Coordination chemistry of sterically encumbered pyrrolyl ligands to chromium( II ): mono( pyrrolyl)- chromium and diazachromocene formation †

A series of diazachromocenes with sterically demanding pyrrolyl ligands, 2,5-(Me 3 C) 2 C 4 H 2 N ( 1 ), 2,5-(Me 3 C) 2 -3,4-Me 2 C 4 N ( 2 ) and 2,3,5-(Me 3 C) 3 C 4 HN ( 3 ), was prepared and investigated by various spectroscopic techniques, and in some cases by X-ray di ﬀ raction and magnetic susceptibility studies. The diaza-chromocenes exhibit an S = 1 ground state; no indication of a spin-equilibrium was obtained. With the same ligands mono(pyrrolyl)chromium( II ) complexes are accessible, [{( κ N -2,5-(Me 3 C) 2 C 4 H 2 N)Cr-(thf)} 2 ( μ -Cl) 2 ] ( 1-CrCl(thf) ), [( κ N -2,5-(Me 3 C) 2 -3,4-Me 2 -C 4 H 2 N)Cr(Cl)(tmeda)] ( 2-CrCl(tmeda ) and [( η 5 - 2,3,5-(Me 3 C) 3 C 4 HN)Cr( μ -Cl) 2 ] ( 3-CrCl ), which show either η 5 - or η 1 -( κ N ) coordination depending on the substitution pattern. 1 H NMR spectroscopy serves as a valuable tool to distinguish between these coordination modes. The Cr( II ) atoms in the mono(pyrrolyl) complexes adopt a high spin con ﬁ guration ( S = 2) and in dimeric species antiferromagnetic coupling between the spin carriers was observed. However, none of these mono(pyrrolyl)chromium complexes is an e ﬀ ective or selective ethylene oligomerization catalyst on activation with MAO or AlMe 3 , supporting the importance of a Cr( I )/Cr( III )-based catalytic cycle.


Introduction
2][3][4][5][6][7] An interesting feature associated with these ligands is their ability to participate in η 5 → η 3 → η 1 ring slippage processes.In homogeneous catalysis this isomerization might represent an advantageous feature.In the case of pyrrolyl, the N atom facilitates this switch, 8 and pyrrolyl complexes with η 5 -and η 1 -(κN) coordination are well documented in the literature. 6,7,9,10However, in many cases the coordination mode is dictated by the Lewisacidity of the metal atom.
Surprisingly, while studies on pyrrolyl chromium(II) and chromium(III) complexes date back to 1966, 11,12 structurally well-characterized species were first reported by Gambarotta in 1990 using pyrrolyl and 2,5-dimethylpyrrolyl as ligands, for which η 1 -(κN)-coordination was observed exclusively. 13Interestingly, chromium pyrrolide catalyst systems generated from Cr[O 2 CCH(Et)(Bu)] 3 , 2,5-dimethylpyrrole and AlEt 2 Cl/AlEt 3 exhibit high activity and excellent selectivity in ethylene trimerization to 1-hexene. 14,15The origin of this selectivity, and also the question as to which intermediates are involved in this process, have been the focus of extensive studies. 14,15One important aspect was to establish which chromium species is responsible for the high selectivity of the trimerization, since under the employed reaction conditions catalytic cycles involving Cr(I)/Cr(III) or Cr(II)/Cr(IV) intermediates are feasible.DFT calculations have been undertaken, but could support either possibility. 16However, more recent in situ EPR monitoring of the trimerization process supports the formation of Cr(I) and Cr(II) species, whereby only Cr(I) is catalytically relevant. 17Furthermore, Gambarotta and Duchateau used the more sterically encumbered 2,5-di-tert-butylpyrrolyl for this reaction and prepared the Cr(III) complex [(η 5  Nevertheless, from a structural point of view the κN-coordination for the latter compound is surprising, since tert-butyl (tBu) groups in 2,5-positions of the pyrrolyl framework generally sterically disfavour η 1 -coordination but instead η 5 -coordination is observed. 6,7,19,202][23] Herein, we report our observations on the coordination chemistry of bulky pyrrolyls (Chart 1) with Cr(II) and briefly discuss the reactivity of pyrrolyl chromium(II) half-sandwich complexes in the polymerization/oligomerization of ethylene.

Synthesis and characterization of diazachromocenes
For Cr(II), η 5 -coordination of pyrrolyl ligands is rarely observed and requires the functionalization of the N-lone pair, e.g., by methylation 24 or coordination by Lewis-acids such as AlMe 3 . 25owever, based on our previous investigations on azatrozircenes 22 and diazaferrocenes 23 we anticipated that the sterically encumbered pyrrolyl ligands 1-3 would behave similarly to their cyclopentadienyl analogues.Indeed, the reaction of pyrrolides 1-Na, 2-K and 3-K with CrCl 2 in a 2 : 1 ratio in THF yields the corresponding red bis( pyrrolyl)chromium(II) complexes 1-Cr, 2-Cr and 3-Cr, respectively (Scheme 1).
These complexes are very soluble in aliphatic hydrocarbons, in which they form red solutions, and crystals of 1-Cr and 2-Cr suitable for X-ray diffraction were grown from concentrated pentane or hexane solutions at −24 °C (Table 1).
However, because of its high solubility, crystals of 3-Cr had to be grown from a saturated hexamethyldisiloxane solution at −24 °C.The 1 H NMR spectra of these complexes feature broad resonances between δ 5 and −1 ppm at ambient temperature.Nevertheless, for 1-Cr and 3-Cr only the tBu resonances were observed, while the methine CH protons are too broad to be detected (see the Experimental section for details).For comparison, in the 1 H NMR spectrum of [(η 5 -1,3-(Me 3 C) 2 C 5 H 3 ) 2 Cr] the tBu-groups resonate at δ 0.2 (ν 1/2 = 250 Hz) and no methine resonances were found. 26Furthermore, the UV/vis spectra of 1-Cr, 2-Cr and 3-Cr resemble each other closely, consistent with the notion that the pyrrolyl ligands 1-3 exhibit similar ligand field strengths (see ESI † for details).
The molecular structures of 1-Cr (which displays crystallographic twofold symmetry) and 2-Cr (which displays approximately twofold symmetry, r.m.s.d.0.20 Å) are shown in Fig. 1  and 3, and selected bond distances and angles are listed in the figure caption.The electron delocalization within a pyrrolyl system can be readily deduced from the individual CC bond distances, which are intermediate between the distances of isolated single (d 0 (C-C) = 1.54 Å) 27 and double (d 0 (CvC) = 1.34 Å) 27 bonds.To quantify the extent of delocalization within the heterocyclic ring, the parameter τ [28][29][30] was defined as the normalized quotient of the single and double bond lengths (eqn (1)).
The introduction of the more sterically encumbered pyrrolyl 2 has only minor effects on the overall molecular structure and the Pyr(cent)-Cr distance (1.82 Å) is only slightly elongated (Fig. 3).Nevertheless, a closer inspection also reveals that the N-atom is better shielded in 2-Cr than in 1-Cr, since the additional methyl substituents in the 3,4-positions push the adjacent tert-butyl groups towards the nitrogen atoms, as shown by the average C(tBu)-C-N angles of 119.3°and 120.6°f or 2-Cr and 1-Cr, respectively.
X-ray investigations also established the qualitative molecular structure for 3-Cr, but this could not be refined satisfactorily because no unambiguous differentiation between the N-and CH-positions within the pyrrolyl ring was possible.

Synthesis and characterization of mono( pyrrolyl)chromium complexes
The next question to be addressed is the influence of the pyrrolyl substitution pattern on the coordination modes in mono-   ( pyrrolyl)chromium(II) complexes.The reaction of 1-Na with 1 equiv. of CrCl 2 in THF results in a blue solution, from which the previously reported complex [{(κN-2,5-( 18 can be isolated in good yield.The complex 1-CrCl(thf ) is only sparingly soluble in aliphatic and aromatic hydrocarbons, but has a good solubility in THF.Alternatively, 1-CrCl(thf ) can be prepared from the reaction of 1-Cr with 1 equiv. of CrCl 2 (Scheme 2).The different coordination modes in 1-CrCl(thf ) and 1-Cr are immediately obvious when the 1 H NMR spectra of both compounds are compared.Whereas the tBu-groups in 1-Cr are observed at δ 1.99 (ν 1/2 = 121 Hz), they experience a dramatic downfield shift to δ 55.4 (ν 1/2 = 2580 Hz).This behaviour can be readily understood when one assumes that the paramagnetic chemical shift is dominated by the pseudo-contact term; that is, resonances closest to the paramagnetic centre display the greatest paramagnetic shifts and are also broader than those further away from the paramagnetic centre. 36This also rationalizes the fact that the methine resonances are now observable at δ −28.4 (ν 1/2 = 399 Hz).Overall, 1 H NMR spectroscopy offers a powerful tool for distinguishing κNand η 5 -coordination in these molecules.When 1-CrCl(thf ) is exposed to a dynamic vacuum for a prolonged period of time, the colour changes from blue to green and a poorly soluble material was obtained, precluding NMR spectroscopic analysis; but on addition of THF the blue colour is re-established.Unfortunately, at present we can only speculate on the identity of the green material, but the colour change might be related to a switch in the pyrrolyl coordination mode (from κNto η 5 -coordination).
Crystals of 1-CrCl(thf ) suitable for X-ray diffraction were grown by vapour diffusion of pentane into a concentrated THF solution at ambient temperature.The complex 1-CrCl(thf ) crystallized in the monoclinic space group P2 1 /n (Table 1), which is different from the previously reported rhombohedric space group R-3. 18The conformation and orientation of the thf ligands are different in both polymorphs, whereas the bond distance and angles vary only slightly (see below).
An ORTEP diagram is shown in Fig. 4   polymorph this displacement is only 0.39 Å. 18 Two relatively short non-bonding Cr⋯C distances (2.91 and 2.97 Å for Cr-C6 and Cr-C10, respectively) between one methyl group of the pyrrolyl tBu-groups are observed, providing additional steric protection of the fifth and the sixth coordination site of the Cr atom.The Cr-N distance of 2.0176(11) Å compares well with those of other reported Cr(II) amides. 13,37,38A slightly shorter Cr-N distance (2.0121(19) Å) was found for the other polymorph. 18In contrast to 1-Cr and 2-Cr the τ value of 0.87 for 1-CrCl(thf ) suggests a more localized electronic structure, as one would expect from κN-coordination, and it can be compared to that of the unsubstituted pyrrole (τ = 0.830). 39The long Cr⋯Cr i distance of 3.49 Å precludes any Cr-Cr bond, and therefore electron exchange between the Cr(II) centres has to occur via the bridging Cl-atoms.Solid state magnetic susceptibility studies revealed that the Cr(II) atoms with a d 4 electron configuration adopt a high spin (S = 2) configuration and only a weak antiferromagnetic coupling is observed at low temperature.Fig. 5 shows the experimental data and the fit to the effective spin Hamilitonian Ĥ = −2J (Ŝ 1 •Ŝ 2 ) (Fig. 5).
As pointed out above, the addition of two methyl groups at the 3,4-positions of the pyrrolyl systems increases the steric demand around the N-atom and therefore reduces its ability to coordinate in a κN-fashion.When 2-K is treated with 1 equiv. of CrCl 2 in THF a steel-blue solution is formed, suggesting the formation of a mono( pyrrolyl) species analogous to 1-CrCl-(thf ).However, when the solvent is removed, an additional colour change to red-brown is observed and only the diazachromocene 2-Cr can be isolated on pentane extraction.However, on tmeda addition to the reaction mixture the monomeric Cr(II) complex 2-CrCl(tmeda) can be crystallized from a concentrated THF solution at −24 °C (Scheme 3).
The 1 H NMR spectrum of 2-CrCl(tmeda) exhibits similar features to that of 1-CrCl(thf ), confirming the κN-coordination.This assumption was further substantiated by X-ray diffraction.The molecular structure of 2-CrCl(tmeda) is depicted in Fig. 6 and selected bond distances and angles are listed in the figure caption.The Cr-atom is coordinated by one Cl-atom, the bidentate tmeda ligand and pyrrolyl 2 in a square planar geometry.Two short non-bonding Cr⋯C distances (2.87 and 2.81 Å for Cr-C8 and Cr-C14, respectively) provide additional steric protection of the Cr atom, and the Cr-N( pyrrolyl) distance (2.0557(13) Å) is slightly longer than in 1-CrCl(thf ).
Nevertheless, the most sterically encumbered pyrrolyl 3 in this series behaves differently.Deep-blue crystals were isolated from a 1 : 1 mixture of CrCl 2 and KPyr tBu3 in THF (Scheme 4), but the 1 H NMR spectrum is distinctly different from those of 1-CrCl(thf ) and 2-CrCl(tmeda).Three resonances attributable to the three inequivalent tBu-groups are observed at δ 2.69, 6.68 and 12.17 ppm with similar line-width at half-height (ν 1/2 ) of 573, 533 and 566 Hz, respectively.This suggests that in 3-CrCl the pyrrolyl ligand is too sterically encumbered to allow κN-coordination; instead it shows η 5 -coordination, as in the diazachromocene 3-Cr.In addition, elemental analysis and EI mass spectrometry confirm the solvent-free, dimeric structure.Complex 3-CrCl can also be prepared from 3-Cr and CrCl 2 .
Whereas X-ray investigations on 3-CrCl confirmed the η 5bonding mode, the N-and CH-positions within the pyrrolyl ring could not be unambiguously assigned during the refinement.To overcome this crystallographic difficulty, we decided to prepare the analogue half-sandwich complex with the 1,2,4- (Me 3 C) 3 C 5 H 2 (4) ligand, which we expected to be isostructural to 3-CrCl.The reaction between CrCl 2 and NaCp tBu 3 forms a green THF solution, and after work-up deep blue crystals are isolated from very concentrated pentane solutions at −24 °C (Scheme 4).Elemental analysis and EI MS data agree with the proposed structure for 4-CrCl, but one particular observation for this complex is a solvent dependence of its solution colour and its 1 H NMR spectrum (see the Experimental section for details).In pentane and aromatic solvents green solutions are obtained, whereas it forms a blue solution in THF and CH 2 Cl 2 .Fig. 7 shows the molecular structure of 4-CrCl, and the figure caption lists some selected bond distances and angles.Key features of the molecular structure include the long Cp(cent)-Cr distances of 1.95 Å and short Cr1⋯Cr2 distance of 3.181 Å, which is in the range of Cr-Cr bond distances (3.185-3.471Å). 40 In principle, the Cr 2 Cl 2 -core can adopt two different structures, either a planar arrangement (A) or a butterfly geometry (B) (Chart 2).In the latter, the displacement of the two Cr atoms towards each other suggests a (weak) bonding interaction between the two Cr atoms.
Both arrangements have literature precedents.The butterfly structure is adopted in [(η 5 -C 5 Me 5 )Cr(μ-X)] 2 (X = Cl, Me), 41 [(η 5 -C 5 Me 5 )Cr(μ-Et)(μ-Ph)] 2 (X = Cl, Me), 41 [(η 5 -C 5 H 5 )Cr(μ-I)] 2 , 42 and [(η 5 -C 5 H 5 )Cr(μ-OtBu)] 2 43 and has direct consequences for the electronic structure of these dimers, which show strong antiferromagnetic coupling between the two Cr-atoms (S = 1). 41In a more recent report, Layfield and Scheer describe a planar arrangement for [(η 5 -C 5 H 5 )Cr(μ-X)] 2 (X = P(SiMe 3 ) 2 , As (SiMe 3 ) 2 ), in which two high spin (S = 2) Cr(II) centres couple antiferromagnetically. 40To investigate the electronic structure in 4-CrCl, magnetic susceptibility studies were undertaken between 5 and 400 K.The χ T vs. T plot is shown in Fig. 8; and χ T is close to zero at 5 K and then slowly increases to 2.6 emu K at 400 K.While this value is significantly less than the spinonly value of 6.0 emu K expected for two non-interacting high spin Cr(II) atoms (assuming g = 2), it is clearly higher than the expected value for two non-interacting S = 1 Cr(II) centers (2.0 emu K), suggesting antiferromagnetic coupling.The magnetic data can also be simulated by the spin-Hamiltonian Ĥ = −2J (Ŝ 1 •Ŝ 2 ) with S 1 = S 2 = 2; g 1 = g 2 = 2.41; J = −65.8cm −1 ; TIP = 2.189 × 10 −4 emu K and ρ = 0.004 (S = 2 impurity).These values are similar to those found for [(η 5 -C 5 H 5 )Cr(μ-As-(SiMe 3 ) 2 )] 2 . 40talytic screening in ethylene oligo-/polymerisation With these Cr(II) complexes in hand, the activity of 2-CrCl-(tmeda), 3-CrCl and 4-CrCl in ethylene oligomerisation and polymerisation was investigated.This study should also provide insights into the influence of the pyrrolyl coordination  mode on the catalytic activity.The complexes were activated by addition of MAO or AlMe 3 in a 1 : 100 and 1 : 30 ratio, respectively.Furthermore 2 equiv. of dodecyltrimethylammonium chloride was added in the AlMe 3 reactions, since an external chloride source has some advantageous effects on the catalytic performance. 44These mixtures were then heated to 50 °C under ethylene (30 bar).As shown in the previous study on 1-CrCl(thf ), 18 these Cr(II) complexes are only poor catalysts in the selective ethylene polymerization and only trace amounts of oligomers are formed.However, there is a marked difference between the η 5 -coordinate complexes 3-CrCl and 4-CrCl and the κN-bound complex 2-CrCl(tmeda).The former two complexes exhibit identical catalytic performance in olefin oligomerization (no selectivity and low activity of ca. 2 g h −1 ) and no polyethylene formation regardless of the activator.In contrast 2-CrCl(tmeda) forms polyethylene (ca.12 g h −1 ) on activation with MAO, but on activation with AlMe 3 no reaction occurs at 50 °C.However, when the temperature is raised to 75 °C a very slow reaction is initiated with very low ethylene consumption (<2 g h −1 ) and most of the ethylene is converted to polyethylene.In conclusion, Cr(II) complexes with sterically encumbered pyrrolyl ligands are not suitable for the selective ethylene oligomerization, supporting the previous in situ EPR study, which established that only Cr(I) is relevant for the ethylene trimerization. 17

Conclusions
Diazachromocenes with an S = 1 ground state are readily accessible from CrCl 2 and sterically encumbered pyrrolyl ligands.No indication of spin equilibrium was found based on solid state magnetic susceptibility studies for 1-Cr.Reaction of the diazachromocenes with an additional equivalent of CrCl 2 forms mono( pyrrolyl) complexes, in which the Cr(II) atoms adopt a high spin (S = 2) ground state.In addition while κNcoordination is observed for 1-CrCl(thf ) and 2-CrCl(tmeda), this coordination mode is strongly disfavoured for the more sterically demanding Pyr tBu 3 ligand and therefore η 5 -coordination is observed in 3-CrCl.To distinguish these coordination modes 1 H NMR spectroscopy proved to be a very valuable tool.None of the mono( pyrrolyl)chromium complexes is a very active or selective ethylene oligomerization catalyst after activation with MAO or AlMe 3 , strongly indicating that Cr(I)/Cr(III) are the important oxidation states in catalysis.
Further studies on the coordination chemistry of these pyrrolyl ligands are ongoing and will be reported in due course.

General
All synthetic and spectroscopic manipulations were carried out under an atmosphere of purified nitrogen, either in a Schlenk apparatus or in a glovebox.Solvents were dried and deoxygenated either by distillation under a nitrogen atmosphere from sodium benzophenone ketyl (THF) or by an MBraun GmbH solvent purification system (all other solvents).NMR data were recorded on a Bruker DPX 200, a Bruker DRX 400, a Bruker Avance III 400 or a Bruker Avance II 300 spectrometer at ambient temperature unless stated otherwise.The residual solvent signal was used as a chemical shift reference (δ H = 7.16 for benzene, 7.26 for chloroform, 3.58 for α-H of THF) for the 1 H spectra and the solvent signal (δ C = 128.06ppm for benzene, 77.17 for chloroform, 67.21 for α-C of THF) for the 13 C spectra.Elemental analyses were performed by combustion and gas chromatographic analysis with an Elementar vario-MICRO instrument.Magnetic measurements were conducted in a 7 T Quantum Design MPMS magnetometer utilizing a superconducting quantum interference device (SQUID).Between 10 and 25 mg of the sample were sealed in evacuated quartz tubes held in place with ∼5 mg of quartz wool.This method provided a very small and reliable container correction, typically of about −2 × 10 −5 emu mol −1 .The data were also corrected for the overall diamagnetism of the molecule using Pascal constants. 45For a more detailed description see ref. 46.The program package JulX was used for spin-Hamiltonian simulations and fitting of the data by a full-matrix diagonalization approach. 47NaPyr tBu2 (1-Na), 48 KPyr tBu2Me2 (2-K), 22 KPyr tBu3 (3-K) 22 and NaCp tBu 3 (4-Na) 49 were prepared according to the literature procedures.
and selected bond distances and angles are listed in the figure caption.The environment around the Cr 2+ atom is square planar (sum of the angles: 360.7(1)°) and the two halves of the dimer are related by an inversion centre located in the middle of the Cr 2 Cl 2 -core.For steric reasons the pyrrolyl ligands are twisted out of the Cr 2 Cl 2 plane by 87.7°.The Cr atom lies 0.8 Å outside the plane defined by the pyrrolyl moiety; in the rhombohedric

Fig. 8 χ
Fig. 8 χ T vs. T plot for 4-CrCl.The red line represents a theoretical fit of the experimental data (see text for parameters).