Amidinato- and guanidinato-cobalt(I) Complexes: Characterization of exceptionally short Co-Co interactions

Full text of DOI:10.1002/anie.200900780

Communications
DOI: 10.1002/anie.200900780
Metal–Metal Interactions
Amidinato– and Guanidinato–Cobalt(I) Complexes: Characterization
of Exceptionally Short Co–Co Interactions**
Cameron Jones,* Christian Schulten, Richard P. Rose, Andreas Stasch, Simon Aldridge,
William D. Woodul, Keith S. Murray, Boujemaa Moubaraki, Marcin Brynda,
Giovanni La Macchia, and Laura Gagliardi
Low-coordinate, carbonyl-free first row transition metal(I)
complexes are relatively rare but are finding increasing use in
the activation of small molecules, as enzyme mimics, and so
forth.[1] These complexes are generally very reactive species
that are stabilized by a variety of sterically bulky, mono-, di-,
tri-, and higher dentate ligands.[2] Perhaps the most versatile
of these are the b-diketiminates (e.g., [{ArNC(Me)}2CH]À
(nacnacÀ; Ar= 2,6-diisopropylphenyl)), which have been
utilized in the preparation of a range of Group 5–12 first
row transition metal(I) complexes that have shown fascinat-
ing chemistry.[3] In recent years, we have employed the bulky
amidinate and guanidinate ligands ([(ArN)2CR]À (R = tBu;
pisoÀ), N(C6H11)2 (gisoÀ), or NiPr2 (prisoÀ))[4] for the stabi-
lization of a variety of Group 2,[5] 13,[6] 14,[7] and 15[8] metal(I)
complexes, and planar four-coordinate lanthanide(II) com-
plexes.[9] These studies have highlighted close analogies (but
also differences) between the stabilizing and ligating proper-
ties of the bulky amidinates and guanidinates, and the
properties of b-diketiminates. With these characteristics in
mind, we extended the coordination chemistry of the bulky
ligand pisoÀ to the preparation of the first amidinato–iron(I)
complex, [(k2-N,N’-piso)Fe(h6-toluene)] (cf. [(k2-N,N’-nac-
nac)Fe(h6-benzene)][3d]), which was shown to weakly activate
dinitrogen to give [{(k1-N-,h6-Ar-piso)Fe(m-N)}2] (cf. [{(k2-
N,N’-nacnac)Fe(m-N)}2][3d]), with an accompanying change in
the coordination mode of the pisoÀ ligand.[10] Subsequent
reports from another research group detailed unprecedented
amidinato–chromium(I) complexes, which included the dia-
magnetic, amidinate bridged species, [{Cr[m-N(Ar’)C(R)N-
(Ar’)]}2] (R = H or Me, Ar’ = Ar or 2,6-Et2C6H3), that contain
very short Cr–Cr quintuple bonds (ca. 1.74 ꢀ).[11]
These results motivated us to extend the coordination
chemistry of bulky amidinate and guanidinate ligands toward
other first row transition metal(I) centers.[12] We were
particularly interested in preparing analogues of the b-
diketiminate-stabilized cobalt(I) system [(k2-N,N’-nacnac)-
Co(h6-toluene)] (1),[2b] which, like other cobalt(I) com-
plexes,[2] has been shown to activate an assortment of small
molecules. In addition, we believed that the previously
demonstrated coordinative flexibility of our ligands relative
to that of b-diketiminates[10,13] could yield varying complex
types, depending on the reaction conditions employed.
Herein, we report the first amidinato– and guanidinato–
cobalt(I) complexes, two dimeric examples of which exhibit
the shortest Co–Co interactions reported to date. Preliminary
further reactivity studies of these complexes are also
reported.
[*] Prof. C. Jones, Dr. C. Schulten, Dr. R. P. Rose, Dr. A. Stasch,
W. D. Woodul, Prof. K. S. Murray, Dr. B. Moubaraki
School of Chemistry, Monash University
PO Box 23, VIC, 3800 (Australia)
Fax: (+61)3-9905-4597
E-mail: cameron.jones@sci.monash.edu.au
Dr. C. Schulten, Dr. R. P. Rose
School of Chemistry, Cardiff University (UK)
Dr. S. Aldridge
Inorganic Chemistry, University of Oxford (UK)
The paramagnetic cobalt(II) precursor complexes 2a–c
(Scheme 1), were readily prepared in good yields by salt-
methathesis reactions between CoIIX2 (X = Br or I) and the
potassium salt of the appropriate ligand.[14] The structural
characterization of one complex, [{(priso)CoI}2], is the first to
be reported for an amidinato– or guanidinato–cobalt(II)
halide complex, and shows the complex to be dimeric with
distorted tetrahedral cobalt centers. Upon reduction of 2a–c
with potassium (or magnesium) in toluene, the amidinato–
and guanidinato–cobalt(I) complexes 3a–c were obtained as
crystalline solids in high yields. It is noteworthy that no
nitrogen-coordinated complexes were obtained when the
reductions were carried out under a dinitrogen atmosphere, as
was the case with the reduction in toluene that gave 1.
Reduction of 2a or 2b with potassium in cyclohexane under a
dinitrogen atmosphere afforded the dimeric cobalt(I) com-
plexes 4a and 4b as extremely air-sensitive solids in good
yields. We have seen no evidence so far for the conversion of
Dr. M. Brynda, G. La Macchia, Prof. L. Gagliardi
Dꢀpartment de Chimie Physique
Universitꢀ de Genꢁve (Switzerland)
Prof. L. Gagliardi
Department of Chemistry and Supercomputing Institute
University of Minnesota (USA)
[**] We thank the Australian Research Council (fellowships for C.J. and
A.S.) and the Engineering and Physical Sciences Research Council
(EPSRC (UK)) (postdoctoral fellowship for R.P.R. and partial
studentship for C.S.). The EPSRC is also thanked for access to the
UK National Mass Spectrometry Facility. Financial support from The
Swiss National Science Foundation (grant no. 200020-120007) is
acknowledged.
Supporting information for this article (synthetic, spectroscopic and
crystallographic details for all compounds; ORTEP diagrams for 2c
(X=I), 3a, 3c, 4b, 7, 8, and [{(priso)Co(m-N3)}2]; and full
descriptions (including references) of the theoretical and magnetic
studies) is available on the WWW under http://dx.doi.org/10.1002/
anie.200900780.
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centers of 3a–c than that of 1. The N centers of 3a–c are,
however, slightly further from the Co center than those of the
nacnac complex (mean N–Co = 2.012 ꢀ).[2b] The room-tem-
perature solution ([D12]cyclohexane) magnetic moments of
3a–c (3.09–3.17 mB; cf. 2.7 mB for 1) suggest that they are high-
spin d8 cobalt(I) complexes. The results of variable-temper-
ature solid-state magnetic studies of 3c are consistent with
this view and revealed the compound to have an essentially
constant meff value of approximately 3.4 mB over the range 300–
À1
65 K. The plot of cM against temperature shows that 3c
almost obeys Curie behavior over the range 300–65 K, but
Curie–Weiss-like behavior over the full span of temperatures
studied (300–2 K, q = À11.80 K, C = 1.50 cm3 molÀ1 K[14]).
These results indicate that 3a–c are paramagnetic with S = 1
ground states. A degree of second-order spin–orbit coupling is
also suggested, as the room-temperature meff values for 3a–c
are higher than expected for the spin-only model.[15]
The solid-state structures of compounds 4a (Figure 2) and
4b are similar and show the complexes to be essentially planar
amidinate- or guanidinate-bridged dimers.[14] No analogous b-
diketiminate cobalt(I) compounds have been reported to
Scheme 1. Preparation of compounds 3a–c, 4a, and 4b (the dashed
line represents a bond of indeterminate order).
4a or 4b to 3a or 3b upon dissolution of the complexes in
toluene.
Compounds 3a–c were crystallographically characterized
and were shown to have similar monomeric structures (for 3b,
see Figure 1).[14] In each complex, the metal is h6-coordinated
Figure 2. Molecular structure of compound 4a (thermal ellipsoids at
25% probability; hydrogen atoms omitted). Relevant bond lengths [ꢂ]
and angles [8]: Co1–N2’ 1.9185(14), Co1–N1 1.9276(14), Co1–Co1’
2.1404(10), N1–C1 1.347(2), C1–N2 1.347(2); N2’-Co1-N1 176.58(6),
N2’-Co1-Co1’ 88.82(5), N1-Co1-Co1’ 93.53(4), N1-C1-N2 113.63(15).
Symmetry operation: ’Àx+1,y,Àz+1/2.
Figure 1. Molecular structure of compound 3b (thermal ellipsoids at
25% probability; hydrogen atoms omitted). Relevant bond lengths [ꢂ]
and angles [8]: Co1–N1 2.046(3), Co1–N2 2.087(3), C1–N1 1.347(5),
C1–N2 1.348(5), C1–N3 1.390(5); N1-Co1-N2 63.96(12), N1-C1-N2
108.6(3).
date, but the structures of 4a and 4b are comparable to the
aforementioned Cr–Cr quintuply bonded systems, [{Cr[m-
N(Ar’)C(R)N(Ar’)]}2].[11] The Co–Co interactions in 4 (4a
2.1404(10) ꢀ; 4b 2.1345(7) ꢀ) are significantly shorter than
any of the more than 5600 other examples (range: 2.185–
3.129 ꢀ) in the Cambridge Crystallographic Database.[16] At
first glance, this result seemed to imply that the compounds
by a toluene ligand and N,N’-chelated by the amidinate or
guanidinate moiety. These structures are closely related to
that of 1, although the Co–centroid distance in 1 (1.747 ꢀ) is
significantly greater than in 3a–c (3a 1.659 ꢀ, 3b 1.662 ꢀ, 3c
1.668 ꢀ). This difference possibly arises because the amidi-
nate and guanidinate ligands are less sterically imposing than
nacnac, therefore the toluene ligand is closer to the metal
=
À
possess strong Co Co bonds, as the normal Co Co single-
bond length is considered to be 2.46 ꢀ.[17] If this hypothesis
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were valid, the Co centers of compounds 4a and 4b are best
and relatively strong Co–Co interactions (bond orders of
ca. 2).
described as having T-shaped coordination geometries. In
both compounds, the “open” cobalt coordination sites have a
number of close contacts with isopropyl hydrogen atoms
In order to ascertain if this ordering is indeed the case for
4a and 4b, theoretical calculations were carried out on the
model complexes [{Co[m-N(Ph)C(R)N(Ph)]}2] (R = Me (5) or
NMe2 (6)). Attempts to optimize the geometries of these
models were not successful and, as a result, non-optimized
geometries constructed from cut-down coordinates taken
from the crystal structures of 4a and 4b were utilized.[14]
Initially, DFT (B3LYP/6-31G(d)) calculations were
employed, but problems with the convergences of these
calculations led to them being abandoned. Given the well
known deficiencies of DFT in describing open-shell sys-
tems,[18] CASPT2 calculations (using appropriate basis sets)
were also carried out on 5 and 6, the geometries of which were
slightly modified to fulfill Ci and C2 symmetries, respectively.
Consistent with the magnetic moments experimentally deter-
mined for 4a and 4b, these calculations showed that the
ground states for both model complexes are quintets (5 5Au , 6
5B), with the first triplet states 9.06 and 11.67 kcalmolÀ1
higher in energy, respectively, than the ground states.
Inspection of the natural orbital occupation numbers indi-
cated highly multiconfigurational characters for the wave-
functions of the model compounds. For example, in the case
of 5, the ground state is characterized by two dominating
electronic configurations, (su*)1(pg*)1(du*)1(du*)1 and (su*)1-
(pg*)1(dg)1(dg)1, with relative weights of 0.45 and 0.29,
respectively. Given the difficulties in optimizing the geo-
metries of the models, and the highly multiconfigurational
nature of their metal–metal bonding, it is impractical to
accurately estimate the absolute Co–Co bond orders of 5 and
6 from the calculations. These bond orders should, therefore,
be considered indeterminate at this stage.[14,19,20]
À
(closest C H···Co distances are ca. 2.62 ꢀ (4a); 2.30 ꢀ (4b)).
The intraligand bond lengths of the complexes are consistent
with significant delocalization over their NCN fragments and
their Co–N separations are considerably shorter than those in
3a and 3b. The presence of terminal or bridging hydride
ligands in both complexes was contemplated, although no
crystallographic or other evidence was found to support this
hypothesis.[14]
The room-temperature solution ([D12]cyclohexane) mag-
netic moments were found to be 5.35 mB (4a) and 5.10 mB (4b),
which indicated four unpaired electrons per dimer. Variable-
temperature solid-state magnetic studies on 4a (Figure 3)
Figure 3. Plot of the effective magnetic moment and inverse molar
magnetic susceptibility (inset) versus temperature for 4a. The open
circles show the experimental results and the solid lines are guides.
In light of their high reactivity, preliminary studies were
carried out to assess the ability of 3a–c and 4a, 4b to activate
small molecules (Scheme 2). Neither compound series
reacted with dihydrogen or dinitrogen, but 3 (R = NiPr2)
was oxidized with dioxygen to give the square-planar co-
balt(III) oxide complex 7 in low yields (cf. the formation of
gave a meff value of 5.25 mB at 300 K, which decreased to
5.15 mB at 50 K. The rapid decrease below 50 K is indicative of
zero-field splitting of the spin ground state, while the plot of
À1
cM versus temperature almost obeys Curie behavior (q =
À3.07 K, C = 3.49 cm3 molÀ1 K). These data appear consistent
with 4a (and, by implication, 4b) having isolated S = 2 ground
states. However, the possibility that the compounds possess
two high-spin d8 metal centers (each S = 1) that have little
magnetic communication cannot be totally ruled out.[14]
From a qualitative point of view, the nature of the metal–
metal bonding in compounds 4a and 4b can be proposed
when comparisons are drawn with the related diamagnetic
chromium(I) dimers, [{Cr[m-N(Ar’)C(R)N(Ar’)]}2] (R = H or
Me, Ar’ = Ar or 2,6-Et2C6H3).[11] DFT calculations on a model
complex suggested that the quintuple bonds of the dimers
arise from filling five molecular orbitals that have predom-
inantly metal–metal bonding character (1 s, 2 p, and 2 d). The
unoccupied frontier molecular orbitals of the model system
are largely metal–metal antibonding in character. If a similar
ordering of molecular orbitals occurs for compounds 4a and
4b, then the extra six electrons that originate from the two
cobalt centers (cf. chromium) could doubly occupy one
molecular orbital, and singly occupy four molecular orbitals
that have metal–metal antibonding character. This ordering
would give rise to compounds that have quintet ground states
Scheme 2. Preparation of compounds 7–9.
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square-planar [{(nacnac)Co(m-O)}2] from 1 and O2[2b]). When
3a or 4a were treated with CO, the unusual diamagnetic
iminocarbamoyl complex 8 was formed in high yields. It is
likely that these reactions occur by initial carbonylations of 3a
(with concomitant loss of toluene) or 4a, followed by
Keywords: cobalt · magnetic properties · metal–
metal interactions · N ligands
.
[1] The Activation of Small Molecules (Ed.: W. B. Tolman), Wiley-
VCH, Weinheim, 2006.
À
migratory insertions of a molecule of CO into the Co N
[2] Examples of cobalt(I) complexes stabilized by ligands of
increasing denticity include a) T. Nguyen, W. A. Merrill, C. Ni,
H. Lei, J. C. Fettinger, B. D. Ellis, G. J. Long, M. Brynda, P. P.
Power, Angew. Chem. 2008, 120, 9255 – 9257; Angew. Chem. Int.
Ed. 2008, 47, 9115 – 9117; b) X. Dai, P. Kappor, T. H. Warren, J.
Am. Chem. Soc. 2004, 126, 4798 – 4799; c) T. A. Betley, J. C.
Peters, J. Am. Chem. Soc. 2003, 125, 10782 – 10783; d) W. A.
Chomitz, J. Arnold, Chem. Commun. 2008, 3648 – 3650; e) W. A.
Chomitz, J. Arnold, Inorg. Chem. 2009, 48, 3274 – 3286.
[3] For examples, see: vanadium: a) Y.-C. Tsai, P.-Y. Wang, K.-M.
Lin, S.-A. Chen, J.-M. Chen, Chem. Commun. 2008, 205 – 207;
chromium: b) W. H. Monillas, G. P. A. Yap, K. H. Theopold,
Angew. Chem. 2007, 119, 6812 – 6814; Angew. Chem. Int. Ed.
2007, 46, 6692 – 6694; manganese: c) J. Chai, H. Zhu, A. C.
Stꢁckl, H. W. Roesky, J. Magull, A. Bencini, A. Caneschi, D.
Gatteschi, J. Am. Chem. Soc. 2005, 127, 9201 – 9206; iron:
d) J. M. Smith, A. R. Sadique, T. R. Cundari, K. R. Rodgers, G.
Lukat-Rodgers, R. J. Lachicotte, C. J. Flaschenreim, J. Vela, P. L.
Holland, J. Am. Chem. Soc. 2006, 128, 756 – 769; Co:
e) Ref. [2b]; nickel: f) G. Bai, P. Wei, D. W. Stephan, Organo-
metallics 2005, 24, 5901 – 5908; Cu: g) X. Dai, T. H. Warren, J.
Am. Chem. Soc. 2004, 126, 10085 – 10094; Zn: h) Y. Wang, B.
Quillian, P. Wei, H. Wang, X.-J. Yang, Y. Xie, R. B. King, P. v. R.
Schleyer, H. F. Schaefer III, G. H. Robinson, J. Am. Chem. Soc.
2005, 127, 11944 – 11945, and references therein.
bonds of the complexes. A similar insertion has been
observed upon carbonylation of a terphenyl cobalt(I) com-
plex.[15] Reaction of 3b with 1-adamantyl azide (AdN3) gave a
good yield of diamagnetic 9 (cf. the formation of [(nac-
nac)Co NAd] from 1 and AdN3[2b]). There are only a handful
ꢀ
of almost-linear terminal cobalt imides known that can be
compared with 9,[2b–c] but the Co N distance in 9 (1.621(3) ꢀ,
ꢀ
see Figure 4) is identical to that in [(nacnac)Co NAd],[2b] and
ꢀ
both distances are shorter than in other examples of four-
coordinate complexes.[16,21]
[4] G. Jin, C. Jones, P. C. Junk, K.-A. Lippert, R. P. Rose, A. Stasch,
New J. Chem. 2009, 33, 64 – 75.
[5] S. P. Green, C. Jones, A. Stasch, Science 2007, 318, 1754 – 1757.
[6] C. Jones, P. C. Junk, J. A. Platts, A. Stasch, J. Am. Chem. Soc.
2006, 128, 2206 – 2207.
[7] S. P. Green, C. Jones, P. C. Junk, K.-A. Lippert, A. Stasch, Chem.
Commun. 2006, 3978 – 3980.
[8] S. P. Green, C. Jones, G. Jin, A. Stasch, Inorg. Chem. 2007, 46, 8 –
10.
[9] D. Heitmann, C. Jones, P. C. Junk, K.-A. Lippert, A. Stasch,
Dalton Trans. 2007, 187 – 189.
[10] R. P. Rose, C. Jones, C. Schulten, S. Aldridge, A. Stasch, Chem.
Eur. J. 2008, 14, 8477 – 8480.
Figure 4. Molecular structure of compound 9 (thermal ellipsoids at
25% probability; hydrogen atoms omitted). Relevant bond lengths [ꢂ]
and angles [8]: Co1–N1 1.942(3), Co1–N2 1.947(3), Co1–N4 1.621(3);
N1-Co1-N2 68.52(12), Co1-N4-C38 172.1(3).
[11] a) Y.-C. Tsai, C.-W. Hsu, J.-S. K. Yu, G.-H. Lee, Y. Wang, T. S.
Kuo, Angew. Chem. 2008, 120, 7360 – 7363; Angew. Chem. Int.
Ed. 2008, 47, 7250 – 7253; b) C.-W. Hsu, J.-S. K. Yu, C.-H. Yen,
G.-H. Lee, Y.-C. Tsai, Angew. Chem. 2008, 120, 10081 – 10084;
Angew. Chem. Int. Ed. 2008, 47, 9933 – 9936. Since submission of
this paper, a related guanidinate-bridged chromium(I) complex
[{Cr[m-N(Ar)C(NMe2)N(Ar)]}2] has been reported: A. Noor, G.
Glatz, R. Mꢁller, M. Kaupp, S. Demeshko, R. Kempe, Z. Anorg.
Allg. Chem. 2009, 635, 1149 – 1152.
[12] The coordination chemistry of amidinates and guanidinates with
higher-oxidation-state 3d metals has been extensively devel-
oped: F. T. Edelmann, Adv. Organomet. Chem. 2008, 57, 183 –
352.
[13] C. Jones, D. P. Mills, A. Stasch, Dalton Trans. 2008, 4799 – 4804.
[14] See the Supporting Information for further details.
[15] A room-temperature meff value of 3.3 mB has been recorded for a
terphenyl cobalt(I) complex, [Ar*Co(h6-toluene)] (Ar* =
C6H(C6H2iPr3-2,4,6)2iPr2-2,6,3,5): H. Lei, B. D. Ellis, C. Ni, F.
Grandjean, G. J. Long, P. P. Power, Inorg. Chem. 2008, 47,
10205 – 10207.
In conclusion, monomeric and dimeric examples of the
first amidinato– and guanidinato–cobalt(I) complexes have
been prepared. The structures and reactivity of the mono-
meric complexes are closely related to those of an analogous
b-diketiminate complex. The paramagnetic, ligand-bridged
dimeric complexes display the shortest known Co–Co inter-
actions, which, according to theoretical calculations, have
highly multiconfigurational characters. We continue to
explore the chemistry of bulky guanidinato and amidinato
complexes of the 3d metals, and the application of these
complexes to small-molecule activation.
[16] As determined from a survey of the Cambridge Crystallographic
Database, February 2009.
Received: February 10, 2009
Published online: August 31, 2009
[17] L. Pauling, Proc. Natl. Acad. Sci. USA 1976, 73, 4290 – 4293.
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Communications
À1
~
[18] F. Rabilloud, J. Chem. Phys. 2005, 122, 134303.
[20] A medium intensity band at n = 277 cm was observed in the
[19] The effective bond orders (EBOs) calculated for the Co–Co
interactions were 0.76 for 5 and 0.40 for 6. For comparison,
CASPT2 calculations show PhCoCoPh to have a quintet ground
state (5Bg) with an EBO of 1.38 and a very short Co–Co distance
of 1.940 ꢀ: G. La Macchia, L. Gagliardi, P. P. Power, M. Brynda,
J. Am. Chem. Soc. 2008, 130, 5104 – 5114. Similarly, calculations
Raman spectrum of 4a. It cannot be sure if this is associated with
a Co–Co stretching mode of that compound. For comparison, a
À1
~
Co–Co stretching absorption is evident at n = 229 cm in the
Raman spectrum of Co2(CO)8 : G. Aullꢂn, S. Alvarez, Eur. J.
Inorg. Chem. 2001, 3031 – 3038.
[21] The reaction of 3c with Me3SiN3 afforded the tetrahedral
cobalt(II) complex [{(priso)Co(m-N3)}2], which was isolated in
low yield and crystallographically characterized.
5
show the metal dimer Co2 to have a Dg ground state with an
interatomic separation of 1.98 ꢀ: G. L. Gustev, C. W. Baus-
chlicher, J. Phys. Chem. A 2003, 107, 4755 – 4767.
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