Binding and activation of small molecules by three-coordinate Cr(I)

Article abstract of DOI:10.1021/ja0725549

[(i-Pr₂Ph)₂nacnacCr]₂(μ-η²:η²-N₂) (2) has been prepared and structurally characterized; it contains a bridging side-on bonded dinitrogen ligand with a N-N distance of 1.249(5) A. Reactions

Full text of DOI:10.1021/ja0725549

Published on Web 06/13/2007
Binding and Activation of Small Molecules by Three-Coordinate Cr(I)
Wesley H. Monillas, Glenn P. A. Yap, Leonard A. MacAdams, and Klaus H. Theopold*
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716
Received April 12, 2007; E-mail: theopold@udel.edu
Continuing our exploration of chromium chemistry enabled by
“nacnac” (i.e., â-diketiminate) ligands,1 we have turned our attention
to low formal oxidation states. More specifically, we wish to explore
the structure and reactivity of monovalent chromium (Cr(I)), a
relatively rare oxidation state of said metal. Herein we report the
synthesis of a versatile precursor moleculesnamely an unusual Cr(I)
dinitrogen complexsand its reactions with various small molecules
of interest.
Reaction of CrI2 with (i-Pr2Ph)2nacnacLi yielded the dinuclear
iodide [(i-Pr2Ph)2nacnacCr(µ-I)]2 (1) as green crystals in high yield
(87%). Magnesium reduction of 1 in THF under a nitrogen
atmosphere resulted in a color change to brown within 24 h.
Crystallization of the reaction product from pentanes produced [(i-
Pr2Ph)2nacnacCr]2(µ-N2) (2) in 67% yield. The structure of 2 has
been determined by X-ray diffraction, and the result is shown in
Figure 1. Chromium dinitrogen complexes are rare,2 and 2 is the
Figure 1. The molecular structure of 2 (30% probability level); all six N
only example featuring side-on bonding of N2 in the µ2-η2:η2
coordination mode.3 The N-N distance of 1.249(5) Å implies a
modest degree of reduction of the N2 molecule. The facile ligand
substitution chemistry exhibited by 2 (vide infra) suggests a
description as a Cr(I) complex containing a dinitrogen ligand that
is not “activated” with respect to hydrogenation to ammonia,3 but
a formal oxidation state of Cr(II) with a N22- ligand could also be
considered. 2 features isotropically shifted and broadened 1H NMR
resonances, and its effective magnetic moment (µeff(293 K) ) 3.9(1)
µB) suggests antiferromagnetic coupling between the two Cr atoms.
The reactivity of 2 is marked by ready displacement of the N2
ligand by a variety of molecules, such as π-acids or potential
oxidants. Scheme 1 shows some representative and interesting
examples. The molecular structures of 3 and 4 are shown in Figure
2; some relevant observations follow.
and two Cr atoms are approximately coplanar. Selected interatomic distances
(Å) and angles (deg): N2-N2A, 1.249(5); Cr1-N1, 2.0264(16); Cr1-
N2, 2.0209(9); Cr1-N2-Cr1A, 144.00(13); Cr1-N2-N2A, 72.00(6).
Scheme 1. Reactions of 2 with CO, C2H4, O2, and PhNdNPh
2 reacts rapidly with molecules that are stronger back-bonders
than N2. Thus, exposure of a THF solution of 2 to CO (1 atm)
produced carbonyl complex [(i-Pr2Ph)2nacnacCr]2(CO)(µ-η1:η1-
CO)2 (3) as green crystals in 63% isolated yield. Remarkably, 3 is
neither symmetric nor diamagnetic. One terminal carbonyl and two
µ-isocarbonyls are C-bonded to square pyramidal Cr1, whereas the
square planar coordination environment of Cr2 is completed by a
nacnac ligand and two carbonyl oxygens. The IR spectrum of 3
features CO stretching bands at 1919, 1616, and 1577 cm-1. 3 is
best thought of as a mixed-valent (Cr0CrII) complex, and its
magnetism (µeff(293 K) ) 4.8(1) µB) is consistent with such a
description, in which Cr1 is diamagnetic (low-spin d6) and Cr2 has
four unpaired electrons (high-spin d4).
Reaction of 2 with ethylene formed another unusual organo-
metallic molecule, namely, [(i-Pr2Ph)2nacnacCr]2(µ-η2:η2-C2H4) (4).
Binuclear 4 features a single ethylene ligand symmetrically
coordinated to two metal centers. The C-C distance of 1.482(6)
Å is consistent with binding of neutral ethylene to two electron-
rich π-basic Cr(I) centers. While the Dewar-Chatt-Duncanson
model can certainly be adapted to fit this situation, the µ-η2:η2
1
bonding mode of ethylene is rare.4 The H NMR spectrum of 4
did not change upon exposure to excess ethylene (1 atm), providing
no evidence for a reversible dissociation into mononuclear ethylene
complexes. We look forward to an investigation of analogues of 4
and their reactivity.
Potential oxidants replace the N2 ligand of 2, resulting in products
of oxidative addition. For example, dioxygensa molecule of some
interest to us5 and many others,6 in the context of aerobic oxidation
catalysissreacted with 2 to yield (i-Pr2Ph)2nacnacCr(O)2 (5), that
is, a mononuclear Cr(V) dioxo complex (µeff(293 K) ) 1.8(1) µB,
consistent with a d1 configuration). This reaction is of note as a
rare example of a four-electron oxidative addition of O2 to a single
metal center. The detailed mechanism of this transformation,
probably involving bi- or mononuclear superoxide and peroxide
intermediates, will be interesting to unravel.
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C O M M U N I C A T I O N S
is reasonably attributed to antiferromagnetic coupling of two Cr(III)
ions (d3) mediated by the bridging ligands.
With dinitrogen complex 2, we have prepared a readily accessible
and reactive Cr(I) synthon. The chemistry described here is merely
the tip of an iceberg; further studies of Cr(I) compounds are in
progress in this laboratory.
Acknowledgment. This research was supported by NSF (CHE-
0616375) and DOE (DE-FG02-92ER14273). We thank P. Tobash
and S. Bobev for help with the preparation of chromous iodide.
Supporting Information Available: Experimental details regarding
the synthesis and characterization of 1-6 (pdf) and the X-ray structure
determinations of 2-6 (cif). This material is available free of charge
via the Internet at http://pubs.acs.org.
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Figure 2. The molecular structures of 3 and 4 (both at 30% probability
level); selected interatomic distances (Å) and angles (deg) for 3: Cr1-C1,
1.833(2); Cr1-C2, 1.797(2); Cr1-C3, 1,797(2); Cr2-O2, 2.0894(15); Cr2-
O3, 2.0905(15); C2-O2, 1.203(2); C3-O3, 1.204(2); Cr1-C2-O2,
162.04(16); Cr1-C3-O3, 160.32(16); C2-O2-Cr2, 122.34(12); C3-O3-
Cr2, 123.59(13); 4: Cr1-C1, 2.151(3); Cr1-C1A, 2.168(3); C1-C1A,
1.482(6); Cr1-C1-Cr1A, 139.87(16); Cr1-C1-C1A, 70.6(2).
Intermediate between O2 (which oxidatively adds) and C2H4
(which merely binds) is diimine (N2H2). Due to the instability of
this simple molecule, we chose its phenyl derivative azobenzene
(Ph-NdN-Ph) as a stand-in. Addition of 1 equiv of azobenzene
to a THF solution of 2 produced [(i-Pr2Ph)2nacnacCr]2(µ-NPh)2 (6).
The structure of 6 (see Supporting Information) showed it to be a
binuclear Cr(III) complex joined by two bridging phenylimido
ligands. Apparently, oxidative addition of the NdN double bond
has taken place,7 halting, in this instance, at the +III formal
oxidation state. Like all other molecules described here, 6 is
paramagnetic and its magnetic moment (µeff(293 K) ) 2.6(1) µB)
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