Low-Valent Aminopyridinato Chromium Methyl Complexes via

Communication pubs.acs.org/Organometallics

Low-Valent Aminopyridinato Chromium Methyl Complexes via Reductive Alkylation and via Oxidative Addition of Iodomethane by a Cr−Cr Quintuple Bond Awal Noor, Stefan Schwarz, and Rhett Kempe* Lehrstuhl für Anorganische Chemie II (Catalyst Design), Universität Bayreuth, 95440 Bayreuth, Germany S Supporting Information *

ABSTRACT: The reaction of 1 equiv of the deprotonated (sterically demanding) aminopyridine Ap*-H (Ap*-H = (2,6diisopropylphenyl)-[6-(2,4,6-triisopropylphenyl)pyridin-2-yl]amine) with [CrCl3(thf)3] led selectively to the monomeric complex [Ap*CrCl2(thf)2]. Methylation of [Ap*CrCl2(thf)2] by methyllithium gave rise to a mixture of a dichromium methylidene and a dichromium methyl complex. Alkylation of [Ap′CrCl2(thf)2] (Ap′-H = (2,6-diisopropylphenyl)-[6-(2,6dimethylphenyl)pyridin-2-yl]amine) afforded selectively the dimethyl complex [Ap′2CrCr(CH3)2]. In addition to reductive alkylation, oxidative addition can be used to synthesize chromium methyl complexes. Selective oxidative addition was observed when CH3I was reacted with the quintuply bonded Cr I dimer [Ap′CrCrAp′]. Here, the Grignard-like Cr 2 compound [Ap′2CrCr(μ-I)(μ-CH3)] was formed. DFT calculations were performed to investigate the electronic structure of the organometallic complexes.

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bonds.6 Stable complexes having a quintuple bond have been reported for a variety of ligands and have been investigated intensely with regard to the controlled electron release of the 10 electrons stored “between” the two metal atoms (quintuple-bond reactivity).7,8 Among all of the ligands used to stabilize Cr−Cr quintuple bonds, Ap ligands provide a unique dimetallic platform for quintuple-bond reactivity studies.7d This unique platform is prone to release two electrons (one per chromium) to reach a relatively stable bimetallic state characterized by a lower bond order. This feature may allow us to oxidatively add alkyl halides and to isolate the oxidative addition product prior to Schlenk equilibrium like ligand redistribution reactions taking place. Here we report the oxidative addition of iodomethane at a Cr− Cr quintuple bond. Furthermore, the synthesis of related dimethyl Ap chromium complexes by reductive alkylation of mononuclear CrIII chloro complexes is described. The formation of such dialkyl complexes emphasizes the relative stability of multiply bonded dichromium-Ap complexes and, thus, may indicate why controlled two electron release is preferred by ApCr5CrAp complexes. The aminopyridines 1 and 2 (Scheme 1) were synthesized according to the published procedures.2f

minopyridinato (Ap) ligands1,2 are bidentate monoanionic N ligands that can coordinate in a η2 or μ fashion3 (Chart 1, top). The ligands are chemically related to amidinato, guanidinato, or β-diketiminato ligands (Chart 1, bottom) with the unique feature of being nonsymmetric. Chart 1. Aminopyridinato (Ap) Ligands and Related Ligands: (Top) Binding Modes of Ap Ligands; (Bottom) Chemically Related (from Left to Right) Amidinato, Guanidinato, and βDiketiminato Ligand Typesa

a R and R′ are for instance alkyl, aryl, and silyl substituents, and M and M′ represent metals.

Ap ligand complexes have been described for nearly all transition metals,4 whereas early transition metals (and lanthanoids) prefer the η2 coordination mode. Bulky Ap ligands give rise to low-coordinated transition-metal complexes with diverse and unique reactivity patterns.5 Such bulky Ap ligands have also been used successfully to stabilize Cr−Cr quintuple © XXXX American Chemical Society

Special Issue: Mike Lappert Memorial Issue Received: December 3, 2014

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DOI: 10.1021/om501230g Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics Scheme 1. Applied Aminopyridines 1 and 2

Reacting 1 equiv of the potassium salt of 12f in THF with [CrCl3(thf)3] leads to the formation of the monomeric CrIII complex 3 (Scheme 2). The coordination of 3 can be best

Figure 1. Molecular structure of 4 (ellipsoids correspond to 50% probability). Hydrogen atoms (except those for the bridging methylidene ligands) have been omitted for clarity. Selected bond lengths (Å): N1−Cr1 2.097(3), N2−Cr1 2.029(4), C1−Cr1 2.143(5), Cr1−C1A 2.223(5), Cr1−Cr1 2.3279(14).

Scheme 2. Synthesis of 3

atoms and the methyl ligands coordinate in a nonbridging manner (Figure 2). The nitrogen atoms of the Ap ligands are in

described as pseudo-octahedral with two THF ligands in cis positions and two chloro ligands in trans positions (Figure S2). In 3, the Cr−N2(pyridine) distance (2.050(2) Å) is only slightly longer than the Cr−N1(amide) distance (2.006(19) Å), suggesting the delocalization of the anionic function of the ligand.3 Similar observations have been made for the chromium complexes with less bulky version of such ligands (silylaminopyridinato ligands) (Cr−N(pyridine) 2.04 Å and Cr−N(amide) 2.06 Å).9 However, siloxane-bridged bis(aminopyridinato) ligands coordinate in the amidopyridine form (Cr−N(pyridine) 2.102(7) Å and Cr−N(amide) 1.999(5) Å).10 Only broad signals can be observed in the 1H NMR spectra of 3. Magnetic susceptibility measurements show a slightly lower than expected magnetic moment of μeff(298 K) = 3.4 μB for 3. The alkylation of 3 using MeLi led to the formation of a mixture of red and green crystals. X-ray analysis showed the red material being a methylidene bridging CrIII complex (4) (Scheme 3). In this binuclear chromium complex, each

Figure 2. Molecular structure of 5 (ellipsoids correspond to 50% probability). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å): Cr1−Cr1 1.8216(17), C1−Cr1 2.097(8), N1−Cr1 2.013(4), N2−Cr1 2.061(4).

one plane with the two Cr atoms (N−Cr−N angle of 166.27(16)°). The Cr−Cr distance of 1.8216(17) Å lies in the range known for supershort chromium−chromium multiple bonds.12 A related guanidinato complex of 5 has been reported by Gambarotta and co-workers.13 The complex was synthesized by treatment of the mononuclear CrII bisguanidinate with [Al2(CH3)6]. An impressively short Cr−Cr distance has been observed (1.773(1) Å) for this compound. When 3 was reacted with an excess of MeLi, we were able to crystallize 4 as a pure material (see the Supporting Information). Compound 4 is paramagnetic and was characterized by magnetic measurements. The room-temperature magnetic moment is 5.01 μB, slightly lower than that for a CrIII compound with two S = 3/2 centers (theoretical value 5.48) due to antiferromagnetic interactions of the two chromium centers. To see if the steric bulk of the Ap ligand is a factor with regard to the formation of the mixture of 4 and 5, we became interested in the alkylation of 6. The steric bulk of the aminopyridinato ligand in 6 is slightly reduced.6b The alkylation of 6 by MeLi affords smoothly the methyl CrII complex 7 (Scheme 4). Complex 7 is diamagnetic. The structural features of 7 are quite similar to those of 5, and again an extremely short Cr−Cr multiple bond (1.7992(17) Å) was observed (Figure S4). In both 5 and 7, the two methyl groups are arranged nearly perpendicular to the N4Cr2 plane. Cr1−Cr1−C1 angles of 83.0(3)° (5) and 87.10(19)° (7) have been observed. The orientation of the methyl group precludes an agostic interaction, as observed for Gambarotta’s complex.13 There, a Cr−Cr−C angle of 78.2° has been observed. The Cr−C bond lengths in 6 (C1−Cr1 2.097(8) Å) and 7 (C1−Cr1 2.057(5) Å) are shorter than that found for

Scheme 3. Synthesis of 4 and 5

chromium atom is bonded to a η2-coordinated aminopyridinato ligand and two bridging methylidene ligands (Figure 1). The observed Cr−C bond distances (2.143(5) and 2.223(5) Å) fall short in comparison to those observed for the Cp* stabilized (Cp* = pentamethylcyclopentadienyl ligand) mixed methylidene methyl CrIII complex [{Cp*Cr(μ-CH3)2(μ-CH2)].11 A Cr−Cr distance of 2.3279(14) Å is observed for 4.12 The green crystals were identified as a CrII methyl complex (5). In 5, the aminopyridinato ligands are bridging the two Cr B

DOI: 10.1021/om501230g Organometallics XXXX, XXX, XXX−XXX

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Organometallics

complexes,14 respectively. Both complexes (dichloro and dibromo) crystallize preferably and shift the Schlenk-like equilibrium away from the direct oxidative addition product. Density functional theory (DFT) calculations were performed using the TURBOMOLE16 program package. The geometries of 4, 7 and 9 were optimized starting from structural data obtained by X-ray single crystal structure analysis. Computational details are given in the Supporting Information. Figure S9 (Supporting Information) shows the orbitals contributing to the metal−metal multiple bonds in 7 and 9 as well as to the bonds between the metals and the bridging methyl and iodine ligands. Figure S10 (Supporting Information) shows the corresponding MOs for 4. Table S4 (Supporting Information) gives the calculated QTAIM17 (quantum theory of atoms in molecules) delocalization indices for the Cr bonds in compounds 4 and 7−9. The delocalization indices give the number of electrons shared between two basins and are used to estimate the formal bond order. The formation of 7 and 9 decreases the Cr−Cr bond order by 1 in comparison to 8. In comparison to the dimethyl complexes 5 and 7, where we observe nonbridging methyl ligands, the methyl ligand in 9 bridges the two chromium centers nearly symmetrically. The nonbridging mode seems to be thermodynamically favored. The difference in 9 may result from a weakly directing influence of the bridging iodo ligand, which itself bridges due to the available lone pairs. Calculations indicate that even 9 prefers a nonbridging methyl ligand. The minimum structure starting from the X-ray analysis coordinates results in a difference of the Cr−C distances of 0.26 Å. This hypothesis is also in accordance with observations made by the Gambarotta group.13 In summary, we describe two novel synthetic pathways to methyl dichromium complexes containing a multiple bond. Dialkyl complexes were made via reductive alkylation of mononuclear CrIII dichloro complexes and mixed alkyl halogenido complexes via oxidative addition starting from a quintuply bonded dichromium complex. The steric demand of the used N ligand seems crucial to obtain selctive reactions.

Scheme 4. Synthesis of 7

the related guanidinato13 complex (2.192(7) Å). When 7 was reacted with ethylene (5 bar, 80 °C), no oligomerization or polymerization activity could be observed. Surprised by the rearrangement during the alkylation of 3 and 6, we concluded that the dialkyls 5 and 7 are thermodynamically rather stable and are expected to isolate mixed methyl halogenido complexes via oxidative addition starting from the corresponding quintuply bonded dichromium complex (8; Scheme 5). The Scheme 5. Synthesis of 9

reaction of 8 with CH3I gave rise to the alkyl and iodo chromium dimer 9 (Scheme 5). In the 1H NMR spectrum (benzene-d6) the CH3 protons of the bridging methyl group appear as a sharp singlet at δ −0.064 ppm. The molecular structure of 9 was determined by X-ray crystal structure analysis. The methyl and iodo ligands in 9 bridge the two Cr atoms (Figure 3). The Cr−I bond length of 2.846(16) Å

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ASSOCIATED CONTENT

* Supporting Information S

Text, figures, tables, and CIF files giving experimental and computational details and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for R.K.: [email protected]. Figure 3. Molecular structure of 9 (ORTEP representation at the 50% probability level) for all non-carbon atoms). Hydrogen atoms and one toluene molecule (per complex) have been deleted for clarity. The iodo and the methyl ligands are disordered (half occupation). Selected bond lengths (Å): N1−Cr1 2.014(5), N2−Cr1 2.010(5), Cr1−Cr1 1.838(2), Cr1−C1 2.308(16), Cr1−C1 2.329(16), Cr1−I1 2.841(16), Cr1−I1 2.852(16).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG KE 756/20-2). We thank Stephan Kümmel for helpful discussion and the reviewers for their valuable suggestions. Furthermore, we thank Birgit Weber for the help with the magnetic measurements of 4.

is significantly longer than that observed for [ApCrI]2 complexes (2.757(2) Å).14 Examples of bridging methyl dichromium complexes are rare,11,15 and mixed μ-halogenido μ-methyl complexes have not yet been reported. The use of CH3I as the substrate allowed us to isolate the direct oxidative addition product. For other alkyl halides, for instance benzyl chloride and bromide, we could only isolate the dichloro and dibromo

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DEDICATION Dedicated to Mike Lappert in honor of his pioneering contribution to metal alkyl chemistry. C

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