Metal-metal distances at the limit: Cr-Cr 1.73 A - The importance of the ligand and its fine tuning

Article abstract of DOI:10.1002/zaac.200900175

The synthesis and structure of a homobimetallic chromium complex is reported. The ligand used to stabilise the quintuply bonded metals is a sterically fine-tuned guanidinate. A chromium - chromium bond length of 1.7293(12) A was observed. It is the shortest metal - metal distance reported for a stable compound yet.

Full text of DOI:10.1002/zaac.200900175

SHORT COMMUNICATION
DOI: 10.1002/zaac.200900175
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Metal؊Metal Distances at the Limit: Cr؊Cr 1.73 A ؊ the Importance of the
Ligand and its Fine Tuning
Awal Noor,[a] Germund Glatz,[a] Robert Müller,[b] Martin Kaupp,*[b] Serhiy Demeshko,[c] and
Rhett Kempe*[a]
Keywords: Chemical bonding; Chromium; MetalϪmetal bonds; Multiple bonds; N ligands
˚
Abstract. The synthesis and structure of a homobimetallic chro-
mium complex is reported. The ligand used to stabilise the quintu-
chromiumϪchromium bond length of 1.7293(12) A was observed.
It is the shortest metalϪmetal distance reported for a stable com-
pound yet.
ply bonded metals is
a sterically fine-tuned guanidinate. A
ecule [6]. Hein and Tille also proposed the dimeric structure
for this compound [5]. The chromiumϪchromium bond
length of Cr2 generated by laser-evaporation of the metal
Quintuple Bonds
The shortest metalϪmetal bond in a stable compound
has been of interest to natural scientists and chemists in
particular for decades. In this context, the element chro-
mium plays a decisive role. Because of their electron con-
figuration, chromium and the higher homologues of group
6 transition metals are able to form metalϪmetal bonds of
high formal bond orders [1]. Within group 6, chromium
exhibits the smallest ion radius and is ideally suited to form
exceptionally short metalϪmetal bonds. The search for
shorter metalϪmetal distances in coordination compounds
has recently been reignited by the pioneering work of Power
and co-workers, reporting quintuple bonding between chro-
mium atoms [2]. Interestingly, the first compound of this kind
ArCrϪCrAr [Ar ϭ C6H3-2,6-(C6H3-2,6-{isopropyl}2)2] exhi-
˚
and flash photolysis of Cr(CO)6 is about 1.68 and 1.71 A,
respectively [7, 8]. Similar distances were calculated for Cr2
[9]. For the Cr2 dimer a formally sixtuple bond is assumed.
Hence, quintuply bonded complexes could be expected to
exhibit bond lengths between that observed for
˚
ArCrϪCrAr and 1.68 A. Consequently, a number of com-
pounds with shorter CrϪCr distance have been synthesised.
Theopold, Landis and co-workers reported on an N ligand
stabilised homobimetallic chromium complex [distance
˚
(CrϪCr) 1.8028(9) A] with quintuple bonding, as suggested
by DFT-based NRT and NBO analyses [10]. The com-
pound was synthesised by subsequent adding of sodium
and a CrIII chloride to a sterically demanding diazadiene
affording a monochloro chromium complex which was
further reduced by potassium graphite. The problem with
the used ligand is its redox ambiguity making it difficult
to judge the oxidation state of the chromium atoms of the
multiply bonded dimer. Multiconfigurational quantum
chemical calculations suggested a formal oxidation state of
1.5ϩ for each of the chromium atoms and a bond order of
3.43, based on natural orbital occupations [11]. Derivatives
of ArCrϪCrAr show variations in the metalϪmetal dis-
˚
bited a CrϪCr distance of 1.8351(4) A [2], which is slightly
longer than the shortest formally quadruply bonded chro-
mium complexes [3]. For almost 30 years an aryl CrII com-
pound, structurally characterised by Cotton and Koch [4]
and first prepared by Hein and Tille more than 40 years
ago [5], was reported to have the shortest experimentally
˚
observed metalϪmetal distance, 1.830(4) A, of a stable mol-
* Prof. Dr. M. Kaupp,
E-Mail: Kaupp@Mail.Uni-Wuerzburg.de
* Prof. Dr. R. Kempe
˚
˚
tances between 1.8077(7) A and 1.8351(4) A. Packing forces
rather than electronic effects seem to dominate the different
E-Mail: Kempe@uni-bayreuth.de
[a] Lehrstuhl Anorganische Chemie II
Universität Bayreuth
˚
distances [12]. Distances of 1.74 and 1.75 A for N ligand
stabilised chromium dimers were reported simultaneously
by Tsai et al. and our own group, respectively [13Ϫ15].
These examples represent the class of ultra-short
metalϪmetal bonded homobimetallics. Calculations indi-
cate quintuple bonding. The CrϪCr bond lengths are
intermediate between the quadruply bonded CrIIϪCrII
complexes and the gas phase Cr2 molecule with its formal
sixtuple bond. An important aspect in the context of ultra-
short metalϪmetal distances is the HeinϪCotton principle
95440 Bayreuth, Germany
[b] Institut für Anorganische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
[c] Institut für Anorganische Chemie
Georg-August-Universität Göttingen
Tammannstraße 4
37077 Göttingen, Germany
Supporting information for this article is available on the
WWW under www.zaac.wiley-vch.de or from the author.
Z. Anorg. Allg. Chem. 2009, 635, 1149Ϫ1152
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Noor, G. Glatz, R. Müller, M. Kaupp, S. Demeshko, R. Kempe
SHORT COMMUNICATION
[13], that is the XϪYϪZ ligand arrangement, which brings two “wings” further down resulting in a shortening of the
the two metal atoms into close proximity in the first place metalϪmetal bond. A ligand introduced by Hessen and
(Figure 1) [13Ϫ15].
co-workers recently seems to be ideal in this regard [16].
Additionally, the potential of guanidinates to stabilise short
metalϪmetal distances was shown by Gambarotta and
co-workers recently [17]. We show here that the strategy
discussed above was successfully applied to synthesise the
coordination compound with the shortest metalϪmetal-
distance observed yet.
The reaction of lithium guanidinate Li[Me2NC(NAr)2]
(1, Ar ϭ 2,6-diisopropylphenyl) with CrCl2 in THF af-
forded, after removal of the solvent and subsequent extrac-
tion with hexane, the CrII complex 2 as blue crystalline ma-
1
terial in good yield (Scheme 1). The H NMR shows only
broad signals and magnetic susceptibility experiments pro-
vide a magnetic moment μeff (298.8 K) ϭ 2.70 μB per chro-
mium. The X-ray crystal structure analysis shows com-
pound 2 to be dimeric in the solid state. The coordination
pattern around each metal centre can be best described as
slightly distorted square planar (Figure 2) [18]. Consider-
able interaction of the lone pair of the Me2N moiety with
Figure 1. The role of the ligand in stabilising ultra-short
metalϪmetal bonds: aminopyridinates (left, top) vs. amidinates
(right, top) vs. guanidinates (bottom).
By comparing the two ligand families which were used to
stabilise ultra-short chromiumϪchromium bonds, aminop-
yridinates and amidinates (Figure 1, top), the two-wings-up
arrangement observed for amidinates (Figure 1, top right)
apparently causes much less inter-ligand repulsion within
the bimetallic complex than the wing-up-wing-down ar-
rangement observed for aminopyridinates (Figure 1, top
left). This allows the generation of a closer NϪCϪN-pincer
and/or an alignment of nitrogen atom based orbitals that
interact with chromium (lone pairs) towards each other re-
sulting in shorter CrϪCr distances. Gradual closing of this
pincer by applying steric pressure via the substituent at the
bridging carbon atom should give rise to a further reduced
distance between the clamped metal atoms (Figure 1, bot-
tom). Guanidinates appear to be ideally suited in this re-
gard. The π-system can be delocalised with a planar ar-
rangement of the three nitrogen atoms and their residues
(Figure 1, bottom) which should become “frozen” between
two bulky 2,6-alkyl phenyl “wings” avoiding an out of
plane arrangement of the substituent of the non-coordinat-
ing nitrogen atom. Consequently, the substituent of the non
Figure 2. Molecular structure of 2 [ORTEP representation (on the
50 % probability level) for all non carbon atoms]; hydrogen atoms
˚
have been omitted for clarity. Selected bond lengths /A and angles /°:
C1ϪN2 1.323(9), C1ϪN3 1.347(8), C1ϪN1 1.361(9), N1ϪCr1
2.030(6), N2ϪCr1 1.995(6), N4ϪCr2 2.012(6), N5ϪCr2 2.017(6),
Cl1ϪCr2 2.3713(6), Cl1ϪCr1 2.383(2), Cl2ϪCr2 2.366(2),
metal-bonded nitrogen atom is ideally suited to push the Cl2ϪCr1 2.389(3); N2ϪCr1ϪN1 65.9(2), Cl1ϪCr1ϪCl2 88.77(8).
Scheme 1. Synthesis of 2 and 3.
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Z. Anorg. Allg. Chem. 2009, 1149Ϫ1152

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MetalϪMetal Distances at the Limit: CrϪCr 1.73 A
the conjugated NCN moiety is indicated by very similar complex with an exceptionally short metalϪmetal distance
˚
CϪN distances [C1ϪN2 1.323(9), C1ϪN3 1.347(8), of 1.7293(12) A (Figure 3) [19]. It is the shortest CrϪCr
˚
C1ϪN1 1.361(9) A], which is also in accordance with the distance as well as the shortest metalϪmetal bond reported
structure of a yttrium complex of the same ligand [17].
for a stable compound. The chromiumϪchromium distance
˚
Subsequent reduction of 2 with KC8 in THF resulted in is 0.1 A shorter than that observed in the two examples that
a sudden colour change from royal blue to orange red. had been the classics in terms of the shortest metalϪmetal
After work up, 3 was isolated as orange crystalline material. bond for nearly 30 years [4, 6]. The CrϪN bond lengths of
1
The H NMR shows four doublets for the non equivalent [N1ϪCr1 2.022(3), N2ϪCr1 2.013(3)] are comparable to
CH3-protons of the isopropyl groups at Ϫ1.08, 1.13, 1.64 the known quintuply bonded amidinate chromium com-
and 1.87 ppm and two septets for the CH-proton of the plexes [14, 15]. The C1ϪN1, C1ϪN2 and C1ϪN3 bond
isopropyl groups at 3.30 and 5.18 ppm. However, a single lengths of the ligand back bone are C1ϪN2 1.352(4),
˚
set of signals was observed for CH3ϪN and aromatic pro- C1ϪN1 1.332(4) and C1ϪN3 1.371(4) A respectively, indi-
tons. This might be attributed to the rigid behaviour of the cating the delocalisation of the π-system over the three ni-
compound arising from the steric crowding. X-ray crystal trogen atoms. The space-filling plot of 3 (Figure 3, bottom)
structure analysis shows 3 to be a bridged homobimetallic shows how the methyl groups of the dimethylamine moiet-
ies are locked between the two isopropyl substituents of the
“wings” and how these methyl groups or moieties put
“steric pressure” on the phenyl groups of that “wings“. The
magnetic susceptibility measurement of 3 in the range of
300 to 2 K are indicative of a S ϭ 0 ground state, as could
be expected for strongly coupled d5Ϫd5 bonding electrons
[2]. Some positive contribution of temperature-independent
paramagnetism (TIP) as characteristic for 3d metal ions
[20] amounts 535 ϫ 10Ϫ6 cm3 ·molϪ1 for 3.
To investigate the electronic structure of 3 we employed
DFT calculations [21] with subsequent natural population
analyses (NPA) [22] and inspection of the electron-localis-
ation function (ELF) [23]. All calculations were carried out
on a model structure of 3, namely 3a, in which the iso-
propyl substituents were replaced with hydrogen atoms [24].
The results show 3a to be almost identical to 4a in terms
of its electronic structure (Figure 4) [13], which supports
the assumption that the observed bond lengths are rather
determined by the sterics of the ligands instead of substan-
tial changes in the electronic structure.
Figure 4. Structure of 3a (left) and 4a (right).
For the CrϪCr bond in 3a, a Wiberg bond index [25] of
4.43 was obtained. By comparison with an index of 4.36 for
Figure 3. Molecular structure of 3 [ORTEP representation top (hy-
complex 4a, a similar bond order is apparent. NPA revealed
no significant differences for the metal charges nor for the
relative 4s and 3d populations. Canonical MO patterns as
well as ELF results (see Supporting Information) also com-
pare the essentially similar electronic structure of the two
complexes.
drogen atoms have been omitted for clarity.) and space filling model
bottom (carbon atom of one of the methyl group of the dimethyl-
amine moiety has been coloured brown and isopropyl residues of
one of the 2,6-diisopropylphenyl moiety has been coloured yellow
for clarity.)]; selected bond lengths /A and angles /°: C1ϪN2
1.352(4), C1ϪN3 1.371(4), C1ϪN1 1.332(4), N1ϪCr1 2.022(3),
N2ϪCr1 2.013(3), Cr1ϪCr1A 1.7293(12); N2ϪC1ϪN1 115.4(3).
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Z. Anorg. Allg. Chem. 2009, 1149Ϫ1152
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A. Noor, G. Glatz, R. Müller, M. Kaupp, S. Demeshko, R. Kempe
SHORT COMMUNICATION
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In conclusion we introduced the coordination compound
showing the shortest metalϪmetal bond observed yet and
emphasise the importance of the ligand and its steric fine
tuning to observe such short CrϪCr bonds.
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calculations.
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˚
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˚
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Received: February 26, 2009
Published Online: May 11, 2009
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Z. Anorg. Allg. Chem. 2009, 1149Ϫ1152