More Electron Rich than Cyclopentadienyl: 1,2-Diaza-3,5-diborolyl as a Ligand in Ferrocene and Ruthenocene Analogs

incorporating cyclopentadienly analogs with CB 2 N 2 ‾ skeletons were characterized. Electrochemical measurements supported by computational studies revealed that in combination with larger 10 metal ions such as Ru the CB 2 N 2 ‾ ligand can be more electron-rich than its organic counterpart

More Electron Rich than Cyclopentadienyl: 1,2-Diaza-3,5-diborolyl as a Ligand in Ferrocene and Ruthenocene Analogs Hanh V. Ly, a Jani Moilanen, b Heikki M. Tuononen, b Masood Parvez, a and Roland Roesler* ,a Ruthenium and iron sandwich complexes incorporating cyclopentadienly analogs with CB2N2‾ skeletons were characterized. Electrochemical measurements supported by computational studies revealed that in combination with larger metal ions such as Ru the CB2N2‾ ligand can be more electronrich than its organic counterpart.
The search for heterocyclic cyclopentadienyl analogs was motivated by the exceptional coordinative properties and numerous applications of the parent compound in 15 organometallic chemistry and catalysis. 1 The incorporation of heteroelements in the ring skeleton aimed to tune the electronic properties of the π-ligand and expand the knowledge of main group elements. In the decades following the discovery and elucidation of the bonding in ferrocene, 2 20 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 25 elements in the ring framework, which display a rich coordination chemistry. 4 Most boron-containing Cp analogs include the B,N, 5a B,S 5b,c or B,O 5d pairs that are isolobal with the C2 fragment. A cyclopentadienyl analog with a GeSi2C2‾ framework, stabilized in a ferrocene-type complex, remains so 30 far the only ligand in this category containing more than one heavier group 14 element. 6

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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 B2N3‾ ligands have not yet been substantiated by a crystal structure. 7 The closest analogs to an "inorganic" ferrocene are compounds 40 featuring phosphorus ligands, such as [Cp*Fe(η 5 -P5)], 4,8a and the fully inorganic titanocene [Ti(η 5 -P5)2] 2-. 8b In fact, prior to our work, the only reported complexes from this class that contained ligands featuring more than three heteroelements in the ring skeleton have been pnictogen derivatives. 45 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, 9a benzene, 9b ethyl, 9c ethylene, 9d,e and 50 propane 9f 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 CB2N2‾ frameworks and characterized their complexes with group 1 and 12 metals. 10 The coordination 55 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 η 1 , η 2 , η 3 and η 4 -coordination modes were observed, with the ring nitrogen atoms displaying 60 considerable pyramidalization (CNNC torsion angles of 17 -44°). Reported herein are the first transition metal sandwich compounds employing ligands with CB2N2‾ skeletons that display a classical, η 5 -coordination of the heterocyclic ring. A new precursor 1 featuring a cyclic, pyrazolidyl backbone was synthesized in a fashion similar to reported procedures, 10 in an attempt to enforce a reduction of the CNNC dihedral angle and hence improve the participation of the nitrogen lone 75 pairs to the π-system of the ligand. 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.  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 C3BN‾ frameworks. 12 Single crystal X-ray diffraction analysis revealed for both 3 (Fig. 1) and 4 (Fig. 2) typical sandwich structures with parallel, η 5 -coordinating π-ligands (Fig. 3). The CB2N2 rings are reasonably planar (sum of the intraannular angles 538.8 -10 539.4°) but their geometry is best described as an envelope conformation with a dihedral angle along the B•••B axis of 8 -11°, 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 15 family, to only 2 -3°. However, the C2N2 planes form dihedral angles of 11 -15° 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 20 bonds. The distance between the metal and the best plane of the CB2N2 ring was 1.67 Å for Fe and 1.84 Å for Ru, nearly identical to the corresponding distances in Cp2M (1.66 Å for Fe and 1.84 Å for Ru) and Cp * 2M (1.66 Å for Fe and 1.80 Å for Ru). 13 25 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.
A cyclovoltammetric study showed that both 3 and 4 30 display reversible oxidation steps at +0.45 and -0.04 V, respectively, vs. SCE in CH2Cl2. The reported values for Cp * 2Ru (+0.55 V), Cp2Fe (+0.46 V), and Cp * 2Fe (-0.11 V) indicate that the diazadiborolidine ligands reported herein are comparable to or better electron donors than the parent 35 cyclopentadienyl, 14 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. 15 However, in the case of 3 and 4 a direct comparison of the ligand skeletons is hindered by the 40 lack of data for identically substituted ligands. Hence, a computational investigation was carried out for a set of model systems (see Supplementary Information). Density functional theory was employed to calculate the first ionization energies of Fe and Ru sandwich compounds. The 45 results show that the ionization energy of Cp * 2Fe is 10 kJ mol -1 lower than that of its CB2N2‾ analog, whereas the trend is reversed for Ru complexes, in which case the difference is also slightly bigger, 16 kJ mol -1 . In addition, the calculated ionization energies decrease consistently by ca. 10 kJ mol -1 if 50 the CB2N2‾ ligand contains a pyrazolidyl backbone. Comparable ionization energies were also calculated for Fe and Ru complexes incorporating methylated ligands based on a C3BN‾ framework. These data correlate well with the experimental results and confirm the importance of the 55 bicyclic ligand design. They indicate that, for an identical substitution pattern, the larger CB2N2‾ ring (av. intraannular bond length 1.49 Å in 3) is a better electron donor than cyclopentadienyl (av. intraannular bond length 1.43 Å in 3) for the larger Ru and a poorer electron donor for the smaller 60 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, 10 these efficient ligands display a classical η 5 65 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.
This work was supported by the Natural Sciences and 70