Cobalt-dinitrogen complexes with weakened N-N bonds

Article abstract of DOI:10.1021/ja808783u

(Chemical Equation Presented) Reported N₂ complexes of cobalt do not have substantial weakening of the N-N bond. Using diketiminate ligands to enforce three-coordinate geometries, we have synthesized several novel CoNNCo complexes. In formally

Full text of DOI:10.1021/ja808783u

Published on Web 06/19/2009
Cobalt-Dinitrogen Complexes with Weakened N-N Bonds
Keying Ding,† Aaron W. Pierpont,‡ William W. Brennessel,† Gudrun Lukat-Rodgers,§
Kenton R. Rodgers,*,§ Thomas R. Cundari,*,‡ Eckhard Bill,*,| and Patrick L. Holland*,†
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14267, Department of Chemistry,
UniVersity of North Texas, Denton, Texas 76203, Department of Chemistry, North Dakota State UniVersity, Fargo,
North Dakota 58105, and Max-Planck-Institut fu¨r Bioanorganische Chemie, Mu¨lheim an der Ruhr, Germany
Received November 9, 2008; E-mail: holland@chem.rochester.edu
Scheme 1. Binding of Nitrogen by Low-Coordinate Cobalt
The binding of N2 to transition metal complexes is a topic of great
interest in the chemistry community.1 While the reactivity of N2
complexes does not always correlate with their ground-state properties,
it has proven useful to estimate “activation” of N2 using the N-N
bond length and stretching frequency, quantities that are readily
measurable. One trend that has emerged is that the “early” transition
metals in groups 4-6 often give N2 complexes with weaker, longer
N-N bonds, whereas N2 complexes of the “late” transition metals in
groups 8-10 have shorter, stronger N-N bonds (dNN < 1.15 Å, νNN
> 1900 cm-1).2 For example, all N-N distances in satisfactory (R <
10%) structures of metal-N2 compounds of group 9 metals are less
than 1.18 Å, suggesting relatively little activation.3 This trend is
consistent with the importance of π-backbonding, where the low-energy
d electrons of the electronegative “late” metals are not transferred
effectively to the π* orbitals of N2. Paradoxically, biological (nitro-
genase) and industrial (Haber-Bosch) catalysts for N2 reduction use
iron, a late metal whose isolated N2 complexes typically have little
N-N weakening. The R form of N2 on elemental Fe surfaces has an
N-N stretching frequency of 1415 cm-1, indicating significant
weakening that has never been achieved in a synthetic complex.4 It is
not clear which properties of the catalysts enable them to achieve N-N
weakening and cleavage.
surements show that LCoNNCoL has a quintet (S ) 2) ground state
and K2LCoNNCoL is a triplet (S ) 1). Interestingly, in each case
the CoNNCo group behaves as a single spin system (see Supporting
Information for details).10
X-ray crystal structures were determined for each of the new
cobalt-dinitrogen complexes. Table 1 compares the CoNNCo cores
to the FeNNFe5 and NiNNNi11 analogues. The Co-N bonds (1.8401(8)
Å) are longer than the Fe-N bonds (1.771(5) Å), and the N-N bond
is significantly shorter in the Co complex than in the Fe complex
(1.1390(15) Å for Co vs 1.189(4) Å for Fe). In the nickel analogue
LNiNNNiL, the N-N bond is even shorter (1.120(4) Å). Because of
the exact correspondence of ligands, it is possible to conclude
unambiguously that LCo and LNi are less effective at weakening the
N-N bond than LFe at this oxidation level. We attribute this difference
to the lower d orbital energies of Co and Ni, which give weaker
backbonding into the π* orbital of N2.
Another difference between the LFeNNFeL and LCoNNCoL is
the geometry at the metal. The iron atoms in LFeNNFeL have local
C2V symmetry (Y-shaped geometry), with the N2 ligand on the C2
axis of the diketiminate-iron unit. However, in the Co complex,
the Co atoms have a significant distortion toward a T shape, with
NL-Co-NN2 angles of 101.01(3)° and 162.69(3)°. An independent
crystal structure in a different space group, though poorly refining,
showed a similar T distortion with N-Co-N angles of 108° and
156°, suggesting that the T distortion is not from crystal packing
effects. There is a low barrier for interconversion of the T shapes
in solution, because the two aryl groups of the diketiminate ligand
are equivalent on the 1H NMR time scale (apparent D2h/D2d
symmetry) at temperatures from 205 to 355 K.
Recently, we described a series of low-coordinate Fe-N2-Fe
complexes that have unprecedented weakening of the N-N bond
(1.18-1.23 Å; 1583-1810 cm-1) and explored their electronic
structure in detail.5 Experimental and theoretical studies showed
that the N2 weakening in these complexes arises from the unusual
low-coordinate geometry at iron, which gives high-energy, singly
occupied d orbitals that transfer electron density into the π* orbitals
of N2.6 Tetrahedral Fe-N2-Fe complexes can also give long N-N
bonds.3g,7 Therefore, it is of interest to learn whether low-coordinate
geometries would also enable cobalt, which has lower-energy d
orbitals than Fe, to weaken the N-N bond of N2 as well. Here, we
report that a dicobalt(I) complex has little N-N weakening ability
but that a formally dicobalt(0) complex has significant weakening
of the N-N bond from backbonding.
Treatment of LCoCl (L ) 2,2,6,6-tetramethyl-3,5-bis(2,6-triiso-
propylphenylimido)hept-4-yl)8 with 1 equiv of KC8 gives the
bimetallic dinitrogen complex LCoNNCoL9 and, with 2 equiv of
reductant, gives K2LCoNNCoL (Scheme 1). Reduction with metallic
Na gives the analogous sodium salt Na2LCoNNCoL. Each deep
purple dinitrogen complex is obtained in a yield of 60-70%, and
1
each has a 7-line H NMR spectrum in C6D6 solution at room
temperature that is consistent with averaged D2h or D2d symmetry
on the NMR time scale. Solid-state magnetic susceptibility mea-
† University of Rochester.
In contrast to these differences between formally cobalt(I) and
iron(I) species, the formally cobalt(0) compounds with an
[LCoNNCoL]2- core have metrical parameters that are similar to
‡ University of North Texas.
§ North Dakota State University.
| Max-Planck-Institut fu¨r Bioanorganische Chemie.
9
10.1021/ja808783u CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 9471–9472 9471

C O M M U N I C A T I O N S
Table 1. Metrical Parameters and Resonance Raman Frequencies for MNNM Cores
N2
L2Fe2N2a
L2Co2N2
L2Ni2N2c
Na2L2Fe2N2b
Na2L2Co2N2
K2L2Fe2N2b
K2L2Co2N2
K2L2Ni2N2c
M-N (Å)
N-N (Å)
νN-N (cm-1
N/A 1.770(4),1.772(4) 1.8401(8) 1.836(3), 1.830(3) 1.749(3), 1.746(3) 1.743(7), 1.735(6) 1.773(7), 1.761(7) 1.750(1)
1.747(4)
1.185(8)
1.098 1.189(4)
1.139(2) 1.120(4)
2164 (2093)
1.213(4)
1583
1.211(3)
1598 (1542)
1.241(7)
1589 (1536)
1.220(2)
1599 (1545) 1696 (1642)
d
2331 1778 (1718)
e
)
a Rerefinement of the original data from ref 5a; see Supporting Information. b Reference 5a. c Reference 11. d Stretching frequencies of 15N2
isotopomers given in parentheses. e No isotope-sensitive Raman bands were observed with 514.5, 413.1, or 406.7 nm excitation.
their Fe analogues, with N-N distances of 1.21-1.22 Å (Table
1). These N-N distances are similar to the N-N double bond in
azobenzene of 1.24(2) Å.12 These are the longest N-N distances
in any cobalt dinitrogen complex, including tetrahedral examples.3
They are also significantly longer than the N-N bond in the nickel
analogue K2LNiNNNiL (1.185(8) Å).11 Resonance Raman spectra
(λex ) 406.7 nm) of the [LCoNNCoL]2- complexes in toluene
solution show bands below 1600 cm-1 that shift 50-60 cm-1 lower
in the 15N2 isotopomer. The very low N-N stretching frequency
confirms the crystallographically observed weakening of the N-N
bond. Thus, even though Co is less activating than Fe in LMNNML
species, it is as effective as Fe at weakening N2 in [LMNNML]2-
compounds. The short Co-N bonds of 1.73-1.75 Å suggest some
multiple bond character from π-backbonding.13
zerovalent compounds. In neutral LMNNML, Fe is significantly
better than Co at backbonding into the π* orbitals of N2. Reduction
of LCoNNCoL by two more electrons gives the first Co complex
with a significantly weakened N2 ligand, and this N-N bond
weakening is similar between the formally zerovalent complexes
(alkali)2[LMNNML] with Fe and Co. These results demonstrate
that the coordination geometry and presence of alkali metals can
have a major impact on the reduction level of bound N2, even with
a more poorly backbonding metal like cobalt.
Acknowledgment. The authors acknowledge a gift of 15N2 from
Cambridge Isotopes and financial support from the NIH (GM065313
to P.L.H., AI072719 to K.R.R.) and the NSF (CHE-0112658 to
P.L.H., CHE-0701247 to T.R.C.).
Is the N-N weakening in the Co2 dianions due to the additional
electron density at cobalt or from the influence of the alkali metal
cations? Because the alkali-metal-free dianions have not been
isolable, DFT calculations are used to elucidate the reasons for the
exceptional N-N bond weakening. Calculations use a simplified
ꢀ-diketiminate ligand (L′ ) C3N2H5-) with a pure functional and
an extended all-electron basis set, BPW91/6-311+G(d). The triplet
dianion [L′CoNNCoL′]2- optimizes to a Y-shape minimum with
Co-N ) 1.76 Å, N-N ) 1.19 Å, and νNN ) 1742 cm-1. This
shows that reduction alone gives significant N-N stretching, but
not as much as the observed compounds. Interestingly, geometry
optimization of triplet K2L′CoNNCoL′ gives a model that has
excellent agreement with the experimental metrical data, with Co-N
) 1.74 Å (expt 1.75), N-N ) 1.22 Å (expt 1.22), and νNN ) 1603
cm-1 (expt 1599). The presence of the potassium ions heightens
the electron transfer from Co to N2; Natural Bond Order (NBO)
analyses show that the N-N bond order changes from 2.05 to 1.82
upon incorporation of the K+ ions. Clearly, both reduction and alkali
metal binding work in concert to weaken N2, whereby the positively
charged alkali metal pulls electrons into N2.
Supporting Information Available: Synthetic, computational,
spectroscopic, and crystallographic details. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
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(9) An iron-cobalt heterobimetallic dinitrogen complex LFeNNCoL is also
accessible, though it has not been crystallographically characterized. Its
1H NMR spectrum is consistent with two inequivalent diketiminate
environments. An N-N stretching vibration is present in the IR spectrum
at 1806 cm-1 (1743 cm-1 with 15N2). The N-N stretching mode is not
observed in infrared spectra of LFeNNFeL or LCoNNCoL, and the
observation of an infrared-active N-N band is strong evidence for the
heterobimetallic complex LFeNNCoL. See the Supporting Information for
characterization of this and other compounds.
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Figure 1. Solid state structures of (a) LCoNNCoL and (b) K2LCoNNCoL
(right), using 50% thermal ellipsoids. Metrical parameters are in
Table 1.
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9472 J. AM. CHEM. SOC. VOL. 131, NO. 27, 2009