More electron rich than cyclopentadienyl: 1,2-Diaza-3,5-diborolyl as a ligand in ferrocene and ruthenocene analogs

Article abstract of DOI:10.1039/c1cc12281a

Ruthenium and iron sandwich complexes incorporating cyclopentadienyl analogs with CB₂N₂^- skeletons were characterized. Electrochemical measurements supported by computational studies revealed that in combination with large

Full text of DOI:10.1039/c1cc12281a

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More electron rich than cyclopentadienyl: 1,2-diaza-3,5-diborolyl as a
ligand in ferrocene and ruthenocene analogsw
Hanh V. Ly,a Jani Moilanen,b Heikki M. Tuononen,b Masood Parveza and Roland Roesler*a
Received 19th April 2011, Accepted 5th June 2011
DOI: 10.1039/c1cc12281a
Ruthenium and iron sandwich complexes incorporating cyclo-
pentadienyl analogs with CB2N2À skeletons were characterized.
Electrochemical measurements supported by computational
studies revealed thÀat in combination with larger metal ions such
as Ru the CB2N2 ligand can be more electron-rich than its
organic counterpart.
The closest analogs to an ‘‘inorganic’’ ferrocene are compounds
featuring phosphorus ligands, such as [Cp*Fe(Z5-P5)],4,9a and
the fully inorganic titanocene [Ti(Z5-P5)2]2À 9b
Computational
.
studies identified Fe(N5)2 as a promising target for synthesis,
however, this exotic complex has yet to be isolated.10 In fact,
prior to our work, all reported sandwich complexes that
contained five-membered cyclopentadienyl analogs featuring
more than three heteroelements in the ring skeleton have been
pnictogen derivatives.
The search for heterocyclic cyclopentadienyl analogs was
motivated by the exceptional coordinative properties and
numerous applications of the parent compound in organo-
metallic chemistry and catalysis.1 The incorporation of hetero-
elements in the ring skeleton aimed to tune the electronic
properties of the p-ligand and expand the knowledge of main
group elements. In the decades following the discovery and
elucidation of the bonding in ferrocene,2 metal complexes
featuring five-membered heterocyclic Cp analogs containing
various main group elements were reported.3 The majority of
these ligands contain only one heteroelement in the ring
skeleton. Notable exceptions include ligands containing up
to five substituent-free group 15 elements in the ring frame-
work, which display a rich coordination chemistry.4 Most
boron-containing Cp analogs include the B,N,5a B,S5b,c or
B,O5d pairs that are isolobal with the C2 fragment. Sandwich
complexes incorporating boron-rich bis(dicarbollide) ligands
showed promise as tumor imaging reagents.6 A cyclopentadienyl
The formal replacement of C2 fragments with isoelectronic
BN moieties in simple organic entities has received considerable
interest recently, resulting in the isolation of several remarkable
molecules with exquisite properties. Analogs of pyrene,11a
benzene,11b ethyl,11c ethylene,11d,e and propane11f incorporating
the BN fragment have been characterized, free or stabilized in
the coordination sphere of transition metals. In this context,
we reported a family of ligands with CB2N2À frameworks and
characterized their complexes with group 1 and 12 metals.12
The coordination chemistry of these ligands was similar to that
of Cp, although substantial differences were observed as well.
The ring carbon atom proved to play a central role in the
binding of the ligand to metals and only Z1, Z2, Z3 and
Z4-coordination modes were observed, with the ring nitrogen
atoms displaying considerable pyramidalization (CNNC torsion
angles of 17–441). Reported herein are the first transitioÀn
metal sandwich compounds employing ligands with CB2N2
skeletons that display a classical, Z5-coordination of the
heterocyclic ring.
À
analog with a GeSi2C2 framework, stabilized in a ferrocene-
type complex, remains so far the only ligand in this category
containing more than one heavier group 14 element.7
A new precursor 1 featuring a cyclic, pyrazolidyl backbone
was synthesized (Scheme 1) in a fashion similar to reported
procedures,12 in an attempt to enforce a reduction of the
CNNC dihedral angle and hence improve the participation
of the nitrogen lone pairs to the p-system of the ligand.
A truly ‘‘inorganic’’ ferrocene containing no carbon atoms
in the ligand skeleton is still unknown, and early claims
regarding the synthesis of a ferrocene featuring B2N3À ligands
have not yet been substantiated by a crystal structure.8
a Department of Chemistry, University of Calgary,
2500 University Drive NW, Calgary, AB, T2N 1N4, Canada.
E-mail: roesler@ucalgary.ca; Fax: +1 (403) 289 9488;
Tel: +1 (403) 220 5366
b Department of Chemistry, University of Jyvaskyla, P.O. Box 35,
¨
¨
FIN-40014 Jyvaskyla, Finland. Fax: +358 (14) 260 2501;
¨
Tel: +358 (14) 260 2618
¨
w Electronic supplementary information (ESI) available: Complete
experimental details for compounds 1–4, including NMR spectra
and CV data, crystallographic data for complexes 3 and 4, as well as
computational details. CCDC 697078 and 697079. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc12281a
Scheme 1 Synthesis of derivatives 1–4.
c
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Fig. 3 Perpendicular projection onto the CB2N2 planes of 3 (left) and
4 (right) revealing the hapticity of the ligand. Ring substituents have
been omitted for clarity.
Fig. 1 Molecular structure of 3 with 50% probability level thermal
ellipsoids. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (A): B–C(1) 1.511(4), 1.522(4), B–N 1.479(3), 1.487(3),
N–N 1.431(3), Ru–C(1) 2.296(2), Ru–B 2.322(3), 2.323(3), Ru–N
2.126(2), 2.127(2), Ru–CCp* 2.154(2)–2.194(2).
A cyclovoltammetric study showed that both 3 and 4 display
reversible oxidation steps at +0.45 and À0.04 V, respectively, vs.
SCE in CH2Cl2. The reported values for Cp* Ru (+0.55 V),
2
Cp2Fe (+0.46 V), and Cp* Fe (À0.11 V) indicate that the
The deprotonation of 1 with formation of 2 was easily
accomplished using LiTMP and the corresponding change in
molecular symmetry from Cs to C2v was obvious in the NMR
spectra. The reaction of 2 with [Cp*RuCl]4 and [FeCl2(thf)2]
yielded complexes 3 and 4, respectively, in good yields. The
chemical shifts for the heterocyclic ring carbon (22.4, 91.6,
80.0 and 65.5 ppm in 1, 2, 3 and 4, respectively) mirror the
shift of the corresponding carbon resonances in Cp* (52.2,
2
diazadiborolidine ligands reported herein are comparable to
or better electron donors than the parent cyclopentadienyl,16
confirming the results of a previous study showing that the
presence of a BN fragment in the cyclopentadienyl framework
generates ligands with superior electron donating capability.17
However, in the case of 3 and 4 a direct comparison of the
ligand skeletons is hindered by the lack of data for identically
substituted ligands. Hence, a computational investigation was
carried out for a set of model systems (see the ESIw).
105.2, 82.9 and 78.4 ppm in Cp*H, Cp*Na, Cp* Ru and
2
Cp* Fe, respectively).12b,13 The 11B resonances (39.3, 31.5,
2
14.7 and 13.6 ppm in 1, 2, 3 and 4, respectively) fall in the
range observed for Ru (14–18 ppm) and Fe (3–22 ppm)
metallocenes incorporating ligands with C3BNÀ frameworks.14
Single crystal X-ray diffraction analysis revealed for both 3
(Fig. 1) and 4 (Fig. 2) typical sandwich structures with
parallel, Z5-coordinating p-ligands (Fig. 3). The CB2N2 rings
are reasonably planar (sum of the intraannular angles
538.8–539.41) but their geometry is best described as an
envelope conformation with a dihedral angle along the BÁ Á ÁB
axis of 8–111, which allows for a larger separation between the
metal and the larger boron atoms. The CNNC torsion angles
were reduced considerably in comparison to other ligands from
this family, to only 2–31. However, the C2N2 planes form
dihedral angles of 11–151 with the B2N2 planes and hence the
nitrogen atoms remain slightly pyramidal. The intraannular C–B
and B–N bonds display distinct multiple bond character, while
the N–N distances are typical of single bonds. The distance
between the metal and the best plane of the CB2N2 ring was
1.67 A for Fe and 1.84 A for Ru, nearly identical to the
corresponding distances in Cp2M (1.66 A for Fe and 1.84 A
Density functional theory was employed to calculate the
first ionization energies of Fe and Ru sandwich compounds.
The results show that the ionization energy of Cp* Fe is
2
10 kJ molÀ1 lower than that of its CB2N2 analog, whereas
À
the trend is reversed for Ru complexes, in which case the
difference is also slightly bigger, 16 kJ molÀ1. In addition, the
calculated ionization energies decrease consistently by ca.
À
10 kJ molÀ1 if the CB2N2 ligand contains a pyrazolidyl
backbone. Comparable ionization energies were also calculated
for Fe and Ru complexes incorporating methylated ligands
based on a C3BNÀ framework. These data correlate well with
the experimental results and confirm the importance of the
bicyclic ligand design. They indicate that, for an identical
substitution pattern, the larger CB2N2À ring (av. intraannular
bond length 1.49 A in 3) is a better electron donor than
cyclopentadienyl (av. intraannular bond length 1.43 A in 3)
for the larger Ru and a poorer electron donor for the smaller
Fe, likely due to differences in orbital overlap.
Derivatives 3 and 4 prove that the BN pair provides a viable
platform for the design of heteroatom-rich cyclopentadienyl
analogs. Unlike other systems we investigated,12 these efficient
ligands display a classical Z5 coordination mode towards Fe
and Ru and are comparable to or, in the case of the latter
metal, even more electron rich than the parent carbon ring,
generating complexes with increased reducing ability.
for Ru) and Cp* M (1.66 A for Fe and 1.80 A for Ru).15
2
This work was supported by the Natural Sciences and
Engineering Research Council of Canada, the Canada Foundation
for Innovation, the Academy of Finland and the Alberta
Science and Research Investments Program.
Notes and references
Fig. 2 Molecular structure of 4 with 50% probability level thermal
ellipsoids. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (A): B–C(1) and B–C(18) 1.510(4)–1.521(4), B–N
1.472(3)–1.489(4), N–N 1.436(3), 1.439(3), Fe–C(1) and Fe–C(18)
2.177(2), 2.192(2), Fe–B 2.198(3)–2.217(3), Fe–N 1.971(2)–1.997(2).
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