Sm(II) reduction chemistry of heteroalkynes: Stable adducts, reductive coupling, reductive C-C/C-N bond cleavage and trapping of the tert-butyl fragment with bulky nitriles, phosphaalkynes and isonitriles

Article abstract of DOI:10.1039/c002778b

Reactions of a dimetallated N,N′-dimethyl substituted porphyrinogen Sm(ii) complex with a series of t-butyl substituted heteroalkynes affords a diverse range of reactivity. The phosphaalkyne t-BuCP gives a dinuclear Sm(iii) P-P reductively coupled complex of (t-BuCPPC-t-Bu)^2- featuring a new μ-η²(1,2-C,P) binding mode. In contrast, the nitrile aza analogue t-BuCN forms Sm(ii) adducts that undergo reductive C-C bond cleavage at elevated temperatures to afford a trimeric Sm(iii) cyanide (μ-CN ^-) complex. The isomeric isonitrile t-BuNC undergoes the related reductive C-N bond cleavage reactivity at milder temperatures, allowing the trapping of the tert-butyl fragment as a Sm(iii) η²-iminoacyl (t-BuCN-t-Bu)^- complex. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c002778b

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Sm(II) reduction chemistry of heteroalkynes: stable adducts, reductive
coupling, reductive C–C/C–N bond cleavage and trapping of the tert-butyl
fragment with bulky nitriles, phosphaalkynes and isonitriles†
Michael G. Gardiner,*a Adam N. James,a Cameron Jonesb and Christian Schultenb
Received 10th February 2010, Accepted 5th May 2010
First published as an Advance Article on the web 3rd June 2010
DOI: 10.1039/c002778b
Reactions of a dimetallated N,N¢-dimethyl substituted porphyrinogen Sm(II) complex with a series of
≡
t-butyl substituted heteroalkynes affords a diverse range of reactivity. The phosphaalkyne t-BuC P
2-
=
=
gives a dinuclear Sm(III) P–P reductively coupled complex of (t-BuC PP C-t-Bu) featuring a new
2
≡
m-h (1,2-C,P) binding mode. In contrast, the nitrile aza analogue t-BuC N forms Sm(II) adducts that
undergo reductive C–C bond cleavage at elevated temperatures to afford a trimeric Sm(III) cyanide
(m-C N ) complex. The isomeric isonitrile t-BuN C undergoes the related reductive C–N bond
cleavage reactivity at milder temperatures, allowing the trapping of the tert-butyl fragment as a Sm(III)
-
≡
≡
2
-
=
h -iminoacyl (t-BuC N-t-Bu) complex.
≡
cyanide [{(C5H5)3U(C N)}n] in reactions of [{(C5H5)3U] with the
Introduction
corresponding nitriles upon heating the stable, crystallographically
5
The range of stoichiometric and catalytic reductive lanthanide
mediated reactions of multiply bonded substrates continues to
grow. In the specific case of organometallic ligand-supported
Sm(II) reactivity with isonitriles and nitriles, a limited number
of Sm(II) adducts have been structurally characterised for each
case, with supporting ligands apparently being influential on their
stability.1,2 Both isonitriles and nitriles are typically reduced under
mild conditions leading to the formation of Sm(III) cyanides
through single electron reduction and homolytic C–N or C–C
single bond cleavage, respectively, with the concomitant formation
of various metal and non-metal containing products derived from
the isonitrile and nitrile substituent.2a,3
This reductive cleavage ties in with the often convenient
synthesis of Ln(III) chloride complexes from the reaction of alkyl
chlorides with Ln(II) precursors.2a This reaction proceeds most
cleanly by reaction with t-BuCl in a 1 : 1 stoichiometry, where
trapping of by-products derived from the alkyl fragment is not
problematic.4 In the isonitrile and nitrile reactions, however, the
stoichiometry is more complex, with a portion of the Sm reagent
being directed towards products containing the heteroalkyne
substituent following C–N/C–C cleavage. These products are
described below.
≡
characterised U(III) adducts [{(C5H5)3U(N CR)]. To the best of
our knowledge, the trapping of alkyl substituents following C–N
bond reductive cleavage of isonitriles has only been reported in
f element chemistry for the reaction of [(C5Me5)2Sm(THF)2] with
cyclohexyl isonitrile, whose characterisation as the iminoacyl com-
plex [(C5Me5)2Sm(C6H11NCC6H11)] rests on microanalysis alone.3
2
=
The U(IV) h -iminoacyl complexes [(C5H5)3U(MeC NCy)] and
=
[(C5Me5)(C8H8)U(PhC N-t-Bu)] are the only f element iminoacyl
compounds to be structurally characterised. They were prepared
from isonitrile 1,1-insertion reactions of the preformed alkyl
≡
complexes [(C5H5)3UMe] and [(C5Me5)(C8H8)UPh] with CyN C
and t-BuN C, respectively.
In comparison, much less has been reported for phos-
phaalkyne reductions with Sm(II). The generally noted reactivity
pattern of phosphaalkynes not simply affording P-analogues
6
≡
≡
of nitrile reactivity is seen in the reaction of t-BuC P with
[(C5Me5)2Sm(THF)2], leading to the dinuclear Sm(III) P–P reduc-
tively coupled (t-BuC PP C-t-Bu) complex.7
2-
=
=
In previous reports, we have discovered a number of unique
f-element reactivity patterns for the dimetallated N,N¢-dimethyl
substituted porphyrinogen Sm(II) complex [(Me2N4)Sm(THF)2],
1. This has included small molecule activations,8 novel binding
modes for p-bound ancillary ligands9 and a rare reversible
Sm(III)/Sm(II) redox process.10
=
Ketimide complexes (LnSm–N C(H)R) have been isolated as
secondary products from the C–C reductive cleavage of nitriles,
arising from hydride 1,2-addition to excess nitrile. The hydride lig-
and is generated from decomposition of an initial alkyl complex.2a
Such alkyl complexes have been isolated in U(IV) chemistry as
[{(C5H5)3UR], R = Me, n-Pr, i-Pr, t-Bu, along with the U(IV)
aSchool of Chemistry, University of Tasmania, Hobart, Tasmania, Australia.
E-mail: Michael.Gardiner@utas.edu.au; Fax: +61 3 62262858; Tel: +61 3
6226 2404
The unusual chemistry of this class of macrocyclic complex
appears, at least in part, to be driven by the increased steric demand
of the macrocycle as a non-participating ligand set relative to
the much studied bis(pentamethylcyclopentadienyl) system, upon
bSchool of Chemistry, PO Box 23, Monash University, VIC 3800, Australia
† CCDC reference numbers 765856–765859 for 2–5, respectively. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/c002778b
6864 | Dalton Trans., 2010, 39, 6864–6870
This journal is
The Royal Society of Chemistry 2010
©

Scheme 1 Synthesis of complexes 2–5.
which many reactivity patterns of the lanthanides have become
based.
In extending our small molecule activation study, we report
The structure of 2 is shown in Fig. 1. All crystal morphologies
were found to be identical. In the solid state, molecules are severely
disordered over sites relating to the inversion centre, which leads
to many fractionally occupied and coincidental, or near so, C/N
atom positions of the overlayed orientations. As such, discussion
is largely limited to connectivity of the complex that is nonetheless
herein the outcome of a series of reactions of the Sm(II) complex 1
with a range of t-Bu substituted heteroalkynes. A diverse range
of products are obtained, which are discussed in relation to
established Sm(II) reduction chemistry, providing evidence for the
mechanism of nitrile and isonitrile reductive cleavage through the
first isolation of the tert-butyl fragment trapped as an iminoacyl
complex. Reactivity trends of the various heteroalkyne types are
also discussed.
unequivocal, revealing the complex to be trimeric through bridging
-
≡
C N ligands. C/N linkage isomerism issues of the cyanide
ligands cannot be established owing to the disorder.11
Results and discussion
Isonitrile reactivity
≡
The reaction of excess tert-butyl isonitrile, t-BuN C, and
[(Me2N4)Sm(THF)2] 1 was studied at various temperatures
in benzene. At 77 ◦C the trimeric Sm(III) cyanide complex
≡
[{(Me2N4)Sm(m-C N)}3] 2 was isolated as the sole Sm containing
product as a result of reductive C–N bond cleavage, Scheme 1.
The reaction proceeded rapidly at this temperature and the
dark purple colour of the Sm(II) starting material was observed
to bleach out within two minutes to give an orange solution.
Heating was continued overnight, during which time the product
had precipitated from solution as small, well formed orange
crystals that were then insoluble in a range of solvents. GC-MS
analysis of the quenched solution showed remaining excess
≡
≡
t-BuN C along with t-BuC N (ca. 1 : 1 ratio; the latter was not
present in the starting material), as well as a small amount of the
≡
Fig. 1 Molecular structure of [{(Me2N4)Sm(m-C N)}3] 2. Atoms are
=
imine t-Bu(H)C N-t-Bu (see below in relation to the isolation
drawn at arbitrary sizes. Lattice benzene is omitted for clarity, as are all
calculated hydrogen atoms. Extensive disorder has also been removed for
clarity.
of the iminoacyl complex 3 when the reaction was conducted
at room temperature) and both isobutane and isobutene as the
dominant by-products derived from the t-Bu substituent. The
identity of complex 2 was established through X-ray structure
determinations of several crystals under various modified reaction
conditions owing to the observation of variable visual crystal
morphologies.
˚
The Sm ◊ ◊ ◊ Sm separations of 6.23 A in 2 are slightly shorter than
those in the more precise structures of the Lewis base adducts
≡
≡
[{(C5Me5)2Sm(m-C N)L}3], L = CyN C, also featuring near
3
˚
≡
linear Sm–C N–Sm units, 6.28–6.30 A. All trimeric adducts of
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6864–6870 | 6865
©

this type have near planar Sm3N3C3 nine-membered ring planes.3,12
The lack of isonitrile/nitrile coordination in complex 2 that was
found for the samarocene analogues is likely to be sterically based;
indeed, the macrocyclic units in 2 are twisted somewhat relative
to the Sm3 plane as a necessity to allow interlocking of the meso-
ethyl groups of adjacent macrocycles. The unsolvated samarocene
cyanide has been accessed via a different cyanide generation route
in the absence of an excess prolate shaped Lewis basic donor
capable of binding to the Sm centre and was shown to be a
Bu)- ligand. However, it is likely that steric restrictions would not
permit the possible Sm(III) alkyl intermediate [(Me2N4)Sm(t-Bu)]
≡
to additionally coordinate t-BuN C based on the monomeric,
Lewis base free structure of the less demanding methyl complex
[(Me2N4)Sm(Me)].15 Thus the C–C bond formation leading to the
-
=
(t-BuC N-t-Bu) ligand may not occur on the Sm centre.
The lack of formation of the tert-butyl fragment trapped
complex 3 at elevated temperature may relate to a number of
factors. These include the shortened life time of the tert-butyl
fragment at higher temperatures leading to its decay via the
observed established routes and/or the lower concentration of
the Sm(II) starting material available to partake in this secondary
reaction due to the more rapid formation of the Sm(III) cyanide
complex 2.
≡
hexamer, [{(C5Me5)2Sm(m-C N)}6], with a chair shaped Sm6C6N6
18-membered ring having a closer average Sm ◊ ◊ ◊ Sm separation
13
˚
of 6.187 A. In the case of 2, retention of the smaller trimeric
unsolvated structure is likely to be dictated by the increased
interactions between adjacent macrocycles that would exist for
the larger oligomer by virtue of the decreased Sm ◊ ◊ ◊ Sm ◊ ◊ ◊ Sm
The structure of 3 is shown in Fig. 2. The Sm centre binds
5
1
5
1
≡
exterior ring angle for expanded (SmC N)n rings.
to the macrocycle in its typical h :h :h :h - binding mode with
the Sm–C/N distances to the macrocycle in accord with previous
reports for Sm(III) centres,16 which are clearly distinguished from
Sm(II) examples featuring the larger metal ion, including that of the
The formation of the cyanide complex 2 is consistent with
isonitrile reactivity with [(C5Me5)2Sm(THF)2] affording trimeric
3
≡
≡
cyanide complexes, [(C5Me5)2Sm(m-C N)(C NR)}3]. Even at
elevated temperatures, the reductive cleavage in the case of 2
proceeds much slower than was reported for the samarocene
system at room temperature. Steric bulk of the supporting ligand
may be influential in this regard; indeed the only known stable
≡
nitrile adduct [(Me2N4)Sm(N C-t-Bu)] 4, see below. The structure
-
2
=
reveals the (t-BuC N-t-Bu) ligand to bind h - to the Sm centre,
as is commonly found for transition metal complexes.17
Sm(II) isonitrile adduct is for the heavily substituted [{C5H2(t-
1
≡
Bu)3}2Sm(C N-2,6-Me2C6H3)].
≡
The reaction of [(Me2N4)Sm(THF)2] 1 and t-BuN C at room
temperature in benzene proceeds to completion slowly overnight,
≡
again giving the trimeric cyanide [{(Me2N4)Sm(m-C N)}3] 2 as
well as variable yields of an additional red insoluble crystalline
product. The latter was identified by X-ray crystal structure
determination to be the C–C coupled Sm(III) iminoacyl complex
=
[(Me2N4)Sm(t-BuC N-t-Bu)] 3, Scheme 1. Complex 3 is insoluble
in benzene, toluene, diethyl ether, hexane and THF.
Complex 3 is the first structurally characterised lanthanide
iminoacyl complex. The synthesis of U(IV) iminoacyl complexes
via 1,1-addition to isonitriles was discussed above.6 In general,
iminoacyl complexes have more commonly been accessed from
lithium precursors prepared by the reaction of the appropriate
isonitrile with an alkyl lithium or by reduction of nitrilium
=
salts, [RC NR¢]X, routes which have obviously allowed access
to unsymmetrically substituted examples.14
=
Fig. 2 Molecular structure of [(Me2N4)Sm(t-BuC N-t-Bu)] 3. Thermal
Direct trapping of the alkyl substituent following nitrile cleavage
as alkyl complexes has been reported for uranium reductions5
and has, to the best of our knowledge, only been reported
once in lanthanide chemistry as a samarocene analogue to 3.3
ellipsoids are shown at the 50% probability level. Disorder of the C/N
centres of the iminoacyl ligand and the t-Bu groups has been removed for
clarity, as have all calculated hydrogen atoms.
=
As noted above, the ketimide complex [(C5Me5)2Sm(N C(H)-t-
≡
Bu)(THF)] was isolated from the reduction of t-BuC N with
In the solid state, molecules of 3 reside on mm sites, leading to
disorder of the C/N centres of the iminoacyl ligand. Rotational
disorder of the t-Bu groups is also observed. As such, detailed
[(C5Me5)2Sm(THF)2] via the formation of the intermediate Sm(III)
alkyl complex [(C5Me5)2Sm(t-Bu)], with b-H elimination giving
the required hydride intermediate.2a The hydride addition to the
nitrile is likely to occur on the Sm centre based on the independent
˚
analysis of the Sm–C/N distance (2.397(13) A) to the iminoacyl
ligand is not warranted; the thermal ellipsoids of the disordered
C/N centres are indicative of inequivalent, but similar Sm–C/N
distances. The Sm–C/N distance compares with the distinct, but
related, Sm(III) methyl15 and bis(trimethylsilyl)amide complexes,16
≡
synthesis of the ketimide from the reaction of t-BuC N with the
Sm(III) hydride [{(C5Me5)2Sm(m-H)}2].
At this stage no detailed mechanistic information is available
for the formation of 3. Thus, the stepwise versus concerted nature,
order of events and specific involvement of the Sm centre are
unknown, with many alternatives being possible in relation to the
single electron transfer, C–C bond formation and Sm coordination
˚
2.424(5) and 2.307(2)–2.314(2) A, respectively, and the U(IV) com-
˚
=
plex [(C5Me5)(C8H8)U(PhC N-t-Bu)], 2.423(6) and 2.390(4) A
(U–C and U–N distances, respectively).6b The C N bond distance
of 1.32(2) A is consistent with the CN multiple bond formalism
=
˚
=
leading to the ultimate formation of the anionic (t-BuC N-t-
of the complex, and is also similar to the same U(IV) complex,
6866 | Dalton Trans., 2010, 39, 6864–6870
This journal is
The Royal Society of Chemistry 2010
©

6b
˚
of the isonitriles used in the initial reports of isonitrile cleavage by
these Sm reagents.3
The structure of 4 is shown in Fig. 3. The asymmetric unit
contains two similar molecules of 4 residing on crystallographic
mirror planes differing mainly in the conformation of meso-
ethyl substituents. Both the Sm–N distances to the nitrile ligand,
=
1.299(4) A. Space filling representations of 3 show that the N C–
◦
=
C(t-Bu)/C N–C(t-Bu) bond angle of 135.8(7) may be influenced
-
=
by steric interactions between the (t-BuC N-t-Bu) ligand and
meso-ethyl groups of the macrocycle, however resonance forms
=
=
of the anion predict bent N C–C and C N–C geometries and
this arrangement has been seen in both transition metal and U(IV)
complexes.6
˚
2.609(6) and 2.612(7) A, and the Sm–C/N distances to the
macrocyclic ligand are in agreement with the Sm(II) formalism of
the complex.16 The nitri◦le ligand is linearly bound (Sm–N C and
≡
Nitrile reactivity
≡
N C–C all ≥ 174.1(12) ), negating interpretation of the complex
≡
=
The reaction of excess tert-butyl nitrile, t-BuC N, and
as the plausible ketimide complex [(Me2N4)Sm{N C(H)t-Bu}]
[(Me2N4)Sm(THF)2] 1 in benzene at room temperature results
initially in a dark green solution, Scheme 1. Removal of solvent and
excess nitrile in vacuo at room temperature gives a green powder
that was not fully characterised. Redissolution of this green solid
that could conceivably form by the generation of a Sm(III) hydride
from decomposition of an initial tert-butyl complex following
nitrile reductive cleavage.
1
in benzene gives a purple solution, whose H NMR spectrum
established the green solid as the bis(adduct) on the basis of
integration of nitrile (1.66 ppm)/macrocyclic proton resonances.
Concentration of this solution gives purple crystals of the Sm(II)
1
≡
1 : 1 adduct [(Me2N4)Sm(N C-t-Bu)] 4. The H NMR spectrum
of 4 confirmed the loss of the second nitrile ligand to give the 1 : 1
adduct, confirming that our visual observations are consistent with
the stability of the 2 : 1 adduct in benzene only in the presence of
excess nitrile. The 1H NMR spectrum of the isolated 1 : 1 adduct
and the dissolved solution of the 2 : 1 adduct in C6D6 are different
(nitrile resonance at 3.55 ppm for the isolated 1 : 1 adduct), and are
indicative of rapidly exchanging nitrile ligands in the latter case.
Heating a C6D6 solution of 4 at 52 ◦C for 16 h effects no
1
change to the H NMR spectrum. However, continued heating
of the same solution for an additional 18 h at 77 ◦C resulted in a
red solution with the Sm(III) cyanide complex [{(Me2N4)Sm(m-
≡
C N)}3] 2 forming as orange crystals through reductive C–C
bond cleavage. Under similar conditions, C6D6 solutions of 4 react
with excess t-BuC N at 77 ◦C also giving orange crystals of 4
≡
≡
along with a complex mixture of t-BuC N oligomers. Due to the
≡
Fig. 3 Molecular structure of [(Me2N4)Sm(N C-t-Bu)] 4. Thermal
large range of observed products, no further attempts were made
to characterise these oligomerisation products. Isobutane and
isobutene are the dominant t-Bu fragment derived by-products
under both conditions.
ellipsoids are shown at the 50% probability level. Lattice benzene is omitted
for clarity, as are all calculated hydrogen atoms. Only one of the two
crystallographically independent molecules is shown.
≡
In reactions of [(C5Me5)2Sm(THF)2] with t-BuC N, the sta-
bility of the initial nitrile adduct has been shown to vary
greatly according to the presence of Lewis basic solvents,
which impede the subsequent reduction of the substrate. Thus,
Phosphaalkyne reactivity
≡
The reaction of excess tert-butyl phosphaalkyne, t-BuC P, and
[(Me2N4)Sm(THF)2] 1 affords the green crystalline dinuclear
≡
[(C5Me5)2Sm(THF)2] is known to react with t-BuC N giving
≡
=
=
the isolable Sm(II) bis(adduct) [(C5Me5)2Sm(N C-t-Bu)2(THF)]
Sm(III) complex [{(Me2N4)Sm}2(m-t-Bu–C PP C-t-Bu)] 5 fea-
turing reductive coupling of the substrate, Scheme 1. The reaction
mixture gave initially soluble products, whereas complex 5 proved
to be insoluble after isolation. The reaction was followed by
only in the presence of THF.2a When the reaction is conducted in
toluene, reductive cleavage occurred more rapidly to the trimeric
≡
≡
cyanide [(C5Me5)2Sm(m-C N)(N C-t-Bu)}3]. Consideration of
the coordination sphere of the Sm centre is thus important in
the outcome of nitrile reduction chemistry. It is likely that the
increased steric demand of the supporting macrocyclic ligand here,
relative to the bis(pentamethylcyclopentadienyl) ligand set, plays a
role in increasing the thermal stability of the Sm(II) nitrile adducts.
This parallel study of the analogous isonitrile and nitrile pair
1
31P{ H} NMR spectroscopy, however no resonances other than
≡
the excess t-BuC P were observed (most likely due to the param-
agnetic nature of the products) and this prevented interpretation
of the reaction as possibly occurring via soluble intermediate/s.
The analogous reaction of [(Me2N4)Sm(THF)2] 1 with an excess of
≡
the l◦ess bulky phosphaalkyne MeC P in toluene was attempted at
t-BuN C and t-BuC N, respectively, has shown clear differences
in the rate of reductive cleavage of the substrates, product out-
comes and stability of the initial adducts. Reactivity distinctions
have also been made in [(C5Me5)2Sm(THF)2] and [(C5Me5)3Sm]
reactions2a,3,12 subsequent to a comment1 in relation to the purity
-90 C, but was shown by 31P{ H} NMR spectroscopy to yield an
1
≡
≡
intractable mixture of phosphorus containing compounds from
which no product could be isolated.
≡
The reductive coupling observed for t-BuC P contrasts with the
nitrile and isonitrile reactivity discussed above. The outcome ties
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6864–6870 | 6867
©

in with the well established alkyne-type reactivity18 of t-BuC P
in relation to the reductive coupling/C–H cleavage observed
for some terminal alkynes with [(C5Me5)2Sm(THF)n] giving, for
Molecules of 5 display considerable distortions from cen-
trosymmetry in the solid state, with variations in the Sm–P
distances being the sole affected geometrical difference between
≡
˚
= = =
example, [{(C5Me5)2Sm}2(m-C(R) C C C(R))] or the alkynyl
the Sm-donor atom geometries (2.934(7) and 3.008(7) A). The
≡
=
=
[(C5Me5)2Sm(C CR)(THF)], with the presence of Lewis basic
Sm–P distances in [{(C5Me5)2Sm}2(m-t-Bu–C PP C-t-Bu)] are
solvents again being influential on the reaction outcomes.19
The structure of 5 is shown in Fig. 4. The structure revealed the
P–P coupled phosphaalkyne unit to bridge the two macrocyclic
bound Sm centres. The h (1,2-C,P) binding mode is unique for
this ligand type. The previously discussed analogous product from
similar at 2.952(2) and 2.945(2) A. The Sm–C distances of
7
˚
˚
=
˚
2.524(12) and 2.520(11) A are similar to 2.557(6)/2.556(6) A in
7
=
[{(C5Me5)2Sm}2(m-t-Bu-C PP C-t-Bu)], and expectedly longer
2
than in the monomeric, unsolvated Sm(III) methyl complex
[(Me2N4)Sm(Me)], 2.424(5) A.15 Unlike the samarocene analogue,
˚
˚
=
=
[(C5Me5)2Sm(THF)2], [{(C5Me5)2Sm}2(m-t-Bu-C PP C-t-Bu)],
where the C4P2Sm2 core of the complex is within 0.015 A of
2
displayed a m-h (1,3-C,P) binding mode.7 This earlier complex still
planarity, 0.60 A deviations are noted here (each C2PSm end is
˚
˚
remains the only structurally characterised example for Ln reactiv-
ity, with various P-substituted cyclopentadienyl and benzene com-
plexes having been reported to have stabilised novel chemistry.20
The different binding modes of these two (m-t-Bu-C PP C-t-
Bu)2- complexes likely relates to the varying steric demand of
the macrocycle relative to the bis(pentamethylcyclopentadienyl)
ligand set which has previously been documented in relation to
steric crowding in complexes featuring 1,4-diazabuta-1,3-diene10
and cyclopentadienyl9 ancillary ligands. As was reported for
planar to within 0.03 A, with C2 symmetric distortions of up to
12◦ in the central CCPP dihedral angles being mainly responsible
˚
˚
=
for this). The C P bond lengths are 1.660(12) A and 1.673(11) A,
=
=
which are slightly shorter than those found in [{(C5Me5)2Sm}2(m-t-
˚
=
=
Bu–C PP C-t-Bu)], 1.694(7) and 1.698(7) A, and are consistent
with double bonds.7 The P–P distance in 5 and [{(C5Me5)2Sm}2(m-
˚
=
=
t-Bu–C PP C-t-Bu)] of 2.267(5) and 2.207(2) A, respectively,
vary substantially but are both typical for single bonds.7
=
the dinuclear complex [{(Me2N4)Sm}2(m-t-Bu-NC(H) C(H)N-t-
Experimental
Bu)],10 the unusual cis,trans,cis-geometry of the dianionic bridging
ligand (in that case, contrasting with the more typical chelating
radical anion geometries well known for f element chemistry21)
is likely to be in response to relieving steric forces. Overall, the
General
All reactions were either carried out under an inert atmosphere
of high purity argon using standard Schlenk techniques or
carried out in a dry glove box (Innovative Technologies) under
a nitrogen atmosphere. Solvents for reactions were of analytical
grade and were dried by passing through columns on an Innovative
Technologies Solvent Purifier. NMR solvents were dried over
2
2
contrasting m-h (1,2-C,P) and m-h (1,3-C,P) binding modes of
the two P–P coupled complexes achieve similar Sm–C/P binding
contacts, see below, but notably the m-h (1,2-C,P) binding mode is
effective in accommodating the t-Bu substituent further away from
the meso-ethyl substituents of the macrocycle that impinge on the
end of the macrocycle’s binding groove open to the coordination
of ancillary ligands to the Sm centre.
2
≡
sodium, distilled and degassed. t-BuC P and the macrocyclic
Sm(II) complex [(Me2N4)Sm(THF)2] 1 were prepared according
to our literature procedures.22,16 t-BuC N and t-BuN C were
≡
≡
purchased from Sigma-Aldrich and degassed by three freeze–
˚
thaw cycles and stored over activated 3 A molecular sieves. All
other reagents were purchased from Sigma-Aldrich and used as
received. NMR spectra were recorded on a Varian Mercury Plus
1
300 spectrometer (1H, 299.9 MHz; 13C{ H}, 75.4 MHz) in C6D6
◦
1
or d8-THF at 20 C unless otherwise noted. H NMR and 13C
NMR spectra were referenced to the residual 1H resonance of the
solvent residual peaks, while 13C NMR spectra were referenced to
the deuterated 13C resonance. J values are given in Hz. Elemental
analyses were carried by the Central Science Laboratory at the
University of Tasmania using a Carlo Erba EA1108 Elemental
Analyser or Medac Ltd, UK. GC-MS analyses were carried by the
Central Science Laboratory at the University of Tasmania using a
Varian 3800 GC coupled to a Varian 1200 triple quadrupole MS.
Syntheses
≡
[{(Me2N4)Sm(C N)}3] (2). tert-Butyl isonitrile (0.536 mmol,
22 mg) was added to a filtered benzene (1 mL) solution of
[(Me2N4)Sm(THF)2] 1 (1.04 ¥ 10-2 mmol, 9 mg). The reaction
was heated at 77 ◦C for 18 h, during which time small orange
≡
crystals of [{(Me2N4)Sm(C N)}3] 2 formed. The supernatant was
=
=
Fig. 4 Molecular structure of [{(Me2N4)Sm}2(t-Bu–C PP C-t-Bu)] 5.
Thermal ellipsoids are shown at the 50% probability level. Disorder has
been removed for clarity, as has lattice hexane and all calculated hydrogen
atoms.
removed and the crystals of 2 (7 mg, 87%) were washed with fresh
benzene (2 ¥ 0.2 mL) and dried in vacuo (Found: C, 64.37; H, 7.06;
N, 8.64. C117H162N15Sm3·C6H6 requires C, 64.01; H, 7.34; N, 9.10).
6868 | Dalton Trans., 2010, 39, 6864–6870
This journal is
The Royal Society of Chemistry 2010
©

Complex 2 was insoluble after initial precipitation, preventing
orientations. This resulted in partial occupancies of the Sm centre,
cyanide ligand and the macrocycle, but full occupancy of the
benzene solvent molecule. A number of overlapping macrocycle
C/N atoms were included in the refinement model, with the
majority able to be separately located. The accuracy of the
structure is clearly limited by this issue, that is particularly
clear in the nonplanarity of the anionic pyrrolide units where
only partial resolution of individual components was possible.
The C/N centres of the cyanide ligand were also disordered by
necessity of the space group, which also likely features as linkage
isomerism in the trimeric species. All hydrogen atoms were placed
≡
NMR characterisation. Similarly, [{(Me2N4)Sm(C N)}3] 2 was
prepared in 87% from the reaction of tert-butyl nitrile (0.536 mmol,
22 mg) in a benzene solution (1 mL) of [(Me2N4)Sm(THF)2] 1
(1.04 ¥ 10-2 mmol, 9 mg) after heating at 77 ◦C for 18 h and
isolated in an analogous manner (Found: C, 63.90; H, 7.08; N,
8.78. C117H162N15Sm3·C6H6 requires C, 64.01; H, 7.34; N, 9.10).
=
[(Me2N4)Sm(t-BuC N-t-Bu)]
(3). tert-Butyl
isonitrile
(0.536 mmol, 22 mg) was added to a filtered benzene (1 mL)
solution of [(Me2N4)Sm(THF)2] 1 (1.16 ¥ 10-2 mmol, 10 mg).
The reaction was allowed to stand at room temperature for
18 h, during which time large red crystals of [(Me2N4)Sm(t-
in calculated positions and refined using a riding model with fixed
2
˚
˚
˚
C–H distances of 0.95 A (sp CH), 0.99 A (CH2), 0.98 A (CH3).
The thermal parameters of all hydrogen atoms were estimated as
=
BuC N-t-Bu)] 3 formed along with small orange crystals of
≡
[{(Me2N4)Sm(C N)}3] 2. The red crystals of [(Me2N4)Sm(t-
U
iso(H) = 1.2Ueq(C) except for CH3 where Uiso(H) = 1.5Ueq(C). A
=
BuC N-t-Bu)] 3 (2 mg, 19% based on total Sm) were separated
summary of crystallographic data is given below.
with a spatula and washed with benzene (0.4 and 0.2 mL) and
dried in vacuo (Found: C, 65.83; H, 8.45; N, 8.25. C47H72N5Sm
requires C, 65.83; H, 8.46; N, 8.17). Complex 3 was insoluble
after initial precipitation, preventing NMR characterisation.
Crystal data for 2: C117H162N15Sm3·C6H6, M = 2307.77, trigonal,
˚
a = 27.638(3), b = 27.638(3), c = 12.423(9) A, a = 90, b = 90, g =
◦
3
˚
120 , V = 8218(6) A , T = 100 K, space group R((no. 148) Z =
3, 20193 reflections measured, 2910 unique (Rint = 0.083), 2465 >
4s(F), R = 0.074 (observed), Rw = 0.198 (all data). Crystal data
≡
[(Me2N4)Sm(N C-t-Bu)] (4). tert-Butyl nitrile (0.536 mmol,
22 mg) were added to
a benzene solution (1 mL) of
for 3: C47H72N5Sm, M = 857.45, monoclinic, a = 21.06(6), b =
[(Me2N4)Sm(THF)2] 1 (1.04 ¥ 10-2 mmol, 9 mg). The solution
was left at room temperature for 18 h and then all volatiles
were removed in vacuo. Fresh benzene was added (0.5 mL)
to effect dissolution and then allowed to evaporate, providing
3
˚
˚
10.493(7), c = 19.3928(15) A, V = 4285(13) A , T = 193 K,
space group Cmcm (no. 63) Z = 4, 1986 reflections measured,
1971 unique (Rint = 0.063), 1814 > 4s(F), R = 0.0832 (observed),
Rw = 0.2174 (all data). Crystal data for 4: C43H63N5Sm·C6H6,
≡
[(Me2N4)Sm(N C-t-Bu)] 4 (9 mg, 98%) as purple crystals which
M = 878.44, orthorhombic, a = 17.460(6), b = 28.000(15), c =
3
˚
˚
were collected; dH(299.9 MHz; C6D6; Me4Si) -26.42 (4 H, s, CH2),
-10.06 (12 H, s, CH3), -8.07 (4 H, s, CH2), -0.62 (12 H, s, CH3),
-0.62 (4 H, s, CH2), 1.73 (4 H, s, CH), 3.55 (9 H, s, CCH3), 5.56
(4 H, s, CH2), 13.77 (4 H, s, CH), 50.28 (6 H, s, NCH3).
18.760(4) A, V = 9171(6) A , T = 100 K, space group Cmc21
(no. 36) Z = 8, 49806 reflections measured, 7138 unique (Rint
=
0.0391), 7124 > 4s(F), R = 0.0306 (observed), Rw = 0.0748
(all data). Flack parameter = 0.359(13). Crystal data for 5:
C86H125N8P2Sm2·1/2(C6H14), M = 1676.67, triclinic, a = 14.941(3),
=
=
≡
[{(Me2N4)Sm}2(t-Bu–C PP C-t-Bu)] (5). t-BuC P (46 mg,
0.46 mmol) was added to a solution of the samarium(II) complex
[(Me2N4)Sm(THF)2] 1 (80 mg, 0.093 mmol) in toluene (10 mL) at
room temperature. After 10 min stirring all volatiles were removed
in vacuo and the residue extracted in hexane (10 mL). The extract
was concentrated in vacuo to 2 mL and stored overnight at -30 ◦C,
˚
b = 17.469(4), c = 18.533(4) A, a = 112.38(3), b = 91.11(3), g =
◦
3
˚
98.02(3) , V = 4415.2(15) A , T = 150 K, space group P((no. 2)
Z = 2, 28983 reflections measured, 15481 unique (Rint = 0.1679),
6948 > 4s(F), R = 0.0819 (observed), Rw = 0.1881 (all data).
=
=
yielding [{(Me2N4)Sm}2(t-Bu-C PP C-t-Bu)] 5 (30 mg, 33%)
as a green crystalline solid (Found: C, 62.79; H, 7.66; N, 6.73.
C86H125N8P2Sm2 requires C, 63.23; H, 7.71; N, 6.86); Dec.: 190 ◦C;
Complex 5 was insoluble after initial precipitation, preventing
NMR characterisation.
Summary and conclusions
A range of reactivities have been demonstrated for Sm(II) re-
ductions of bulky heteroalkynes. Our findings have widened the
scope of the complex types that have been obtained through
this chemistry and have also established a more varied structural
chemistry of the obtained products. We have provided additional
mechanistic evidence for Sm(II) isonitrile reductions via the first
structurally characterised iminoacyl lanthanide complex resulting
from trapping of the t-Bu fragment. We are continuing our
small molecule activation studies to include a broader range of
substrates. In particular regard to heteroalkynes, the investigation
of less bulky systems have already commenced, where more
complex stoichiometric and catalytic modifications have been
identified.
X-ray crystallography
◦
Data for 2 and 4 were collected at -173 C on crystals mounted
on a Hampton Scientific cryoloop at the MX1 beamline (l =
23
˚
0.775056 A) of the Australian Synchrotron. Data for 3 was
collected at -80 ◦C with an Enraf Nonius Turbo CAD4 with
˚
Mo-Ka radiation (l = 0.71073 A) on crystals mounted on glass
fibres. Data for 5 was collected at -123 ◦C with a Nonius Kappa
˚
CCD with Mo-Ka radiation (l = 0.71073 A). The structures
were solved by direct methods with SHELXS-97, refined using
full-matrix least-squares routines against F2 with SHELXL-97,24
and visualised using X-SEED.25 All non-hydrogen atoms were
refined anisotropically except for a N-Me carbon in 2, the methyl
carbons of the disordered t-Bu group of 3 and the hexane solvent
molecules for 5. The structure of 2 was severely disordered on
crystallographic sites resulting in a 50 : 50% occupancy of two
Acknowledgements
We thank the Australian Research Council for initial financial
support. CJ thanks the donors of The American Chemical Society
Petroleum Research Fund. Data for the structures of 2 and 4 were
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6864–6870 | 6869
©

obtained on the MX1 beamline at the Australian Synchrotron,
Victoria, Australia.
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≡
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2
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6870 | Dalton Trans., 2010, 39, 6864–6870
This journal is
The Royal Society of Chemistry 2010
©