Synthesis and Characterization of Super Bulky β -Diketiminato Group 1 Metal Complexes

: Sterically bulky β -diketiminate (or Nacnac) ligand systems have recently shown the ability to kinetically stabilize highly reactive low-oxidation state main group complexes. Metal halide precursors to such systems can be formed via salt metathesis reactions involving alkali metal complexes of these large ligand frameworks. Herein, we report the synthesis and characterization of lithium and potassium complexes of the super bulky anionic β -diketiminate ligands, known [ TCHP Nacnac] − and new [ TCHP/Dip Nacnac] − ( Ar Nacnac = [(ArNCMe) 2 CH] − ) (Ar = 2,4,6-tricyclohexylphenyl (TCHP) or 2,6-diisopropylphenyl (Dip)). The reaction of the proteo-ligands, Ar NacnacH, with n BuLi give the lithium etherate compounds, [( TCHP Nacnac)Li(OEt 2 )] and [( TCHP/Dip Nacnac)Li(OEt 2 )], which were isolated and characterized by multinuclear NMR spectroscopy and X-ray crystallography. The unsolvated potassium salts, [{K( TCHP Nacnac)} 2 ] and [{K( TCHP/Dip Nacnac)} ∞ ], were also synthesized and characterized in solution by NMR spectroscopy. In the solid state, these highly reactive potassium complexes exhibit differing alkali metal coordination modes, depending on the ligand involved. These group 1 complexes have potential as reagents for the transfer of the bulky ligand fragments to metal halides, and for the subsequent stabilization of low-oxidation state metal complexes.

The activation of β-diketiminate coordinated Mg-Mg bonded fragments by the addition of a sub-stoichiometric amount of a Lewis base is known to generate unsymmetrical magnesium(I) compounds with elongated Mg-Mg bonds. These have been shown to increase the reactivity of the magnesium(I) compound towards inert molecules such as CO and ethylene [19,20]. Recently, we showed that a TCHP-substituted magnesium(I) dimer, upon irradiation, was able to reduce benzene to generate "Birch-like" cyclohexadienyl bridged products, e.g., [{( TCHP Nacnac)Mg} 2 (µ-C 6 H 6 )] [21]. Further to this, Harder and co-workers have shown that Nacnac ligands incorporating the DIPeP aryl moiety are able to stabilize molecular magnesium(0) compounds [22,23], as well as calcium complexes, which activate dinitrogen, as in [{( DIPeP Nacnac)Ca} 2 (µ-N 2 )] [24].
Routes to low-oxidation state metal complexes bearing Nacnac ligands typically proceed via the reduction of β-diketiminato metal halide precursors, which can be synthesized by the reaction of an alkali metal salt of the β-diketiminate ligand with a metal halide (Scheme 1). As such, the synthesis of alkali metal complexes of new very bulky ligands is desirable, with the view to using such complexes as ligand transfer reagents in the preparation of metal halide precursors to highly reactive low-valent metal complexes. Potentially, compounds previously inaccessible using the archetypal Dip Nacnac ligand system can be realized using this approach. Herein, we describe the synthesis and characterization of a new unsymmetrical, bulky β-diketimine pro-ligand incorporating the TCHP aryl moiety, as well as four alkali metal complexes derived from this, and a previously reported TCHP-substituted β-diketimine. The activation of β-diketiminate coordinated Mg-Mg bonded fragments by the addition of a sub-stoichiometric amount of a Lewis base is known to generate unsymmetrical magnesium(I) compounds with elongated Mg-Mg bonds. These have been shown to increase the reactivity of the magnesium(I) compound towards inert molecules such as CO and ethylene [19,20]. Recently, we showed that a TCHP-substituted magnesium(I) dimer, upon irradiation, was able to reduce benzene to generate "Birch-like" cyclohexadienyl bridged products, e.g., [{( TCHP Nacnac)Mg}2(µ-C6H6)] [21]. Further to this, Harder and coworkers have shown that Nacnac ligands incorporating the DIPeP aryl moiety are able to stabilize molecular magnesium(0) compounds [22,23], as well as calcium complexes, which activate dinitrogen, as in [{( DIPeP Nacnac)Ca}2(µ-N2)] [24].
Routes to low-oxidation state metal complexes bearing Nacnac ligands typically proceed via the reduction of β-diketiminato metal halide precursors, which can be synthesized by the reaction of an alkali metal salt of the β-diketiminate ligand with a metal halide (Scheme 1). As such, the synthesis of alkali metal complexes of new very bulky ligands is desirable, with the view to using such complexes as ligand transfer reagents in the preparation of metal halide precursors to highly reactive low-valent metal complexes. Potentially, compounds previously inaccessible using the archetypal Dip Nacnac ligand system can be realized using this approach. Herein, we describe the synthesis and characterization of a new unsymmetrical, bulky β-diketimine pro-ligand incorporating the TCHP aryl moiety, as well as four alkali metal complexes derived from this, and a previously reported TCHP-substituted β-diketimine. Scheme 1. Generic reaction of a β-diketiminato alkali metal salt with a metal/metalloid halide.

Results and Discussion
The β-diketimine, TCHP NacnacH, 1, was recently reported to be prepared by a condensation reaction between acacH, H2C(MeC=O)2 and two equivalents of (TCHP)NH2 [17]. Here, the new, unsymmetrical β-diketimine, TCHP/Dip NacnacH 2, was similarly synthesized by the condensation of one equivalent of (TCHP)NH2 with Dip NacacH, {DipN(H)CMe}CHC(O)Me, which gave 2 in a 43% isolated yield [25]. It is of note that, in the current study, during one preparation of 1, a low yield of the β-ketimine, TCHP NacacH, {(TCHP)N(H)CMe}CHC(O)Me 3, was obtained as a by-product. This likely results from the use of slightly less than two equivalents of (TCHP)NH2 in the synthesis. A rational synthesis of 3 was not attempted, but it was spectroscopically characterized. The molecular structure of 2 is shown in Figure 1a, which reveals it to exist as its ene-imine conjugated tautomer in the solid state, as is typical for β-diketimines. The proton on N2 is hydrogen bonded to N1, which leads to the two double bonds within the C3N2 backbone (viz. N1-C1 and C3-C4) adopting a cis-configuration, relative to each other. A similar structure is adopted by β-ketimine, 3 (Figure 1b), in which the double bonds within the C3NO backbone are C1-O1 and C3-C4. Scheme 1. Generic reaction of a β-diketiminato alkali metal salt with a metal/metalloid halide.

Results and Discussion
The β-diketimine, TCHP NacnacH, 1, was recently reported to be prepared by a condensation reaction between acacH, H 2 C(MeC=O) 2 and two equivalents of (TCHP)NH 2 [17]. Here, the new, unsymmetrical β-diketimine, TCHP/Dip NacnacH 2, was similarly synthesized by the condensation of one equivalent of (TCHP)NH 2 with Dip NacacH, {DipN(H)CMe}CHC(O)Me, which gave 2 in a 43% isolated yield [25]. It is of note that, in the current study, during one preparation of 1, a low yield of the β-ketimine, TCHP NacacH, {(TCHP)N(H)CMe}CHC(O)Me 3, was obtained as a by-product. This likely results from the use of slightly less than two equivalents of (TCHP)NH 2 in the synthesis. A rational synthesis of 3 was not attempted, but it was spectroscopically characterized. The molecular structure of 2 is shown in Figure 1a, which reveals it to exist as its ene-imine conjugated tautomer in the solid state, as is typical for β-diketimines. The proton on N2 is hydrogen bonded to N1, which leads to the two double bonds within the C 3 N 2 backbone (viz. N1-C1 and C3-C4) adopting a cis-configuration, relative to each other. A similar structure is adopted by β-ketimine, 3 (Figure 1b), in which the double bonds within the C 3 NO backbone are C1-O1 and C3-C4.
The bulky β-diketimines, 1 and 2, were deprotonated by treating diethyl ether solutions of the compounds with a slight excess of nBuLi at −78 • C. Upon warming to room temperature and stirring the reaction solutions overnight, they were concentrated and stored at −30 • C to yield colorless crystals of lithium β-diketiminate complexes [( TCHP Nacnac)Li(OEt 2 )], 4, and [( TCHP/Dip Nacnac)Li(OEt 2 )], 5, in moderate isolated yields. The solution state NMR spectroscopic data for the compounds are consistent with them retaining their solid-state structures (see below) in solution. However, it is noteworthy that the 13 C{ 1 H} NMR spectrum of compound 4 exhibits 10 cyclohexyl carbon signals (δ = 25-45 ppm), which suggests restricted rotation around one of the sterically more encumbered ortho-cyclohexyl rings of the TCHP substituents.
Compounds 4 and 5 have very similar solid-state structures, as is evident from Figure 2. The molecular structure of these compounds are typical of previously reported β-diketiminate lithium etherate complexes, e.g., [( Dip Nacnac)Li(OEt 2 )] [26]. That is, they are monomeric and their lithium centers possess trigonal planar coordination geometries with one coordinated ether ligand. The β-diketiminate ligand binds as an N,N-chelate to the lithium, with Li-N bond distances between 1.921 and 1.935 Å, within the normal range for such complexes [26]. The Li-O distances are slightly longer when compared to the previously reported [( Dip Nacnac)Li(OEt 2 )] complex (1.953(4) Å for 4 and 1.975(4) Å for 5; cf. 1.911(4) Å for [( Dip Nacnac)Li(OEt 2 )]), which is likely caused by steric repulsion from its bulky TCHP aryl groups. This also leads to a slight narrowing of the N-Li-N angle for 4 and 5 (97.67(18) • and 97.79(17) • , respectively) compared to the angle in [( Dip Nacnac)Li(OEt 2 )] (99.9(2) • ). The bond lengths within the C 3 N 2 backbones of both compounds imply significant electronic delocalizations over those fragments. The bulky β-diketimines, 1 and 2, were deprotonated by treating diethyl ether solutions of the compounds with a slight excess of nBuLi at −78 °C. Upon warming to room temperature and stirring the reaction solutions overnight, they were concentrated and stored at −30 °C to yield colorless crystals of lithium β-diketiminate complexes [( TCHP Nacnac)Li(OEt2)], 4, and [( TCHP/Dip Nacnac)Li(OEt2)], 5, in moderate isolated yields. The solution state NMR spectroscopic data for the compounds are consistent with them retaining their solid-state structures (see below) in solution. However, it is noteworthy that the 13 C{ 1 H} NMR spectrum of compound 4 exhibits 10 cyclohexyl carbon signals (δ = 25-45 ppm), which suggests restricted rotation around one of the sterically more encumbered orthocyclohexyl rings of the TCHP substituents.
Compounds 4 and 5 have very similar solid-state structures, as is evident from Figure 2. The molecular structure of these compounds are typical of previously reported β-diketiminate lithium etherate complexes, e.g., [( Dip Nacnac)Li(OEt2)] [26]. That is, they are monomeric and their lithium centers possess trigonal planar coordination geometries with one coordinated ether ligand. The β-diketiminate ligand binds as an N,N-chelate to the lithium, with Li-N bond distances between 1.921 and 1.935 Å, within the normal range for such complexes [26].  Unsolvated potassium salts of the TCHP-substituted β-diketiminates could also be synthesized using a similar methodology to that previously reported for the synthesis of [K( Dip Nacnac)] [27]. That is, β-diketimine, 1, was combined with benzyl potassium in toluene, pro-ligand 2 was combined with potassium bis(trimethylsilyl)amide (KHMDS) in toluene and both reactions were monitored by NMR spectroscopy. No reaction could be observed after 16 h at room temperature for either pro-ligand, contrary to the synthesis of [K( Dip Nacnac)], which showed good conversion under these conditions. This can be attributed to the increased steric bulk from the TCHP aryl group, perturbing deprotonation Unsolvated potassium salts of the TCHP-substituted β-diketiminates could also be synthesized using a similar methodology to that previously reported for the synthesis of [K( Dip Nacnac)] [27]. That is, β-diketimine, 1, was combined with benzyl potassium in toluene, pro-ligand 2 was combined with potassium bis(trimethylsilyl)amide (KHMDS) in Inorganics 2021, 9, 72 4 of 9 toluene and both reactions were monitored by NMR spectroscopy. No reaction could be observed after 16 h at room temperature for either pro-ligand, contrary to the synthesis of [K( Dip Nacnac)], which showed good conversion under these conditions. This can be attributed to the increased steric bulk from the TCHP aryl group, perturbing deprotonation of 1 and 2 by the base. However, heating these solutions at 60 • C overnight led to clean conversion to the potassium salts [{K( TCHP Nacnac)} 2 ], 6, and [{K( TCHP/Dip Nacnac)} ∞ ], 7, as determined by 1 H NMR spectroscopy. It should be noted that compound 6 could also be formed using KHMDS as the base, though this route gave variable yields of product. Attempts to isolate compound 6 repeatedly led to a degree of decomposition of the complex to 1 upon manipulation of the reaction mixture. Moreover, in the presence of silicone grease, compound 6 decomposed to unknown products in solution over time. This suggests that compound 6 is highly reactive, and as such could only be characterized by NMR spectroscopy after formation in situ. As monitoring the formation of 6 from 1 by NMR spectroscopy showed complete conversion, we believe this compound could be used as a ligand transfer reagent after its formation in situ, and in the absence of silicone grease.
Crystals of 6 suitable for an X-ray diffraction study were formed by the addition of n-hexane to a concentrated toluene solution of 6 at room temperature. The molecular structure (Figure 3) shows the compound crystallizing as a dimer, with the potassium centers having trigonal planar geometries, being N,N-chelated by a β-diketiminate, as well as possessing an intermolecular C···K interaction with a meta-carbon of an opposing TCHP group of the second [K( TCHP Nacnac)] monomer unit. The N-K distances (2.633(3) and 2.619(3) Å) are in the expected range and correspond well to those previously reported for [K( Dip Nacnac)] (2.681 Å mean) [27]. The C···K distance of 3.309(3) Å is outside the sum of the covalent radii for the two elements (2.71 Å [28]), but well within the sum of their van der Waals radii (4.45 Å [29]).  Compound 7 crystallized as a colorless material from the toluene reaction solution, upon cooling it to room temperature from 60 °C. However, it should be noted that, like 6, compound 7 undergoes decomposition to the proteo-ligand 2 in the presence of silicone grease. Crystals of 7 suitable for an X-ray diffraction were obtained from a saturated benzene solution, and its molecular structure is shown in Figure 4. It is polymeric, similar to previously reported [K( Dip Nacnac)] [27]. The polymer is propagated via intermolecular C•••K interactions between the potassium centers and para-and meta-carbons of a Dip group on an adjacent monomer unit. The K•••C bond distances are 3.178(4) Å for K-Cpara  Compound 7 crystallized as a colorless material from the toluene reaction solution, upon cooling it to room temperature from 60 • C. However, it should be noted that, like 6, compound 7 undergoes decomposition to the proteo-ligand 2 in the presence of silicone grease. Crystals of 7 suitable for an X-ray diffraction were obtained from a saturated benzene solution, and its molecular structure is shown in Figure 4. It is polymeric, similar to previously reported [K( Dip Nacnac)] [27]. The polymer is propagated via intermolecular C···K interactions between the potassium centers and paraand meta-carbons of a Dip group on an adjacent monomer unit. The K···C bond distances are 3.178(4) Å for K-C para Inorganics 2021, 9, 72 5 of 9 and 3.254(3) Å for K-C meta . These separations are slightly shorter than the equivalent C···K interactions in [K( Dip Nacnac)], which are 3.214(3) Å for K-C para , and 3.351(3) Å and 3.276(3) Å for K-C meta interactions. The K-N distances in the compound are comparable to those in 6.
Compound 7 crystallized as a colorless material from the toluene reaction solution, upon cooling it to room temperature from 60 °C. However, it should be noted that, like 6, compound 7 undergoes decomposition to the proteo-ligand 2 in the presence of silicone grease. Crystals of 7 suitable for an X-ray diffraction were obtained from a saturated benzene solution, and its molecular structure is shown in Figure 4. It is polymeric, similar to previously reported [K( Dip Nacnac)] [27]. The polymer is propagated via intermolecular C•••K interactions between the potassium centers and para-and meta-carbons of a Dip group on an adjacent monomer unit. The K•••C bond distances are 3.178(4) Å for K-Cpara and 3.254(3) Å for K-Cmeta. These separations are slightly shorter than the equivalent C•••K interactions in [K( Dip Nacnac)], which are 3.214(3) Å for K-Cpara, and 3.351(3) Å and 3.276(3) Å for K-Cmeta interactions. The K-N distances in the compound are comparable to those in 6.

General Considerations
All manipulations were carried out using standard Schlenk and glove box techniques under an atmosphere of high purity dinitrogen. Toluene and hexane were distilled over molten potassium, while diethyl ether was distilled over 1:1 Na/K alloy. Benzene-d 6 was stored over a mirror of sodium and degassed three times via freeze-pump-thawing before use. The 1 H, 13 C{ 1 H} and 7 Li NMR spectra were recorded on a Bruker AvanceIII 400 spectrometer and were referenced to the residual resonances of the solvent used or external 1 M LiCl. Mass spectra were collected using an Agilent Technologies 5975D inert MSD with a solid-state probe. FTIR spectra were collected as Nujol mulls on an Agilent Cary 630 attenuated total reflectance (ATR) spectrometer. Microanalyses were carried out at the Science Centre, London Metropolitan University or using a PerkinElmer-2400 CHNS/O Series II System. Melting points were determined in sealed glass capillaries under dinitrogen and are uncorrected. The starting materials, TCHP NacnacH 1 [17], Dip NacacH [30] (TCHP)NH 2 [31] and benzyl potassium [32], were prepared by literature procedures. All other reagents were used as received.

Syntehsis of TCHP/Dip NacnacH 2 and Data for TCHP NacacH 3
Preparation of TCHP/Dip NacnacH, 2: 2,4,6-Tricyclohexylaniline (4.57 g, 13.5 mmol), Dip NacacH (3.50 g, 13.5 mmol) and p-toluenesulfonic acid (2.32 g, 13.5 mmol) were dissolved in toluene (100 mL), and the mixture was heated at reflux for 3 days in a Dean-Stark apparatus. Volatiles were then removed in vacuo, the resultant oil was dissolved in dichloromethane (100 mL), the extract washed with a saturated Na 2 CO 3 solution (100 mL, 1 M) and the organic layer was separated. The organic layer was dried over MgSO 4 and volatiles were removed in vacuo. Methanol (5 mL) was added to the oily residue, which was then sonicated and filtered to yield compound 2 as a crude white powder. This was dried and recrystallized from hot ethyl acetate to yield pure TCHP/Dip NacnacH (2.25 g). A further crop of the product could be obtained by concentration of the mother liquor, and letting it stand at room temperature (1.00 g). Total yield: 3.25 g, 42%. X-ray quality crystals could be grown from the slow evaporation of a diethyl ether solution of TCHP/Dip NacnacH. Preparation of [{K( TCHP Nacnac)} 2 ], 6: TCHP NacnacH (400 mg, 0.54 mmol) and benzyl potassium (72 mg, 0.55 mmol) were dissolved in toluene (10 mL) and the mixture stirred overnight at 60 • C. The solution was then cooled to room temperature and volatiles were removed in vacuo. The subsequent solid was dissolved in n-hexane (5 mL) and the extract left to stand at room temperature, yielding colorless crystals of 6 suitable for X-ray diffraction studies. Analysis of the crystalline material by NMR spectroscopy consistently showed contamination of the product with TCHP NacnacH, which could not be removed, despite repeated recrystallizations.
In situ preparation of 6: TCHP NacnacH (10 mg, 0.013 mmol) and benzyl potassium (3 mg, 0.023 mmol) were dissolved in C 6 D 6 (0.6 mL) in a J. Young's NMR tube equipped with a Teflon screw cap and the mixture heated overnight at 60 • C. The subsequent reaction solution was analyzed by NMR spectroscopy, showing near complete conversion to compound 6. 1 (8 mL) and the mixture heated overnight at 60 • C. The solution was cooled to room temperature, after which time micro-crystalline 7 was deposited. The suspension was filtered, and the solid was dried in vacuo giving compound 7 as a spectroscopically near pure colorless solid (500 mg, 91%). Crystals suitable for X-ray diffraction were grown by slow cooling a saturated solution of 7 in C 6 D 6 from 60 • C to room temperature. M.p. > 260 • C: 1 H NMR (400 MHz, C 6 D 6 , 298 K) N.B. integration for cyclohexyl groups are estimated due to complex overlapping signals: δ = 1.14-1.17 (d, 3 J HH = 6.9 Hz, 6H, CH(CH 3 ) 2 ), 1.24-1.30 (m, 2H, Cyc-CH 2 ), 1.32-1.34 (d, 3

Crystallographic Details
Crystals of 2-7 suitable for X-ray structural determination were mounted in silicone oil. Crystallographic measurements were carried out at 123(2) K, and were made using a Rigaku Synergy diffractometer using a graphite monochromator with Cu Kα radiation (λ = 1.54184 Å). The structures were solved by direct methods and refined on F 2 by full matrix least squares (SHELX16) [33] using all unique data. All non-hydrogen atoms were anisotropic with hydrogen atoms typically included in calculated positions (riding model). Crystal data, details of the data collection and refinement are given in Table S1 (in Supplementary Materials). Crystallographic data for the structures have been deposited with the Cambridge Crystallographic Data Centre (CCDC no. 2105059-2105064). Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; email: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).

Conclusions
In summary, the syntheses of four alkali metal complexes of two super bulky βdiketiminate ligand systems are described. These complexes were characterized by X-ray crystallography and multinuclear NMR spectroscopy. The solid-state structures of the lithium etherate complexes are similar to previously reported examples. The molecular structures of the potassium salts show varied coordination environments in potassium, depending on the accompanying ligand framework. That is, [{K( TCHP Nacnac)} 2 ] 6 is dimeric with C···K interactions involving a meta-aryl carbon of the opposing monomer unit. In contrast, [{K{( TCHP/Dip Nacnac)} ∞ ] 7 is polymeric, and shows comparable connectivity between monomer units to that seen in the previously reported complex [K( Dip Nacnac)]. These alkali metal compounds have significant potential for use as ligand transfer reagents in the synthesis of metal complexes incorporating large TCHP-substituted β-diketiminates. We are currently exploring the use of these ligands for the kinetic stabilization of lowoxidation state metal complexes.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/inorganics9090072/s1, Table S1: Summary of crystallographic data for 2-7; Figures S1-S14: 1 H, 13  Data Availability Statement: NMR spectra and crystal data are given in the supporting information. Crystal data, details of the data collection and refinement are given in Table S1. Crystallographic data for the structures have been deposited with the Cambridge Crystallographic Data Centre (CCDC no. 2105059-2105064). Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; email: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).