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43703-AC1
C-C-C N-Heterocyclic Carbene Pincer Complexes: A General Method of Preparation Critical for Applications in Catalysis

T. Keith Hollis, The University of Mississippi

Abstract: Upon heating Zr(NMe₂)₄ (1.1 equiv), 1,3-bis(N-butyl-imidazolium)benzene diiodide (1.0 equiv) and toluene, analytically pure [Zr(CCC-NHC-pincer ligand)I2(NMe2)] was obtained upon cooling, which was found to be an efficient intramolecular hydroamination/cyclization catalyst.

            Our previous methodology (Scheme 1) exploited the basicity and electrophilicity of Zr(NMe₂)₄ to activate the three C-H bonds of ligand precursor 1 for coordination to the Zr metal.  Although this method rapidly produced the desired complex, it employed an excess of Zr(NMe₂)₄, produced a mixture of coordination spheres at Zr (2a and 2b) and did not lead to ready separation of the Zr pincer product from the excess reagent.

Scheme 1.  Original synthesis with excess reagent.

The use of a stoichiometric amount of Zr reagent was highly desired.  Subliming Zr(NMe₂)₄ prior to use, employing higher boiling solvents and longer reaction times provided the desired complex in high yield and purity.  Combining the bis(imidazolium) salt 1, Zr(NMe₂)₄ (1.1 eq), toluene and heating in a sealed vessel produced a homogeneous reaction mixture (Scheme 2).  Metallation was found to proceed efficiently and quantitatively as observed by ¹H NMR spectroscopy.  Metallation was incomplete at 150 minutes, but longer reaction times were found to be satisfactory.  Upon scaling up the reaction an additional advantage was found.  At the completion of the reaction the desired complex 3 precipitated upon cooling in analytically pure form (1.8 g, 67%) as shiny yellow crystals.  The only by-product of reaction was the volatile and soluble Me₂HN.  The slight excess of Zr(NMe₂)₄ remained in solution.  

Scheme 2.  Improved metallation procedure for triple C-H activation.

            ¹H and ¹³C NMR spectroscopic analysis of the precipitated yellow crystals confirmed that pure CCC-NHC pincer 3 was synthesized.  X-ray quality crystals of 3 were selected from the precipitate.  An ORTEP® view of complex 3 is presented in Fig 1.  In the solid state zirconium adopts a distorted octahedral geometry. The Zr-C(carbene) distances, 2.367(3) and 2.362(3)Å, are similar to previous reports of pincer NHC complexes of Zr,10b,  and shorter than non-chelated NHC – Zr distances (2.43 - 2.46 Å).  The Zr-C12(aryl) distance is 2.310(3) Å, slightly shorter than the complex 2b, which contained an equatorial amido group.  The axial Zr-I1 distance, which is trans to NMe2, is 3.0038(4) Å, demonstrating a stronger trans influence for the amido group compared to the aryl group,  since the equatorial Zr-iodide distance, trans to Ar, is 2.8431(4) Å. The Zr-N (amido) distance is 1.986(3) Å.  The crystal shows evidence of a slight axial/equatorial disorder that was modeled at a 94:6 occupancy.  Additional data are listed in Fig 1. 

Fig. 1. ORTEP® representation of the molecular structure of 3 (major isomer illustrated, 94%). Selected geometric data: Zr-C12, 2.310(3) Å; Zr-C3, 2.367(3) Å, Zr-C4, 2.362(3) Å; Zr-I1, 3.0038 Å; Zr-I2, 2.8431(4) Å; Zr-N5, 1.986(3) Å C12-Zr-C3, 68.20(11)°; C12-Zr-C4, 68.28(11)°; C12-Zr-I1, 79.88(7)°; C12-Zr-I2, 162.66(7)°; C12-Zr-N5, 107.11(12)°; C3-Zr-C4, 136.40(11)°; I1-Zr-I2, 83.278(11)°; N5-Zr-I1, 172.86(10)°.

            CCC-NHC zirconium pincer complex 3 was assayed for hydroamination activity against 2,2-diphenyl-4-pentenamine 4. The results of these experiments are summarized in Table 1.  All of the catalytic reactions were conducted in deuterated solvent, and the reaction progress was monitored by 1H NMR spectroscopy.  At 80°C the CCC-NHC pincer zirconium complex 3 did not show any catalytic activity (Table 1, entry 1).  Changing the solvent to toluene and increasing the temperature from 80° to 100°C (entry 2) revealed excellent catalytic activity with 2,2-diphenyl-4-pentenamine 4 quantitatively converted into the corresponding pyrrolidine 5 after 100 min.  Conducting the reaction in a sealed NMR tube and increasing the temperature to 120°C (entry 3) and 160°C (entry 4) yielded the cyclized pyrrolidine 5 in quantitative yield more rapidly (100 min and 50 min).  The catalyst loading could be reduced to 2.5 and 1 mol% (entry 5 and 6) with success. 

Table 1. Optimization of hydroamination conditions.

a All conversions were determined by ¹H NMR. b solvent = C₆D₆.  All others: solvent = C₇D₈.

            From the series of hydroaminations with 4, the optimized condition of 5 mol% catalyst loading in toluene-d8 under reflux was selected to study the scope and limitation of catalyst 3 (Table 2). 

Table 2. Substrate survey of hydroamination catalytic activity.a

^a 5 mol% of 3, Toluene-d8, 160 °C.  ^b Conversion determined by ¹H NMR spectroscopy.  ^c No reaction. ^d Diastereomeric ratio: ~1:1.

            In conclusion, we report herein an efficient method for preparing analytically pure and on large-scale Zr CCC-NHC pincer complex 3.  Complex 3 has been shown to have excellent catalytic activity in the hydroamination/cyclization of unactivated alkenyl-amine yielding pyrrolidines and piperidines. 

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