bpy2,2'bipyridine

1
X-ray structures
New ytterbium(II) indenyl complexes with ligands
containing sp2-nitrogen donors have recently been
reported in the literature1,2,3. One of them in
particular, the diazabutadiene derivative
Yb(C9H7)2(tBu-NCCN-tBu), appears to show
evidence of paramagnetism (?3.4 ??) which seems
to point to a change in the formal oxidation
state of Yb (Yb2 has a Xe4f14 electron count).
Nevertheless, the magnetic moment is still below
the typical 4.8?B that would be expected from a
formal Yb(III) complex. The explanation for this
behaviour has been tentatively assigned to a
redox tautomerism (below) between the
closed-shell and the 4f13 complex. The related
Yb(C9H7)2(bpy), on the other hand, is
diamagnetic.
Calculated structures
The following computational strategy was adopted
The structures were optimised using the Amsterdam
density functional code. The scalar ZORA
relativistic hamiltonian was used employing
triple zeta uncontracted Slater type orbitals
with two polarisation functions. Smearing was
used to overcome difficulties in convergence with
the triplet state. Integration was set to 6.0.
For Yb a frozen core of 4d10 was chosen and C,N
and O had a frozen 1s2 shell.
Kohn-Sham orbitals for the spin paired (left),
and the triplet (right) models of the complex
Yb(C9H7)2(tBu-NCCN-tBu)
Structure A Yb(C9H7)2(tBu-NCCN-tBu)
CCDC 401205 Structure B Yb(C9H6SiMe2NHtBu)2(bpy)
CCDC 195746 Structure C
Yb(C9H7)2(tBu-NCCN-tBu) BP86/TZ2P S0 Structure
D Yb(C9H7)2(bpy) BP86/TZ2P S0 Structure E
Yb(C9H7)2(pyca) BP86/TZ2P S0
Table 2 Spin transition energies
bpy2,2'-bipyridine pyca(E)-N-(pyridin-2-ylmethyl
ene)benzenamine
Despite not having used a proper multi-reference
method, the x-ray bond distances in the
diazabutadiene complex come closer to the triplet
state model rather than the singlet state. The
Yb-N bond is shorter with S1 (2.38Å) and
comparable to the average crystallographic Yb-N
distance (2.33 Å), and a better agreement is
observed for the other bond distances in the
metallacycle (N-C,C-C). One reason for these
differences arises from the occupation of orbital
91a, antibonding between N and C, but C-C
bonding, weakening the C-N bond and strengthening
the C-C bond. The spin density map is almost
identical with 91a where the spin asymmetry lies
in case of S1. Although no triplet state was
calculated for the bpy complex, the experimental
and calculated distances are in good agreement,
supporting the diamagnetic behaviour. The
HOMO-LUMO gap of the singlets (C,D,E) is
indicative of the trend followed by each compound
for their magnetic properties. The complex
Yb(C9H7)2(pyca) is on this basis predicted to be
paramagnetic and show a formal Yb(III) oxidation
state with a slightly higher value of ? than
Yb(C9H7)2(tBu-NCCN-tBu). This is also consistent
with structural data from recent experimental
findings (unpublished material). As is usual for
BP86, the spin-paired form is overstabilised and
the hybrid functionals provide more reasonable
estimates for the spin state energy difference.
The calculated broken spin magnetic exchange
between both spins is fairly low. The redox
tautomerism hypothesis is thus confirmed.
1Occupations ? 90a1 ? 90a0 91a1 and ? 90a0
91a1 ? 90a1

• References
• Trifonov, A.A. Fedorova, E.A. Ikorskii, V.N.
  Dechert, S. Schumann, H. Bochkarev, M. Eur. J.
  Inorg. Chem. 2005, 2812.
• Trifonov, A. A. Spaniol, T.P. Okuda, J. Eur.
  J. Inorg. Chem. 2003,926
• Bochkarev, M. N.  Trifonov, A. A. Cloke,
  F.G.N. Dalby, C.I.  Matsunaga, P.T. Andersen,
  R.A.  Schumann, H.  Loebel, J.  Hemling, H. J.
  Organomet. Chem. 1995,486,177
• http//www.scm.com

NAGB wishes to thank FCT for funding
SFRH/BD/17095/2004 and project
POCTI/QUI58925/2004