A systematic route to mixed imido complexes of rhenium

Article abstract of DOI:10.1039/b004938g

The synthesis and structure of Re(VII) complexes with mixed imido ligands are described. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b004938g

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A systematic route to mixed imido complexes of rhenium
Anthony K. Burrell* and Andrew J. Steedman
IFS-Chemistry, Massey University, Private Bag 11222, Palmerston North, New Zealand.
E-mail: A.K. Burrell@massey.ac.nz
Received 20th June 2000, Accepted 28th June 2000
Published on the Web 12th July 2000
The synthesis and structure of Re(VII) complexes with
mixed imido ligands are described.
The imido ligand’s ability to stabilise high-oxidation state
metals and their utility as ancillary ligands in ring opening
metathesis polymerisation (ROMP)1 has spurred interest in the
synthesis and reactivity of complexes containing these ligands.2
More recently, compounds containing multiple imido ligands
have exhibited the ability to take part in C–H bond activation.3
One of the most important features of the π-donor imido
ligand is the ability to vary the size of the alkyl or aryl func-
tional group in the nitrogen. A change in the steric pressures
of the imido ligands results in dramatic effects on both the
structure and reactivity of their complexes.4 It is therefore
surprising that there are only a few publications that analyse
this effect and even less that deal with internal comparisons
of imido reactivity.5 We have recently been investigating the
reactivity of rhenium() tris(imido) complexes and in particu-
lar how the steric demands of the imido groups influence
Fig.
1
Molecular structure of ReCl(NAr)2(NAr*) (Ar* = 2-tert-
reactivity both at the metal and at the imido ligand. To evaluate
this reactivity in a logical way we have developed a systematic
synthetic route to mixed imido complexes.
butylphenyl). H atoms are omitted and thermal ellipsoids are plotted at
the 50% level. Selected dimensions (Å and Њ): Re(1)–N(1) 1.744(3),
Re(1)–N(2) 1.751(3), Re(1)–N(3) 1.757(3), Re(1)–Cl(1) 2.2990(8); N(1)–
Re(1)–N(2) 111.46(13), N(1)–Re(1)–N(3) 110.56(13), N(2)–Re(1)–N(3)
111.62(12), N(1)–Re(1)–Cl(1) 107.54(9), N(2)–Re(1)–Cl(1) 107.37(8),
N(3)–Re(1)–Cl(1) 108.10(9).
Mixed imido complexes are rare, with only a handful of
reports in the literature.5,6 In general, the method for preparing
bis- and tris-(imido) complexes require that all of the imido
ligands be added to the metal during a single reaction. While
it is possible to prepare mixed ligand complexes by employing
mixtures of imido ligand precursors, this is not always satisfac-
tory.6 We required a general method for the formation of mixed
imido complexes that could be carried out in large scale with
minimal purification.
Here, we outline a route that provides a systematic method
for forming mixed imido complexes of rhenium. The method-
ology is based upon chemistry reported by Schrock and
Williams.7 Treatment of ReO4NH4 with H2NAr or H2NArЈ
(Ar = 2,6-diisopropylphenyl, ArЈ = 2,6-dimethylphenyl) in the
presence of SiMe3Cl and pyridine gives the bis(imido) com-
plexes ReCl3(py)(NAr)2 and ReCl3(py)(NArЈ)2, respectively.
Interestingly, replacing the pyridine with triethylamine results
in the formation of tris(imido) complexes. This difference
can be attributed to the relative pKa values of the pyridine
and triethylamine. The bis(imido) complexes provide excellent
routes to mixed imido complexes.
Treatment of ReCl3(py)(NAr)2 with an aniline in the presence
of triethylamine results in the formation of a mixed tris(imido)
complex. Examples of the range of mixed complexes available
from this route are shown in Scheme 1. The yields for these
reactions are generally good (60–85%), although all are very
soluble and crystallisation proves difficult in some cases leading
to losses upon purification. The compounds are all red/orange
and produce simple NMR spectra consistent with their form-
ulation.† It is possible that ligand exchange can occur for com-
plexes containing smaller ligands. Solutions were generally
handled at room temperature and below, and no exchange was
observed.
Scheme 1 Reagents and conditions: i, H2NAr*, 3NEt3, benzene,
room temp, 30 min (Ar* = 2,6-dimethylphenyl, 2,6-diisopropylphenyl,
p-tolyphenyl, p-nitrophenyl, 2-tert-butylphenyl, 2-chlorophenyl, 3-
chlorophenyl, 4-fluorophenyl).
molecular structure is shown in Fig. 1 and selected bond lengths
and angles are given in the caption.‡ The metal–nitrogen bond
distances 1.744(3) [Re(1)–N(1)], 1.751(3) [Re(1)–N(2)] and
1.757(3) [Re(1)–N(3)] are consistent with bond lengths gener-
ally found for arylimido ligands bound to rhenium.8 The M–N–
C angles for the NAr ligands are very similar at 172.4(2)Њ
[Re(1)–N(1)–C(1a)] and 171.2(2)Њ [Re(1)–N(2)–C(1b)]. In con-
trast the 2-tert-butylphenyl ligand has a Re–N–C angle at
160.8(2)Њ due to unfavourable steric interactions caused by the
Crystals of ReCl(NAr)2(NAr*) (Ar* = 2-tert-butylphenyl)
were obtained by cooling a pentane solution to Ϫ35 ЊC. The
DOI: 10.1039/b004938g
J. Chem. Soc., Dalton Trans., 2000, 2501–2503
This journal is © The Royal Society of Chemistry 2000
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Fig. 2 Molecular structure of Re(Me)(NAr)2(NArЈ). H atoms are
omitted and thermal ellipsoids are plotted at the 50% level. Selected
dimensions (Å and Њ): Re–N(1) 1.763(4), Re–N(2) 1.757(3), Re–N(3)
1.753(4), Re–C 2.113(5); N(3)–Re–N(2) 115.63(17), N(3)–Re–N(1)
115.23(18), N(2)–Re–N(1) 114.75(17), N(3)–Re–C 102.81(19), N(2)–
Re–C 101.85(19), N(1)–Re–C 103.9(2).
Scheme 2 Reagents and conditions: i, 2pyHCl, THF, room temp, 30
min. ii, H2NAr*, 3NEt3, benzene, room temp, 30 min (Ar* = p-toly-
phenyl, p-nitrophenyl, 2-tert-butylphenyl).
Notes and references
1
tert-butyl group. In all other respects the structure is similar to
previously reported rhenium tris(imido) complexes.8
† Data for ReCl(NAr)2(NArЈ): H NMR (C6D6): δ 7.10 (d, 4, J = 7.1,
Ar), 6.96 (t, 2, J = 7.1, Ar), 6.88 (d, 2, J = 7.1, ArЈ), 6.72 (t, 1, J = 7.1,
ArЈ), 3.84 (sept, 4, J = 6.8, CHMe2), 2.32 (s, 6, CH3), 1.13 (d, 24, J = 7.0,
CHCH3). 13C{1H} NMR (C6D6): δ 152.8 (Ar), 152.7 (ArЈ), 143.0 (Ar),
142.5 (ArЈ), 132.1 (Ar), 127.1 (ArЈ), 122.5 (Ar), 122.4 (ArЈ), 28.1
(CHCH3), 23.5 (CHCH3), 17.9 (CH3). Found: C 55.97; H 6.12; N 5.96.
Calc: C 55.59; H 6.27; N 6.08%.
Reaction of the mixed imido complexes with Grignard
reagents (MeMgBr, p-CH3C6H4MgBr) results in clean conver-
sion to the respective organometallic complex. Thus treatment
of ReCl(NAr)2(NArЈ) with one equivalent of MeMgBr, pro-
duces the methyl complex Re(Me)(NAr)2(NArЈ).† Crystals
of Re(Me)(NAr)2(NArЈ) were obtained by cooling a pentane
solution to Ϫ35 ЊC. The molecular structure is shown in
Fig. 2 and selected bond lengths and angles are given in the
caption.‡ As with ReCl(NAr)2(NAr*) (Ar* = 2-tert-butyl-
phenyl) the metal–nitrogen bond distances 1.763(4) [Re–N(1)],
1.757(3) [Re–N(2)] and 1.753(4) [Re–N(3)] are consistent with
bond lengths generally found for arylimido ligands bound to
rhenium. The M–N–C angles for all of the imido ligands are
very similar at 169.1(3)Њ [Re–N(1)–C(11)] and 171.2(3)Њ [Re–
N(2)–C(21)] and 168.5(3)Њ [Re–N(3)–C(31)], indicating that the
steric effects are less pronounced when the smaller NArЈ ligand
is present.
We envisioned that the nitrogen atoms of the smaller imido
ligands would be selectively protonated in preference to those
of the larger imido ligands. Indeed, we found that in all cases
the nitrogen of the least sterically demanding ligand is proton-
ated upon treating the mixed imido complex with 2 equivalents
of pyHCl. In particular, the reaction of ReCl(NArЈ)2(NAr)
gives ReCl3(NArЈ)(NAr)(py) in 70% yield. The synthesis and
reactions of this new mixed bis(imido) complex are shown in
Scheme 2. This complex provides a clean specific route to
a tris(mixed imido) complex. Treatment of ReCl3(NArЈ)-
(NAr)(py) with H2NAr* (Ar* = 2-tert-butylphenyl) in the
presence of triethylamine results in the formation of a racemic
mixture of ReCl(NArЈ)(NAr)(NAr*) enantiomers. The chem-
istry of technetium and rhenium tris(imido) complexes with the
1
Data for ReCl(NArЈ)2(NAr): H NMR (C6D6): δ 7.03 (d, 2, J = 7.1,
Ar), 6.89 (t, 1, J = 7.1, Ar), 6.79 (d, 4, J = 7.1, ArЈ), 6.65 (t, 2, J = 7.1,
ArЈ), 3.79 (sept, 2, J = 6.7, CHMe2), 2.25 (s, 12, CH3), 1.10 (d, 12,
J = 7.0, CHCH3). 13C{1H} NMR (C6D6): δ 159.2 (Ar), 152.2 (ArЈ),
142.9 (Ar), 142.6 (ArЈ), 132.2 (Ar), 126.9 (ArЈ), 121.9 (Ar), 121.8 (ArЈ),
28.1 (CHCH3), 23.0 (CHCH3), 17.8 (CH3). Found: C 52.67; H 5.85; N
6.33. Calc: C 52.95; H 5.55; N 6.61%.
Data for ReCl(NAr)2(NAr*) (Ar* = 2-tert-butylphenyl): 1H NMR
(C6D6): 7.19 (d, 1, J = 7.2, Ar*), 7.01 (d, 4, J = 7.0, Ar), 6.64–6.91 (m, 5,
Ar/Ar*), 3.72 (sept, 4, J = 6.8, CHMe2), 1.48 (s, 9, tert-butyl), 1.04 (d,
24, J = 7.0, CHCH3). 13C NMR (C6D6): δ 155.2 (Ar), 151.9 (C6H5C-
(CH3)3), 143.0 (Ar), 142.9 (C6H5C(CH3)3), 128.6 (C6H5C(CH3)3), 125.4
(Ar), 125.1 (C6H5C(CH3)3), 125.0 (C6H5C(CH3)3), 124.8 (C6H5C-
(CH3)3), 122.6 (Ar), 35.2 (C6H5C(CH3)3), 30.6 (C6H5C(CH3)3), 27.6
(CCH3), 23.1 (CCH3). Found: C 57.06; H 6.78; N 6.50. Calc: C 56.76; H
6.58; N 5.84%.
Data for Re(Me)(NAr)2(NArЈ): 1H NMR (C6D6): δ 7.11 (d, 4, J = 7.1,
Ar), 7.02 (t, 2, J = 7.1, Ar), 6.96 (d, 2, J = 7.1, ArЈ), 6.81 (t, 1, J = 7.1,
ArЈ), 3.82 (sept, 4, J = 6.8, CHMe2), 2.68 (s, 3, CH3), 2.29 (s, 6, ArЈ-
CH3), 1.08 (d, 24, J = 7.0, CHCH3). 13C{1H} NMR (C6D6): δ 156.2
(Ar), 152.9 (ArЈ), 141.9 (Ar), 141.7 (ArЈ), 129.5 (Ar), 125.6 (ArЈ), 123.1
(Ar), 122.8 (ArЈ), 28.8 (CHCH3), 23.1 (CHCH3), 18.6 (ArЈCH3),
7.6 (ReCH3). Found: C, 58.96; H, 6.77; N, 6.51. Calc: C 59.07; H 6.91; N
6.26%.
Data for ReCl(NAr)(NArЈ)(NAr*) (Ar* = 2-tert-butylphenyl): 1H
NMR (C6D6): δ 7.21 (d, 1, J = 7.2, tert-butyl), 7.02 (d, 2, J = 7.0, Ar),
6.61–6.92 (m, 7, Ar/tert-butyl/ArЈ), 3.78 (sept, 2, J = 7.0, CHMe2), 2.29
(s, 6, CH3), 1.48 (s, 9, tert-butyl), 1.05 (d, 12, J = 7.0, CHCH3).
13C NMR (C6D6): δ 155.8 (Ar), 154.6 (ArЈ), 152.1 (C6H4CMe3), 143.2
(Ar), 143.0 (C6H4CMe3), 142.4 (ArЈ), 133.2 (Ar), 129.9 (ArЈ), 128.0
(C6H4CMe3), 127.4 (C6H4CMe3), 126.4 (C6H4CMe3), 126.2 (C6H4-
CMe3), 125.4 (Ar), 122.8 (ArЈ), 35.4 (CMe3), 30.2 (C(CH3)3), 28.6
(CHMe2), 23.0 (CH(CH3)2), 18.2 (CH3). Found: C, 54.26; H, 5.87; N,
6.58. Calc: C, 54.32; H, 5.93; N, 6.33%.
formula XRe(᎐NR) (R = tert-butyl, 2,6-dimethylphenyl and
᎐
3
2,6-diisopropylphenyl) have been investigated in some detail
and both reactivity and structure in these complexes is influ-
enced by the size of the imido ligands.9 The methodologies
reported here provide access to a π-loaded rhenium() core
where the steric demands at the metal can be tuned in a system-
atic and well defined fashion, hopefully providing control over
the reactivity both at the metal centre and at the ligands.
‡ Crystal data for ReCl(NAr)2(NAr*) (Ar* = 2-tert-butylphenyl): C34-
H47ClN3Re, M = 719.40; crystal size 0.38 × 0.17 × 0.06 mm, triclinic,
¯
P1, a = 10.6026(2), b = 10.6126(2), c = 15.93340(10) Å, α = 80.3510(10),
β = 72.8540(10), γ = 81.98Њ, V = 1681.19(5) Å3, Z = 2, Dc = 1.421 Mg
m
Ϫ3, F(000) = 728, µ(Mo-Kα) = 3.718 mmϪ1, λ = 0.71073 Å, 16481
reflections were collected (7301 unique, Rint = 0.0205), R( f ) = 0.0214
[I > 2σ(I)] and wR(F2) = 0.0707.
Acknowledgements
Crystal data for Re(Me)(NAr)2(NArЈ): C33H46N3Re, M = 670.93;
crystal size 0.53 × 0.40 × 0.26 mm, monoclinic, P2(1)/c, a = 9.3838(3),
b = 39.3038(11), c = 9.1197(2) Å, β = 107.789(1), V = 3202.7(15) Å3,
We are grateful to the Massey University Research Fund and
the Marsden Fund of New Zealand (MAU610) for supporting
this work.
Z = 4, Dc = 1.391 Mg mϪ3, F(000) = 1360, µ(Mo-Kα) = 3.818 mmϪ1
,
λ = 0.71073 Å, data were collected at 203 K on a Siemens SMART
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CCD area-detector. 19520 reflections were collected (7073 unique,
Rint = 0.0279), R(F) = 0.0361 [I > 2σ(I)] and wR(F2) = 0.0812. CCDC
reference number 186/2063. See http://www.rsc.org/suppdata/dt/b0/
b004938g/ for crystallographic files in .cif format.
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