Expected and Unexpected Reactivities of Homoleptic LiNacNac and Heteroleptic NacNacMg(TMP) β-Diketiminates toward Various Small Unsaturated Organic Molecules

Article abstract of DOI:10.1021/acs.inorgchem.1c00549

Homoleptic LiNacNac forms simple donor-acceptor complexes with N,N′-dicyclohexylcarbodiimide (CyN=C=NCy), triphenylphosphine oxide (Ph3P=O), and benzophenone (Ph2CO). These crystallographically characterized compounds could be regarded as model intermediates en route to reducing the N=C, P=O, and C=O bonds of unsaturated substrates. Heteroleptic NacNacMg(TMP) intriguingly functions as a TMP nucleophile both with t-BuNCO and t-BuNCS, producing a urea or thiourea derivative respectively attached to Mg, though the NacNac ligand in the former reaction also engages noninnocently with a second t-BuNCO molecule via insertion at the reactive NacNac backbone γ-carbon site.

Full text of DOI:10.1021/acs.inorgchem.1c00549

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Article
Expected and Unexpected Reactivities of Homoleptic LiNacNac and
Heteroleptic NacNacMg(TMP) β‑Diketiminates toward Various Small
Unsaturated Organic Molecules
Richard M. Gauld,∥ Jennifer R. Lynch,∥ Alan R. Kennedy, Jim Barker, Jacqueline Reid,
and Robert E. Mulvey*
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ABSTRACT: Homoleptic LiNacNac forms simple donor−accept-
or complexes with N,N′-dicyclohexylcarbodiimide (CyNC
NCy), triphenylphosphine oxide (Ph3PO), and benzophenone
(Ph2CO). These crystallographically characterized compounds
could be regarded as model intermediates en route to reducing the
NC, PO, and CO bonds of unsaturated substrates.
Heteroleptic NacNacMg(TMP) intriguingly functions as a TMP
nucleophile both with t-BuNCO and t-BuNCS, producing a urea
or thiourea derivative respectively attached to Mg, though the
NacNac ligand in the former reaction also engages noninnocently
with a second t-BuNCO molecule via insertion at the reactive
NacNac backbone γ-carbon site.
extent that it is still the most popular β-diketiminate proligand
utilized today and the subject of this present paper. β-
Diketiminate ligands in general have made a particularly strong
impact in Group 2.13−20 The same β-diketiminate ligand
(NacNac) was behind the opening of an exciting new area of
Group 2 chemistry through Jones and Stasch’s pioneering of
thermally stable magnesium(I) compounds of type (NacNac)-
MgMg(NacNac).21 Other highlights of magnesium β-diketi-
minate chemistry include advances made in the ring-opening
polymerization (ROP) of lactides.22−25 First synthesized
independently in 2002 by Roesky26 and Gibson,27 NacNacMg-
(n-Bu) has recently taken on greater significance through Hill’s
catalytic and insertion applications.28 Surprisingly, given the
prominence of n-Bu and TMP s-block reagents,29−32 the
analogous amide, NacNacMg(TMP), was only introduced as
recently as 2013 (TMP is 2,2,6,6-tetramethylpiperidide).33
This study by Hevia established that NacNacMg(TMP) is a
more effective metallating (C−H deprotonating) agent than its
alkyl analogue NacNacMg(n-Bu), reversing the normal order
of alkyl versus amide basicity.
INTRODUCTION
■
When attached to organic ligands, lithium and magnesium
represent the luminaries of the s-block of the periodic table,
arguably of the whole of the periodic table, in terms of their
phenomenal synthetic utility. It is not surprising therefore that
both metals have been involved in the proliferation of the
chemistry of β-diketiminate (NacNac or BDI) ligands that has
taken place over the past 20 or so years. Though originating in
the 1960s,1−4 β-diketiminate chemistry took about 30 years
before its tunable spectator ligand credentials rose to the fore5
in the mold of its cyclopentadienyl (Cp and substituted Cp)
predecessors.
Befitting the “masters of mediation” eminence of organic
alkali metal compounds in general,6 alkali metal β-diketimi-
nates are generally utilized for transferring their dinitrogen
ligands to other metals. This was first established with lithium
and tin in 1994 by Lappert, with the report also recording one
of the first crystal structures of a lithium β-diketiminate.7
Today this utilization continues as for example by Kretschmer,
who detailed an elegant but simple approach to obtaining
aluminum and gallium β-diketiminate complexes from the
sodium congener,8 as well as by the groups of Schaper and
Shen, who used the lithium or sodium β-diketiminate complex
in the convenient preparation of zirconium and lanthanide β-
diketiminate complexes, respectively.9−11
Received: February 23, 2021
Published: April 8, 2021
Power’s straightforward high-yielding synthesis of the
aminoimine 2,6-diisopropylphenyl-β-methyldiketiminate [Nac-
Nac (Me, Dipp) but labeled here for brevity as NacNac(H)]12
opened up access to this ligand to a wider community, to the
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Despite these developments, studies probing the behavior of
these s-block NacNac species with small molecules have been
relatively scarce, especially for Group 1 examples.34−38 Our
first expedition in this area found that the electrophilic species
CO2, t-BuNCO, and i-PrNCO all reacted with lithium NacNac
at the γ-C site of the ligand backbone, instead of at the
“frontal” polar Li−N bonds, expelling the innocent, spectator
image of alkali-metal-attached NacNac ligands.39 Here, the
picture becomes more complicated when the outcomes of
exposing LiNacNac to carbodiimide and phosphine oxide
molecules are revealed. Moreover, bringing NacNacMg(TMP)
into this study for the first time by treating it with isocyanate
and isothiocyanate molecules reveals surprising reactivities out
of kilter with those of conventional Mg TMP-containing
compounds.
Its structure (Figure 2) revealed a simple donor−acceptor
arrangement with DCC datively attached to the frontal Li atom
Since our earlier work on reactions of LiNacNac with
unsaturated organic molecules invariably showed backbone γ-
carbon reactivity with concomitant redistribution of the
NCCCN unit into a diimine, as displayed in Figure 1, we
Figure 2. Molecular structure of [{(MeCN-2,6-iPr2C6H3)2CH}Li·
N(Cy)CN(Cy)] (1). H atoms and disorder are omitted and the
NacNac Dipp groups are shown as a wire frame for clarity. Thermal
ellipsoids are displayed at the 40% probability level.
via one of its terminal atoms (N3). Repeating the reaction but
Figure 1. ChemDraw schematic showing general diimine function-
ality seen when LiNacNac was previously reacted with a series of
small molecules, where Dipp is 2,6-diisopropylphenyl.
heating the mixture to reflux temperature still afforded 1 as
1
confirmed by NMR spectra of the isolated product, with a H
resonance at 5.02 ppm, corresponding to the still intact
backbone γ-hydrogen atom in contrast to the sigmatropic
rearrangement of this CH atom to the NC bond of the
previously studied isocyanate systems.39
decided to investigate a larger sample of small molecules. Since
the NacNac NCC bonds can be regarded as joined
enamido units, this reactivity at the γ-carbon atoms is not that
unexpected.
The planarity of the NCCCN ring in 1 remains intact like
those in Power’s Et2O and THF LiNacNac structures, while
the two CC and two CN bond lengths [CC bond
lengths of 1.399(4) Å (C13−C14) and 1.420(4) Å (C14−
C15) compared to 1.387(4) Å and 1.418(4) Å respectively in
NacNac(H), and CN bond lengths of 1.324(3) (C13−N1)
and 1.318(3) Å (C15−N2) compared to 1.318(4) Å and
1.341(4) Å respectively in NacNac(H)] within the ring
become slightly more symmetrical upon lithium substitution
compared to NacNac(H), signifying a degree of delocalization
of the π-bonding. Lithium exhibits a trigonal planar geometry,
with bond angles markedly distorted from idealized values
RESULTS AND DISCUSSION
■
Our first choice was N,N′-dicyclohexylcarbodiimide, CyN
CNCy, DCC, which has a linear interior akin to that of
isoelectronic CO2. Thus, a 1:1:1 stoichiometric mixture of
LiNacNac, TMEDA (N,N,N′,N′-tetramethylethylenediamine),
and DCC in hexane solution produced crystals in 83% yield
identified by X-ray crystallography as [{(MeCN-2,6-
iPr2C6H3)2CH}Li·N(Cy)CN(Cy)], 1 (Scheme 1).
Scheme 1. Small Molecule Fixation Reactions Carried out in This Work
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ranging from 100.9(2)° (N2−Li1−N1) to 135.6(3)° (N2−
Li1−N3), while also lying effectively equidistant between the
two NacNac N atoms [Li1−N1, 1.898(5) Å and Li1−N2,
1.902(5) Å]. Lithium coordination of DCC results in
asymmetry in the NCN bonds since C36N3
[1.206(4) Å] is significantly shorter than C36N4 [1.39(1)
Å ]. This distortion is also reflected in the inequivalence of the
DCC bond angles [C33N4C36, 134.6(8)°] and C37
N3C36, 121.4(3)°], the former being notably more obtuse
than the CNC bond angles approaching 120° normally
seen in carbodiimide ligands.40 The N3−C36−N4 bond angle
in 1 also shows a distortion from linearity [165.4(5)°]. These
features suggest that the major resonance structure is polarized
(Cy)NCN+(Cy) rather than (Cy)NCN(Cy). Re-
actions of DCC with organolithium compounds generally
follow nucleophilic addition pathways as exemplified by the
lithium amidinate FcC(NCy)2Li formed when DCC is treated
with bulky ferrocenyllithium (FcLi).41 There are also several
articles referencing such addition reactions between amidinates
derived from DCC and organolithium reagents.42−45 For
example, reacting DCC with LiHMDS leads to amidinate
[(Cy)NC{N(SiMe3)}N(Cy)·Li], with addition of the N-
(SiMe3) group seen at the central DCC carbon.46 However,
to the best of our knowledge, there are no crystalline examples
of DCC or any other carbodiimide interacting with lithium
centers or any other metal centers as a Lewis donor such as
that seen in 1. Thus, 1 can be considered a model intermediate
en route to forming an amidinate from a carbodiimide and an
alkali metal nucleophilic source. Such κ1-RNCNR metal
coordinations have been implicated in various catalytic
heterofunctionalizations of carbodimides.47−52 It has previ-
ously been shown by the Harder group that activation of
carbodiimides is possible under metal-free conditions, although
it was noted that in this case the “extent of activation is less
than that in carbodiimide···Li+ complexes”, with harsh
conditions necessary under a metal-free environment.53 It is
likely that the bulk of the DCC molecule prevents formation of
an amidinate in this system, with the DCC being too sterically
congested to allow for nucleophilic addition of the NacNac in
this case. As alluded to earlier, solution NMR data are in
agreement with the solid-state structure (see the Supporting
Information for full details), suggesting that its composition is
Figure 3. Molecular structure of [{(MeCN-2,6-iPr2C6H3)2CH}Li·
OP(Ph)3] (2). H atoms, disorder, and cocrystallized hexane solvent
are omitted and the NacNac Dipp groups are shown as a wire frame
for clarity. Thermal ellipsoids are displayed at the 40% probability
level.
its identity confirmed by NMR characterization. Occupying a
highly distorted trigonal planar NNO coordination, with bond
angles ranging from 99.8(2)° (N1−Li1−N2) to 131.4(2)°
(O1−Li1−N1), Li lies equidistant between the NacNac N
atoms [N1−Li1, 1.925(3) Å and N2−Li1, 1.911(3) Å]. The
P−O−Li unit sits exactly within the NCCCN plane, which in
turn is not disturbed from planarity, with bond lengths [N1−
C13, 1.318(2) Å; C13−C14, 1.407(2) Å; C14−C15, 1.412(2)
Å; N2−C15, 1.316(2) Å] indicating a degree of π-
delocalization. Multinuclear NMR spectroscopic data on 2
concur with the solid-state structure, particularly showing that
coordination between the phosphine oxide and LiNacNac is
7
maintained in solution. This is deduced from the Li NMR
spectrum, which shows a resonance at 2.70 ppm, in contrast to
that seen for LiNacNac at 0.73 ppm, indicating a change in the
lithium environment. The 31P{1H} NMR also indicates
coordination, with the resonance at 23.2 ppm for uncoordi-
nated triphenylphosphine oxide contrasting to that at 30.80
ppm seen in 2. Though a CSD search revealed 20 hits for
Ph3PO → Li dative bonds, no hits were found for any
LiNacNac scaffold, with 7 of the 20 hits featuring a cationic
(Ph3PO)4Li+ unit with a balancing counteranion present. Of
note is work by Lichtenberg, who structurally characterized a
lithium aminotroponiminate (LiATI) solvated by Ph3PO,
exhibiting a Li chelated by two ATI N atoms in a similar
arrangement to that of 2 but with an additional O (THF)
ligation.54 Significantly, a search of the CSD revealed no hits
for a phosphine oxide unit bonded to LiNacNac, with the
closest match being a 1,8-C10H6{NHSiMe3}2-supported
dilithium compound; however, the lack of delocalization over
the backbone of this ligand limits comparison.55 Attempted
reactions with the sulfur analogue Ph3PS failed to produce a
complex with LiNacNac as determined via NMR studies,
contrasting with Nikonov’s report of NacNacAl(I) which
forms NacNacAl(S)SPPh3 via a complexation/oxidative
cleavage process.56
7
maintained in solution. Of note is the Li spectrum, which
showed a single resonance corresponding to the single lithium
environment present within 1 at 2.61 ppm. This contrasts with
7
the Li spectrum of LiNacNac, which shows a resonance at
0.73 ppm, confirming that the lithium in 1 remains in a
different environment, due to the donating role of the DCC
molecule being retained in solution.
Next, we studied triphenylphosphine oxide, Ph3PO. A
1:1:1 stoichiometric mixture of LiNacNac, Ph3PO and
PMDETA (N,N,N′,N″,N″-pentamethyldiethylenetriamine;
added to enhance solubility) in hexane solution deposited
crystals (78% yield) identified by X-ray crystallography as
[{(MeCN-2,6-iPr2C6H3)2CH}Li·OP(Ph)3], 2. Matching that
of 1, the structure of 2 (Figure 3) is a donor−acceptor complex
connected via a frontal Li−O bond [1.489(1) Å], with no
backbone insertion present.
PMDETA is also absent from 2, though subsequent
experiments showed that PMDETA addition is necessary in
order to obtain crystals, although why PMDETA should aid
the crystallization process is not yet clear. Without adding
PMDETA, 2 can still be made as an amorphous powder, with
Our third and final homoleptic LiNacNac structure was
obtained from the reaction between the ketone benzophenone
and LiNacNac. A mixture of LiNacNac and a slight
stoichiometric excess of benzophenone (1:1.25) was reacted
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in hexane, with two drops of PMDETA (N,N,N′,N″,N″-
pentamethyldiethylenetriamine) added for crystallization pur-
poses. This solution deposited a crop of large red crystals (in a
59% yield) identified by X-ray crystallography as [{(MeCN-
2,6-iPr2C6H3)2CH}Li·OC(Ph)2], 3. Following the trend set by
1 and 2, the structure of 3 (Figure 4) is yet another donor−
LiNacNac is maintained in solution. This is deduced from the
7Li NMR spectrum, which shows a resonance at 3.64 ppm, in
contrast to that seen for LiNacNac at 0.79 ppm, indicating a
significant change in the lithium environment.
Upon searching the CSD, it was discovered there were no
examples of bonding between Ph2CO and Li involving the
extensively studied Dipp2NacNac scaffold. In fact, there were
no examples at all of an aldehyde/ketone CO unit
coordinating to either Li, Na, or K within the Dipp2NacNac
scaffold in this way, with examples of benzophenone
coordinating to lithium metal in any environment being
rare.57,58 However, there was a single hit for dative Ph2CO
→ LiNacNac bonding featuring a much less commonly studied
variant of NacNac, [Me3SiNC(Ph)CHC(Ph)NSiMe3]Li] by
Tong and Liu.59 This unusual ligand (displayed in Scheme 2)
could be viewed as an inversion of the ubiquitous
Dipp2NacNac ligand, as now we see a rich electron donor
sitting on the α-nitrogen position while an electron with-
drawing group sits on the β-carbon positions. The presence of
phenyl rings on the backbone, acting as electron withdrawing
groups, may also alter the nucleophilicity of the γ-carbon
position, a key feature in several recent small molecule
activations using the Dipp2NacNac scaffold.37
The contrast with this 2008 structure can help rationalize
the dominance of the Dipp2NacNac ligand, with the Dipp
groups flanking the metal allowing for coverage of the
coordination sphere to be maximized which helps to prevent
coordination to multiple reactant molecules, unwanted solvent
interactions, or dimerization. The dual coordination of
benzophenone units in [[Me3SiNC(Ph)CHC(Ph)NSiMe3]Li]
can thus be rationalized both by the reduction of steric bulk
around the metal center decreasing the activation barrier and
the loss of steric coverage of the coordination sphere.
Interestingly, Sen et al. recently reported that LiNacNac
could catalyze hydroboration reactions of aldehydes and
ketones using pinacolborane, theorizing a catalytic cycle
involving a donor−acceptor Ph(H)CO → LiNacNac
intermediate.60 While complex 3 incorporates benzophenone
rather than benzaldehyde, it exemplifies that the RCO →
LiNacNac unit predicted by Sen et al. using DFT calculations
is achievable. In view of the aforementioned inverted NacNac
structure reported by Liu, this may also highlight the advantage
of Dipp2NacNac in preventing saturation of the metal
coordination sphere, facilitating the formation of catalytically
active intermediates such as this. Meanwhile complex 2 could
Figure 4. Molecular structure of [{(MeCN-2,6-iPr2C6H3)2CH}Li·
OC(Ph)2] (3). H atoms are omitted and the NacNac Dipp groups are
shown as a wire frame for clarity. Thermal ellipsoids are displayed at
the 40% probability level.
acceptor complex, connected via a slightly elongated frontal
Li−O bond [1.843(3) Å], when compared to that observed in
compound 2. Of note is that once again no backbone insertion
at the γ-carbon is observed, in contrast to that noted in
previous work.39
In a similar manner to 2, PMDETA addition was shown to
be required for crystallization of compound 3, despite it not
being part of the crystal structure obtained. The lithium atom
of compound 3 sits in an approximately trigonal planar
coordination environment, with bond angles ranging from
101.22(14)° (N1−Li1−N2) to 133.92(18)° (O1−Li1−N1).
The lithium center lies equidistant between the NacNac N
atoms [N1−Li1, 1.908(3) Å and N2−Li1, 1.914(3) Å]. Akin
to that in compound 2, the C−O−Li unit of compound 3 sits
within the NCCCN plane, which retains its planarity. Bond
lengths [N1−C5, 1.326(2) Å; C5−C6, 1.410(2) Å; C6−C4,
1.408(2) Å; N2−C4, 1.317(2) Å] are also indicative of a
degree of π-delocalization. Multinuclear NMR spectroscopic
data on 3 concurred with the solid-state structure, particularly
showing that coordination between the benzophenone and
Scheme 2. Selection of Products Found Involving CO Bonds Interacting with Various β-Diketiminate Scaffolds
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be viewed as a model intermediate of this catalysis with PO
as opposed to CO coordination. The Mair group similarly
predicted an intermediate containing the RCO → LiNacNac
unit in their 2003 paper on reversible CC bond formation.61
On this occasion NMR evidence was cited to suggest the
presence of the adamantanone-coordinated intermediate
structure, supported by a 1,5-diazapentadienyllithium complex,
a close cousin of Dipp2NacNac, with only one less iso-propyl
arm per phenyl ring. This new structure is evidence that such
structures are obtainable in the solid state, as well as being
observed by NMR in the solution state.
These three results with DCC, Ph3PO, and Ph2CO
show that coordination of bulky nitrogen and oxygen donors
to LiNacNac is possible, although for these donor systems
coordination occurs preferentially at the frontal site of the
molecule, with the NacNac backbone remaining undisturbed;
this is in contrast with results obtained with isocyanates and
CO2, where nucleophilic attack occurs via the backbone γ-
carbon position.39
Figure 5. Molecular structure of [{(MeCN-2,6-iPr2C6H3)2CH(CON-
(t-Bu)}Mg(CON(t-Bu))TMP] (4). H atoms and cocrystallized
hexane solvent are omitted and NacNac Dipp groups and organic
TMP scaffold are shown as a wire frame for clarity. Thermal ellipsoids
are displayed at the 40% probability level.
As alluded to earlier, Hevia’s work has established that
heteroleptic NacNacMg(TMP) is an efficient TMP base for
selectively deprotonating sensitive organic molecules such as
1,3-benzoazoles or fluorinated aromatic compounds, capturing
the emergent anionic molecules and concomitantly releasing
TMP(H), as shown in Scheme 3.33
of a hydrogen from the γ-carbon to the nitrogen of either
isocyanate unit, as seen in the lithium NacNac case, though
there is concomitant redistribution of the NacNac NCCCN
unit into a diimine as seen previously. In 4, Mg occupies a
distorted trigonal bipyramidal site comprising two Mg−(axial)
O bonds (O1−Mg1−O2 bond angle, 169.7(1)°) and three
Mg−(equatorial)N bonds (average N−Mg−N bond angle,
119°). The C−N and C−O bond lengths of approximately 1.3
(Å) support the hypothesis of two highly delocalized systems
within the NCO unit, with both bonds existing as intermediate
between single and double bonds. Solution-state NMR
characterization of 4 was also carried out (see the SI for full
details).
Scheme 3. Trapping of Benzothiazole Using
NacNacMg(TMP), with Concomitant Formation of
TMP(H), as Reported by Hevia33
The final small molecule studied was isothiocyanate t-
BuNCS, prompted by a recent report by Ma et al., who found
that the NacNacMg(n-Bu) dimer exhibited different reac-
tivities depending on the ratio of PhNCS used.34 Exposure to
one equivalent led to deaggregation of the dimer and bidentate
(N, S) coordination to Mg via a thioamidinate NC(n-Bu)S
unit, while exposure to two equivalents showed this
coordination again but combined with coordination at the
NacNac backbone, similar to that found in 4. Since our
attempts to grow a crystalline product from NacNacMg(TMP)
and PhNCS failed, we turned to t-BuNCS. Surprisingly, neither
of these aforementioned coordinations were found in
[{(MeCN-2,6-iPr)2C6H3)2CH}Mg(TMP)(t-BuNCS)] 5, the
product of the 1:1 NacNacMg(TMP) and t-BuNCS reaction.
Instead, the structure of 5 (Figure 6) shows addition of the
Mg−N(TMP) bond to the NC bond of the NCS unit to form
a four-membered MgNCN ring, leaving the sulfur uncoordi-
nated to the Mg.
Thus, the TMP unit again plays an unfamiliar addition role
as in 4 but in a different way as it is now bridging as opposed
to terminal, a distinction dictated by the different heteroatoms
on the cyanate units with Mg preferring to bind to O over N in
the case of 4 and to N over S in the case of 5, keeping with the
HSAB concept.67 This basic concept can also rationalize the
different N, S coordination observed in the Ma structure, as
Mg has less affinity for the adding n-Bu group than the S
heteroatom. There are two crystallographically independent
molecules within the unit cell of 5; however, as the metrics for
each molecule are essentially identical, only one is discussed.
Centralized magnesium occupies a highly distorted tetrahedral
Mg(TMP) systems are good bases in general though they
tend to work best in synergistically operative bimetallic
mixtures.62−65 These reactions piqued our curiosity so we
decided to also investigate the small molecule chemistry of
NacNacMg(TMP) with molecules bereft of acidic bonds,
namely with isocyanate t-BuNCO and isothiocyanate t-BuNCS
(Scheme 1). Surprisingly, with t-BuNCO, a 2-fold insertion
was seen, with one isocyanate molecule inserting into the γ-C
site, while another inserted into the Mg−N(TMP) bond. No
insertion was seen at a frontal Mg−N(NacNac) site. Evident
from the crystal structure of the isolated urea-type product,
[{(MeCN-2,6-iPr2C6H3)2CH(CON(t-Bu)}Mg(CON(t-Bu))-
TMP] 4 (Figure 5), this 2-fold reactivity contrasts with that of
LiNacNac with t-BuNCO, where a single insertion occurred at
the γ-C site. When this NacNacMg(TMP) reaction was
repeated rationally with two equivalents of t-BuNCO instead
of one, the same product was formed, proving the
reproducibility of the reaction, but only in a similarly small
yield. These low isolated yields can be attributed to the high
solubility of 4 in nonpolar aprotic solvents. The standout
feature of 4 is TMP acting as a nucleophile, as its inherent low
nucleophilicity is its prized asset with regard to its popularity as
a selective base with unsaturated substrates, though rare
examples of TMP nucleophilicity exist.66 Another feature of
interest is that there is no sigmatropic hydride rearrangement
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also consider the development of new NacNac-derived ligands
with extra functionality such as that displayed in the urea-like
compound 4.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00549.
Experimental methods, crystallographic information,
NMR spectra, IR data, and melting point data (PDF)
Accession Codes
CCDC 2061662−2061666 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 6. Molecular structure of [{(MeCN-2,6-iPr)2C6H3)2CH}Mg-
(TMP)(t-BuNCS)] (5). H atoms, disorder, and a second molecule
are omitted and the NacNac Dipp groups and organic TMP scaffold
are shown as a wire frame for clarity. Thermal ellipsoids are displayed
at the 40% probability level.
AUTHOR INFORMATION
■
environment, with bond angles ranging from 63.4(2)° for N3−
Mg1−N4 to 128.0(2)° for N1−Mg1−N3. In 3, the N1−
Mg1−N2 chelation angle is 88.2(1)°, while in 5 it increases to
94.8(2)°. It also lies equidistant between the chelating nitrogen
atoms of the NacNac ligand in a similar situation to that seen
in 4 [N1−Mg1, 2.097(5) Å; N2−Mg1, 2.105(5) Å; cf. N1−
Mg1 2.162(3) Å; N2−Mg1 2.163(2) Å in 3]. The NacNac
ligand is essentially planar, with the root-mean-square (RMS)
deviation from linearity being less than 1 Å [0.075(4) Å], with
Mg lying 0.744(6) Å outside this plane. Bonding in the
NC(NTMP)S unit appears to be that of an TMP-based imine
unit, based on the bond lengths of N3−C39 [1.506(9) Å) and
N4−C39 (1.297(9) Å], with a C−S bond length of 1.701(7)
Å. These lengths suggest considerable delocalization within
this NC(NTMP)S unit (specifically across S1−C39−N3),
which adopts a thiourea-like arrangement.
Corresponding Author
Robert E. Mulvey − WestCHEM, Department of Pure and
Applied Chemistry, University of Strathclyde, Glasgow G1
1XL, U.K.; orcid.org/0000-0002-1015-2564;
Email: r.e.mulvey@strath.ac.uk
Authors
Richard M. Gauld − WestCHEM, Department of Pure and
Applied Chemistry, University of Strathclyde, Glasgow G1
1XL, U.K.
Jennifer R. Lynch − WestCHEM, Department of Pure and
Applied Chemistry, University of Strathclyde, Glasgow G1
1XL, U.K.
Alan R. Kennedy − WestCHEM, Department of Pure and
Applied Chemistry, University of Strathclyde, Glasgow G1
1XL, U.K.; orcid.org/0000-0003-3652-6015
Jim Barker − Innospec Ltd., Cheshire CH65 4EY, U.K.
Jacqueline Reid − Innospec Ltd., Cheshire CH65 4EY, U.K.
CONCLUSIONS
■
We have demonstrated that there remains a significant and
wide variety of potential reactivities of homoleptic LiNacNac
and heteroleptic NacNacMg(TMP) β-diketiminates toward
small unsaturated molecules that are yet to be fully explored. In
this study we have structurally characterized what is, to the
best of our knowledge, the first example of a carbodiimide
acting as a Lewis donor toward an alkali metal center, as well as
the first structural example of a phosphine oxide binding to
LiNacNac. In contrast to our previous work, both products
display reactivity at the front of the NacNac scaffold, a feature
that was also found with benzophenone. The benzophenone
structure could act as a model compound for the binding of an
aldehyde or ketone to the LiNacNac scaffold. Further
exemplifying the unusual reactivity that we have come across
in NacNac chemistry, we have shown the unconventional
behavior of the typically selective base TMP, in which we see a
rare case of TMP acting as a nucleophile in order to play an
addition role, e.g., in one example in a terminal fashion and
bridging in another, in a manner which is rarely seen, though in
keeping with classical HSAB theory. One facet of future work
is to determine what effect changing the alkali metal of the
whole of Group 1 (Li−Cs) will have on the structure and
reactivity of NacNac and related compounds.6 Future work will
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00549
Author Contributions
∥R.M.G. and J.R.L. contributed equally. The manuscript was
written through contributions of all authors. All authors have
given approval to the final version of the manuscript.
Notes
A data set containing the raw crystallographic and X-ray data
(cif, fid) can be found at 10.15129/a79e0465-457a-40cd-81f6-
fd2b90df2a3d.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the EPSRC (DTP award EP/
N509760/1 for R.M.G. and EP/T517938/1 for J.R.L.) and
Innospec Ltd. for funding the studentships of R.M.G. and
J.R.L. We also thank Craig Irving for support and advice with
regards to the NMR aspects of this work and Prof. Eva Hevia
(University of Bern) and Dr. Catherine Weetman (University
of Strathclyde) for helpful discussions.
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pubs.acs.org/IC
Article
Carbamateand Carboxylate Isosteres. Angew. Chem., Int. Ed. 2020,
59, 13628−32.
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