Syntheses and characterization of phenyldiazenido and mixed phenyldiazenido-isocyanide complexes of rhenium. Crystal structure of [ReBr2(NNPh)2(PPh3)2]

Article abstract of DOI:10.1039/a800898a

The complex [ReBr₂(NNPh)₂(PPh₃)₂]Br 1, prepared by bromination of [ReCl(NNPh)₂(PPh₃)₂], was reduced, in thf or acetone, to the paramagnetic phenyldiazenido-complex [ReBr₂(NNPh)₂(PPh₃)₂] 2 and its acetone solvate 2·Me₂CO (formed in the presence of HSC₆H₂Prⁱ₃-2,4,6 or CNMe), and converted spontaneously (via a conceivable nucleophilic displacement at phenyldiazenide) into [ReBr₃(NNPh)(PPh₃)₂] 3 from which 2 can also be derived. The molecular structure of 2 has been determined by X-ray diffraction analysis which indicates that one of the diazenide ligands is doubly bent and the other singly bent, each being trans to a bromide ligand and exerting a significant trans influence. Complexes 1-3 react with isocyanides to give the reduced mixed diazenido-isocyanide complexes [ReBr₂(NNPh)(CNR)(PPh₃)₂] (R = Me 4 or C₆H₄Cl-4 5), [ReBr₂(NNPh)(CNMe)₂(PPh₃)] 6 or [ReBr(NNPh)(CNMe)₂(PPh₃)₂] 7.

Full text of DOI:10.1039/a800898a

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Syntheses and characterization of phenyldiazenido and mixed
phenyldiazenido–isocyanide complexes of rhenium. Crystal structure
of [ReBr2(NNPh)2(PPh3)2]
M. Teresa A. R. S. da Costa,a Jonathan R. Dilworth,b M. Teresa Duarte,a João J. R. Fraústo da
Silva,a Adelino M. Galvãoa and Armando J. L. Pombeiro*,†,a
a Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais,
1096 Lisboa codex, Portugal
b ICL, South Parks Road, Oxford, UK OX1 3QR
The complex [ReBr2(NNPh)2(PPh3)2]Br 1, prepared by bromination of [ReCl(NNPh)2(PPh3)2], was reduced, in
thf or acetone, to the paramagnetic phenyldiazenido-complex [ReBr2(NNPh)2(PPh3)2] 2 and its acetone solvate
2ؒMe2CO (formed in the presence of HSC6H2Pri3-2,4,6 or CNMe), and converted spontaneously (via a conceivable
nucleophilic displacement at phenyldiazenide) into [ReBr3(NNPh)(PPh3)2] 3 from which 2 can also be derived.
The molecular structure of 2 has been determined by X-ray diffraction analysis which indicates that one of the
diazenide ligands is doubly bent and the other singly bent, each being trans to a bromide ligand and exerting
a significant trans influence. Complexes 1–3 react with isocyanides to give the reduced mixed diazenido–isocyanide
complexes [ReBr2(NNPh)(CNR)(PPh3)2] (R = Me 4 or C6H4Cl-4 5), [ReBr2(NNPh)(CNMe)2(PPh3)] 6 or
[ReBr(NNPh)(CNMe)2(PPh3)2] 7.
The co-ordination chemistry of organodiazenides is a matter of
Results and Discussion
Phenyldiazenido-complexes
current and growing interest1,2 with well established importance
in fields such as nitrogen fixation3,4 and with recognized
potential significance5 in radiopharmaceutical development.
As an entry to the study of diazenido-complexes of rhenium
However, in spite of their versatile binding ability which allows
them to exhibit different co-ordination geometries and to
donate different numbers of electrons to the metal centre,1,2
paramagnetic complexes with such ligands are still almost
unknown.6,7 Also only very rare examples have been reported8
of organodiazenido-complexes with isocyanides which are
quite versatile co-ordination reagents9 and known10 substrates
of nitrogenases.
in medium/high oxidation states we have selected the bis-
(diazenido)-complex [ReBr2(NNPh)2(PPh3)2]Br 1 which we
found to be conveniently prepared (isolated as a green solid
in ca. 85% yield) by bromination in CH2Cl2 (ca. 15 min)
of [ReCl(NNPh)2(PPh3)2], a known14 compound. This last
complex, which has low solubility possibly due to polymeric
structure, would not be expected to be a promising starting
material. It was obtained in a high yield (ca. 85%) by adapting a
method taken from the literature14 for related aryldiazenide
complexes, i.e. by refluxing a suspension of [ReOCl3(PPh3)2] in
MeOH with PhNHNH2 and PPh3.
In compound 1 the bromide counter ion can easily be
replaced by tetraphenylborate, as expected, and the corre-
sponding species [ReBr2(NNPh)2(PPh3)2][BPh4] 1Ј (greyish
green) was obtained upon treatment of a CH2Cl2 solution of
1 with Na[BPh4], thus confirming the cationic nature of the
complex 1. The perchlorate salt has previously been reported,15
although without indication of the preparative method or of
any characterization data apart from IR ν(NN) frequencies. In
the IR spectra (KBr disc) of complexes 1 or 1Ј (see Experi-
mental section), the bands assigned to ν(NN) of the diazenide
ligands are observed in the 1845–1565 cmϪ1 range, i.e. at values
which are higher than those exhibited by [ReCl(NNPh)2(PPh3)2]
(1540 and 1510 cmϪ1), in accord with the higher metal oxidation
state in the cationic complexes with a resulting lower π-electron
releasing ability of the metal.
Within our interest on extending to higher metal oxidation
states our studies11 on the activation, by electron-rich transition-
metal centres (typically ReI, Mo0, W0 or FeII), of biologically
significant small molecules with unsaturated nitrogen or car-
bon, we have also investigated the reactions of isocyanides
with organodiazenido-complexes such as [ReCl2(NNCOPh)-
(PPh3)x{P(OR)3}3 Ϫ x] (x = 0 or 1, R = Me or Et)12,13 in the pres-
ence of MeOH, and [ReBr2(NNPh)2(PPh3)2]Br 1.7 In the former
cases a variety of low-oxidation-state rhenium isocyanide
products, i.e. [ReCl(N2)(CNR)(PPh3)x{P(OR)3}3 Ϫ x]
12 or [ReCl-
(CNR)3{P(OMe)3}2],13 were obtained with loss of the organo-
diazenide ligand which decomposed to N2 and PhCO2Me via
nucleophilic attack by methoxide ion. On attempted reaction of
[ReBr2(NNPh)2(PPh3)2]Br 1 with CNMe in thf or acetone the
mono(organodiazenido) complex [ReBr3(NNPh)(PPh3)2] was
isolated and structurally characterized, as quoted7 in a pre-
liminary communication.
We now report this reaction in detail and a further invest-
igation of the conversion of [ReBr2(NNPh)2(PPh3)2]Br 1 into
other organodiazenido-complexes and of their reactions
with isocyanides to form mixed organodiazenido–isocyanide
products. Some of the diazenido-complexes obtained are
paramagnetic with the rhenium in an unusual oxidation
state.
In solution, compounds 1 and 1Ј appear to occur as a mix-
ture of three isomers (one of them predominating over the
others), as suggested by the detection, in the 31P-{1H} NMR
spectra (CDCl3) (see Experimental section) of three singlet
resonances [the main one at δ ca. 148 and the other two at δ ca.
144 and 153 relative to P(OMe)3] which are maintained even
after repeated recrystallizations of the sample. This can be
accounted for by considering the possibility of occurrence of
† E-Mail: pombeiro@alfa.ist.utl.pt
J. Chem. Soc., Dalton Trans., 1998, Pages 2405–2410
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[ReCl(NNPh)2(PPh3)2]
(i )
(iv )
[ReBr2(NNPh)2(PPh3)2]
2
(ii )
[ReBr2(NNPh)2(PPh3)2]Br
[ReBr2(NNPh)(CNR)(PPh3)2]
(iii )
1
R = Me 4 or C6H4Cl-4 5
(iv )
[ReBr3(NNPh)(PPh3)2]
3
(v )
(vi )
(vii )
[ReBr(NNPh)(CNMe)2(PPh3)2]
[ReBr2(NNPh)(CNMe)2(PPh3)]
7
6
Scheme 1 (i) CH2Cl2, Br2, 15 min; (ii) thf, heat, Htipt, 4 h (for 2); thf (3–4 d) or acetone (2 d) (for 3); acetone, CNMe (2 h) (for 2ؒMe2CO ϩ 3); (iii)
thf, heat, PPhCPh᎐CPh, 2 d; (iv) thf, CNR (3:1), 1 d; (v) thf (for 4) or acetone (for 5), CNR (3:1), 1 d; (vi) thf, CNMe (3:1), 18 h; (vii) thf or
᎐
acetone, CNMe (6:1), 3 d
three geometrical isomers, two of them with the diazenide
ligands mutually cis (each of them trans to a bromide or to a
phosphine, forms a or b, respectively) and the other one (form
c) with the diazenide ligands in trans positions. However, the
presence of isomers with different relative conformations of
the phenyldiazenide ligands cannot be ruled out, as suggested16
[ReBr2(NNPh)2(PPh3)2]Br
1
(A)
Br –
+
(Ph3P)2(PhNN)Br2Re
N
N
– Ph•
– N2
(B)
– PhBr
– N2
solv
1
for [TcCl(NNC6H4OMe-4)2(PPh3)2] the H NMR spectrum of
which displays three resonances assigned to the methoxide
protons.2
+
PhBr
+
+
ReBr3(NNPh)(PPh3)2
ReBr2(NNPh)(PPh3)2(solv)
– solv, – Ph•
PPh3
PPh3
PPh3
Re
Br
Br
Br
PPh3
Br
PhNN
Br
3
PPh3
NNPh
NNPh
Scheme 2 (A), Nucleophilic displacement; (B), CN bond homolysis
Re
Re
NNPh
PhNN
which involves the extensive overall conversion of a bis- into a
mono-phenyldiazenido-complex.
Br
NNPh
PPh3
c
b
a
The extent of conversion of complex 1 into 3 follows the
Lewis basicity of the solvent,17 i.e. thf > acetone > CH2Cl2 (no
reaction), thus pointing towards the involvement of solvent co-
ordination. The presence of the BrϪ counter ion is also relevant,
since no reaction was observed for 1Ј, but the replacement of
one of the diazenides by BrϪ is not expected in view of their
inertness to displacement, and since the association of the two
ions (the cationic complex and BrϪ) to form an ion pair should
be faster18 in a solvent of low relative permittivity such as
CH2Cl2.
For compounds 1 and 1Ј the 1H NMR resonances (see
Experimental section) of the phenyl protons of the phenyl-
diazenide ligands are observed as the expected multiplets at
higher fields than those of free PPh3 and BPh4Ϫ (the latter spe-
cies in the case of 1Ј).
Complex 1 can act as a good starting material for a variety of
other phenyldiazenido-complexes, as shown in Scheme 1 which
summarizes the reactions we have investigated. Complex 1 is
reduced by a thiol, HSC6H2Pri3-2,4,6 (Htipt), to form, after
heating in thf under reflux for 4 h [see reactions (ii), Scheme 1],
the corresponding paramagnetic neutral complex [ReBr2(N-
NPh)2(PPh3)2] 2 (isolated as a red crystalline solid in ca. 20%
yield) [ν(NN) (KBr pellet) 1625m, 1585w and 1560s cmϪ1] the
crystal structure of which was determined by X-ray diffraction
analysis (see below).
The direct nucleophilic attack of BrϪ at the ipso-carbon of
NNPh forming PhBr [route (A), Scheme 2] would be a conceiv-
able step in view of the high ν(NN) frequencies in 1 indicating
a weak π-electron acceptance of the organodiazenides, as
observed19 in the reactions of [M(η5-ZC5H4)(NNR)(CO)2]ϩ
[M = Mn or Re, Z = H or Me, R = substituted phenyl;
ν(NN) у 1750 cmϪ1] with a nucleophile (XϪ) such as a halide
or a pseudo-halide, to form the corresponding N2 complexes
and RX. This process, assisted by replacement of the generated
N2 ligand (the rhenium oxidation state would not be sufficiently
low to bind N2)20 by solvent (solv) would lead to the postulated
intermediate [ReBr2(NNPh)(PPh3)2(solv)]. This is strongly cor-
roborated by the fact that the acetonitrile complex [ReBr2-
(NNPh)(PPh3)2(NCMe)] was observed21 to react with PhBr
to give 3. Moreover, metal halogenation by organohalides
with homolysis of the carbon–halogen bond is a known22 reac-
tion that could account for the formation of 3 from that
intermediate.
Complex 2 was also obtained as one of the products from the
attempted reaction of 1 with the phosphirene PPhCPh᎐CPh,
᎐
in thf under reflux. Its acetone solvate 2ؒMe2CO was isolated
(ca. 20% yield) upon reduction of 1 by CNMe in acetone (for
ca. 2 h at ambient temperature), with concomitant formation
of another paramagnetic complex, [ReBr3(NNPh)(PPh3)2] 3
[ν(NN) (KBr disc) 1700ms and 1575s cmϪ1] (isolated as a green
solid in ca. 50% yield). Complex 3 can be obtained, in a more
convenient way, by the spontaneous conversion of 1 in thf or
acetone, for extended periods (typically 3 to 4 d in thf, ca. 85%
yield), at ambient temperature and without requiring the add-
ition of any other reagent. The molecular structure of 3, with a
singly bent diazenide ligand, has already been reported by us,7
although without any details of its remarkable formation,
The alternative route (B) (Scheme 2), involving the homolysis
of a C᎐N bond, is less favoured. It has been reported4 for
[Re(η5-ZC5H4)L(LЈ)(NNC6H4OMe-4)]ϩ [Z = H or Me; L and/
2406
J. Chem. Soc., Dalton Trans., 1998, Pages 2405–2410

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phenyldiazenide ligands are essentially coplanar, suggesting
ؒؒ
an extensive π-electron delocalization over the Ph᎐N(2)᎐
ؒؒ ؒؒ
ؒؒ
N(1)᎐Re᎐N(3)᎐N(4)᎐Ph system.
The doubly bent diazenide ligand in 2 has Re᎐N(1)᎐N(2) and
N(1)᎐N(2)᎐C(21) angles of 130.6(6) and 120.3(8)Њ, whereas
the Re᎐N(1) and N(1)᎐N(2) bond lengths are 1.908(7) and
1.304(10) Å, respectively. This doubly bent geometry is much
less common than the singly bent one,2 and in our complexes
the angles fall in the expected range (115–135Њ) for sp2 hybrid-
ization,2 in spite of being significantly greater (i.e. the diazenide
ligand is less bent at both N atoms) than the corresponding
ones for other complexes such as [RhCl(NNPh){PhP(CH2CH2-
CH2PPh2)2}][PF6] [125.1(6) and 118.9(8)Њ, respectively].24
The singly bent diazenide exhibits a bending angle, 120.9(9)Њ,
at the β-N atom, N(3)᎐N(4)᎐C(41), which is similar to that
observed for the doubly-bent ligand, whereas the Re᎐N(3)᎐N(4)
angle is 171.2(7)Њ, showing the approach to linearity of this
ligand at the α-N atom. In comparison with the doubly bent
ligand, the Re᎐N distance in the singly bent diazenide [Re᎐N(3)
1.827(9) Å] is significantly shorter, consistent with the expected
multiple Re᎐N bond character for the singly bent ligand (three-
electron donor) and single Re᎐N bond character for the doubly
bent diazenide (one-electron donor) (see the valence bond res-
onance form d). A lengthening of the M᎐N distance as a result
of bending at this N atom is commonly observed.2
Fig. 1 Molecular structure of [ReBr2(NNPh)2(PPh3)2] 2 with number-
ing scheme
N(2)
Ph
Re
N(1)
Table 1 Selected bond lengths (Å) and angles (Њ) for [ReBr2(N-
NPh)2(PPh3)2] 2
N(3)
N(4)
Re᎐Br(1)
Re᎐Br(2)
Re᎐P(1)
Re᎐P(2)
Re᎐N(1)
Re᎐N(3)
2.596(1)
2.563(2)
2.481(2)
2.487(2)
1.908(7)
1.827(9)
P᎐C(average)
N(1)᎐N(2)
N(3)᎐N(4)
N(2)᎐C(21)
N(4)᎐C(41)
1.83
1.304(10)
1.120(10)
1.430(12)
1.428(14)
•
•
Ph
d
Br(1)᎐Re᎐Br(2)
P(1)᎐Re᎐P(2)
N(1)᎐Re᎐N(3)
Re᎐N(1)᎐N(2)
90.15(4)
176.34(8)
94.0(3)
Re᎐N(3)᎐N(4)
N(1)᎐N(2)᎐C(21)
N(3)᎐N(4)᎐C(41)
171.2(7)
120.3(8)
120.9(9)
The N᎐N bond length for the singly bent diazenide,
N(3)᎐N(4) 1.120(10) Å, is shorter than for the doubly bent one,
N(1)᎐N(2) 1.304(10) Å, in contrast to the usual2 behaviour.
Although theoretical calculations25 indicate that a bending at
Nα would commonly lead to a weakening of the N᎐N bond, in
the M᎐N᎐N 135–115Њ range (in which our complexes fall) the
effect cannot be clearly established.
The Re᎐Br bond lengths, in the 2.563(2)–2.596(1) Å range,
are comparable with that, 2.564(2) Å, which we have observed7
in the isoelectronic complex [ReBr3(NNPh)(PPh3)2] 3 for the
bromide ligand trans to the singly bent phenyldiazenide which
presents a significant trans influence. Hence, in complex 2, both
the singly and the doubly bent diazenides also appear to exhibit
an appreciable trans influence on the bromide ligand. Interest-
ingly, in 2 both diazenides, although having different geom-
etries, exert approximately the same trans influence. The
trans influence of the singly bent species {with a rather limited
number of exceptions for some thiolate complexes14b,26 such
as [HNEt3][Re2(NNPh)2(SPh)7]} is usually small, although
quite a perceptible trans influence has been recognized27 for the
doubly bent geometry, e.g. in [IrCl2(NNC6H4NO2-2)(CO)-
(PPh3)2].
The average Re᎐P bond distance, 2.484(2) Å, is close to that
for complex 3, 2.514(3) Å,7 and identical to the reported28 aver-
age value for triphenylphosphine complexes of rhenium-(),
-() and -().
It is also noteworthy to mention that the structural data
for complex 2 are quite similar to those reported14a for the
related diazenido–hydrazide complex [ReBr2(NNPh)(NNHPh)-
(PPh3)2], indicating that the presence of the hydrogen atom at
the β-N of the hydrazide ligand does not result in a significant
structural alteration. The absence of such an H atom in 2, i.e.
the presence of the doubly bent diazenide NNPh rather than
130.6(6)
or LЈ = CO, PMe3 or P(OMe)3] but only after reduction of the
metal to the ϩ2 oxidation state which conceivably is lower
than that of 1. In contrast with the expected promotion of that
homolysis by reduction, 2 (the reduced form of 1) does not
convert into 3.
No evidence has been found for the involvement of phenyl-
ation of the metal which was reported23 for the synthesis of
some arylgold() complexes derived from arylhydrazine. This
would lead, upon reductive elimination of PhBr, to a less bro-
minated product than the tribromo complex 3.
The conversion of [ReBr3(NNPh)(PPh3)2] 3 into [ReBr2-
(NNPh)2(PPh3)2] 2 involves a formal replacement of a bromide
by a diazenide ligand and could occur via the formation of
diazenide and bromide bridges between two rhenium metal
centres, but the overall yield in this reaction is low and the
mechanism may be very complex.
Crystal structure of [ReBr2(NNPh)2(PPh3)2] 2
The crystal structure of [ReBr2(NNPh)2(PPh3)2] 2 was authen-
ticated by single-crystal X-ray diffraction analysis (Fig. 1), and
selected bond distances and angles are given in Table 1. It pres-
ents an octahedral-type geometry with the two phosphines in
trans positions and the four charged ligands in the equatorial
sites, the phenyldiazenides being mutually cis and each of
them trans to a bromide ligand. One of the phenyldiazenide
ligands has a trans doubly bent geometry (1e donor) whereas
the other one is singly bent (three-electron donor), thus provid-
ing the complex with a formal 17-electron configuration. The
J. Chem. Soc., Dalton Trans., 1998, Pages 2405–2410
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᎐
the hydrazide(2Ϫ) NNHPh ligand, is consistent with the para-
ν(C᎐N) band for the diisocyanide complexes 6 and 7 suggests a
᎐
magnetism of the complex (as clearly indicated by EPR and 1H
trans arrangement of the two CNMe ligands.
1
NMR) and with the failure to detect it by X-rays and by H
Proton and 31P-{1H} NMR data were collected (in CDCl3)
for the diamagnetic complexes 4, 5 and 6 (see Experimental
section). Each of them exhibits a singlet resonance in its 31P-
{1H} NMR spectrum [δ Ϫ150.93, Ϫ154.01 or Ϫ136.34 relative
to P(OMe)3, respectively], thus showing, in the cases of the
diphosphine complexes 4 and 5, the equivalency of the two
PPh3 ligands which therefore should be either mutually trans or
each of them in the trans position to a bromide ligand (forms e
or f, respectively).
NMR (the resonance at δ 12.3 assigned14a to NNHPh was not
observed for 2).
The crystal structure of the acetone-solvated complex
2ؒMe2CO was also determined, by single-crystal X-ray diffrac-
tion. Probably as a result of solvation, it crystallizes in a differ-
ent crystal system from that of the unsolvated compound 2:
monoclinic, space group P21/c (no. 14) with a = 10.401(1),
b = 23.639(3) and c = 19.379(3) Å, β = 96.35(2)Њ. The molecular
structure of the complex with acetone as solvate is similar to
that of 2, but the lower quality of the crystallographic data
(R = 0.089), as well as a disorder in the linear phenyldiazenide
ligand, precluded an accurate comparison.
PPh3
Re
PPh3
Re
Br
PhNN
Ph3P
Br
CNR
CNR
NNPh
Br
Phenyldiazenido–isocyanide complexes
PPh3
Br
f
e
As indicated above, [ReBr2(NNPh)2(PPh3)2]Br 1, in acetone or
thf, is reduced by CNMe (3:1) to give [ReBr2(NNPh)2(PPh3)2]
2, isolated after ca. 2 h reaction (complex 3 also formed in the
absence of the isocyanide). However, extending the reaction
time leads to the formation of the isocyanide complex
[ReBr2(NNPh)(CNMe)(PPh3)2] 4 [reaction (v), Scheme 1] which
was isolated after 1 d (ca. 55% yield) as a red solid. The analo-
gous aryl isocyanide complex [ReBr2(NNPh)(CNC6H4Cl-4)-
(PPh3)2] 5 was obtained in a similar way, by using CNC6H4Cl-4
instead of CNMe, and was isolated also as a red crystalline
solid (ca. 60% yield).
The equivalency of the two CNMe ligands in complex 6 (as
suggested by IR spectroscopy, see above) was confirmed by the
detection in its 1H NMR spectrum of a single resonance
assigned to the methyl protons (singlet at δ 3.56). For complex 4
the resonances due to the phenyl protons of the diazenide
ligand are clearly detected with the expected patterns at higher
fields than those of PPh3.
The H and 31P-{1H} NMR spectra of complex 7 are broad
1
and the former spread along an extended δ range, thus clearly
indicating its expected paramagnetism. The formulation was
confirmed by its FAB MS spectrum in which the molecular ion
(m/z 977) was very clearly observed with the expected isotopic
pattern.
Complex 4 can also be obtained from [ReBr2(NNPh)2-
(PPh3)2] 2 or [ReBr3(NNPh)(PPh3)2] 3 [route (iv), Scheme 1],
under similar experimental conditions to those used in the reac-
tion of 1 with the isocyanide for 1 d. However, it was isolated in
a lower yield (ca. 30%) and the diisocyanide complex [ReBr-
(NNPh)(CNMe)2(PPh3)2] 7 was also an isolated product
formed upon further reaction of 4 with CNMe. In fact 7 was
also isolated (ca. 30%) from the reaction of 4 with CNMe (3:1),
in thf, a process in which another diisocyanide complex,
[ReBr2(NNPh)(CNMe)2(PPh3)] 6, was also obtained (orange
solid, ca. 20% yield). The latter complex of ReIII was formed by
simple replacement of PPh3 in 4 by CNMe [route (vi), Scheme
1]. A better method [route (vii), Scheme 1] for the synthesis of
the diisocyanide complex [ReBr(NNPh)(CNMe)2(PPh3)2] 7 is
the direct treatment of [ReBr2(NNPh)2(PPh3)2]Br 1, in thf or
acetone, with a higher excess of CNMe (6:1) for a prolonged
period (7 was isolated after 3 d as a green solid, ca. 40% yield).
In the formation of 4 or 5, 6 and 7 from 1, complexes 2 and/or 3
are detected intermediates and the reduction of Re is in accord
with the reducing ability of the isocyanide. In their IR spectra
(KBr discs) the isocyanide complexes exhibit medium/strong
Conclusion
This study shows that [ReBr2(NNPh)2(PPh3)2]Br can act as a
convenient starting material for a variety of diazenido- and
mixed diazenido–isocyanide complexes, a number of them
being paramagnetic. The susceptibility of the rhenium metal to
ready reduction by a reagent such as a thiol or an isocyanide
is evident, as is the versatility of the phenyldiazenide ligand
towards formal displacement by bromide or isocyanide (its loss
being interpreted, in the former case, in terms of a nucleophilic
displacement of the phenyl group) or transfer to another metal.
The potential synthetic interest of these unusual types of reac-
tions of organodiazenides should be explored, as well as the
establishment of the conditions to induce their reactions with
isocyanides.
intensity bands in the 2220–2110 cmϪ1 range, assigned to
Experimental
᎐
ν(C᎐N), whereas ν(NN) of the phenyldiazenide ligand occurs
᎐
as two strong bands at ca. 1650 and ca. 1560 cmϪ1.
The solvents were dried and degassed by using standard tech-
niques. All reactions were performed under an inert atmosphere
(N2). Phenylhydrazine and HBr were commercially available
and used without further purification. The compounds
HSC6H2Pri3-2,4,6,30 CNMe31 and [ReOCl3(PPh3)2]32 were pre-
pared according to published methods, and [ReCl(NNPh)2-
(PPh3)2] by adapting a method taken from the literature14 for
related aryldiazenido-complexes. All of the equipment used is
that existing at the Instituto Superior Técnico.
Infrared spectra were run with a Perkin-Elmer 683 spectro-
photometer and NMR spectra on a Varian Unity 300 spec-
trometer; δ values are in ppm relative to SiMe4 (1H) or to
P(OMe)3 (31P); the dominant 31P resonance for mixtures of
isomers is given in italics. The FAB MS spectrometric meas-
urements were performed on a Trio 2000 spectrometer, positive-
ion spectra being obtained by bombarding 3-nitrobenzyl
alcohol matrices of the samples with 8 keV (ca. 1.28 × 10Ϫ15 J)
Xe atoms.
The complexes [ReBr2(NNPh)(CNMe)(PPh3)2]
4
and
[ReBr2(NNPh)(CNMe)2(PPh3)] 6, which differ only on the rel-
ative numbers of CNMe and PPh3 ligands, display a common
Ϫ1
᎐
ν(C᎐N) value, 2170 cm . This suggests that CNMe and PPh
᎐
3
present similar electron donor/acceptor abilities as those metal
centres which are not strong π-electron releasers as indicated by
᎐
the relatively high ν(C᎐N) frequency (slightly above that shown
᎐
by the free isocyanide, 2150 cmϪ1). The CNMe and PPh3 lig-
ands exhibit then comparable electrochemical PL values (Ϫ0.43
and Ϫ0.35 V, respectively),29 i.e. behave as similar net electron
᎐
donor/acceptors. The higher ν(C᎐N) value for [ReBr(NNPh)-
᎐
(CNMe)2(PPh3)2] 7 (2220 cmϪ1) results from the replacement,
relatively to 4 and 6, of a bromide ligand (a very strong net
electron donor, PL ca. Ϫ1.2 V)29 by an isocyanide or a phos-
phine ligand, a much weaker net electron donor, with a result-
ing decrease of the π-electron releasing ability of the metal
centre to the isocyanide ligands. The observation of a single
2408
J. Chem. Soc., Dalton Trans., 1998, Pages 2405–2410

View Article Online
Syntheses
below) (ca. 50% yield) which was filtered off. Concentration of
the mother-liquor led to the formation of 2ؒMe2CO as a red
crystalline solid (ca. 20% yield). One of the crystals was
analysed by X-ray diffraction.
[ReCl(NNPh)2(PPh3)2]. Phenylhydrazine (2.0 cm3, 20.3
mmol) was added to a stirred suspension of [ReOCl3(PPh3)2]
(2.0 g, 2.4 mmol) and PPh3 (1.0 g, 3.8 mmol) in MeOH (10 cm3)
and the system refluxed for 2 h. The dark orange product was
filtered off, washed with MeOH–Et2O and CH2Cl2 and dried in
vacuo (ca. 85% yield) (Found: C, 58.8; H, 4.4; N, 5.7. Calc. for
C48H40ClN4P2Reؒ0.5CH2Cl2: C, 58.3; H, 4.1; N, 5.6%). IR (KBr
disc): 1540s and 1510s cmϪ1 [ν(NN)]. 1H NMR (CDCl3):
δ 7.73–6.99 (m, 30 H, PPh3) and 6.88–6.44 (m, 10 H, NNPh).
31P-{1H} NMR (CDCl3): δ Ϫ127.30 (s) and Ϫ130.24 (s).
[ReBr3(NNPh)(PPh3)2] 3. Although this complex could be
obtained from [ReBr2(NNPh)2(PPh3)2] 1 in acetone, either in
the presence (see above preparation of 2ؒMe2CO) or in the
absence of CNMe (other experimental conditions as for the
preparation of 2ؒMe2CO, but extending the reaction time to
2 d), a better route (with a higher yield) involves the use of thf
as the solvent, in the following way. A suspension of [ReBr2-
(NNPh)2(PPh3)2]Br 1 (2.0 g, 1.7 mmol) in thf (100 cm3) was
stirred until a clear dark green solution formed (ca. 3 to 4 d)
which was then concentrated in vacuo to give 3 as a green crys-
talline solid. This was filtered off, washed with pentane and
dried in vacuo. Further crops could be obtained from the
mother-liquor upon concentration and addition of pentane
(yield ca. 85%) [Found (when prepared in acetone): C, 48.4; H,
3.7; N, 2.5. Calc. for C42H35Br3N2P2ReؒOCMe2: C, 48.5; H, 3.7;
N, 2.5%]. IR (KBr pellet): 1700ms and 1575s cmϪ1 [ν(NN)].
[ReBr2(NNPh)2(PPh3)2]Br 1. To a stirred suspension of
[ReCl(NNPh)2(PPh3)2] (2.0 g, 2.1 mmol) in CH2Cl2 (10 cm3)
was added a 0.5  Br2 solution in CH2Cl2 (16.8 cm3, 8.4 mmol
Br2). After 15 min the dark green solution was taken to dryness
in vacuo. Methanol was then added and the residual green solid
(complex 1) was filtered off, washed with MeOH–Et2O and
dried in vacuo (ca. 85% yield) (Found: C, 50.4; H, 3.5; N, 4.7.
Calc. for C48H40Br3N4P2Re: C, 49.7; H, 3.5; N, 4.8%). IR (KBr
pellet): 1845 (sh), 1755ms (br), 1575m and 1565m cmϪ1
1
[ν(NN)]. H NMR (CDCl3): δ 7.90–7.77 (m, 12 H, o-H of
[ReBr2(NNPh)(CNMe)(PPh3)2] 4. A suspension of [ReBr2-
(NNPh)2(PPh3)2]Br 1 (0.15 g, 0.13 mmol) in thf (40 cm3), in
the presence of CNMe (20 µl, 0.39 mmol), was stirred for 1 d.
Concentration of the solution followed by addition of pentane
resulted in the precipitation of 4 as a red solid. A further crop
was obtained from the mother-liquor, upon concentration and
addition of pentane. These fractions were recrystallized
together from CH2Cl2–Et2O, and the resulting solid was filtered
off, washed with Et2O and dried in vacuo (ca. 55% yield). Com-
plex 4 could also be obtained, although in lower yield (ca. 30%)
and with concomitant formation of [ReBr(NNPh)(CNMe)2-
(PPh3)2] 7 (see below), by using [ReBr2(NNPh)2(PPh3)2] 2 or
[ReBr3(NNPh)(PPh3)2] 3 instead of 1, under similar experi-
mental conditions (Found: C, 52.8; H, 4.3; N, 3.9. Calc. for
C44H38Br2N3P2ReؒEt2O: C, 52.9; H, 4.4; N, 3.9%). IR (KBr
PPh3), 7.35–7.27 (m, 18 H, m,p-H of PPh3), 7.18–7.08 (m, 6 H,
m,p-H of NNPh) and 6.75 [t, J(HH) = 7.1 Hz, 4 H, o-H of
NNPh]. 31P-{1H} NMR (CDCl3): δ Ϫ143.80 (s), Ϫ148.40 (s)
and Ϫ152.98 (s).
[ReBr2(NNPh)2(PPh3)2][BPh4] 1Ј. The salt Na[BPh4] (160 mg,
0.47 mmol) was added to a CH2Cl2 solution (20 cm3) of
[ReBr2(NNPh)2(PPh3)2]Br 1 (0.50 g, 0.43 mmol) and the system
was stirred for 1 h. Filtration followed by concentration of the
solution and addition of methanol resulted in the precipitation
of 1Ј as a greyish green solid (ca. 70% yield) (Found: C, 61.4; H,
3.9; N, 4.3. Calc. for C72H90BBr2N4P2Re: C, 61.8; H, 4.0; N,
4.3%). IR (KBr disc): ν(NN) as for 1. 1H NMR (CDCl3): δ 7.77–
Ϫ
6.96 (m, 49 H, PPh3, BPh4 ), 6.83 [t, J(HH) = 7.14, 6 H, m,p-H
of NNPh] and 6.35 [d, J(HH) = 7.14 Hz, 4 H, o-H of NNPh].
Ϫ1
1
᎐
pellet): 2170ms [ν(C᎐N)], 1630s and 1560s cm [ν(NN)]. H
31P-{1H} NMR: δ Ϫ142.74 (s), Ϫ147.41 (s) and Ϫ152.28 (s).
᎐
NMR (CDCl3): δ 7.81–6.20 (m, 30 H, PPh3), 6.97 [t,
J(HH) = 7.3, 2 H, m-H of NNPh], 6.77 [t, J(HH) = 7.3, 1 H, p-
H of NNPh], 6.49 [d, J(HH) = 7.3 Hz, 2 H, o-H of NNPh] and
3.14 (s, 3 H, CNCH3). 31P-{1H} NMR (CDCl3): δ Ϫ150.93 (s).
[ReBr2(NNPh)2(PPh3)2] 2. This complex was obtained upon
reduction of [ReBr2(NNPh)2(PPh3)2]Br 1 by HSC6H2Pri3-2,4,6
(Htipt) in thf or from the attempted reaction of [ReBr3(N-
NPh)(PPh ) ] 3 (see below) with PPhCPh᎐CPh in thf.
᎐
3
2
[ReBr2(NNPh)(CNC6H4Cl-4)(PPh3)2] 5. A suspension of
[ReBr2(NNPh)2(PPh3)2]Br 1 (0.15 g, 0.13 mmol) in acetone (30
cm3) in the presence of CNC6H4Cl-4 (54 mg, 0.39 mmol) was
stirred for 1 d. Concentration of the solution followed by add-
ition of Et2O resulted in the precipitation of 5 as a red solid.
Further crops could be obtained from the mother-liquor upon
concentration and addition of Et2O. Recrystallization from
CH2Cl2–Et2O formed a red crystalline solid of 5 which was
filtered off, washed with Et2O and dried in vacuo (ca. 60% yield)
(Found: C, 53.8; H, 3.6; N, 3.8. Calc. for C49H39Br2ClN3P2Re:
First method. The compound Htipt (0.13 cm3, 0.52 mmol)
was added to a suspension of complex 1 (0.15 g, 0.13 mmol) in
thf (20 cm3) and the system left under reflux for 4 h. Concen-
tration of the red solution in vacuo followed by addition of
Et2O led to the precipitation of 2 as a red microcrystalline solid
which was filtered off, washed with Et2O and dried in vacuo (ca.
20% yield).
Second method. A suspension of [ReBr3(NNPh)(PPh3)2] 3
(see below) (0.20 g, 0.19 mmol) in thf (60 cm3), in the presence
᎐
C, 52.9; H, 3.5; N, 3.8%). IR (KBr pellet): 2110ms [ν(C᎐N)],
of PPhCPh᎐CPh (60 mg, 0.21 mmol), was refluxed for 2 d.
᎐
᎐
1650s and 1565s cmϪ1 [ν(NN)]. 1H NMR (CDCl3): δ 7.51–6.94
(m, C6H5 and C6H4). 31P-{1H} (CDCl3): δ Ϫ154.01 (s).
Concentration in vacuo and addition of pentane led to the
formation of an orange solid which was filtered off. Concen-
tration of the mother-liquor and addition of pentane resulted
in the precipitation of 2 as a red crystalline solid which was
filtered off, washed with Et2O and dried in vacuo (ca. 15%
yield). One of the crystals was analysed by X-ray diffraction
(Found: C, 53.7; H, 3.6; N, 5.0. Calc. for C48H40Br2N4P2Re:
C, 53.3; H, 3.7; N, 5.2%). IR (KBr pellet): 1625m, 1585w and
1560s cmϪ1 [ν(NN)].
[ReBr2(NNPh)(CNMe)2(PPh3)] 6. The compound CNMe
(31 µl, 0.60 mmol) was added to a thf solution (60 cm3) of
[ReBr2(NNPh)(CNMe)(PPh3)2] 4 (0.20 g, 0.20 mmol) and the
solution was stirred for 18 h. Filtration followed by concen-
tration in vacuo and addition of pentane resulted in the precipi-
tation of a green solid (complex 7, see below, ca. 30% yield)
which was filtered off and dried in vacuo. Concentration of the
remaining filtered solution followed by addition of pentane led
to the precipitation of 6 as an orange crystalline solid which
was filtered off, washed with pentane and dried in vacuo
(ca. 20% yield) (Found: C, 53.5; H, 3.9; N, 5.2. Calc. for
C28H26Br2N3PRe: C, 53.9; H, 4.0; N, 5.5%). IR (KBr pellet):
[ReBr2(NNPh)2(PPh3)2]ؒMe2CO 2ؒMe2CO. This acetone
solvate was obtained with [ReBr3(NNPh)(PPh3)2] 3 (see below)
in the following way. To a suspension of complex 1 (0.50 g, 0.43
mmol) in acetone (60 cm3) was added CNMe (56 µl, 1.1 mmol)
and the system stirred for 2 h to give a clear green solution. This
was concentrated in vacuo to give a green precipitate of 3 (see
Ϫ1
1
᎐
2170ms [ν(C᎐N)], 1645s and 1570m cm [ν(NN)]. H NMR
᎐
J. Chem. Soc., Dalton Trans., 1998, Pages 2405–2410
2409

View Article Online
1993, p. 89; G. J. Leigh, Acc. Chem. Res., 1992, 25, 177; R. L.
(CDCl3): δ 7.65–6.95 (m, 20 H, C6H5) and 3.56 (s, 6 H,
CNCH3). 31P-{1H} NMR (CDCl3): δ Ϫ136.34 (s).
Richards, in Biology and Biochemistry of Nitrogen Fixation, eds.
M. J. Dilworth and A. R. Glenn, Elsevier, Amsterdam, 1991, p. 58.
4 A. Cusanelli and D. Sutton, Organometallics, 1995, 14, 4651.
5 T. Nicholson, J. Cook, A. Davison, D. J. Rose, K. P. Maresca, J. A.
Zubieta and A. G. Jones, Inorg. Chim. Acta, 1996, 252, 427.
6 J. Chatt, J. R. Dilworth, G. J. Leigh and V. D. Gupta, J. Chem. Soc.
A, 1971, 2631.
[ReBr(NNPh)(CNMe)2(PPh3)2] 7. Although this complex
could also be obtained (ca. 30% yield) in the above procedure
for 6, its direct synthesis from [ReBr2(NNPh)2(PPh3)2]Br 1 con-
stitutes a better method, as follows. The compound CNMe (40
µl, 0.78 mmol) was added to a suspension of 1 (0.15 g, 0.13
mmol) in thf or acetone (30 cm3) which was then stirred for 3 d.
The green solid of 7 was filtered off, washed with thf (or
acetone)–Et2O and dried in vacuo. A further crop was obtained
upon concentration of the mother-liquor and addition of
pentane. Recrystallization from CH2Cl2–Et2O gave 7 as a green
microcrystalline solid which was filtered off, washed with Et2O
and dried in vacuo (ca. 40% yield) (Found: C, 53.1; H, 4.2;
7 M. T. A. Ribeiro, A. J. L. Pombeiro, J. R. Dilworth, Y. Zheng and
J. R. Miller, Inorg. Chim. Acta, 1993, 211, 131.
8 B. L. Haymore and J. A. Ibers, J. Am. Chem. Soc., 1973, 95, 3052;
J. W. Dart, M. K. Lloyd, R. Mason and J. A. McCleverty, J. Chem.
Soc., Dalton Trans., 1973, 2039; R. B. King and M. S. Saran, Inorg.
Chem., 1974, 13, 364.
9 E. Singleton and H. E. Oosthuizen, Adv. Organomet. Chem., 1983,
22, 209.
10 B. K. Burgess, in Molybdenum Enzymes, ed. T. G. Spiro, Wiley, New
York, 1985, p. 161.
11 A. J. L. Pombeiro, New J. Chem., 1994, 18, 163; A. J. L. Pombeiro
and R. L. Richards, Trends Organomet. Chem., 1994, 1, 263;
R. Herrmann and A. J. L. Pombeiro, Química, 1995, 59, 16; A. J. L.
Pombeiro, in Transition Metal Carbyne Complexes, ed. F. R. Kreissl,
NATO ASI Series, Kluwer, Dordrecht, 1993, p. 105; A. J. L.
Pombeiro and R. L. Richards, Coord. Chem. Rev., 1990, 104, 13.
12 M. F. N. N. Carvalho, A. J. L. Pombeiro, U. Schubert, O. Orama,
C. J. Pickett and R. L. Richards, J. Chem. Soc., Dalton Trans., 1985,
2079.
13 M. F. N. N. Carvalho and A. J. L. Pombeiro, J. Organomet. Chem.,
1990, 384, 121.
14 (a) J. R. Dilworth, S. A. Harrison, D. R. M. Walton and E. Schweda,
Inorg. Chem., 1985, 24, 2594; (b) T. Nicholson and J. Zubieta, Inorg.
Chim. Acta, 1985, 100, L35; (c) T. Nicholson, P. Lombardi and
J. Zubieta, Polyhedron, 1987, 7, 1577.
N, 4.6. Calc. for C46H41BrN3P2ReؒCH2Cl2: C, 53.2; H, 4.1; N,
᎐
5.3%). IR (KBr pellet): 2220ms [ν(C᎐N)], 1630s and 1560ms
᎐
cmϪ1 [ν(NN)]. FAB mass spectrum: m/z 977 (Mϩ).
Crystallography
Crystal data. C48H40Br2N4P2Re 2, Mr = 1080.83, triclinic,
a = 12.334(2), b = 12.146(2), c = 17.844(3) Å, α = 62.02(1),
β = 91.99(1), γ = 71.55(1)Њ, U = 2185.1(7) Å3, T = 293 K, space
Ϫ3
¯
group P1 (no. 2), Z = 2, Dc = 1.64 g cm , µ(Mo-Kα) = 4.69
mmϪ1, specimen 0.4 × 0.5 × 0.3 mm, 5987 reflections measured,
5581 unique (Rint = 0.0185), 5418 (F2 > 0) which were used in all
calculations. The final wR(F2) was 0.113, R1 = 0.042.
The unit cell and orientation matrix were obtained by least-
squares refinement of 25 automatically centred reflections
14 < θ < 18Њ, in an Enraf-Nonius TURBO CAD4 diffract-
ometer equipped with a rotating anode, using graphite-
monochromated radiation. Three standard reflections were
monitored during data collection (1.5 < θ< 25Њ), but no decay
or instrumental instability was detected. Using the CAD4 soft-
ware, data were corrected for Lorentz-polarization effects and
empirically for absorption. 5418 Reflections were used in struc-
ture solution and refinement of 554 parameters. The position of
the Re atom was obtained by a three-dimensional Patterson
synthesis, and all the other non-hydrogen atoms were located in
subsequent Fourier-difference maps and refined with aniso-
tropic thermal motion parameters. The hydrogen atoms were
inserted in calculated positions and refined isotropically with
fixed distances to the parent carbon atom. The program
SHELXS 8633 was used in the structure solution and SHELXL
9334 in the refinement of the crystal structure; the illustration
was drawn with ORTEP II.35 The atomic scattering factors and
anomalous scattering terms were taken from ref. 36.
15 B. L. Haymore, unpublished work (ref. 400 cited in ref. 2).
16 T. Nicholson, N. de Vries, A. Davison and A. G. Jones, Inorg.
Chem., 1989, 28, 3813.
17 V. Gutmann and R. Schmid, Coord. Chem. Rev., 1974, 12, 263; P. C.
Maria and J. F. Gal, J. Phys. Chem., 1985, 89, 1296; J. C. Scaiano
and L. C. Stewart, J. Am. Chem. Soc., 1983, 105, 3609.
18 R. G. Wilkins, Kinetics and Mechanism of Reactions of Transition
Metal Complexes, VCH, Weinheim, 1991, p. 116; K. Conners,
Chemical Kinetics, the Study of Reaction Rates in Solution, VCH,
New York, 1990, p. 390; S. H. Pine, J. B. Hendrickson, D. J. Cram
and G. S. Hammond, Organic Chemistry, McGraw-Hill,
Kogakusha, 1981, p. 387.
19 C. F. Barrientos-Penna, F. W. B. Einstein and D. Sutton, Inorg.
Chem., 1980, 19, 2740.
20 J. Chatt, J. R. Dilworth and R. L. Richards, Chem. Rev., 1978, 78,
589.
21 M. T. A. R. S. Costa and A. J. L. Pombeiro, unpublished work.
22 J. Chatt, R. A. Head, G. J. Leigh and C. J. Pickett, J. Chem. Soc.,
Dalton Trans., 1978, 1638.
23 P. Braunstein, J. Chem. Soc., Chem. Commun., 1973, 851;
P. Braunstein and R. J. H. Clark, Inorg. Chem., 1974, 13, 2224.
24 A. P. Gaughan, B. L. Haymore, J. A. Ibers, W. H. Myers, T. E.
Nappier and D. W. Meek, J. Am. Chem. Soc., 1973, 95, 6859.
25 E. R. Moller and K. A. Jorgensen, Acta Chem. Scand., 1991, 45,
68.
26 T. Nicholson and J. Zubieta, Inorg. Chem., 1987, 26, 2094.
27 R. E. Gobbledick, F. W. B. Einstein, N. Farrel, A. B. Gilchrist and
D. Sutton, J. Chem. Soc., Dalton Trans., 1977, 373.
28 A. G. Orpen, L. Brammer, F. H. Allen, O. Kennard, D. G. Watson
and R. Taylor, J. Chem. Soc., Dalton Trans., 1989, S1.
29 J. Chatt, C. T. Kan, G. J. Leigh, C. J. Pickett and D. R. Stanley,
J. Chem. Soc., Dalton Trans., 1980, 2032.
CCDC reference number 186/995.
See http://www.rsc.org/suppdata/dt/1998/2405/ for crystallo-
graphic files in .cif format.
Acknowledgements
This work has been partially supported by the Junta Nacional
de Investigação Científica
e Tecnológica (JNICT), the
Fundação para a Ciência e Tecnologia (FCT) and the PRAXIS
XXI programme, Portugal. We also thank Professor J. Nixon,
University of Sussex, UK, for the generous gift of a sample of
phosphirene.
30 P. J. Blower, J. R. Dilworth, J. Hutchinson, T. Nicholson and J. A.
Zubieta, J. Chem. Soc., Dalton Trans., 1985, 2639.
31 R. E. Schuster, J. E. Scott and J. Casanova, jun., Org. Synth., 1966,
46, 75.
32 J. Chatt and G. J. Rowe, J. Chem. Soc., 1962, 4019.
33 G. M. Sheldrick, SHELXS 86, Acta Crystallogr., Sect. A, 1990, 46,
467.
34 G. M. Sheldrick, SHELXL 93, A computer program for crystal
structure determination, University of Göttingen, 1993.
35 C. K. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge
National Laboratory, Oak Ridge, TN, 1976.
References
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3 R. A. Henderson, G. J. Leigh and C. J. Pickett, Adv. Inorg. Chem.,
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Received 2nd February 1998; Paper 8/00898A
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J. Chem. Soc., Dalton Trans., 1998, Pages 2405–2410