Stable adducts of a dimeric magnesium(I) compound

Article abstract of DOI:10.1002/anie.200803960

(Figure Presented) A bit of a stretch! A series of stable Lewis base adducts of a dimeric magnesium(I) complex has been prepared and shown to possess exceptionally long Mg-Mg bonds. Theoretical studies on model compounds suggest the elongations of these bonds are low-energy processes. The structures of the magnesium(I) adducts are compared with those of related magnesium(II) hydride complexes (see picture; Mg pink, N blue, O red, H green).

Full text of DOI:10.1002/anie.200803960

Angewandte
Chemie
DOI: 10.1002/anie.200803960
À
Mg Mg Bonds
Stable Adducts of a Dimeric Magnesium(I) Compound**
Shaun P. Green, Cameron Jones,* and Andreas Stasch*
The chemistry of compounds containing metal–metal bonds is
an extensive and fundamentally important field that has
greatly added to our understanding of chemical bonding.[1]
Traditionally, activity in the area has focused on the d-block
metals, yielding landmark results which include Cottonꢀs
have revealed them to act as facile two-center/two-electron
reductants towards a range of unsaturated substrates. In our
initial report this was demonstrated with the facile insertion of
À
a carbodiimide, CyNCNCy (Cy = cyclohexyl), into the Mg
Mg bond of 2 to give the unusual magnesium magnesioami-
dinate complex [(nacnac)Mg{m-C(NCy)2}Mg(nacnac)], which
we proposed was formed via an intermediate carbodiimide–
Mg complex.[9] Herein we show that less reducible Lewis
bases readily complex 2 to give adducts that are surprisingly
stable towards disproportionation reactions and that have
quadruply bonded dianion, [Re2Cl8]2À [2]
and more recently,
,
Powerꢀs quintuply bonded chromium(I) dimer, [Ar’CrCrAr’]
(Ar’ = C6H3(C6H3iPr2-2,6)2-2,6).[3] In the past three decades
rapid progress has also been made by many groups working
with the p-block metals and metalloids. Now, a vast array of
dimeric compounds with heavier p-block element–element
bonds possessing orders of up to 3 are known.[4] At the
interface of the p and d blocks, undoubtedly the most
important breakthrough in the new millenium has been
Carmonaꢀs synthesis of the dimeric zinc(I) compound
À
exceptionally long Mg Mg bonds. For comparison, closely
related magnesium(II) hydride complexes have been pre-
pared and studied.
Although we originally noted that the addition of
tetrahydrofuran (THF) to yellow C6D6 solutions of 2 did not
lead to any changes in its NMR spectra, it did lead to the
solutions taking on an orange color. One explanation for this
observation is that a transient coordination of the magnesium
centers of 2 by THF was occurring. To assess the possibility
that 2 could form stable adducts with cyclic ethers, it was
dissolved in either neat THF or dioxane to yield red-orange
and orange solutions, respectively. When volatiles were
removed from these solutions in vacuo, uncoordinated 2
was quantitatively recovered. However, concentration and
cooling of the solutions afforded good yields of the crystalline
adducts, red-orange 3 and orange 4 (Scheme 1).[11] Placing
crystalline samples of 3 or 4 under vacuum leads to the loss of
[Cp*ZnZnCp*] (Cp* = C5Me5 ).[5] This has led to a flurry of
À
activity in the area and the preparation of a handfull of other
[6]
À
Zn Zn bonded complexes.
Conspicuously absent from the arena of metal–metal
bonded complexes have been those involving the s-block
metals.[7] However, given the unexpected stability of zinc(I)
dimers, and the chemical similarities between zinc and the
Group 2 metals, a number of theoretical studies have
predicted that thermally stable compounds of the type
RMMR (M = Be, Mg, or Ca) should be accessible.[8] For
magnesium, we have shown that this is the case with the
preparation of the bulky guanidinate or b-diketiminate
coordinated magnesium(I) dimers, [LMgMgL] (L =
[(ArN)2CNiPr2]À, prisoÀ (1) or [(ArNCMe)2CH]À, nacnacÀ
(2) (Ar= C6H3iPr2-2,6).[9,10] These are remarkably thermally
stable compounds (1 decomp. 170–1738C; 2 decomp. 301–
À
3038C) with Mg Mg distances of 2.8508(12) Š (1) and
2.8457(8) Š (2). Theoretical studies on a model of 1,
[{Mg{[(C6H3Me2-2,6)N]2CNMe2}}2], showed it to contain a
2+
high-s-character, covalently bonded Mg2
core, having
largely ionic interactions with its guanidinate ligands. Pre-
liminary reactivity studies on the magnesium(I) compounds
Scheme 1. The preparation of the magnesium(I) adducts 3–6.
[*] S. P. Green, Prof. C. Jones, Dr. A. Stasch
School of Chemistry, Monash University
PO Box 23, VIC, 3800 (Australia)
their coordinated ethers and the regeneration of 2. This
process is considerably more rapid for 4 than for 3. The same
outcome occurs if they are heated to greater than ca. 708C (3)
or 608C (4) under dinitrogen atmospheres. These properties
indicate that the ether ligands of 3 and 4 are only weakly
coordinated. Moreover, the lower decomposition temper-
ature of 4, and the fact that it can co-crystallize with 2 from
neat dioxane solutions, signifies that dioxane is a weaker
donor towards 2 than THF.[12] It is noteworthy, however, that
treatment of THF solutions of 4 with the carbodiimide
CyNCNCy led to no reaction. This shows that a large excess
of THF successfully competes with the carbodiimide for
Fax: (+61)3-9905-4597
E-mail: cameron.jones@sci.monash.edu.au
andreas.stasch@sci.monash.edu.au
S. P. Green
School of Chemistry, Cardiff University
Main Building, Park Place, Cardiff, CF10 3AT (UK)
[**] We thank the Australian Research Council (fellowships for C.J. and
A.S.), the Engineering and Physical Sciences Research Council
(partial studentship for S.P.G.) and the US Air Force Asian Office of
Aerospace Research and Development for funding.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803960.
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coordination to 2 and adds weight to our original proposal
that the formation of [(nacnac)Mg{m-C(NCy)2}Mg(nacnac)]
proceeds via an intermediate complex such as [2(h1-N-
CyNCNCy)].
In attempts to prepare more robust adducts of 2, toluene
solutions of the compound were treated with an excess of
either quinuclidine or tmeda (tetramethylethylenediamine),
or the compound was dissolved in neat diethylether or 1,2-
dimethoxyethane. In each instance, no reaction or color
change occurred, and 2 was recovered intact. Attention then
turned to the highly Lewis basic substituted pyridines 4-
dimethylaminopyridine (DMAP) and 4-tert-butylpyridine (4-
tBuPy), which when reacted with 2 in non-coordinating
solvents gave good yields of the deep red-brown compounds 5
and 6, respectively (Scheme 1). These compounds are very
thermally stable and display no evidence for the loss of their
pyridine ligands up to their decomposition temperatures
(159–1608Cand 248–250 8C, respectively), or when they are
placed under vacuum. Indeed, a molecular ion peak envelope
was observed in the accurate-mass EI mass spectrum of 6.
These results suggest that the pyridine ligands are
significantly stronger donors towards 2 than either THF or
dioxane. This is also borne out by the results of NMR
spectroscopic studies on 3–6 which imply that in C6D6 (or
[D8]toluene), 3 and 4 exist in equilibria that heavily favor 2
and the free ether, whereas resonances for the free pyridine
ligands were not seen in the spectra of 5 and 6. Despite this,
their NMR spectra, and those of 3 and 4 (recorded in
[D8]THF and [D8]dioxane, respectively) are more symmet-
rical than would be expected if their solid-state structures are
retained in solution. A reasonable explanation for these
observations is that fluxional ligand dissociation/coordination
processes are occurring for the complexes, which are rapid
compared to the NMR timescale. Attempts to investigate
these processes by variable-temperature NMR studies were
thwarted by the low solubility of the complexes at temper-
atures below 08Cor, in the case of 4, the melting point of
[D8]dioxane (118C).
Figure 1. Molecular structure of [{Mg(nacnac)(THF)}2] (3) (symmetry
operation: ’ Àx+1, Ày, Àz). Relevant bond lengths [Š] and angles [8]
À
À
À
for 3: Mg Mg 3.0560(12), Mg O 2.1733(13), Mg N 2.159 (mean), N-
À
À
À
Mg-N 87.08(5); 4: Mg Mg 3.1499(18), Mg O 2.2438(18), Mg N
À
À
2.152 (mean), N-Mg-N 87.79(8); 5: Mg Mg 3.1962(14), Mg N-
À
(DMAP) 2.2353(18), Mg N(nacnac) 2.178 (mean), N-Mg-N 86.11(6);
6: Mg Mg 3.1260(15), Mg N(4-tBuPy) 2.2257(18), Mg N(nacnac)
2.162 (mean), N-Mg-N 86.30(7).
À
À
À
The most remarkable features of the structures of com-
À
pounds 3–6 are their Mg Mg distances. Although each
compound co-crystallizes with small amounts of the corre-
sponding hydroxide complex, [{Mg(nacnac)(L)(m-OH)}2]
(L = THF, dioxane, DMAP, or 4-tBuPy), the apparent Mg
À
Mg distances vary over more than 0.14 Š and are from ca.
0.21 to 0.35 Š larger than that in 2 (2.8457(8) Š).[9,14,15] To put
this into context, the revised sum of two divalent Mg covalent
radii is 2.82 Š,[16] and the Mg Mg distances in elemental and
À
The X-ray crystal structures of compounds 3–6 were
determined and show the compounds to have broadly similar
structural features. As a result, only the molecular structure of
3 is depicted in Figure 1 (see the Supporting Information for
the molecular structures of 4–6), though relevant metrical
parameters for all compounds can be found in the figure
caption. Although the Mg(nacnac) heterocycles are signifi-
cantly distorted from planar, the delocalized backbones of
both nacnac ligands in each compound are close to planar and
effectively parallel to each other. This contrasts to the
situation in 2 in which these planes are close to orthogonal.
The magnesium centers of the compounds all exhibit heavily
distorted tetrahedral coordination geometries with consider-
diatomic magnesium are 3.20 Š and 3.890 Š, respectively.[17]
Moreover, there seems to be little correlation between the
À
ligand donor strength and Mg Mg separation in the com-
pounds. This is best illustrated by the fact that, although
compound 4 readily loses its weakly donating dioxane ligands,
À
it has the second largest Mg Mg distance of the four
À
compounds. This suggests that the origin of the large Mg
Mg distances in 3–6 has less to do with electronics than other
factors, for example, sterics. For sake of comparison, we are
unaware of any p-block compound incorporating a metal–
metal single bond which increases in length by more than
0.2 Š upon coordination by one or more neutral Lewis base
ligands.[13] That said, it is noteworthy that the Ge Ge distance
À
À
ably larger Mg N(nacnac) distances than those in 2 (2.060 Š
of a singlet diradicaloid digermyne, [Ar’GeGeAr’], increases
by ca. 0.38 Š upon coordination by two isonitrile molecules
(to give [Ar’Ge(CNMes)Ge(CNMes)Ar’], Mes = mesityl).
mean), which has three-coordinate Mg centers. In addition,
À
the ether or pyridine O/N Mg distances in all complexes are
À
significantly larger than any previously reported examples
involving these ligands coordinated to four-coordinate Mg
centers.[13] Furthermore, there is no structural (or spectro-
scopic) evidence for the reduction of the pyridine ligands in 5
and 6, or, indeed, the nacnac ligands in all complexes.
This is, however, accompanied by a reduction in the Ge Ge
bond order from approximately 2 to 1.[18]
In order to provide insight into the exceptional length-
À
ening of the Mg Mg bond of 2 upon coordination, DFT
calculations were carried out on a simplified model of it,
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[{Mg[(HNCH)2CH]}2] 7, and its THF adduct, [{Mg-
[(HNCH)2CH](THF)}2] 8, using the B3LYP method, and
appropriate basis sets (see the Supporting Information for full
details). The optimized geometries of the two compounds are
similar to those of 2 and 3, albeit with Mg heterocycles that
are closer to being planar, a consequence of the minimal steric
bulk of the model b-diketiminate ligands.[19] In both 7 and 8,
the HOMO largely corresponds to a high-s-character (NBO
analysis for 7: 92.0% s, 7.5% p character; 8: 92.2% s, 7.3%
bond distance, so that a 0.20 Š increase in the metal–metal
bond increases the energy of the system by only 1.2 kcalmolÀ1
(an 0.35 Š elongation requires 3.5 kcalmolÀ1).
In order to confirm the absence of bridging hydride
ligands in 3, and for purposes of comparison, efforts were
made to prepare the magnesium(II) hydride analogue of this
compound. This was eventually achieved using a synthetic
methodology similar to that developed by Harder et al. for
the preparation of [{Ca(nacnac)(THF)(m-H)}2].[21] That is, the
uncoordinated, colorless magnesium hydride complex 9 was
prepared in moderate yield by reaction of a magnesium alkyl
precursor[22] with PhSiH3, according to Scheme 2. Treatment
À
p character), single covalent Mg Mg interaction with a bond
order close to unity (Wiberg bond indices for 7: 0.97; 8: 0.99).
In addition, their b-diketiminate–Mg interactions are pre-
dominantly ionic (natural charges for 7: Mg + 0.87 mean, N
À1.01 mean; 8: Mg + 0.91 mean, N À0.99 mean) which is
indicative of them possessing anion-stabilized Mg22+ cores. A
similar view has previously been established for models of 1
[8–10]
À
and other Mg Mg bonded species.
In contrast to the
model of 1, [{Mg{[(C6H3Me2-2,6)N]2CNMe2}}2], the LUMO
À
and LUMO+1 of which correspond to Mg Mg p-bonding
orbitals, the low-lying unoccupied orbitals of 7 and 8 are
ligand-based. It is of note that the HOMO–LUMO gaps of
the model compounds (7: 3.87 eV, 89.5 kcalmolÀ1; 8: 3.02 eV,
69.7 kcalmolÀ1) are less than that of [{Mg{[(C6H3Me2-
2,6)N]2CNMe2}}2] (4.02 eV, 93.0 kcalmolÀ1), which might
explain why 2 and 3 are colored, while 1 is colorless.
Scheme 2. The preparation of the magnesium(II) hydride complexes 9
and 10.
The most obvious difference between the experimentally
observed complexes and their calculated models is the much
À
smaller change in Mg Mg bond length upon THF coordina-
tion for the model pair, 7 and 8 (difference between 2.865 and
2.945 Š, respectively = 0.08 Š), as compared with the exper-
imental pair, 2 and 3 (difference = 0.21 Š). This could be due
to increased steric interactions between the monomeric units
of 3, as compared with those of 8. If this is the case, the
of this with an excess of THF then led to colorless 10 in good
yield. Attempts to form crystalline dioxane, DMAP, and 4-
tert-butylpyridine complexes of 9 were not so far successful.
Although the THF ligand appears to be more strongly bound
in 10 than in 3, dissolution of 10 in toluene and subsequent
removal of volatiles in vacuo did lead to the regeneration of 9.
The NMR spectroscopic patterns for 9 and 10 are similar to
those of 2 and 3, with the exception of hydride resonances
À
elongation of the Mg Mg bond of 2 upon THF coordination
would still have to be a relatively low-energy process. Saying
this, the calculated metal–metal bond dissociation energy for
the model, 7, is not insignificant at 45.4 kcalmolÀ1, though this
is considerably lower than the value of 65.2 kcalmolÀ1
calculated for its zinc(I) analogue, [{Zn[(HNCH)2CH]}2].[20]
In order to quantify the energy required to elongate the
metal–metal bond of 7, a partial potential energy curve for the
1
being present in the H NMR spectra of the former pair (9:
d = 4.03 ppm (sharp); 10: d = 4.21 ppm (broad)).
The molecular structures of 9 and 10 are depicted in
Figures 3 and 4 and represent the first examples for neutral
compounds of the type [{LnMg(m-H)}2] (n = 1 or 2).[23,24] The
symmetrically bridging hydride ligands of both structures
were located from difference maps and their positional
parameters freely refined. As we have previously predicted,[9]
the magnesium heterocycles of 9 are close to co-planar with
each other, which contrasts to the nearly orthogonal hetero-
cycles in 2 (and 1). The Mg···Mg separation in 9 is slightly
larger (2.890(2) Š) than that in 2 (2.8457(8) Š), but signifi-
cantly greater than that calculated (2.795 Š, B3LYP; Wiberg
bond index: 0.34) for the sterically unencumbered model
system, [{Mg[(HNCH)2CH](m-H)}2], in which the magnesium
heterocycles are also close to co-planar. Comparisons of the
structures of 3 and 10 show that they have similar molecular
À
compound (as a function of Mg Mg separation) was calcu-
lated. Figure 2 shows this to be shallow about the equilibrium
À
geometries, though the Mg Mg separation is slightly smaller
À
in the latter. Similarly, the Mg O/N distances in the
magnesium(II) hydride complex are significantly less than
those of 3, which might be expected considering the lower
metal oxidation state of that compound.
À
Figure 2. Potential energy curve as a function of Mg Mg distance d
for 7.
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Theoretical studies have been carried out which indicate
that these elongations may arise from a combination of steric
buttressing between the monomeric fragments of the adducts
À
and from shallow potential energy curves for their Mg Mg
bonds. The absence of hydride ligands in one complex has
been confirmed by the preparation of its magnesium(II)
hydride analogue. We are currently investigating the ability of
[{Mg(nacnac)}2] (2) to coordinate functionalized unsaturated
substrates, prior to their reduction. We will report on this in a
future publication.
Experimental Section
Full synthetic, spectroscopic and crystallographic details for 3–6, 9
and 10, crystallographic details for a polymorph of 2, and full details
and references for the DFT calculations on 7,
8 and [{Mg-
[(HNCH)2CH](m-H)}2] can be found in the Supporting Information.
Received: August 11, 2008
Published online: October 16, 2008
Keywords: low oxidation states · magnesium · metal hydrides ·
metal–metal bonding · structure elucidation
Figure 3. Molecular structure of [{Mg(nacnac)(m-H)}2] (9) (25% ellip-
soids; non-hydride H atoms omitted for clarity). Relevant distances [Š]
.
À
À
and angles [8]. Mg1···Mg1’ 2.890(2), Mg1 N1 2.064(2), Mg1 N2
À
À
2.065(2), Mg1 H1 1.95(3), Mg1’ H1 1.97(3); N1-Mg1-N2 93.14(9),
Mg1-H1-Mg1’ 95(1). Symmetry operation: ’ 1Àx, Ày, Àz
[1] F. A. Cotton, C. A. Murillo, R. A. Walton, Multiple Bonds
Between Metal Atoms, 3rd ed., Springer, New York, 2005.
[2] F. A. Cotton, N. F. Curtis, C. B. Harris, B. F. G. Johnson, S. J.
Lippard, J. T. Mague, W. R. Robinson, J. S. Wood, Science 1964,
145, 1305 – 1307.
[3] T. Nguyen, A. D. Sutton, M. Brynda, J. C. Fettinger, G. J. Long,
P. P. Power, Science 2005, 310, 844 – 847.
[4] See for example: a) T. Sasamori, N. Tokitoh, Dalton Trans. 2008,
1395 – 1408; b) E. Rivard, P. P. Power, Inorg. Chem. 2007, 46,
10047 – 10064; c) W. Uhl, Adv. Organomet. Chem. 2004, 51, 53 –
108; d) A. Sekiguchi, R. Kinjo, M. Ichinohe, Science 2004, 305,
1755 – 1757, and references therein.
[5] I. Resa, E. Carmona, E. Gutierrez-Puebla, A. Monge, Science
2004, 305, 1136 – 1138.
[6] E. Carmona, A. Galindo, Angew. Chem. 2008, 120, 6626 – 6637;
Angew. Chem. Int. Ed. 2008, 47, 6526 – 6536, and references
therein.
[7] The structures of several weakly associated alkalide dimers, for
example, Na22À, have been determined. See for example: M. Y.
Redko, R. H. Huang, J. E. Jackson, J. F. Harrison, J. L. Dye, J.
Am. Chem. Soc. 2003, 125, 2259 – 2263.
[8] See for example: a) A. Velazquez, I. Fernandez, G. Frenking, G.
Merino, Organometallics 2007, 26, 4731 – 4736; b) Q. S. Li, Y.
Xu, J. Chem. Phys. A 2006, 110, 11898 – 11902; c) Y. Xie, H. F.
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421.
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[10] M. Westerhausen, Angew. Chem. 2008, 120, 2215 – 2217; Angew.
Chem. Int. Ed. 2008, 47, 2185 – 2187.
[11] In contrast, colorless THF solutions of compound 1 slowly
decompose at room temperature, depositing magnesium metal.
[12] Dioxane is known to be a weaker Lewis base than THF with
respect to its coordination to BF3. C. Reichardt, Solvents and
Solvent Effects in Organic Chemistry, 3rd ed., Wiley-VCH,
Weinheim, 2003.
[13] As determined from a survey of the Cambridge Crystallographic
Database, August, 2008.
[14] During the course of this study, a polymorph of 2 was crystallized
and crystallographically characterized (see the Supporting
Figure 4. Molecular structure of [{Mg(nacnac)(THF)(m-H)}2] (10)
(25% ellipsoids; non-hydride H atoms omitted for clarity). Relevant
À
distances [Š] and angles [8]. Mg1···Mg1’ 3.0332(18), Mg1 N1
À
À
2.1432(14), Mg1 O1 2.0996(16), Mg1 H1 1.947(19), N1-Mg1-N1’’’
93.14(9), Mg1-H1-Mg1’ 102(1). Symmetry operations: ’ Àx, Ày, Àz; ’’
Àx+1, Ày, Àz; ’’’ x, Ày, z.
In conclusion, a series of remarkably stable Lewis base
adducts of a dimeric magnesium(I) complex have been
prepared and shown to possess markedly larger Mg Mg
À
separations than the uncoordinated precursor molecule.
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À
Information). Its Mg Mg distance was measured at
2.8624(15) Š.
previously: P. B. Hitchcock, M. F. Lappert, D.-S. Liu, J. Chem.
Soc. Chem. Commun. 1994, 1699 – 1700.
[15] Despite considerable efforts to exclude moisture from the
reactions that gave complexes 3–6, each co-crystallized from
the reaction mixture with small amounts (ca. 5–9%) of the
corresponding hydrolysis product, [{Mg(nacnac)(L)(m-OH)}2]
(L = THF, dioxane, DMAP, or 4-tBuPy). It is believed that the
presence of these co-crystallized contaminants has a significant,
[20] Y. Wang, B. Quillian, P. Wei, H. Wang, X. Yang, Y. Xie, R. B.
King, P. v. R. Schleyer, H. F. Schaefer III, G. H. Robinson, J. Am.
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[21] a) J. Spielmann, S. Harder, Chem. Eur. J. 2007, 13, 8928 – 8938;
b) S. Harder, J. Brettar, Angew. Chem. 2006, 118, 3554 – 3558;
Angew. Chem. Int. Ed.. 2006, 45, 3474 – 3478.
À
though not substantial, effect on the Mg Mg bond lengths
observed for 3–6. See the Supporting Information for a full
discussion on this point.
[22] A. P. Dove, V. C. Gibson, P. Hormnirum, E. L. Marshall, J. A.
Segal, A. J. P. White, D. J. Williams, Dalton Trans. 2003, 3088 –
3097.
[16] B. Cordero, V. Gomez, A. E. Platero-Prats, M. Reves, J.
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[18] G. H. Spikes, P. P. Power, Chem. Commun. 2007, 85 – 87.
[19] The effect of the steric bulk of b-diketiminate ligands upon the
planarity of their metallacyclic complexes has been noted
[23] Anionic examples have been structurally characterized. See for
example: D. J. Gallagher, K. W. Henderson, A. R. Kennedy,
C. T. OꢀHara, R. E. Mulvey, R. B. Rowlings, Chem. Commun.
2002, 376 – 377.
[24] A variety of dimeric magnesium hydride complexes of the type
[{RMg(m-H)}2] (R = amide, alkyl, etc.) have been previously
prepared, though not structurally characterized. See for exam-
ple: E. C. Ashby, A. B. Goel, Inorg. Chem. 1978, 17, 1862 – 1866.
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