray diffraction to
have a stereochemical arrangement identical to that of 3 where
the dien ligand adopts a facial orientation with the central
nitrogen trans to the triphenylphosphine (Figure 2). The
stereochemistry of the second isomer (5B) has not been
determined.
Figure 2. ORTEP drawings of the cations of [Re(N2)(PPh3)(PF3)(dien)]-
(OTf) (3; top) and [Re(η1-acetone)(PPh3)(PF3)(dien)](OTf)2 (15; bot-
tom). Selected bond lengths (Å) for 3 Re-N1, 1.927(9); Re-P1,
2.346(3); Re-P2, 2.175(3); Re-N3, 2.241(7); Re-N4, 2.185(8); Re-
N5, 2.195(8); N1-N2; 1.13(1). Selected bond lengths for 15: Re-
P1, 2.433(2); Re-P2, 2.178(2); Re-N1, 2.181(6); Re-N2, 2.211(6);
Re-N3, 2.162(6); Re-O1, 2.098(5).
complexes show a higher energy dinitrogen band and a more
positive reduction potential, features that are consistent with a
more electron-deficient metal center. A preliminary X-ray
diffraction study for 6 confirms the stereochemistry shown in
Figure 1. A product similar to 7 results when 1,2-diaminocy-
clohexane is used instead of ethylenediamine, and according to
When the diazenido phosphite complex, 4, is combined with
2,2′-bipyridine (bpy) or with ethylenediamine (en), dinitrogen
complexes are isolated (6 and 7) with both arylphosphines intact.
Compared to its dien analog (5), both the bpy (6) and en (7)

ReI and ReII Amine Dinitrogen Complexes
Inorganic Chemistry, Vol. 36, No. 16, 1997 3555
Table 2. Infrared and Electrochemical Data for Rhenium
Dinitrogen Amine Complexes
ν(NN)
E1/2
compound
(KBr, cm-1) (ReI/ReII; V, NHE)
Rhenium(I)
[Re(N2)(PPh3)(en)2]OTf (10)
[Re(N2)(PPh3)(ampy)2]OTf (11)
[Re(N2)(PPh3)(tmpa)]OTf (12)
[Re(N2)(PPh3)(tbpy)(ampy)]Cl (9)
[Re(N2)(PPh3)(etpb)(dien)]OTf (5B)
[Re(N2)(PPh3)(etpb)(dien)]OTf (5A)
[Re(N2)(PPh3)2(en)(etpb)]OTf (7)
[Re(N2)(PPh3)2(bpy)(etpb)]OTf (6)
[Re(N2)(PPh3)(PF3)(dien)]OTf (3)
1872
1915
1920
1940
1969
1986
2004
2008
2033
-0.50
-0.20
0.18
-0.02
0.27
0.26
0.78
0.84
0.86
Rhenium(II)
[Re(N2)(PPh3)(tmpa)](OTf)2
[Re(N2)(PPh3)(ampy)2](OTf)2
[Re(N2)(PPh3)(en)2](OTf)2
2054
2030
1998
0.18
-0.20
-0.50
reduction potentials range from -0.50 to 0.86 V, indicating the
electron-rich nature of the metal center. For several of the most
reducing metal systems (10-12), oxidation by Ag+ or [FeCp2]+
generates a stable ReII dinitrogen complex (characterized by IR
and cyclic voltammetry), and the infrared data for these
complexes are reported in Table 2. For other cases, however,
the one-electron oxidation of the ReI dinitrogen complex causes
the displacement of dinitrogen by solvent or counterion, thus
creating a useful synthon. For example, in a recent communica-
tion,10 the compound cis-[Re(PPh3)(ampy)(tbpy)(OTf)](OTf)
was shown to be a useful precursor to a number of ReI
complexes including an η2-benzaldehyde system.
Preparation and Properties of a Stretched Dihydrogen
Complex. In the present study, our hope was to find a
compound suitable for use in the preparation of a dihydrogen
complex. As shown in Figure 3, the electrochemical potentials
and dinitrogen stretching frequencies for complex 3, [Re(N2)-
(PPh3)(PF3)(dien)]OTf, are the closest to the prescribed “stabil-
ity” range for a dihydrogen complex.11 Thus, treatment of 3
with AgOTf in acetone followed by recrystallization from DME
results in the formation of fac-[Re(PPh3)(PF3)(dien)(OTf)]-
(OTf)‚DME (13), a ReII complex containing either a triflate or
solvento ligand that is readily exchanged (Figure 4). Compound
13 is found to be a versatile synthon to a number of ReI
complexes with unsaturated organic ligands.12 Moreover, when
13 is reduced (Mg0) under an atmosphere of dihydrogen, the
product (14) is found to be a 20:3 mixture of two rhenium(I)-
hydride species fitting the stoichiometry Re(PPh3)PF3(dien)-
(H2)(OTf).
A sample of 14 in acetone-d6 gives two signals in the hydride
region (Figure 5a): a pseudotriplet at -5.97 ppm corresponding
to the major isomer and a doublet of doublets of quartets at
-4.04 ppm for the minor species. The equal H-P couplings
in the pseudotriplet imply that both phosphorus ligands are cis
to the hydrogen and, thus, that the dien ligand is required to
have a fac configuration. Therefore, the pseudotriplet is
assigned to fac-14 with a stereochemistry similar to that of
compounds 3 and 15 (Figure 4). The JPP coupling of 25 Hz in
the 31P resonances of the major species is also indicative of
mutually cis phosphorus ligands. The existence of a single
hydride signal for the fac species implies that both hydrogen
atoms occupy one site in the coordination polyhedron, that they
are rapidly exchanging, or that they are coupled by quantum
exchange.13,14 With regard to the second isomer (mer-14), the
multiplicity (ddq) of the -4.04 ppm peak indicates that this
Figure 3. Correlation of dinitrogen stretching frequency (cm-1) with
ReII/ReI reduction potential for ReI (circle) and ReII (square) dinitrogen
amine complexes.
preliminary X-ray diffraction data, this compound has stereo-
chemistry identical to that of compound 6. Thus, the stereo-
chemistry of 7 is tentatively assigned to be that shown in Figure
1.
The ammonia diazenido complex Re(NH3)(NNC(O)Ph)(PPh3)2-
Cl2 originally reported by Dilworth9 may be prepared directly
from compound 1 and ammonia and may be used as a synthon
to other amine complexes. As an example, when this product
is treated with the tetradentate ligand tris(2-pyridylmethyl)amine
(tmpa) along with NaOTf in methanol, the dinitrogen complex
12 is formed (Figure 1). Alternatively, a bipyridyl ligand can
be coordinated to rhenium at the diazenido stage. As we have
previously reported,10 treatment of 1 with 4,4′-di-tert-butyl-2,2′-
bipyridine results in the diazenido compound 8, and this material
may be combined with 2-(aminomethyl)pyridine (ampy) to give
the tetraamine dinitrogen complex 9. A similar reaction may
be carried out with ethylenediamine to form the complex [Re-
(N2)(PPh3)(tbpy)(en)]Cl.
Finally, amine-rich dinitrogen complexes may be prepared
directly from compound 1. When a solution of 1, ethylenedi-
amine, sodium triflate, benzene, and methanol is refluxed for 3
h, spectral data for the isolated product are consistent with the
formation of cis-[Re(N2)(PPh3)(en)2]OTf (10). Although at-
tempts to obtain an analytically pure material were unsuccessful,
1H and 13C NMR data reveal a single diamagnetic compound
with two asymmetric en groups and one PPh3 ligand. In
addition, infrared data indicate an extremely low-energy N-N
stretch at 1872 cm-1 and cyclic voltammetric data show a
reversible ReII/ReI reduction potential at -0.50 V. These
observations are consistent with an extraordinarily electron-rich
dinitrogen complex (Figure 3). Similarly, the reaction of 1 with
ampy forms the bis(ampy) (11) analog, and X-ray diffraction
data for this compound indicate a cis-stereochemistry for the
cation as shown in Figure 1.
In Figure 3, infrared and electrochemical data are summarized
for the series of ReI amine dinitrogen complexes described
above. Dinitrogen stretching frequencies for the complexes span
a range of 1872-2033 cm-1 while the corresponding d5/d6
(9) Dilworth, J. R.; Jobanputra, P.; Miller, J. R.; Parrott, S. J.; Chen, Q.;
Zubieta, J. Polyhedron 1993, 12, 513.
(10) Orth, S. D.; Barrera, J.; Sabat, M.; Harman, W. D. Inorg. Chem. 1994,
33, 3026.
(11) Morris, R. H. Inorg. Chem. 1992, 31, 1471.
(12) Harman, W. D.; Chin, R. M.; Brooks, B. C. Manuscript in preparation.
(13) Heinekey, D. M.; Payne, N. G.; Sofield, C. D. Organometallics 1990,
9, 2643-2645.

3556 Inorganic Chemistry, Vol. 36, No. 16, 1997
Chin et al.
Figure 4. Reaction scheme for the preparation of dihydrogen
complexes mer-14 and fac-14.
signal arises from the hydride trans to PF3. Thus, the second
isomer mer-14 is assigned the stereochemistry in Figure 4, where
the dien ligand adopts a meridional geometry.
The minimum T1 time for the 1H resonance at -5.97 ppm of
fac-14 was found to be 43 ms at 300 MHz (226 K). After
correcting the relaxation rate corresponding to this T1 value for
the dipolar contributions of the rhenium and the ortho phenyl
hydrogens15 and assuming a static HH ligand,16 an H-H
distance of 1.38 ( 0.03 Å was calculated.
Figure 5. (a) Hydride region of a 500 MHz 1H NMR spectrum
(acetone-d6) of mer-14 and fac-14. (Offset shows 14-d1 spectrum.) (b)
Simulation at 300 MHz of fac-14 spectrum using JHP ) 22.5 Hz, JHF
) 3.5 Hz, with a line width of 5.5 Hz (exptl, s; calcd, ‚‚‚). (c)
Simulation of fac-14-d1 (HD) spectrum at 500 MHz using JHP ) 22.5
Hz, JHF ) 3.5 Hz, JHD ) 6.7 Hz, with a line width of 5.5 Hz (exptl, s;
calcd, ‚‚‚). The fac-14 (HH) resonance is nulled out as described in
the Experimental Section.
The preparation of 14 with HD gas generates the sample 14-
d1. Two sets of signals are observed for the 14-d1 sample in
the hydride region. The signal arising from the mer-14-d1
isomer (ddm, -4.05 ppm) is similar to that of the mer-14 (ddq,
-4.04 ppm). However, the fac-14 signal has become a set of
four multiplets, assigned as two broad, overlapping triplets. The
broader upfield triplet (-6.03 ppm) arises from fac-14-d1, where
the deuterium is in a hydride position (Vide infra). The
downfield triplet (-5.97 ppm) arises from an impurity (in ∼1:1
ratio) that is thought to be an isomer of fac-14-d1, where the
deuterium has exchanged with a hydrogen in another part of
the complex to produce an HH ligand and an N-D bond (e.g.,
Hz, with estimated errors of (1 Hz. The H-H distance
calculated from the H-D coupling constant17 is 1.31 ( 0.03
Å, approximating the distance of 1.38 Å calculated from the
minimum T1 value. Such good agreement in the estimate of
dHH using these two methods suggests that a severely elongated
dihydrogen ligand is present in fac-[Re(H‚‚H)(dien)(PF3)-
(PPh3)]+ (fac-14) and that it is not rotating fast enough to
2
as verified by H NMR).
In order to obtain a clean hydride signal for the HD isomer
of fac-14-d1, it was necessary to “null out” the overlapping fac-
14 HH isomer impurities by use of an inversion-recovery NMR
pulse sequence. The complex pattern appears to be a 1:2:1 (H-
P) triplet of 1:1:1 (H-D) triplets of 1:3:3:1 H-F quartets (Figure
5c). The coupling constants obtained by simulation at 300 and
500 MHz were 2JHP ) 22.5 Hz, 3JHF ) 3.5 Hz, and 1JHD ) 6.7
1
influence H relaxation; therefore, its rotational frequency is
much less than 300 MHz.
The hydrides in fac-14 could in principle occupy two
inequivalent sites and yet still give rise to a single observable
proton resonance through the extreme second-order effects of
quantum mechanical exchange coupling.13 However, such a
coupling cannot occur for an HD complex. Thus, if quantum
exchange coupling were responsible for the single hydride signal
of fac-14, fac-14-d1 would be expected to show two signals,
(14) (a) Sabo-Etienne, S.; Chaudret, B.; el Marakim, H. A.; Barthelat, J.-
C.; Daudey, J.-P.; Ulrich, S.; Limbach, H.-H.; Mo¨ıse, C. J. Am. Chem.
Soc. 1995, 117, 11602-11603 and references therein. (b) Jalo´n, F.
A.; Otero, A.; Manzano, B. R.; Villasen˜or, E.; Chaudret, B. J. Am.
Chem. Soc. 1995, 117, 10123-10124.
(15) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173-4184.
(16) Earl, K. A.; Jia, G.; Maltby, P. A.; Morris, R. H. J. Am. Chem. Soc.
1991, 113, 3027-3039.
(17) An empirical relationship between JHD and dHH has been derived: dHH
) -0.0167JHD + 1.42 Å. Maltby, P. A.; Schlaf, M.; Steinbeck, M.;
Lough, A. J.; Morris, R.; Klooster, W. T.; Koetzle, T. F.; Srivastava,
R. C. J. Am. Chem. Soc. 1996, 118, 5396-5407.

ReI and ReII Amine Dinitrogen Complexes
Inorganic Chemistry, Vol. 36, No. 16, 1997 3557
Table 3. NMR Coupling Constants and Calculated H-H Distances for Re Dihydrogen Complexes and Some Os Dihydrogen Complexes and
Electrochemical Potentials of Corresponding Dinitrogen Complexes
dHH (Å)
d
HH (Å) calcd
from JHD
calcd from
E1/2 of
ReN2 (V vs NHE)
a
complex (X ) H or D)
ref
JHD (Hz)
T1(min)b
trans-Re(H‚‚H)(Cl)(PMePh2)4
Re(H‚‚H)(Cl)(dppe)2
22
23
1.58/1.25
1.55/1.23
1.38/1.09
1.03/0.81
1.02/0.81
1.01/0.80
0.98/0.78
0.90/0.71
0.85/0.68
1.34/1.06
0.5c
0.5322
0.8
fac-[Re(H‚‚X)(dien)(PF3)(PPh3)]+
cis-[Re(H2)(CO)(PMe3)4]+
[Re(H2)(CO)2{CH3C(CH2PPh2)3}]+
[Re(H2)(CO)2(PMe3)3]+
6.7
27
31
33.7
33
33
27
9.1
15
13.9
25.5
1.31
0.96
0.91
0.86
0.87
0.87
0.96
1.27
1.17
1.19
0.99
25
26
24
27
26
28
6a
6c
17
17
1.3c
1.8c
1.7c
2.2c
2.2c
[Re(H2)(CO)3(PMe3)2]+
[Re(H2)(CO)3(PiPr3)2]+
[Re(H2)(CO)(NO)(C5Me5)]+
trans-[Os(H‚‚X)(en)2(OAc)]+
[Os(H‚‚X)(NH3)5]2+
0.5c,d
1.0
trans-[Os(H‚‚X)(dppe)2(Cl)]+
trans-[Os(H‚‚X)(dppe)2(H)]+
1.35/1.08
1.3/1.02
1.4c
1.3c
a dHH ) -0.0167JHD + 1.42. dHH (slow spinning/fast spinning) calculated from the reported T1(min) data. E1/2(ReII(N2)L5/ReI(N2)L5) estimated
by use of a ligand additivity method (see ref 11). d Based on E ) 0.83 V for the related compound [Os(NH3)4(N2)(NH2Pr)]2+, modified according
to the ligand parameters put forth by Lever.27
b
c
arising from the two inequivalent hydrides.14b Since only one
signal is observed (other than the fac-14-d1 HH isomer), we
can rule out this explanation.
complexes of Table 3 when the effects of the chloride and
hydride trans ligands are compared.17 The other complexes
listed in Table 3 have short H-H distances with predicted E1/2
values above 1 V as expected and with the dihydrogen ligand
trans to less electronegative donor atoms P, C, or H.
Previously, no rhenium complexes with such small JHD values
have been reported (Table 3). The two neutral complexes of
ReI in Table 3 are thought to have elongated dihydrogen
complexes on the basis of T1 measurements and X-ray crystal-
lography, but unfortunately, the JHD coupling constants have
not been obtained. Such elongated ligands are expected to have
small couplings of less than 25 Hz.4b The other known cationic
rhenium(I) dihydrogen complexes have large JHD values and
short dHH values (Table 3). Yet, couplings of the same
magnitude (4-10 Hz) as that of fac-14 have been reported in
some osmium complexes.6a,b The best characterized of these
complexes is trans-[Os(H‚‚H)(en)2(OAc)]PF6 with dHH ) 1.34-
(2) Å and JHD ) 9.1 Hz, values comparable to those of fac-14.
The tendency of a d6 metal complex to have a dihydride or
dihydrogen structure has been discussed in terms of the
electrochemical potential, E1/2(d5/d6), of the corresponding
dinitrogen complex.11 If E1/2 lies much below 0.5 V vs NHE,
then a dihydride structure may be expected (see also Figure 3).
The two tetrakis(phosphine)rhenium compounds listed in Table
3 with E1/2 values close to 0.5 V feature an elongated dihydrogen
ligand, with H-H distances intermediate between those of
dihydrogen and dihydride species. Similarly, trans-[Os(H‚‚H)-
(en)2(OAc)]PF6 has a H-H distance of 1.34 Å (neutron
diffraction) and an estimated E1/2(OsIII/OsII) of ∼0.5 V (NHE)
for the hypothetical dinitrogen complex [Os(N2)(en)2(OAc)]PF6.
The JHD values for fac-14-d1 and [Os(HD)(NH3)5]2+ (see Table
3) indicate the presence of H‚‚H distances of 1.31 Å for fac-14
and 1.17 Å for [Os(NH3)5(H2)]2+, respectively. This is in
reasonable agreement with the measured E1/2 values of fac-14
(0.86 V) and [Os(NH3)5(N2)]2+ (1 V), which are more positive
than the dihydrides mentioned above. This trend breaks down
for the complex trans-[Os(H‚‚H)(dppe)2(Cl)]+,17 which has an
observed H-H distance of 1.22(3) Å and a predicted E1/2 value
of 1.4 V (Table 3). A common factor in all of the dihydrogen
complexes with elongated H-H distances is that the dihydrogen
ligand is trans to an electronegative N, O, or Cl donor atom. A
recent theoretical study concludes that weak-field ammine or
chloride ligands when trans to dihydrogen cause an increase in
dHH and a decrease in JHD values relative to the strong-field
ligands hydride or cyanide (or CO, we assume), when trans to
dihydrogen.18 This is clearly observed in the osmium dppe
In an attempt to prepare a ReII dihydrogen complex that was
isostructural to 14, H2 gas at atmospheric pressure was bubbled
through an acetone solution of the ReII precursor (13) for 30
min. Crystals of the product were grown from the degassed
reaction mixture. The complex identified by single-crystal
X-ray diffraction (Table 1; Figure 2) is fac-[Re(η1-acetone)-
(PF3)(PPh3)(dien)](OTf)2. Evidence indicating that the desired
ReII dihydrogen complex analogous to 14 is thermodynamically
unstable comes from the electrochemical data for 14. A cyclic
voltammogram recorded at 100 mV/s in CH3CN shows a
chemically irreversible wave at Ep,a ) 1.18 V suggesting that
an ErCi mechanism is operative.19
In Figure 2, drawings resulting from the structure determina-
tions of the ReII dinitrogen complex 3 and the ReII acetone
complex 14 are compared. Apparently, the process of oxidation,
replacement of dinitrogen with triflate, and substitution of triflate
by acetone does not alter the stereochemistry of the metal center.
The one-electron oxidation of rhenium results in a lengthening
of the Re-P bonds consistent with a decreased amount of
π-back-bonding, but interestingly, the ReII-PF3 bond distance
is still very short, compared to that for the triarylphosphine,
evidence suggesting that PF3 participates as a π-acid even on
rhenium(II).
Conclusions
Synthetic routes to a variety of new rhenium complexes
containing dinitrogen and nitrogen-donor ligands have been
developed, and a linear correlation between E1/2(ReII/ReI) and
(19) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications; John Wiley & Sons: New York, 1980.
(20) Cotton, F. A.; Luck, R. L. Inorg. Chem. 1991, 30, 767-774.
(21) Kohli, M.; Lewis, D. J.; Luck, R. L.; Silverton, J. V.; Sylla, K. Inorg.
Chem. 1994, 33, 879-883.
(22) Hussain, W.; Leigh, G. J.; Ali, H. M.; Pickett, C.; Rankin, D. A. J.
Chem. Soc., Dalton Trans. 1984, 1703-1708.
(23) Gusev, D. G.; Nietlispach, D.; Eremenko, I. L.; Berke, H. Inorg. Chem.
1993, 32, 3628-3636.
(24) Bianchini, C.; Marchi, A.; Marvelli, L.; Peruzzini, M.; Romerosa, A.;
Rossi, R.; Vacca, A. Organometallics 1995, 14, 3203-3215.
(25) Heinekey, D. M.; Schomber, B. M.; Radzewich, C. E. J. Am. Chem.
Soc. 1994, 116, 4515-4516.
(26) Chinn, M. S.; Heinekey, D. M.; Payne, N. G.; Sofield, C. D.
Organometallics 1989, 8, 1824-1826.
(27) Lever, A. B. P. Inorg. Chem. 1991, 30, 1980; (b) 1990, 29, 1271.
(18) Bacskay, G. B.; Bytheway, I.; Hush, N. S. J. Am. Chem. Soc. 1996,
118, 3753.

3558 Inorganic Chemistry, Vol. 36, No. 16, 1997
Chin et al.
ν(N2) is noted. As expected, the ReII(N2) complexes have higher
ν(N2) than corresponding ReI species. The complex fac-[Re-
(H‚‚H)(dien)(PF3)(PPh3)]+ has been characterized as a severely
elongated dihydrogen structure, and the corresponding HD
complex has one of the lowest JHD values measured for such a
structure. This distortion of the hydrogen ligand is attributed
in part to the electron-donating ability of the saturated nitrogen
ligands, but also to the weak-field nature of the trans amine
ligand.
(∼11 H, overlapping CH2 and NH2 signals), 2.84-2.78 (m, 1 H, mer
2
2
3
CH2), -4.04 (d d qrt, JHP(t) ) 67 Hz, JHP(c) ) 18 Hz, JHF ) 4 Hz,
0.25 H, mer hydride), -5.97 ppm (t, 2JHP ) 22 Hz, 2 H, fac hydride).
Temp/T1 (HRe, 300 MHz) (°C/ms): 24/116 ( 6, 4/90 ( 5, -13/67 (
6, -30/49 ( 2, -47/43 ( 1, -63/47 ( 1, -80/62 ( 2. 31P NMR
(acetone-d6, referenced externally): The major species (fac-14 with the
hydride at -6 ppm in the 1H spectrum) shows phosphorus resonances
2
1
at 45.4 ppm (PPh3; dq, JPP ) 25 Hz, JPF ) 8 Hz) and at 86.4 ppm
(PF3; qd, 1JPF ) 1166 Hz, 2JPP ) 25 Hz). The minor species (mer-14;
hydride at -4 ppm) shows analogous resonances at 32.8 ppm (PPh3;
2
3
1
dq, JPP ) 39 Hz, JPF ) 9 Hz) and at 89.9 ppm (q, JPF ) 1171 Hz,
2JPP unresolved).
Experimental Section
Abbreviations: dien ) diethylenetriamine; DME ) 1,2-dimethoxy-
ethane; DMA ) N,N-dimethylacetamide; OTf ) trifluoromethane-
sulfonate (triflate); TBAH ) tetrabutylammonium hexafluorophosphate;
ampy ) 2-(aminomethyl)pyridine; en ) ethylenediamine; tmpa ) tris-
(2-pyridylmethyl)amine; bpy ) 2,2′-bipyridine; tbpy ) 4,4′-di-tert-
butyl-2,2′-bipyridine; etpb ) 4-ethyl-2,6,7-trioxa-1-phosphabicyclo-
[2.2.2]octane.
fac-[Re(H‚‚D)(PPh3)(PF3)(dien)](OTf) (14-d1). HD gas was gener-
ated by adding D2O (2 mL) dropwise to a NaH (1.5 g) suspension in
DME (10 mL). The HD gas was passed through a CaCl2 drying tube
prior to use. Compound 13 (161 mg, 0.157 mmol) was dissolved in
DME (12 mL), and excess Mg0 (763 mg) was added to the Schlenk
flask. A balloon containing ∼450 mL of HD gas was placed on top
of the Schlenk flask, and ∼200 mL of HD was used to purge the
Schlenk flask. The reaction mixture was stirred for 2 h at room
temperature. The remaining Mg0 was filtered off through Celite, and
the filtrate was collected. The solvent was removed, and the resulting
white residue was washed with Et2O (3 × 10 mL) to yield compound
14-d1 (58 mg, 0.073 mmol, 47%). The D2 complex (14-d2) was
prepared similarly to the H2 complex, in a 49% yield.
fac-[Re(PPh3)(PF3)(dien)(N2)](OTf) (3). Compound 2 (57.1 g, 56.9
mmol) was suspended in DME (1 L). Dien (15.73 g, 152.4 mmol)
was added to the suspension, and the reaction mixture was refluxed
for 3 h. NaOTf (35 g, 203 mmol) in 600 mL of MeOH was then added
to the reaction mixture, and the mixture was refluxed for an additional
2 h. The mixture was held at 21 °C and then stirred for 18 h. The
solvent was removed under reduced pressure and the resulting brown
residue collected. The brown residue was washed with H2O (3 × 200
mL) to yield an off-white solid. The solid was dried for 24 h under
vacuum. The crude dinitrogen mixture was washed with Et2O (2 ×
200 mL) to remove any free PPh3 and then chromatographed on a silica
gel column, using CH3CN as the eluent. The first yellow band was
collected and the solvent removed to yield a yellow solid. The solid
was washed with CH2Cl2 (200 mL) and then Et2O (3 × 100 mL) to
yield pure dinitrogen complex 3 (21.3 g, 26.1 mmol, 46%). 1H NMR
(CD3CN): δ 7.49-7.37 (m, 15H, PPh3), 5.91 (br s, 1H), 3.43-2.53
(m, 12H, dien). 13C{1H} NMR (CD3CN): δ 138.14 (d, JCP ) 45.5
Hz), 133.60 (d, JCP ) 11.4 Hz), 130.60 (d, JCP ) 2.1 Hz), 129.58 (d,
JCP ) 9.3 Hz), 54.36 (d, JCP ) 4.1 Hz), 53.81 (s), 46.11 (d, J ) 4.1
Hz), 42.38 (br s). CV (DMA, TBAH, 100 mV/s): Ep,a ) 0.86 V. IR
(KBr): ν(N2) ) 2033 cm-1. Anal. Calcd for C23H28F6N5O3P2ReS:
C, 33.83; H, 3.45; N, 8.58. Found: C, 34.22; H, 3.58; N, 8.44.
fac-[Re(PPh3)PF3(dien)(OTf)](OTf)‚DME (13). Compound 3
(1.041 g, 1.27 mmol) was dissolved in acetone (10 mL). AgOTf (328
mg, 1.28 mmol) was dissolved in acetone (5 mL) and slowly added to
the stirring rhenium solution over a 5 min period. The mixture was
then filtered through Celite to remove the Ag0, and the filtrate was
collected. The solvent was removed under reduced pressure, yielding
a yellow oil. The yellow oil was then redissolved in DME (5 mL) and
the reaction mixture stirred for 3 h at room temperature. The resulting
yellow precipitate was collected and washed with DME (3 × 2 mL)
and Et2O (3 × 2 mL) to yield complex 13 with 1 equiv of DME (508
mg, 49.4 mmol, 39%). CV (DMA, TBAH, 100 mV/s): E1/2 ) -0.12
V. Anal. Calcd for C28H38F9N3O8P2ReS2: C, 32.72; H, 3.73; N, 4.09.
Found: C, 32.66; H, 3.55; N, 4.35. The EPR spectrum of 13 was run
on the powder at room temperature with a sweep width of 5000 G and
a center field of 3480.01 G. A very strong, broad signal (g ) 1.7-
3.4) resulted, but no coupling was resolved. An acetone solution of
13 at -105 °C gives a weaker signal, but it was still impossible to
resolve any fine structure.
fac-[Re(H‚‚H)(PPh3)(PF3)(dien)](OTf) (14). Compound 13 (380
mg, 0.369 mmol) was dissolved in DME (15 mL), and excess Mg0
(1.23 g) was added to the Schlenk flask. A balloon was placed over
the top of the flask and purged three times with H2. The balloon was
then inflated to ∼1 L volume with H2 gas. The reaction mixture was
stirred at room temperature for 100 min. The remaining Mg0 was
filtered off through Celite, and the filtrate was collected. The solvent
was removed under reduced pressure, and the resulting white residue
was collected and washed with CH2Cl2 (2 mL) and Et2O (5 mL) to
yield compound 14 (150 mg, 0.190 mmol, 47%). CV (CH3CN, TBAH,
100 mV/s): Ep,a ) 1.15 V. Anal. Calcd for C23H30F6O3N3P2ReS: C,
34.94; H, 3.82; N, 5.31. Found: C, 35.49; H, 3.98; N, 5.39. 1H NMR
(acetone-d6, 500 MHz): δ 7.70-7.66 (m, 1 H, Ph), 7.57-7.53 (m, 5.6
H, Ph), 7.52-7.41 (m, 9.25 H, Ph), 6.86 (br s, 1H, fac NH), 6.7 (br s,
0.13 H, mer NH), 4.45 (br s, 0.25 H, mer NH2), 4.23 (br s, 2 H, fac
NH2), 4.1 (br s, 0.25 H, mer NH2), 3.72 (br s, 1 H, fac NH2), 3.4-2.9
1H NMR (500 MHz, acetone-d6): δ 7.70-7.66 (m, 1 H, Ph), 7.57-
7.53 (m, 6 H, Ph), 7.52-7.41 (m, 8.5 H, Ph), 6.86 (br s, 1H, fac NH),
6.7 (br s, 0.25 H, mer NH), 4.45 (br s, 0.5 H, mer NH2), 4.23 (br s, 2
H, fac NH2), 4.1 (br s, 0.5 H, mer NH2), 3.72 (br s, 1 H, fac NH2),
3.4-2.9 (∼11 H, overlapping CH2 and NH2 signals), 2.84-2.78 (m, 1
2
2
H, mer CH2), -4.05 (ddm, JHP(t) ) 68 Hz, JHP(c) ) 18 Hz, 0.25 H,
mer hydride), -6.0 ppm (m, 1.5 H, fac hydride). When the HH signal
at -6 ppm is successfully nulled out in inversion-recovery experi-
ments, a broad, inverted triplet at -6.01 ppm results. Reasonably
“clean” HD signals were obtained both at 500 MHz and at 300 MHz
by use of null times of 78 ms in both cases and of spectral windows
that bracketed the resonance at -6 ppm.
The 31P NMR (acetone-d6, referenced externally) spectrum of 13-d1
was indistinguishable from that of 13.
2
2D NMR (sample of 14-d2 in acetone): 3.25, -4.01 (d, JDP ) 11
Hz), and -5.90 ppm.
[Re(PF3)(PPh3)(dien)(acetone)](OTf)2 (15). [ReOTf(PF3)(PPh3)-
(dien)]OTf‚DME (13, 70 mg, 0.068 mmol) was dissolved in acetone
(10 mL) to give a deep yellow solution in a Schlenk tube in the N2
glovebox. The tube was brought outside and exposed to Ar, and H2
was bubbled through the solution. No color change was observed. After
30 min, the volume had been reduced to 3-5 mL, and hexanes were
added under Ar to precipitate out a deep yellow powder. X-ray quality
crystals were grown directly from the degassed reaction mixture by
slow diffusion of degassed hexanes. The complex identified by single-
crystal X-ray diffraction is fac-[Re(acetone)(PF3)(PPh3)(dien)](OTf)2.
MS (acetone, FAB/NBA matrix): 789, 638, 556, 551. Calcd: Re-
(OTf)(PF3)(PPh3)(dien), 789; Re(PF3)(PPh3)(dien) - 2H+, 638; Re-
(PPh3)(dien) - H+, 551. An X-ray crystal structure determination
reveals a fac-stereochemistry identical to that of the dinitrogen complex
3.
Acknowledgment is made of the National Science Founda-
tion (W.D.H.; NYI program) and the Alfred P. Sloan Foundation
(W.D.H.) for their generous support of this work. NSERC
Canada is thanked for an operating grant to R.H.M. and a
postgraduate scholarship to T.Y.B. We thank Mr. Nick Plavac
for running some NMR spectra on the Varian Unity 400 MHz
and 500 MHz spectrometers. Dr. Alex Young ran the mass
spectra, and Mr. Scott Kirkby and Dr. Natasha Varaksa
performed the EPR experiments.
Supporting Information Available: Detailed synthesis and char-
acterization for all compounds presented herein, complete crystal-
lographic data for 3 and 15, and ORTEP drawings for 5A, 6, and 11
(25 pages). Ordering information is given on any current masthead
page.
IC9700866