Binding and activation of small molecules by a quintuply bonded chromium dimer

Article abstract of DOI:10.1039/c3cc48746f

The quintuply bonded [^HL^iPrCr]₂ reacts with various small molecules, revealing a pattern of two kinds of transformations. Unsaturated molecules that are neither polar nor oxidizing form binuclear [2+n] cycloaddition products retaining Cr-Cr quadruple bonds. In contrast, polar or oxidizing molecules effect the complete cleavage of the Cr-Cr bond. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc48746f

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Binding and activation of small molecules
by a quintuply bonded chromium dimer†
Cite this: Chem. Commun., 2014,
50, 2579
Jingmei Shen,‡ Glenn P. A. Yap and Klaus H. Theopold*
Received 15th November 2013,
Accepted 8th January 2014
DOI: 10.1039/c3cc48746f
www.rsc.org/chemcomm
The quintuply bonded [HLiPrCr]2 reacts with various small molecules,
revealing a pattern of two kinds of transformations. Unsaturated mole-
cules that are neither polar nor oxidizing form binuclear [2+n] cyclo-
addition products retaining Cr–Cr quadruple bonds. In contrast, polar or
oxidizing molecules effect the complete cleavage of the Cr–Cr bond.
Occasioned by the discovery of a dinuclear chromium complex
featuring a sterically accessible quintuple metal–metal bond, we
have begun to explore the reactivity of this novel functional group
unique to transition metal chemistry. Recent studies indicate that
M–M quintuple bonds have a remarkable reaction chemistry.1–16
Herein we describe the products of reactions between quintuply
bonded [HLiPrCr]2 (1, where HLiPr = Ar–NQC(H)–(H)CQN–Ar, with
Ar
=
2,6-diisopropylphenyl)17 and various small molecules
(Scheme 1). These reactions are of interest in their own right
and make for fascinating comparisons with the reactivities of
other binuclear metal complexes.
1 reacts rapidly with molecules containing multiple bonds.
For example, we have previously described [2+2] cycloaddition
reactions between 1 and alkynes.18 While the analogous reaction
with ethylene is apparently reversible, 1 adds to the destabilized
CQC double bond of 1,1-dimethylallene, yielding another
isolable [2+2] cycloaddition product, namely [HLiPrCr]2(m-Z1:Z1-
H2CCCMe2) (2, see Fig. 1). The terminal CQC bond of the allene
ligand has added across the two metal centers, forming a four-
membered dimetallacycle. The C53–C54 distance of 1.466(5) Å
and the Cr–Cr distance of 1.9462(8) Å are consistent with a two-
electron reduction of allene and concomitant oxidation of the
Cr–Cr center, which, however, retains the short Cr–Cr distance
characteristic of a quadruple bond (see Table 1). The other CQC
bond of the allene remains essentially unperturbed (1.346(5) Å).
Scheme 1 Reactions of 1 with alkyne, allene, sulfur, PhNQNPh, AdN3,
CO, benzophenone and benzylideneaniline.
Department of Chemistry and Biochemistry, University of Delaware, Newark,
DE 19716, USA. E-mail: theopold@udel.edu
† Electronic supplementary information (ESI) available: Preparative and crystallo-
graphic data. CCDC 971178–971183. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3cc48746f
‡ Current address: Department of Chemical and Biological Engineering,
Northwestern University, USA.
Fig. 1 The molecular structure of 2 (30% probability level). Ligand i-Pr
groups and H-atoms have been omitted for clarity.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 2579--2581 | 2579

View Article Online
Communication
ChemComm
Table 1 Selected interatomic distances (Å) and angles (1)
Cr–Cr
C–Cc
C–Nc
ya
db
1
2
3
4
5
6
7
1.8028(9)
1.9462(8)
1.9305(8)
2.498(4)
1.9575(11)d
3.1667(15)
N/A
1.350(5)
1.337(5)
1.367(3)
1.395(11)
1.346(6)
1.360(6)
1.383(6)
1.352(4)
1.368(3)
1.380(4)
1.360(3)
1.380(9)
1.385(6)
1.336(6)
1.355(5)
1.370(4)
N/A
N/A
24.31
15.61
N/A
N/A
N/A
N/A
23.71
1511
1431
N/A
1421d
N/A
N/A
1461
1-Butyne18
1.9248(7)
a
b
Twist angle (X–X)–(Cr–Cr) (X = C or S). Dihedral angle between two
c
ligand planes (see the ESI for details). Average bond lengths in the
a-diimine backbones. Average.
d
The core of 2 adopts an almost planar geometry with a (C–C)–(Cr–Cr)
twist angle of 24.31, similar to the aforementioned alkyne adducts.18
The 1H NMR spectrum of 2 exhibited sharp resonances consistent
with a diamagnetic ground state of the molecule.
Oxygen atom sources, such as O2, N2O, and NO led to
decomposition of 1 accompanied by loss of the diimine ligand.
This motivated us to extend the exploration to less oxidizing
chalcogens. Thus, treatment of an Et2O–toluene solution of 1
with elemental sulphur, at room temperature, caused the
initially green solution to turn deep blue. A standard work-up
of the reaction and recrystallization from diethyl ether yielded
the simple binuclear adduct, [HLiPrCr]2(S2) (3) in modest yield (20%).
The molecular structure of 3 is depicted in Fig. S1 (ESI†); it features
a four-membered Cr2S2 ring. The ‘‘supershort’’ (Cr–Cr o 2.0 Å)
Cr–Cr bond of 3 (1.9305(8) Å) is appreciably longer than that in 1
(1.8028(9) Å), indicating an oxidation from Cr(I) to Cr(II) and hence a
bond order reduced to 4. The S–S bond length of 2.0513(10) Å
approximates that of Kempe’s disulfide analog (2.058(4) Å),2 which,
Fig. 2 The molecular structure of 4 and 5 (both at 30% probability level).
2À
however, features perpendicular coordination of the S2 unit and
that of Cp2Cr2(m-S)2(m-Z1-Z1-S2) (2.028(2) Å).19 As is typical of the
[2+2] cycloaddition products of 1, the Cr2S2 core is not perfectly was added to a solution of (m-Z1:Z1-HLiPr)2Cr2 (1) in diethyl
planar. The (S–S)–(Cr–Cr) twist angle for the core is 15.61, somewhat ether, subsequent work-up and recrystallization produced red-
smaller than the analogous angles in the alkyne adducts and 2.
brown crystals of dinuclear complex [HLiPrCr(m-NPh)]2 (4) in
Table 1 contains selected bond lengths and angles for 40% isolated yield. 4 is a dinuclear complex with bridging
compounds 2–7. All the ‘cycloaddition’ products of 1 that imido ligands (Fig. 2, top). This reaction may well go through
maintain Cr–Cr bonds, i.e. 2, 3, and 1-2-butyne, exhibit the an unstable [2+2] cycloaddition intermediate, which suffers
twisted m-Z1:Z1 bonding mode for the X2 ligands (X = C, S); this oxidative addition, due to the high electronegativity of nitrogen.
differs from the perpendicular (i.e. m2-Z2:Z2) bonding motif The molecular structure of 4 features four-coordinate chromium
more typically observed for complexes with metal–metal bonds, (ignoring the rather long Cr–C interactions) adopting pseudo-
e.g. in Kempe’s aminopyridinato dichromium complexes.2–4,20 tetrahedral geometry, which is the preferred geometry of
At the same time, the dihedral angles (d) between the a-diimine 4-coordinate Cr(III). The NQN double bond has been severed
ligand planes are significantly larger than those of the amino- completely (NÁ Á ÁNavg = 2.695 Å). Similarly, the distance between
pyridinato complexes (e.g. 1071 for both the disulfide and the the two chromium atoms in 4 is 2.498(4) Å, indicating the
tolylacetylene adduct). In other words, the [L2Cr2] fragments of absence of any significant bonding interactions.
the a-diimine complexes are considerably flatter than those
The average bond lengths of C–C, C–N bonds in the backbone
with aminopyridinato ligands. The near preservation of the of the a-diimine ligand are 1.395(11) and 1.380(9) Å, characteristic
planar geometry of 1 and the formation of unsaturated four- of a diimine radical anion; accordingly, chromium is in the formal
membered Cr2X2 rings as opposed to tetrahedrane-like structures oxidation state +III (S = 3/2). The effective magnetic moment of 4
is unlikely to be steric in origin. An electronic explanation may at room temperature was 2.4(1) mB, consistent with antiferromag-
be rooted in the electronic flexibility afforded by the redox-active netic coupling, both between the metal and its radical ligand as
a-diimine ligands; this remains to be explored.
well as between the chromium atoms.
An isoelectronic – but less oxidizing – analog of O2 is
The reaction between (m-Z1:Z1-HLiPr)2Cr2 (1) and sterically
azobenzene (PhNQNPh). When one equivalent of the latter demanding Ad–N3 afforded another imido complex, namely
2580 | Chem. Commun., 2014, 50, 2579--2581
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
[HLiPrCr]2(NAd) (5), as shown in Fig. 2 (bottom). Only one imido with the a-diimine apparently being in the singly reduced state
group has been added across the Cr–Cr bond. Once again, we (see Table 1). The room temperature effective magnetic moment
suggest that a five-membered [2+3] cycloaddition product may of 7 was found to be 2.9(1) mB, consistent with a Cr(III) metal
be formed first, which rapidly extrudes N2. The bond distances center (S = 3/2) strongly coupled to a ligand radical (S = 1/2).
and angles of 5 are comparable to those of other known
In summary, reactivity studies on a quintuply bonded dichro-
bridging imido complexes of chromium.22–26 Similar to the mium complex supported by a-diimine ligands have been
geometries of the [2+2] cycloaddition products, the elongated extended to a variety of molecules. The products are varied and
Cr–Cr distance of 1.9575(11) Å is consistent with the two- their structures differ from those established for quintuply
electron oxidation of the Cr2 unit (to Cr(II)). 5 is also diamagnetic, bonded complexes supported by other ligands. A pervasive feature
presumably due to metal–metal quadruple bonding.
of 1 seems to be the formation of [2+n] cycloaddition products
Finally, we were interested in studying the reactivity of 1 with nonpolar substrates. Polar, heteroatomic multiple bonds on
toward unsaturated molecules featuring X–Y bonds (X, Y = C, N, O). the other hand effect complete cleavage of the metal–metal bond.
Exposure of a benzene solution of 1 to CO (1 atm) produced the
dark blue carbonyl HLiPrCr(CO)4, as confirmed by 1H NMR spectro-
scopy.21 The reaction of 1 with benzophenone resulted in dinuclear
[HLiPrCr(m-OPh2)]2 (6). The structure of 6 (shown in Fig. S2, ESI†)
reveals a benzophenone-bridged dimer with square planar Cr
centers. The average carbon–oxygen bond length of the benzo-
phenone is 1.355(5) Å, which is much longer than the 1.230(3) Å
in benzophenone,27 suggesting some degree of reduction of the
CQO bonds. The average bond lengths of C–C, C–N bonds of
the backbone of the a-diimine ligand are 1.360(6) and 1.336(6) Å,
consistent with those of a monoanionic diimine ligand.21 These
structural features suggest that 6 is a Cr(II) complex. Like
[HLiPrCr(m-Cl)]2,17 6 exhibited a simple isotropically shifted and
broadened 1H NMR spectrum in C6D6, with chemical shifts at 96,
14.6, 3.2, 1.56, and À13.0 ppm. meff(RT) of this complex was found
to be 5.1(2) mB (3.6(1) mB per chromium), which is consistent with
two antiferromagnetically coupled Cr(II) metal centers (S = 2)
coordinated by ligand radicals (S = 1/2).
This work was supported by the NSF (CHE-0911081).
Notes and references
1 A. Noor, T. Bauer, T. K. Todorova, B. Weber, L. Gagliardi and
R. Kempe, Chem.–Eur. J., 2013, 19, 9825–9832.
2 E. S. Tamne, A. Noor, S. Qayyum, T. Bauer and R. Kempe,
Inorg. Chem., 2012, 52, 329–336.
3 C. Schwarzmaier, A. Noor, G. Glatz, M. Zabel, A. Y. Timoshkin,
B. M. Cossairt, C. C. Cummins, R. Kempe and M. Scheer,
Angew. Chem., Int. Ed., 2011, 50, 7283–7286.
4 A. Noor, E. S. Tamne, S. Qayyum, T. Bauer and R. Kempe, Chem.–Eur. J.,
2011, 17, 6900–6903.
5 A. Noor and R. Kempe, Chem. Rec., 2010, 10, 413–416.
6 F. R. Wagner, A. Noor and R. Kempe, Nat. Chem., 2009, 1, 529–536.
7 A. Noor, G. Glatz, R. Mu¨ller, M. Kaupp, S. Demeshko and R. Kempe,
Nat. Chem., 2009, 1, 322–325.
8 A. Noor, F. R. Wagner and R. Kempe, Angew. Chem., Int. Ed., 2008,
47, 7246–7249.
9 P.-F. Wu, S.-C. Liu, Y.-J. Shieh, T.-S. Kuo, G.-H. Lee, Y. Wang and
Y.-C. Tsai, Chem. Commun., 2013, 49, 4391–4393.
10 H.-G. Chen, H.-W. Hsueh, T.-S. Kuo and Y.-C. Tsai, Angew. Chem.,
Int. Ed., 2013, 52, 10256–10260.
In contrast to 6, reductive coupling of CQN double bonds
was observed upon exposure of 1 to four equivalents of trans-
benzylideneaniline. The reaction was found to form the coupling
product, HLiPrCr(k2-N2C26H22) (7). The crystal structure is shown
in Fig. 3. 7 adopts tetrahedral coordination about chromium
11 S.-C. Liu, W.-L. Ke, J.-S. K. Yu, T.-S. Kuo and Y.-C. Tsai, Angew. Chem.,
Int. Ed., 2012, 51, 6394–6397.
12 Y. L. Huang, D. Y. Lu, H. C. Yu, J. S. Yu, C. W. Hsu, T. S. Kuo,
G. H. Lee, Y. Wang and Y. C. Tsai, Angew. Chem., Int. Ed., 2012, 51,
7781–7785.
13 Y.-C. Tsai, H.-Z. Chen, C.-C. Chang, J.-S. K. Yu, G.-H. Lee, Y. Wang
and T.-S. Kuo, J. Am. Chem. Soc., 2009, 131, 12534–12535.
14 Y. C. Tsai, C. W. Hsu, J. S. Yu, G. H. Lee, Y. Wang and T. S. Kuo,
Angew. Chem., Int. Ed., 2008, 47, 7250–7253.
15 Y.-C. Tsai, Y.-M. Lin, J.-S. K. Yu and J.-K. Hwang, J. Am. Chem. Soc.,
2006, 128, 13980–13981.
16 C. Ni, B. D. Ellis, G. J. Long and P. P. Power, Chem. Commun., 2009,
2332–2334.
17 K. A. Kreisel, G. P. Yap, O. Dmitrenko, C. R. Landis and
K. H. Theopold, J. Am. Chem. Soc., 2007, 129, 14162–14163.
18 J. Shen, G. P. Yap, J. P. Werner and K. H. Theopold, Chem. Commun.,
2011, 47, 12191–12193.
19 L. Y. Goh and T. C. W. Mak, J. Chem. Soc., Chem. Commun., 1986,
1474–1475.
20 M. J. Calhorda and R. Hoffmann, Organometallics, 1986, 5, 2181–2187.
21 K. A. Kreisel, G. P. Yap and K. H. Theopold, Inorg. Chem., 2008, 47,
5293–5303.
22 W. H. Monillas, G. P. A. Yap and K. H. Theopold, Inorg. Chim. Acta,
2011, 369, 103–119.
23 W. H. Monillas, G. P. Yap, L. A. MacAdams and K. H. Theopold,
J. Am. Chem. Soc., 2007, 129, 8090–8091.
24 A. A. Danopoulos, D. M. Hankin, G. Wilkinson, S. M. Cafferkey, T. K. N.
Sweet and M. B. Hursthouse, Polyhedron, 1997, 16, 3879–3892.
25 A. A. Danopoulos, G. Wilkinson, T. K. N. Sweet and
M. B. Hursthouse, J. Chem. Soc., Dalton Trans., 1995, 2111–2123.
26 B. Moubaraki, K. S. Murray, P. J. Nichols, S. Thomson and
B. O. West, Polyhedron, 1994, 13, 485–495.
27 E. B. Fleischer, N. Sung and S. Hawkinson, J. Phys. Chem., 1968, 72,
4311–4312.
Fig. 3 The molecular structure of 7 (30% probability level).
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 2579--2581 | 2581