A two-coordinate manganese(0) complex with an unsupported Mn-Mg bond: Allowing access to low coordinate homo- and heterobimetallic compounds

Article abstract of DOI:10.1021/ja5021348

This study details the synthesis and characterization of an unprecedented two-coordinate, high-spin manganese(0) complex that incorporates an unsupported Mn-Mg bond, viz. L^?MnMg(^MesNacnac) (L ^? = -N(Ar^?)(SiPrⁱ₃), Ar^? = C₆H₂{C(H)Ph₂} ₂Prⁱ-2,6,4; ^MesNacnac = [(MesNCMe) ₂CH]^-; Mes = mesityl). This compound has been utilized as an inorganic Grignard reagent in the preparation of the first two-coordinate manganese(I) dimer, L^?MnMnL* (L* = -N(Ar*)(SiMe₃), Ar* = C₆H₂{C(H) Ph₂}₂Me-2,6,4), and the related mixed valence, bis(amido)-hetereobimetallic complex, Mn^II(μ-L^?) (μ-L*)Cr⁰. It is also shown to act as a two-electron reducing agent in reactions with unsaturated substrates.

Full text of DOI:10.1021/ja5021348

Communication
pubs.acs.org/JACS
A Two-Coordinate Manganese(0) Complex with an Unsupported
Mn−Mg Bond: Allowing Access to Low Coordinate Homo- and
Heterobimetallic Compounds
Jamie Hicks,† Chad E. Hoyer,‡ Boujemaa Moubaraki,† Giovanni Li Manni,‡ Emma Carter,§
Damien M. Murphy,§ Keith S. Murray,† Laura Gagliardi,‡ and Cameron Jones*,†
†School of Chemistry, Monash University, Melbourne, Victoria, 3800, Australia
‡Department of Chemistry and Minnesota Supercomputing Institute, University of Minnesota 207 Pleasant St, SE Minneapolis,
Minnesota 55455-0431, United States
§School of Chemistry, Cardiff University, Cardiff, CF10 3AT, United Kingdom
S
* Supporting Information
great advantage to be able to access representatives that exhibit
ABSTRACT: This study details the synthesis and
characterization of an unprecedented two-coordinate,
high-spin manganese(0) complex that incorporates an
unsupported Mn−Mg bond, viz. L†MnMg(MesNacnac) (L†
= −N(Ar†)(SiPri3), Ar† = C6H2{C(H)Ph2}2Pri-2,6,4;
MesNacnac = [(MesNCMe)2CH]−; Mes = mesityl). This
compound has been utilized as an “inorganic Grignard
reagent” in the preparation of the first two-coordinate
manganese(I) dimer, L†MnMnL* (L* = −N(Ar*)-
(SiMe3), Ar* = C6H2{C(H)Ph2}2Me-2,6,4), and the
related mixed valence, bis(amido)-hetereobimetallic com-
plex, MnII(μ-L†)(μ-L*)Cr0. It is also shown to act as a
two-electron reducing agent in reactions with unsaturated
substrates.
their lowest possible metal coordination number, namely two. In
this regard, while one diamagnetic terphenyl-zinc(I) example,
Ar′ZnZnAr′, has been reported,9 two-coordinate, open-shell
metal(I) dimers remain unknown.10 This paucity likely stems
from a lack of suitably bulky monodentate ligands for the kinetic
stabilization of such species. We have recently developed an
extremely bulky class of monodentate amide ligand, e.g.,
−N(Ar†)(SiPri3) (Ar† = C6H2{C(H)Ph2}2Pri-2,6,4 (L†),11
which we have utilized in the preparation of a variety of very
reactive one- and two-coordinate p-block metal(I) com-
pounds.12 It seemed reasonable that these amides might also
prove their worth in the stabilization of novel low-valent/low-
coordinate first-row d-block systems. To this end, here we report
that the reduction of a bulky amido-manganese(II) halide with a
magnesium(I) dimer yields an unprecedented two-coordinate,
high-spin manganese(0) complex bearing an unsupported Mn−
Mg bond. This has been utilized as an “inorganic Grignard
reagent” to access the first two-coordinate manganese(I) dimer
and a related mixed valence heterobimetallic (MnII/Cr0) species,
both of which incorporate two amides as their only ligands. The
reactivity of the Mn−Mg bonded species toward several
unsaturated substrates is also reported.
At the outset of this study the reaction of the dimeric
manganese(II) bromide complex, {L†Mn(THF)(μ-Br)}2 (see
Supporting Information (SI)), with 1 equiv of the magnesium(I)
dimer, {(MesNacnac)Mg}2 (MesNacnac = [(MesNCMe)2CH]−;
Mes = mesityl),13 was carried out with the expectation that the
manganese(I) compound, L†MnMnL†, would be formed.
Instead, the reaction returned the intensely royal blue colored
manganese(0) compound, L†MnMg(MesNacnac) 1, and un-
reacted {L†Mn(THF)(μ-Br)}2 in an ∼2:1 ratio. Repeating the
reaction in a 1:2 ratio led to the complete consumption of
{L†Mn(THF)(μ-Br)}2 and afforded a good isolated yield of 1
(Scheme 1). Consequently, it is likely that the mechanism of
these reactions involves the generation of the transient
manganese(I) fragment, “L†Mn”, which, because of its high
steric loading, is incapable of dimerizing (to give L†MnMnL†)
etal−metal bonded compounds have been intensively
M
studied for many decades, not only due to their
fundamental appeal but also because of the many applications
they have found in areas such as small molecule activations,
catalysis, enzyme mimicry, etc.1 With respect to the d-block
metals, the vast majority of efforts in this field have lain with the
second- and third-row elements, though the development of
highly reactive, and often open-shell, first-row metal−metal
bonded complexes has been rapid in recent years.1 Arguably, the
most impressive advances here stem from the kinetic stabilization
of low coordinate, carbonyl free, metal(I)−metal(I) bonded
dimers using very bulky mono- and higher dentate ligands.
Landmark compounds from this work include Carmona’s zinc(I)
2
−
dimer, Cp*ZnZnCp* (Cp* = C5Me5 ), Power’s quintuply
bonded, terphenyl-coordinated chromium(I) dimer, Ar′CrCrAr′
(Ar′ = C6H3Dip2-2,6, Dip = C6H3Pri2-2,6),3 Roesky’s β-
diketiminate chelated manganese(I) dimer, (DipNacnac)MnMn-
(
DipNacnac) (DipNacnac = [(DipNCMe)2CH]−),4 and a variety
of systems with extremely short MI−MI (M = Cr,5 Fe6 or Co7)
multiple bonds bridged by bulky amidinate, guanidinate, or
reduced diazabutadiene ligands. Given the highly reactive nature
of such compounds, it is not surprising that they are finding
synthetic applications in a variety of areas.5a,8
In order to significantly enhance the reactivity scope of neutral
first-row d-block metal−metal bonded systems, it would be a
Received: March 1, 2014
Published: March 24, 2014
© 2014 American Chemical Society
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a
Scheme 1. Synthesis of Compounds 1−4
a
The metal(II) halide precursor complexes are dimeric in the solid
state.
and instead is further reduced by the magnesium(I) reagent,
yielding 1.
Compound 1 is of considerable fundamental interest for a
number of reasons. First, while a handful of complexes
containing d-block metal-Mg bonds have been reported,14
none incorporate manganese, and all are higher coordinate
closed-shell species. Furthermore, compound 1 is the first
bimetallic system to contain two-coordinate manganese, formally
in the zero oxidation state.15 These novel features strongly
suggested that 1 should exhibit potent reactivity, which was
initially explored by examining its use as an ″inorganic Grignard
reagent″ for the transfer of the L†Mn fragment. In this regard, its
reaction with the amido-manganese(II) halide, {L*Mn(THF)-
(μ-Br)}2 (L* = -N(Ar*)(SiMe3), Ar* = C6H2{C(H)Ph2}2Me-
2,6,4), afforded a good yield of the unsymmetrically substituted
manganese(I) dimer, L†MnMnL* 2, as red-purple crystals
(Scheme 1). The formation of 2, in combination with the fact
that 1 does not readily react with the bulkier precursor complex,
{L†Mn(THF)(μ-Br)}2, is consistent with our proposed
mechanism for the generation of 1. To add further weight to
this, the reduction of the less bulky precursor, {L*Mn(THF)(μ-
Br)}2, with {(MesNacnac)Mg}2 was carried out, and this
proceeded directly to the symmetrical manganese(I) dimer,
L*MnMnL* 3, with no evidence for the formation of a Mn−Mg
bonded species.
Low-valent heterobimetallic systems incorporating electroni-
cally distinct first-row d-block metals have proved valuable as
reagents/catalysts in a variety of organic transformations.1 It
seemed reasonable that 1 could be used as a synthon to access
novel examples of such systems incorporating one Mn center. To
probe this possibility, the compound was reacted with the
chromium(II) halide complex, {L*Cr(THF)(μ-Cl)}2,16 which
yielded the mixed-valence bis(amido)-MnII/Cr0 compound, 4
(Scheme 1). Based on the chemistry described above, it is feasible
that this reaction proceeds via a metal−metal bonded
intermediate, L†Mn-CrL*, which undergoes an internal redox
process to give 4. This compound is an unprecedented example
of a first-row d-block heterobimetallic complex, the coordination
sphere of which encompasses only two amide ligands.17 The
formation of 4 implies that the synthesis of a variety of related
systems should be achievable, especially given that groups 6−9
and group 12 metal(II) halide complexes incorporating bulky
amide ligands (e.g., L* and L†) have recently been made available
as potential precursors.16
Figure 1. Thermal ellipsoid plots (20% probability surface) of (a) 1, (b)
2, and (c) 4.
magnesium bonds (range: 2.481−2.633 Å),14 all of which belong
to higher-coordinate, closed-shell systems. It is, however, well
within the sum of the covalent radii of Mg and high-spin Mn
(3.02 Å).18 Compounds 2 and 3 represent the first open-shell,
two-coordinate d-block metal(I) dimers and do not exhibit any
close interactions between their metal centers and the
benzhydryl phenyl groups of their amide ligands (closest Mn···
CPh = 2.970(3) Å 2; 3.043(3) Å 3, cf. 3.033 Å 1). They have
similar, mildly trans-bent structures (N−Mn−Mn = 153.05°
(mean) 2; 148.39(5)° 3) and their Mn−Mn bonds (2.7431(7) Å
2; 2.7224(6) Å 3) are comparable to that in three-coordinate
(
DipNacnac)MnMn(DipNacnac) (2.721(1) Å).4 The heterobi-
metallic system, 4, possesses a two-coordinate, bis(amido)-MnII
center (N−Mn−N = 140.00(9)°), while its Cr0 atom is η6-
coordinated by the central phenyl ring of the L† ligand and a
flanking phenyl group of the other amide, L* (centroid−Cr−
centroid =164.42(6)°). Although the separation between the two
metal centers (3.0443(6) Å) is close to the sum of the covalent
radii for Cr and high-spin Mn (3.00 Å),18 computational studies
(vide infra) indicate a negligible metal−metal interaction.
All of 1−4 are paramagnetic and exhibit solution-state
magnetic moments (Evans method: 1 5.3 μB, 2 3.4 μB (per
Mn), 3 3.4 μB (per Mn), 4 5.5 μB) indicative of them containing
high-spin Mn centers. So as to gain some insight into the nature
of their metal−metal interactions, variable-temperature solid-
state magnetic susceptibility measurements were carried out.
Those for 1 (see Figure 2a) show it to exhibit a χMT value at 300
K of 4.14 cm3 mol−1 K (μeff = 5.76 μB), which slowly decreases, in
a Curie−Weiss fashion, to reach ∼3.9 cm3 mol−1 K (μeff = 5.59
μB) at ∼6 K then, more rapidly due to zero-field splitting,
reaching 3.0 cm3 mol−1 K (μeff = 4.90 μB) at 2 K. Such data are
suggestive of an isolated S = 5/2 ground state, as would be
expected for a compound containing high-spin Mn0 (“4s23d5”).
The low-temperature (10 K) X-(9 GHz) and Q-(35 GHz) band
EPR spectra of frozen solutions of 1 are largely consistent with
this conclusion, in that their simulations yielded spin
Hamiltonian parameters characterized by g = 2.004, S = 5/2,
and anisotropic zero-field parameters of D = 0.1 cm−1 (2850
MHz) and E = 0.04 cm−1 (1250 MHz) (see SI).
Compounds 1−4 were crystallographically characterized, and
the molecular structures of 1, 2 and 4 are depicted in Figure 1.
Compound 1 is monomeric and exhibits a nonlinear, two-
coordinate Mn center (N−Mn−Mg = 160.85(9)°). Its
unsupported Mn−Mg bond (2.8244(13) Å) is markedly longer
than the handful of previously reported first-row d-block metal−
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Figure 3. Representations of (a) the Mn−Mg σ-bonding orbital of 1Me
and (b) the Mn−Mn σ-bonding orbital of 3, derived from the CASSCF
calculations.
calculated to be similar and positive for both metals (Mn +0.69,
Mg +0.83).20 It is noteworthy that the calculations on 1Me also
indicate that the intense blue color of 1 is not due to Mn d−d
electronic transitions and more likely originates from charge-
transfer excitations involving the ligands. Calculations on 3
revealed a comparable bonding picture to that reported for
(Piso)MnMn(Piso).7 That is, the low-energy states are highly
multiconfigurational, with the singlet spin state being the ground
state. In the ground state, the compound has a Mn−Mn single
bond (EBO = 0.90, Figure 3b), and each high-spin Mn center
encompasses five singly occupied 3d orbitals that are essentially
nonbonding. Again in line with the experimental magneto-
chemical studies, compound 4 was calculated to have a high-spin
MnII center and a dominant sextet ground state. The calculations
showed effectively no MnII−Cr0 bonding in the compound.
In light of the reducing abilities displayed by 1 in its reactions
with metal halide precursors, preliminary studies of its reactivity
toward several unsaturated molecules were also carried out. First,
while it reacts with dioxygen to give an intractable mixture of
products, its treatment with stoichiometric or excess N2O in
THF afforded a good yield of the colorless μ-oxo-bridged
manganese(II) complex, L†MnOMg(THF)(MesNacnac) 5.21
Like 1, this compound is high spin in solution (μeff = 5.9 μB)
and the solid state (see SI), and its molecular structure (Figure 4)
shows it to have a two-coordinate Mn center (N−Mn−O
163.85(8)°), which is unprecedented for an oxo-manganese
complex. Similarly, reaction of 1 with the carbodiimide,
PriNCNPri, led to a two-electron reduction of the substrate,
and its insertion into the Mn−Mg bond of the bimetallic
Figure 2. Plot of χMT vs T for (a) 1 and (b) 3 (per Mn center). The solid
line in (a) is a guide to the eye, while that in (b) is the calculated best fit
using the parameters in the text.
The solid-state magnetic behavior of 4 is similar to that of 1
(see SI), again implying an isolated S = 5/2 ground state, though
originating from high-spin MnII (3d5) in this case. Quite different
magnetic data were obtained for 3, which exhibits an χMT value of
1.28 cm3 mol−1 K per Mn center at 300 K (μeff = 3.20 μB, the
Curie temperature is significantly higher than 300 K). This
decreases almost linearly down to ∼50 K (reaching ∼0.1 cm3
mol−1 K), before plateauing at close to 0 cm3 mol−1 K below that
(Figure 2b). The corresponding χM vs T plot (see SI) is typical of
an antiferromagnetic coupled system, the data for which were
fitted to an S = 5/2 (per Mn) dimer model (−2JS1·S2) using g =
2.18, J = −47.5 cm−1. Theoretical calculations (vide infra)
confirm an S = 0 coupled ground state for the compound. This
magnetic profile is similar to those reported for the higher
coordinate dimers, (DipNacnac)MnMn(DipNacnac)4 and (Piso)-
MnMn(Piso) (Piso = [(DipN)2CBut]−),6 which were both
shown to have Mn−Mn single bonds derived from the 4s
electron at each antiferromagnetically coupled, high-spin MnI
center.
In order to further examine the metal−metal bonding in the
compounds reported here, calculations (CASSCF/CASPT2)
were carried out to determine the ground-state electronic
structures of a model of 1, with its isopropyl substituents replaced
with methyl groups (viz. 1Me), and the full molecules of 3 and 4
(see SI for full details). Consistent with the magnetochemical
studies on 1, a sextet ground state was determined for 1Me, arising
from single occupation of its five, nonbonding Mn 3d orbitals.
Moreover, the compound possesses a Mn−Mg single bond
(effective bond order, EBO = 0.97), which largely originates from
the 4s and 3s orbitals on Mn and Mg, respectively (Figure 3a). As
manganese is more electronegative than magnesium (1.55 vs
1.31 on the Pauling scale),19 the compound formally contains
Mn0 and MgII centers. However, this electronegativity difference
is not great, and therefore, the Mn−Mg bond would not be
expected to be heavily polarized and should have significant
covalent character. Indeed, the LoProp atomic charges were
Figure 4. Thermal ellipsoid plot (20% probability surface) of 5.
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(9) Zhu, Z.; Brynda, M.; Wright, R. J.; Fischer, R. C.; Merrill, W. A.;
Rivard, E.; Wolf, R.; Fettinger, J. C.; Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 2007, 129, 10847.
(10) (a) In all terphenyl substituted metal(I) dimers, e.g., Ar′MMAr′
(M = Cr, Fe, or Co), the ligands bridge the metal−metal bond, see ref
8b; (b) One compound containing a Mn−Fe bond, with a two-
coordinate, open-shell MnII center has been described: Lei, H.; Guo, J.-
D.; Fettinger, J. C.; Nagase, S.; Power, P. P. J. Am. Chem. Soc. 2010, 132,
17399.
compound, giving a good yield of the high-spin (μeff = 5.1 μB)
magnesium manganesio-amidinate complex, L†Mn{κ1-C,κ2-
N,N′−C(NPri)2}Mg(MesNacnac) 6 (see SI for further details).22
It is of note that this reactivity is broadly similar to that displayed
by β-diketiminato magnesium(I) dimers, e.g., (DipNacnac)-
MgMg(DipNacnac),13,23 which suggests that 1 may well prove
of comparable utility to such compounds as a reducing agent in
organic synthesis and small molecule activations.
In conclusion, a novel two-coordinate, high-spin manga-
nese(0) complex, bearing an unsupported Mn−Mg bond, has
been prepared, and its reactivity explored. Its versatility as an
“inorganic Grignard reagent” in the preparation of bimetallic
systems is evidenced by the synthesis of an unprecedented two-
coordinate manganese(I) dimer and a mixed valence, bis-
(amido)-hetereobimetallic (MnII/Cr0) complex. Moreover,
preliminary reactivity studies suggest it will prove a powerful
reducing agent in organic synthesis. We are currently
investigating this possibility, in addition to developing the
chemistry of other low-valent d-block metal−Mg bonded
systems.
(11) e.g. (a) Wong, E. W. Y.; Dange, D.; Fohlmeister, L.; Hadlington,
T. J.; Jones, C. Aust. J. Chem. 2013, 66, 1144. (b) Li, J.; Stasch, A.;
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̈
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(14) Blake, M. P.; Kaltsoyannis, N.; Mountford, P. Chem. Commun.
2013, 49, 3315 and refs therein.
(15) A two-coordinate “bis(carbene)Mn” complex has been reported,
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ASSOCIATED CONTENT
* Supporting Information
Synthesis details and characterizing data. This material is
available free of charge via the Internet at http://pubs.acs.org
■
S
Frenking, G.; Demeshko, S.; Meyer, F.; Stuckl, A. C.; Christian, J. H.;
̈
Dalal, N. S.; Ungur, L.; Chibotaru, L. F.; Propper, K.; Meents, A.;
̈
Dittrich, B. Angew. Chem., Int. Ed. 2013, 52, 11817.
AUTHOR INFORMATION
Corresponding Author
cameron.jones@monash.edu
(16) Hicks, J.; Jones, C. Inorg. Chem. 2013, 52, 3900.
■
(17) The first Mn-Cr bonded complex has recently been reported.
Clouston, L. J.; Siedschlag, R. B.; Rudd, P. A.; Planas, N.; Hu, S.; Miller,
A. D.; Gagliardi, L.; Lu, C. C. J. Am. Chem. Soc. 2013, 135, 13142.
(18) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.;
Echeverria, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans.
2008, 2832.
Notes
The authors declare no competing financial interest.
(19) Emsley, J. The Elements, 2nd ed.; Clarendon: Oxford, 1995.
(20) Although compound 1 could alternatively be viewed as containing
an MnI−MgI covalent bond, its formulation as incorporating a Mn0−
MgII dative covalent, or polar covalent, bond is more appropriate on
electronegativity grounds. For a review on metal−metal dative covalent
bonding, see: Bauer, J.; Braunschweig, H.; Dewhurst, R. D. Chem. Rev.
2012, 112, 4329.
ACKNOWLEDGMENTS
■
Financial support from the Australian Research Council (C.J.,
K.S.M.) is acknowledged. The EPSRC National Mass Spectrom-
etry Facility is also thanked. The computational work (C.E.H.
and G.L.M.) was supported by the National Science Foundation
under grant number CHE-1212575.
(21) A small amount of L†MnOMnL† crystallized from one reaction;
see SI for details.
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