Magnesium, calcium and zinc [N₂N′] heteroscorpionate complexes

Abstract

A new family of sterically demanding N₂N′ heteroscorpionate pro-ligands (HC(^tBu₂pz)₂SiMe₂N(H)R (R = ⁱPr, ^tBu, Ph, Xyl)) has been prepared via a straightforward modular synthetic route. An extensive study into the synthesis and characterisation of lithium, magnesium, calcium and zinc complexes supported by both 3,5-^tBu and 3,5-Me substituted N₂N′ ligand families has been conducted. Attempted deprotonation of the pro-ligands with ⁿBuLi afforded the corresponding lithium salts Li{HC(^tBu₂pz)₂SiMe₂NR} (R = ⁱPr (1), ^tBu (2), Ph (3) and Xyl (4)) but air- and thermal-sensitivity limited the yields of these potentially useful precursors; only the sterically encumbered ligand system allowed clean reactivity. Magnesium methyl complexes Mg{HC(^tBu₂pz)₂SiMe₂NR}Me (R = ⁱPr (5) and R = Ph (6)) were prepared using an excess of the Grignard reagent MeMgCl. Magnesium butyl complexes were synthesised in good yields using the dialkyl precursor MgⁿBu₂ to afford Mg{HC(R′₂pz)₂SiMe₂NR}ⁿBu (R′ = Me; R = ⁱPr (7), ^tBu (8), Ad (9), Ph (10). R′ = ^tBu; R = ⁱPr (11), Ph (12)). Protonolylsis reactions were used to synthesise magnesium and calcium amide complexes Mg{HC(R′₂pz)₂SiMe₂NR}{N(SiHMe₂)₂} (R′ = Me; R = ⁱPr (13), ^tBu (14), Ph (15). R′ = ^tBu; R = Ph (16)) or Mg{HC(R′₂pz)₂SiMe₂NR}{N(SiMe₃)₂} (R′ = Me; R = ⁱPr (17), ^tBu (18), Ph (19). R′ = ^tBu; R = Ph (20)), and Ca{HC(R′₂pz)₂SiMe₂NR}{N(SiMe₂)₂} (L) (R′ = Me; L = thf; R = ⁱPr (21), ^tBu (22), Ph (23). R′ = ^tBu; L = none; R = Ph (24). Zinc methyl complexes Zn{HC(R′₂pz)₂SiMe₂NR}Me (R′ = Me; R = ⁱPr (25), ^tBu (26), Ph (27). R′ = ^tBu; R = Ph (28)) were prepared by reaction of the N₂N′ heteroscorpionate pro-ligands with ZnMe₂. In preliminary studies, magnesium amide complexes 16 and 20 were evaluated as initiators for the ring-opening polymerisation (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA). Although the overall polymerisation control was poor, 16 and 20 were found to be active initiators.

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Introduction

Heteroscorpionates, E(R_npz)₂(X),¹ are a versatile class of tridentate poly(pyrazolyl) ligand where one of the pyrazolyl groups of a classical homoscorpionate, E(R_npz)₃,² is replaced by a different C, N, O or S donor. Chiral classes of heteroscorpionate ligands have also been developed.³ They have demonstrated a wide range of uses in homogeneous catalysis⁴ and supramolecular^3c,5 and bioinorganic chemistry,⁶ as well as a supporting ligand set for studying fundamental stoichiometric transformations.⁷

Substitution of the 3-position of the pyrazolyl ring has been demonstrated to clearly influence complex nuclearity; incorporation of sterically demanding tert-butyl substituents is an efficient way to favour mononuclear systems (Chart 1).⁸ This is particularly important in the case of group 2 and related metal complexes which are prone to redistribute via the Schlenk equilibrium and makes the synthesis of well-defined heteroleptic compounds L_nMX (L and X = a monoanionic ligand or ligand set) challenging.

Chart 1 Scorpionate complexes with 3-^tBu substituents.

Scorpionate ligands have found particular use in the ring-opening polymerisation (ROP) of cyclic esters; Chisholm et al. first reported bulky tris(pyrazolyl)hydroborate metal alkoxide complexes M{HB(^tBu₂pz)₃}OR (M = Mg, Zn, Ca) as active polymerisation initiators where stereoselectivity was ascribed to chain-end control.⁹ Otero and co-workers have extensively developed heteroscorpionate complexes based on Mg^8a,d,10 and Zn,^10b,11 (κ³ coordination), demonstrating moderate to good levels of heterotacticity (P_s = 0.78–0.85) or isotacticity (P_i = 0.77), and Al^11c,12 (κ² coordination). Cui and co-workers have also reported zwitterionic, achiral N₂N′ heteroscorpionate complexes to obtain highly isotactic-biased polylactide (P_i up to 0.85).¹³

Use of alkaline earth metals is particularly desirable in homogeneous catalysis as they are earth-abundant and possess low toxicity.¹⁴ Furthermore, Hill and co-workers have recently demonstrated that the organometallic calcium complexes [(BDI)CaR]₂ are capable of nucleophilic alkylation of benzene, opening new possibilities for catalytic transformations with suitable ligand design.¹⁵

Our group has previously reported on the structures and reactivity of group 2 and group 12 scorpionate complexes.¹⁶ Of particular relevance to our current contribution is our previous communication on the preparation of the N₂N′ heteroscorpionate rare earth complex Sc{HC(Me₂pz)₂SiMe₂NⁱPr}(CH₂SiMe₃)₂ and its ability to polymerise ethylene.¹⁷ Now we have developed a N₂N′ heteroscorpionate ligand family with sterically demanding tert-butyl substituents in the 3-position to ensure that that only ligation of a single scorpionate ligand occurs, leaving one reactive M–X bond for further controlled reactivity. Herein, we report a study into the synthesis and structure of group 1, 2 and 12 complexes supported by this ligand type.

Results and discussion

Synthesis of a new family of sterically demanding N₂N′ heteroscorpionate pro-ligands

We have previously reported the modular synthesis of the N₂N′ heteroscorpionate pro-ligand HC(Me₂pz)₂SiMe₂N(H)ⁱPr based on the 3,5-Me pyrazolyl substitution motif.¹⁷ This family has now been extended to include HC(Me₂pz)₂SiMe₂N(H)R (R = ^tBu, Ad, Ph). These new pro-ligands were prepared by the same methodology of nucleophilic attack of the desired amine at electropositive silicon in HC(Me₂pz)₂SiMe₂Cl in 85–96% yield. For R = ^tBu and Ad, an excess of amine was used in order to neutralise the HCl produced by generating the ammonium salt, [RNH₃]Cl. For R = Ph, NEt₃ is added to act as a superior Brønsted base and quench the HCl produced; the use of an excess of NH₂Ph is also unfavourable due to its low volatility (b.p. 184 °C at 1 atm).

The 3,5-^tBu and parent 3,5-H systems were initially targeted in order to investigate how modifying the stereoelectronic properties of the pyrazolyl rings would affect the subsequent properties of metal–ligand complexes. The ligand precursor HC(^tBu₂pz)₂SiMe₂Cl was prepared by lithiation of the appropriate bis(pyrazolyl)methane followed by in situ addition of Li{HC(^tBu₂pz)₂} to an excess of SiMe₂Cl₂ (Scheme 1). The specific order of addition and non-stoichiometric conditions are used to prevent the double displacement of Cl⁻. The solid state structure was determined by a single crystal X-ray diffraction study and is depicted in Fig. S10 of the ESI.‡

Scheme 1 Synthesis of a new N₂N′ heteroscorpionate pro-ligand family. See main text for further details of the second step.

Attempts to prepare HC(pz)₂SiMe₂Cl using the same methodology resulted in an intractable mixture of products; deprotonation at the apical sp³ C is stabilised by the inductive effect of the two adjacent α-N atoms whereas the pyrazolyl ring C(5) is sp²-hybridised and is inductively stabilised by a single α-N atom. Use of a base such as ⁿBuLi facilitates both pathways; coordination to N(1) favours C(5) deprotonation and coordination to N(2) favours apical carbon deprotonation. The mixture of lithiated species is trapped out by the hard SiMe₂Cl₂ electrophile to afford the mixture of substituted products where ring substitution is the major product.¹⁸

New pro-ligands HC(^tBu₂pz)₂SiMe₂N(H)R (R = ⁱPr, ^tBu, Ph, Xyl (Xyl = 2,6-C₆H₃Me₂)) were then prepared using procedures related to the 3,5-Me system in excellent yields of 94–99%. The influence of the sterically demanding 3,5-^tBu groups becomes apparent with all but one reaction (that for R = ⁱPr) taking place at a higher temperature and over a longer time period than for the analogous 3,5-Me substituted homologues. For R = Xyl, lithiation afforded LiN(H)Xyl which is a potent nucleophile and reacts with 1 equivalent of HC(^tBu₂pz)₂SiMe₂Cl in a salt elimination reaction.

The ¹H and ¹³C{¹H} NMR spectra of HC(^tBu₂pz)₂SiMe₂N(H)R (R = ⁱPr, ^tBu, Ph, Xyl) all displayed the same type of diagnostic resonances. For example, where R = ⁱPr, singlets at 6.99 and 6.01 ppm correspond to the apical H̲C(^tBu₂pz)₂ and pyrazolyl N₂C₃H̲^tBu₂ protons, respectively, and are in a 1 : 2 ratio, a consequence of C_s molecular symmetry. Singlets at 1.40 and 1.15 ppm are attributed to the tert-butyl groups in the 3- and 5-positions. This assignment is analogous to the 3,5-Me system and is supported by two dimensional ¹³C–¹H correlation experiments. The SiMe singlet resonance at 0.37 ppm appears at a typically low frequency, as expected for methyl protons shielded by an electropositive Si atom. The molecular structure of HC(^tBu₂pz)₂SiMe₂N(H)Ph was also determined by a single crystal X-ray crystallographic study (see Fig. S11, Table S1 of the ESI‡ for further details). This structure shows an intramolecular interaction between the N(Ph)–H and pyrazolyl N atom, and contrasts that for HC(^tBu₂pz)₂SiMe₂Cl where the Si atom is in a pseudo trigonal bipyramidal environment, apparently as a result of electron donation from the lone pair of the pyrazolyl N(1) into the σ* orbital of the Si–Cl bond.

Synthesis and characterisation of N₂N′ heteroscorpionate lithium complexes

The preparation of alkali metal complexes of the N₂N′ heteroscorpionate ligands was initially targeted in order to offer the potential of reactions via salt metathesis with appropriate metal salts as well as protonolysis routes. Several heteroscorpionate lithium salts from the NNO,¹⁹ NNS,^8b,19a,20 NNN′^11a,21 and NNCp²² heteroscorpionate classes have previously been reported; their syntheses all involve deprotonation at the methylene bridge of the appropriate bis(pyrazolyl)methane followed by nucleophilic attack upon a heterocumulene, α,β unsaturated system, aldehyde or imine to afford the desired lithium salt.

Treatment of the N₂N′ pro-ligands HC(^tBu₂pz)₂SiMe₂N(H)R with an excess of ⁿBuLi at −78 °C afforded complexes formulated as [Li{HC(^tBu₂pz)₂SiMe₂NR}]_n (R = ⁱPr (1), ^tBu (2), Ph (3) and Xyl (4)) in poor to good isolated yields (5–70%) as colourless solids (eqn (1)). When thf is used as a reaction solvent, removal of the volatiles under reduced pressure served to leave no coordinated thf. These complexes are all air- and thermally-sensitive, showing degradation in both solution after a few hours and the solid state after 2 weeks when kept at −30 °C. This sensitivity was also noted in the reported thf-stabilised compound, Li{HC(^tBu₂pz)₂CH₂NXyl}(thf), which, on standing in the solid state, showed decomposition to the pro-ligand, HC(^tBu₂pz)₂CH₂N(H)Xyl.²³ NNE heteroscorpionate lithium salts have most commonly been isolated as donor solvent-stabilised adducts. Examples of dimeric and oligomeric complexes have also been reported but are much rarer; Bassanetti et al. described the preparation and characterisation of [Li{HC(^tBu₂pz)₂CS₂}]₃.^20b Otero and co-workers reported dimeric (thio)acetamidinate NNN′ complexes [Li{HC(R′₂pz)₂CNR′′E}]₂ (R′ = Me, Ph, ^tBu, R′′ = achiral or chiral moiety, E = O or S) complexes which provided a versatile route into the synthesis of enantiopure NNN′ ligands.

The ¹H NMR spectra of complexes 1–4 all show resonances clearly shifted from those of the pro-ligand values, and the singlet attributed to the NH̲ proton is no longer present. In addition, ⁷Li NMR spectra were recorded at 23 °C and −80 °C for the most thermally robust complex, [Li{HC(^tBu₂pz)₂SiMe₂NXyl}]_n (4), to investigate the possibility that the lithium salts existed as stabilised aggregates. However, at both high and low temperature a lone singlet was observed at 2.1 ppm. EI-MS data indicated a molecular ion peak at m/z = 555. The IR data were also diagnostic with the absorption attributed to ν(N–H) stretch in the pro-ligand being absent. All complexes were also characterised by elemental analysis. The steric demands of the 3,5-^tBu ligand can stabilise complexes 1–4 with respect to sandwich complexes but it is most likely that these complexes exist as oligonuclear species. Analogous reactions with the corresponding 3,5-Me systems did not afford tractable, well-defined products.

Synthesis and characterisation of magnesium, calcium and zinc alkyl and amide complexes supported by N₂N′ ligands

Treatment of HC(^tBu₂pz)₂SiMe₂N(H)R (R = ⁱPr, Ph) with a two-fold excess of MeMgCl afforded Mg{HC(^tBu₂pz)₂SiMe₂NR}Me (R = ⁱPr (5) and R = Ph (6)) in low isolated yields (24 and 7%, respectively) which is partially attributed to the high solubility of these complexes in aromatic and aliphatic solvents. EI-MS data of the crude reaction mixtures were also consistent with the presence of Mg{HC(^tBu₂pz)₂SiMe₂NR}Cl (e.g. for R = Ph, a molecular ion peak at m/z = 579 was observed). We have previously reported the reaction of HC(Me₂pz)₂(3,5-^tBu₂C₆H₃OH) with ⁿBuMgCl which afforded Mg{HC(Me₂pz)₂(3,5-^tBu₂C₆H₃O)}₂ alongside insoluble precipitates of MgCl₂ and [Mg{HC(Me₂pz)₂(3,5-^tBu₂C₆H₃O)}Cl]_n.^16b Formation of similar homoleptic sandwich complexes has been reported;²⁴ in our system, the bulky 3-^tBu substituents act as ‘tetrahedral enforcers’ preventing sandwich formation.²⁵ Parkin and co-workers reported a detailed study of the competitive alkyl and halide bond metathesis of a variety of Grignard reagents (RMgX) with tris(pyrazolyl)hydroborate (tpb) complexes M{HB(3-^tBupz)₃} (M = Tl, K) to afford Mg{HB(3-^tBupz)₃}R or Mg{HB(3-^tBupz)₃}X.²⁶ The preference for alkyl or halide complexes was found to be dependent on the nature of Grignard reagent, steric profile of the tpb ligand, the Schlenk equilibrium and the molar ratio of M{HB(3-^tBupz)₃} to RMgX. Combination of Grignard reagents with alkali metal salts of ligands can be used to prepare metal alkyl complexes successfully.^8d,10a,27

MgR₂ are therefore more appropriate starting materials for the preparation of the alkyl complexes Mg{HC(^tBu₂pz)₂SiMe₂NR′}R. The reaction of MgⁿBu₂ with HC(R′₂pz)₂SiMe₂N(H)R (R′ = Me, ^tBu) at 23 °C afforded the corresponding magnesium alkyl complexes Mg{HC(R′₂pz)₂SiMe₂NR}ⁿBu (R′ = Me; R = ⁱPr (7), ^tBu (8), Ad (9), Ph (10). R′ = ^tBu; R = ⁱPr (11), Ph (12)) in moderate yields (41–66%) (Scheme 2). Consistent with previous observations, the small scale reaction of HC(^tBu₂pz)₂SiMe₂N(H)ⁱPr with ⁿBuMgCl afforded complex 11 alongside MgCl₂.

Scheme 2 Synthesis of N₂N′ supported magnesium alkyl complexes.

In the ¹H NMR spectra of complexes 7–12, the same diagnostic resonances were observed and demonstrated clearly shifted ligand resonances with respect to the pro-ligands. Four multiplets define the ⁿBu protons at approximately 2.1, 1.8, 1.3 and 0.4 ppm in all complexes. Two dimensional COSY experiments show that these multiplets represent the 2-ⁿBu, 3-ⁿBu, 4-ⁿBu and 1-ⁿBu protons respectively.

The molecular structures of complexes 5 and 10 were elucidated by single crystal X-ray diffraction studies and are depicted in Fig. 1 with selected metric parameters in Table 1. In both complexes, the magnesium is in a distorted tetrahedral environment with the N₂N′ heteroscorpionate ligand coordinated facially in a κ³ binding mode. The increased steric demands of 3,5-^tBu substituents in complex 5 with respect to 3,5-Me groups in complex 10 are observed through longer M–N_pz bond lengths (5: 2.1799(16) and 2.1911(16) Å. 10: 2.1699(19) and 2.138(2) Å) and a more acute N_amide–Mg–C bond angle (5: 122.09(8)° and 10: 138.18(9)°). The M–N_pz and M–C bond lengths are comparable to previously reported four-coordinate tris(pyrazolyl)hydroborate (tpb) magnesium methyl complexes,²⁸ the NNN′ heteroscorpionate complex Mg{HC(^tBu₂pz)₂CNEtN^tBu}Me reported by Otero and co-workers,^8d and the NNO magnesium butyl complex Mg{HC(Me₂pz)₂(3,5-^tBu₂C₆H₃O)}ⁿBu reported by our group.^16b

Fig. 1 Thermal displacement ellipsoid drawings (20% probability ellipsoids) of Mg{HC(^tBu₂pz)₂SiMe₂NⁱPr}Me (5, top) and Mg{HC(Me₂pz)₂SiMe₂NPh}ⁿBu (10, bottom). All hydrogen atoms have been omitted for clarity.

Table 1 Selected bond lengths (Å) and angles (°) for complexes 5 and 10

	5	10
Mg(1)–N(1)	2.1799(16)	2.1699(19)
Mg(1)–N(3)	2.1911(16)	2.138(2)
Mg(1)–N(5)	2.0083(17)	2.0436(19)
Mg(1)–C(20)/(29)	2.145(2)	2.144(2)
N(1)–Mg(1)–C(20)/(29)	123.01(7)	112.98(9)
N(3)–Mg(1)–C(20)/(29)	123.88(7)	114.15(9)
N(5)–Mg(1)–C(20)/(29)	122.09(8)	138.18(9)

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Straightforward protonolysis reactions with 1 equivalent of the N₂N′ heteroscorpionate ligands with 1 equivalent of Mg{N(SiHMe₂)₂}₂ or Mg{N(SiMe₃)₂}₂ yielded Mg{HC(R′₂pz)₂SiMe₂NR}{N(SiHMe₂)₂} (R′ = Me; R = ⁱPr (13), ^tBu (14), Ph (15). R′ = ^tBu; R = ^tBu (16)) or Mg{HC(R′₂pz)₂SiMe₂NR}{N(SiMe₃)₂} (R′ = Me; R = ⁱPr (17), ^tBu (18), Ph (19). R′ = ^tBu; R = Ph (20)), respectively, in moderate to good isolated yields (49–77%) (Scheme 3). Homoleptic sandwich complexes of the 3,5-Me substituted ligand can be isolated when a 2 : 1 ligand : metal stoichiometry is used, and the molecular structure of Mg{HC(Me₂pz)₂SiMe₂NPh}₂ has been determined by a single crystal X-ray diffraction study (see ESI Fig. S12 and Table S3‡). Using the more sterically demanding 3,5-^tBu substituted ligand, the reactions proceeded cleanly at 60 °C and there was no evidence of sandwich complex formation; indeed, complexes 16 and 20 were the only products of 2 : 1 NMR scale reactions under identical conditions.

Scheme 3 Synthesis of N₂N′ supported magnesium amide complexes.

The NMR spectra of complexes 13–20 are consistent with C_s molecular symmetry on the NMR timescale, assuming fast rotation about the Mg–N(SiHMe₂)₂ and Mg–N(SiMe₃)₂ bonds. Complexes 16, 17, 19 and 20 have been structurally authenticated in the solid state by single crystal X-ray diffraction studies and are the first examples of N₂N′ heteroscorpionate magnesium amide molecular structures. The molecular structures of complex 16 and 20 are depicted in Fig. 2 with selected metric parameters in Table 2 (see ESI Fig. S13 and S14 and Table S2‡ for complexes 17 and 19). The approximately tetrahedral magnesium centres are κ³ bound to the N₂N′ heteroscorpionate ligand. The average Mg–N_pz bond lengths (16: 2.192 Å. 20: 2.230 Å) are longer than those in Mg{C(Me₂pz)₃}{N(SiMe₃)₂} (2.115 Å) consistent with the changing steric profiles of the three compounds; the Mg–N(SiRMe₂)₂ distances are within the expected ranges. The largest N–Mg–N(SiRMe₂)₂ angle and longest Mg–N_pz bond in both 16 and 20 are associated with N(5) due to steric repulsion between the methyl groups attached to Si(2) and the phenyl group attached to N(5). The Si(3)Me₃ group has less steric impact as it is able to lodge into the cleft between the pyrazolyl rings. The smaller average N_pz–Mg–N(SiRMe₂)₂ angle in 16 (114.2°) compared to that in 20 (120.4°) is consistent with the presence of the less bulky SiHMe₂ group with respect to SiMe₃ group.

Fig. 2 Thermal displacement ellipsoid drawings (20% probability ellipsoids) of Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiHMe₂)₂} (16, top) and Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiMe₃)₂} (20, bottom). C-Bound hydrogen atoms have been omitted for clarity.

Table 2 Selected bond lengths (Å) and angles (°) for complexes 16 and 20

	16	20
Mg(1)–N(1)	2.167(4)	2.254(2)
Mg(1)–N(3)	2.217(4)	2.206(2)
Mg(1)–N(5)	2.027(4)	2.023(2)
Mg(1)–N(6)	2.002(4)	2.023(2)
N(1)–Mg(1)–N(6)	109.39(15)	123.88(8)
N(3)–Mg(1)–N(6)	119.10(15)	116.93(8)
N(5)–Mg(1)–N(6)	138.33(16)	129.76(8)

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Treatment of HC(R′₂pz)₂SiMe₂N(H)R (R′ = Me, ^tBu) with 1 equivalent of Ca{N(SiMe₃)₂}₂(thf)₂ afforded Ca{HC(Me₂pz)₂SiMe₂NR}{N(SiMe₂)₂}(thf) (R = ⁱPr (21), ^tBu (22), Ph (23)) or, in the case of the more sterically demanding ligand system, thf-free Ca{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiMe₂)₂} (24) in good yields (60–74%), alongside 1 equivalent of HN(SiMe₃)₂ (eqn (2)). As previously observed, the reaction of the 3,5-Me pro-ligand proceeded at 23 °C whilst the bulkier 3,5-^tBu pro-ligand required elevated temperatures of 60 °C to proceed.

Due to the similar ionic radii of Mg^II and Zn^II (Mg^{II,4 C.N.} = 0.570 Å, Zn^{II,4 C.N.} = 0.600 Å), zinc alkyl complexes were targeted as a comparison to magnesium alkyl complexes 5–11. Protonolysis of the N₂N′ heteroscorpionate pro-ligands with ZnMe₂ afforded Zn{HC(R′₂pz)₂SiMe₂NR}Me (R′ = Me; R = ⁱPr (25), ^tBu (26), Ph (27). R′ = ^tBu; R = Ph (28)) in good yields (70–85%) (eqn (3)). The preparation of complex 28 required an elevated temperature of 60 °C, extended reaction time of 72 h and an excess of ZnMe₂ in order to drive the reaction cleanly to completion.

The molecular structures of complexes 25, 27 and 28 were elucidated by single crystal X-ray diffraction studies and are depicted in Fig. 3 with selected metric parameters in Table 3 (see ESI Fig. S15, Table S3‡ for data regarding complex 25). The zinc centre in complexes 27 and 28 is in a distorted tetrahedral coordination environment. The more acute N(5)–Zn(1)–C_CH3 bond angle (27: 135.78(11)° and 28: 125.94(14)°), longer Zn(1)–N_pz (27: 2.144(2) and 2.155(2) Å and 28: 2.171(4) and 2.202(4) Å) and Zn(1)–C_CH3 bond distances (27: 1.981(3) and 28: 2.041(3) Å) of complex 28 with respect to 27 clearly reflect the differing steric demands of the 3,5-Me and 3,5-^tBu pyrazolyl substituents. This trend is also observed in other examples of structurally characterised NNN′, NNO,^8e,16b,29 NNCp^4f heteroscorpionate and tris(pyrazolyl)hydroborate^28b,30 zinc methyl complexes. Otero and co-workers reported the only other example of a structurally characterised NNN′ zinc methyl complex, Zn{HC(^tBu₂pz)₂CN₂ⁱPr₂}Me.^11a Here some asymmetry in the Zn–N_pz bond lengths is observed (2.236(3) and 2.118(3) Å). The M–N_pz and M–C_CH3 bond lengths in 28 (2.171(4), 2.202(4) and 1.976(3) Å, respectively) are shorter than those in Mg{HC(^tBu₂pz)₂SiMe₂NⁱPr}Me (1) (2.1799(16), 2.1911(16) and 2.145(2) Å, respectively) in accord with zinc being more polarising and covalent.^16b

Fig. 3 Thermal displacement ellipsoid drawings (20% probability ellipsoids) of (top) Zn{HC(Me₂pz)₂SiMe₂NPh}Me (27, top) and Zn{HC(^tBu₂pz)₂SiMe₂NPh}Me (28, bottom). All hydrogen atoms have been omitted for clarity.

Table 3 Selected bond lengths (Å) and angles (°) for complexes 27 and 28

	27	28
Zn(1)–N(1)	2.144(2)	2.171(4)
Zn(1)–N(3)	2.155(2)	2.202(4)
Zn(1)–N(5)	1.979(2)	1.976(3)
Zn(1)–C(20)	1.981(3)	—
Zn(1)–C(32)		2.041(3)
N(1)–Zn(1)–C(20)	114.48(11)	—
N(1)–Zn(1)–C(32)	—	122.59(14)
N(3)–Zn(1)–C(20)	116.86(11)	—
N(3)–Zn(1)–C(32)	—	120.12(14)
N(5)–Zn(1)–C(20)	135.78(11)	—
N(5)–Zn(1)–C(32)	—	125.94(14)

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Ring-opening polymerisation of ε-caprolactone and rac-lactide

One of the aims of this work was to assess the performance of the NNN′ heteroscorpionate amide complexes for the catalytic ring-opening polymerisation (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA) monomers, in light of the success of related complexes and our previous studies into the ROP of cyclic esters.^16b,31 An initial study was undertaken using ε-CL owing to the relative ease of its polymerisation with respect to LA due to the favourable release of 7-membered ring strain.

Initial studies at room temperature in thf or toluene showed the amide complexes 16 and 20 to be highly active for the ROP of ε-CL, reaching full conversion of 100 equivalents of ε-CL in 20–30 minutes in either solvent. The results are summarised in Table 4. Both complexes afforded much higher than expected M_n values by gel permeation chromatography (GPC) (66 620–89 740 g mol⁻¹), consistent with a much faster rate of propagation than rate of initiation in a coordination-insertion mechanism.³² The observed M_n values are in slightly better agreement with calculated M_n for polymerisations performed in thf. This is likely to be due to the effect of thf competing with the monomer for coordination to the metal centre, thus reducing the relative rate of propagation (compared to that in non-coordinating toluene solvent).

Table 4 Solution polymerisation of ε-CL by Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiHMe₂)₂} (16) and Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiMe₃)₂} (20)^a

	Solvent	Yield^b (%)	Time^c	M n ^d^,^e	M w/Mn
a [ε-CL] : [catalyst] = 100 : 1, 3.4 mL solvent at 23 °C. b Isolated yield at 100% conversion. c Minutes. d Molecular weights (g mol^−1) determined from GPC using the appropriate Mark–Houwink corrections. e Expected Mn (g mol^−1) for 1 chain growing per metal centre at 100% conversion, ignoring end groups = 11 414.
16	thf	76	25	71 450	1.86
16	Toluene	79	25	89 740	3.17
20	thf	81	30	66 620	1.67
20	Toluene	73	20	84 840	2.79

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The PDIs (polydispersity indices, M_w/M_n) for polymerisations performed in thf are also narrower than corresponding polymerisations performed in toluene (although these are collectively all in the moderate to poor range, 1.67–3.17). This is also likely to be a direct consequence of the coordinative nature of thf compared to toluene. Broad PDIs are well-documented to be the result of intra- and inter-molecular transesterification reactions.³² It is reasonable to assume that the presence of a thf molecule bound to a metal coordination site would reduce the likelihood of ‘backbiting’ by a growing polymer chain at the metal centre. Due to the viscous nature of the polymer formed and the short times involved, it was not possible to acquire full kinetic data over the course of each polymerisation, nor was it possible to collect MALDI-ToF analysis of polymer samples due to the high observed M_n values. Coordination-insertion has been deduced to be the most likely mechanism of polymerisation with these complexes.

There are a relatively small number of metal complexes supported by heteroscorpionate ligands used as initiators for polymerisation of ε-CL. Examples include zinc amides and alkyls,^11a,b,16b,33 rare earth dialkyls,³⁴ aluminium alkyls and alkoxides,³⁵ and magnesium alkyls and amides.^8d,16b,24a We have previously reported the N₂O heteroscorpionate complex Mg{HC(Me₂pz)₂(3,5-^tBu₂C₆H₃O)}{N(SiHMe₂)₂} which afforded complete conversion of 100 equivalents of ε-CL in toluene (but not thf) with high M_n (30 870 g mol⁻¹) and broad PDI (2.76). The related N₂N′ magnesium alkyl complexes described by Otero and co-workers, Mg{HC(Me₂pz)₂CN₂R₂}R′, afforded rapid polymerisation (e.g. R = Et, R′ = CH₂SiMe₃, 97% conversion of 500 equivalents in 1 minute at 20 °C and R = ⁱPr, R′ = CH₂SiMe₃, 62% at −60 °C) with narrow PDIs (1.41 and 1.12, respectively). The steric demands of the amidinate group were found to be important, with increased bulk resulting in slower initiators.

Polymerisation studies were next extended to the more challenging monomer, rac-LA. Due to the more encouraging results with ε-CL observed using thf as a solvent, it was decided to focus solely on thf-mediated polymerisations. The results are summarised in Table 5.

Table 5 Solution polymerisation of rac-LA by Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiHMe₂)₂} (16) and Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiMe₃)₂} (20)^a

	Conv.^b (%)	Time (h)	M n ^c	M n (calcd)^d	M w/Mn	k obs (min^−1)
a Conditions: [rac-LA] : [catalyst] = 100 : 1, 6.0 mL thf at 70 °C. b % conversion by ^1H NMR spectroscopy. c Molecular weights (g mol^−1) determined from GPC using the appropriate Mark–Houwink corrections.^36 d Expected Mn (g mol^−1) for 1 chain growing per metal centre at the % conversion by ^1H NMR spectroscopy.
16	90	1.5	55 080	12 972	1.48	0.03(2)
20	89	0.75	51 480	12 799	1.45	0.04(3)
Ref 8a	96	1.0	14 000	13 800	1.09	—
Ref 8d	91	0.03	12 800	13 100	1.09	—

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One of the several interesting contrasts with the ε-CL polymerisation results is that the rac-LA polymerisations all required elevated temperatures of 70 °C and extended reaction times, largely due to the unfavourable release of the stable 6-membered ring strain in rac-LA when compared to the less thermodynamically stable 7-membered ring strain in ε-CL. The observed M_n values (51 080–51 480 g mol⁻¹) are again large, and the relatively broad PDIs (1.45–1.48) are also evident in the rac-LA polymerisations, no doubt due in part to the increased tendency of transesterification side reactions at elevated temperatures. Disappointingly, no stereochemical enrichment in the PLA polymer produced was observed (P_r ≈ 0.5).

Despite the relatively poor polymerisation control demonstrated by the complexes tested, kinetic studies were carried out in some cases to investigate further the polymerisation process. All of the catalysts tested showed first order consumption of rac-LA after an initial induction period. Similar induction periods have also been observed in the ROP of cyclic esters using aluminium,³⁷ yttrium,³⁸ iron,³⁹ tin alkoxides and amides. In these instances the induction period was thought to correspond to a period of ligand rearrangement around the metal centre, in order to allow the monomer access to the coordination sphere.⁴⁰ An example semi-logarithmic plot is shown in Fig. 4 for the catalyst Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiHMe₂)₂} (16). The apparent rate constant for the polymerisation, k_obs, was 0.03(2) min⁻¹. This compares with a k_obs value of 0.04(3) min⁻¹ for the related catalyst Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiMe₃)₂} (20).

Fig. 4 First order semi-logarithmic plot for rac-LA consumption using Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiHMe₂)₂} (16). Conditions [rac-LA₀] : [16] = 100 : 1, 6 mL thf, 70 °C, 0.1 mL aliquots taken at the given intervals.

Though there are no other specific examples of scorpionate magnesium amide complexes for lactide polymerisation, complexes 16 and 20 compare well with initiators with respect to the pioneering homoscorpionate tris(pyrazolyl)hydroborate (tpb) magnesium and zinc alkoxides for lactide polymerisation reported by Chisholm.^9c Kinetic studies under pseudo first order conditions resulted in a value of k_obs = 0.039 min⁻¹ for Mg{HC(3-^tBupz)₃}OEt. When extended to calcium, initiators were found to be two orders of magnitude faster.^9b Complexes 16 and 20 prove to be slow initiators with respect to some related heteroscorpionate systems.^10a,29a,41 The N₂N′ heteroscorpionate magnesium alkyl complex, Mg{HC(^tBu₂pz)₂CNEtNⁿBu}CH₂SiMe₃, reported by Otero and co-workers was shown to polymerise 200 equivalents of rac-LA at 20 °C, reaching with 83% conversion in 2 minutes, to afford heterotactically-enriched PLAs (P_r = 0.70–0.79) with narrow PDIs (1.01–1.05) and no induction period (k_app = 2.0 min⁻¹).^8d These values compare well with the most active magnesium catalysts reported in the literature.⁴²

Conclusions

A new family of sterically demanding N₂N′ heteroscorpionate pro-ligands (HC(^tBu₂pz)₂SiMe₂N(H)R (R = ⁱPr, ^tBu, Ph, Xyl)) has been developed and the synthesis and characterisation of lithium, magnesium, calcium and zinc complexes, M{HC(R′₂pz)₂SiMe₂NR}(X) (M = Mg, Zn, Ca; R′ = Me or ^tBu, X = amide or alkyl), supported by both 3,5-^tBu and 3,5-Me substituted N₂N′ ligand families has been conducted.

The 3,5-^tBu substituents enabled isolation and characterisation of highly sensitive oligomeric lithium salts, [Li{HC(^tBu₂pz)₂SiMe₂NR}]_n, which could not be identified with the 3,5-Me ligand system. Furthermore, they have also successfully ensured that ML₂ bis(ligand) complexes do not form even when the M : L reaction stoichiometry is 2 : 1; retaining structural integrity and avoiding formation of such sandwich complexes is particularly desirable when considering further potential applications in homogeneous catalysis. Preliminary investigations into the polymerisation of ε-caprolactone and rac-lactide indicated poor control of polymerisation but nonetheless active catalysts.

In general, the N₂N′ ligand set seems well suited to stabilising a variety of reactive group 2 and 12 complexes with alkyl and amide substituents and provides a platform for further reactivity or catalytic studies.

Experimental details

General considerations

All manipulations were carried out under an inert atmosphere of argon or dinitrogen using standard Schlenk-line or drybox procedures. Solvents were pre-dried over activated 4 Å molecular sieves and refluxed over sodium (toluene), sodium/potassium (pentane, diethyl ether), or calcium hydride (dichloromethane) under a dinitrogen atmosphere and collected by distillation. Alternatively, solvents were degassed by sparging with dinitrogen and dried by passing through a column of activated alumina. Deuterated solvents were dried over potassium (C₆D₆) or P₂O₅ (CD₂Cl₂), distilled under reduced pressure, and stored under dinitrogen in J. Young Teflon valve ampoules. Solution NMR samples were prepared under a dinitrogen atmosphere in a drybox, in 5 mm Wilmad NMR tubes possessing Young's Teflon valves. ¹H and ¹³C NMR spectra were recorded on a Varian Mercury 300 spectrometer or a Bruker AVII 500 spectrometer. ¹H and ¹³C{¹H} NMR spectra were referenced internally to residual protio-solvent (¹H) or solvent (¹³C) resonances, and are reported relative to tetramethylsilane (δ = 0 ppm). ⁷Li NMR spectra are referenced relative to LiCl. Chemical shifts are quoted in δ (ppm) and coupling constants in Hz. Where necessary, ¹H and ¹³C assignments were assisted by the use of two-dimensional ¹H–¹H and ¹H–¹³C correlation experiments. IR spectra were recorded on a PerkinElmer 1710 or Nicolet magna 560 FTIR spectrometer. Samples were prepared in a drybox as Nujol mulls between NaCl plates. IR data are quoted as wavenumbers (cm⁻¹) within the range 4000–400 cm⁻¹. Mass spectra were recorded by the mass spectrometry service of the University of Oxford Inorganic Chemistry Laboratory. Elemental analyses were carried out by the Elemental Analysis Service at the London Metropolitan University. The compounds H₂C(^tBu₂pz)₂,^8f HC(Me₂pz)₂SiMe₂N(H)ⁱPr,¹⁷ Ca{N(SiMe₃)₂}₂(thf)₂,⁴³ Mg{N(SiMe₃)₂}₂ ⁴³ and Mg{N(SiHMe₂)₂}₂ ⁴³ were prepared according to literature procedures. ε-Caprolactone was dried over freshly ground CaH₂, stored over molecular sieves (4 Å) at 4 °C and distilled before use. rac-Lactide was recrystallized twice from toluene and subsequently sublimed twice prior to use. All other reagents were purchased and used without further purification. Polymer molecular weights (M_n, M_w) were determined by GPC using a Polymer Laboratories Plgel Mixed-D column (300 mm length, 7.5 mm diameter) and a Polymer Laboratories PL-GPC50 Plus instrument equipped with a refractive index detector. Tetrahydrofuran (HPLC grade) was used as an eluent at 30 °C with a rate of 1 mL min⁻¹. Linear polystyrenes were used as primary calibration standards, and Mark–Houwink corrections for poly(ε-CL) or poly(rac-LA) in thf were applied for the experimental samples.³⁶

X-ray crystallography

Crystal data collection and processing parameters for HC(^tBu₂pz)₂SiMe₂Cl, HC(^tBu₂pz)₂SiMe₂N(H)Ph, Mg{HC(Me₂pz)₂SiMe₂NPh}₂, Mg{HC(^tBu₂pz)₂SiMe₂NⁱPr}Me (5), Mg{HC(Me₂pz)₂SiMe₂NPh}ⁿBu (10), Mg{HC(^tBu₂pz)₂SiMe₂NPh}-{N(SiHMe₂)₂} (16), Mg{HC(Me₂pz)₂SiMe₂NⁱPr}{N(SiMe₃)₂} (17), Mg{HC(Me₂pz)₂SiMe₂NPh}{N(SiMe₃)₂} (19), Mg{HC(^tBu₂pz)₂-SiMe₂NPh}{N(SiMe₃)₂} (20), Zn{HC(Me₂pz)₂SiMe₂NⁱPr}Me (25), Zn{HC(Me₂pz)₂SiMe₂NPh}Me (27), Zn{HC(^tBu₂pz)₂SiMe₂NPh}Me (28) are given in Tables S1–3 and Fig. S10–15 of the ESI.‡ The structures have been deposited in the Cambridge Structural Database as CCDC 1885899–1885910.‡ Crystals were mounted on glass fibres using perfluoropolyether oil and cooled rapidly under a stream of cold N₂ using an Oxford Cryosystems Cryostream unit. Diffraction data were measured using a Enraf-Nonius KappaCCD diffractometer with Mo Kα radiation (λ = 0.71073 Å). As appropriate, absorption and decay corrections were applied to the data and equivalent reflections merged.⁴⁴ The structures were solved with SIR92 ⁴⁵ or Superflip,⁴⁶ and further refinements and all other crystallographic calculations were performed using the CRYSTALS program suite.⁴⁷ Other details of the structure solution and refinements are given in the ESI.‡

Synthetic details and characterising data

HC(Me₂pz)₂SiMe₂N(H)R (R = ^tBu, Ad) illustrated for R = Ad. To a stirred solution of HC(Me₂pz)₂SiMe₂Cl (4.00 g, 13.5 mmol) in Et₂O (60 mL) at 0 °C, was added a solution of NH₂Ad (5.09 g, 33.7 mmol) in Et₂O (20 mL). The resultant white suspension was allowed to warm to 23 °C and stirred for 14 h. The suspension was filtered, the solid washed with Et₂O (3 × 10 mL) and the volatiles removed under reduced pressure to give a white oily solid. The oily solid was recrystallised from hexane (20 mL) at −80 °C, filtered and dried under reduced pressure to afford HC(Me₂pz)₂SiMe₂N(H)Ad as a colourless solid. Yield: 4.71 g (85%). ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 6.14 (1 H, s, H̲C(Me₂pz)₂), 5.67 (2 H, s, N₂C₃H̲Me₂), 2.21 (6 H, s, 3-N₂C₃HM̲e̲₂), 1.94 (6 H, s, 5-N₂C₃HM̲e̲₂), 1.90 (3 H, m, Ad), 1.67 (6 H, app. d, app. ³J_HH = 3 Hz, Ad), 1.51 (6 H, m, Ad), 0.54 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 146.5 (3-N₂C̲₃HMe₂), 140.2 (5-N₂C̲₃HMe₂), 106.4 (4-N₂C̲₃HMe₂), 69.9 (HC̲(Me₂pz)₂), 50.0 (1-Ad), 47.6 36.6 (2,4,6,8,9,10-Ad), 30.4 (3,5,7-Ad), 13.8 (3-N₂C₃HM̲e̲₂), 11.0 (5-N₂C₃HM̲e̲₂), 2.2 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 3342 (m) 2951 (s), 2853 (m), 1560 (s), 1461 (w), 1376 (m), 1258 (m), 1096 (m), 1018 (w), 967 (m), 822 (w), 799 (m), 721 (w). EI-MS: m/z = 411 (100%) [M]⁺, 316 (10%) [M − Me₂pz]⁺. Anal. found (calcd. for C₂₃H₃₇N₅Si): C, 67.0 (67.1); H, 9.1 (9.1); N, 17.0 (17.0)%.

HC(Me₂pz)₂SiMe₂N(H)Ph. To a stirred solution of NH₂Ph (1.23 mL, 13.5 mmol) in Et₂O (20 mL), was added NEt₃ (3.75 mL, 26.9 mmol). This mixture was slowly added to a stirred solution of HC(Me₂pz)₂SiMe₂Cl (4.00 g, 13.5 mmol) in Et₂O (50 mL) at 0 °C. The resultant suspension was allowed to warm to 23 °C and stirred for 14 h. The suspension was filtered, the solid washed with Et₂O (3 × 10 mL) and the volatiles removed under reduced pressure to afford a yellow oily solid which was extracted into pentane (3 × 10 mL) and dried under reduced pressure. The colourless solid was recrystallised from pentane (20 mL) at −30 °C, filtered and dried under reduced pressure to afford HC(Me₂pz)₂SiMe₂N(H)Ph as a colourless solid. Yield: 4.52 g (95%). ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.08 (2 H, t, ³J_HH = 7.9 Hz, 3-C₆H₅), 6.74 (1 H, t, ³J_HH = 7.5 Hz, 4-C₆H₅), 6.61 (2 H, d, ³J_HH = 7.5 Hz, 2-C₆H₅), 6.02 (1 H, s, H̲C(Me₂pz)₂), 5.61 (2 H, s, N₂C₃H̲Me₂), 4.34 (1 H, s, NH), 2.16 (6 H, s, 3-N₂C₃HM̲e̲₂), 1.73 (6 H, s, 5-N₂C₃HM̲e̲₂), 0.60 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 147.7 (3-N₂C̲₃HMe₂), 142.9 (1-C₆H₅), 139.9 (5-N₂C̲₃HMe₂), 129.8 (3-C₆H₅), 118.7 (4-C₆H₅), 117.5 (2-C₆H₅), 106.7 (4-N₂C̲₃HMe₂), 67.0 (HC̲(Me₂pz)₂), 13.9 (3-N₂C₃HM̲e̲₂), 10.6 (5-N₂C₃HM̲e̲₂), −0.2 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 3345 (m), 2930 (s), 2731 (m), 1602 (w), 1552 (s), 1463 (m), 1376 (w), 1296 (m), 1246 (m), 1145 (w), 1110 (w), 1025 (s), 972 (m), 907 (m), 807 (s), 787 (s), 747 (m), 691 (m). EI-MS: m/z = 353 (100%) [M]⁺. Anal. found (calcd for C₁₉H₂₇N₅Si): C, 64.5 (64.5); H, 7.7 (7.7); N, 19.7 (19.7)%.

HC(^tBu₂pz)₂SiMe₂Cl. To a stirred solution of H₂C(^tBu₂pz)₂ (15.0 g, 40.3 mmol) in THF (150 mL) at −78 °C, was added ⁿBuLi (27.7 mL, 44.3 mmol; 1.6 M in hexanes) over a period of 20 minutes. The resultant yellow slurry was stirred for 12 h at −78 °C to afford LiHC(^tBu₂pz)₂ which was reacted on in situ. To a stirred solution of SiMe₂Cl₂ (7.28 mL, 60.4 mmol) in thf (100 mL) at −78 °C was added LiHC(^tBu₂pz)₂ over a period of 20 minutes. The resultant pale-yellow solution was stirred for 8 h at −78 °C. The volatiles were removed under reduced pressure to give a colourless, oily solid which was extracted into pentane (3 × 40 mL) and dried under reduced pressure. The oily solid was recrystallised from pentane (120 mL) at −80 °C, filtered and dried under reduced pressure to afford HC(^tBu₂pz)₂SiMe₂Cl as a colourless powder. Yield: 18.0 g (96%). Diffraction-quality crystals were grown from a pentane solution. ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 6.80 (1 H, s, H̲C(^tBu₂pz)₂), 6.01 (2 H, s, N₂C₃H̲^tBu₂), 1.27 (18 H, s, 3-N₂C₃H^t̲B̲u̲₂), 1.25 (18 H, s, 5-N₂C₃H^t̲B̲u̲₂), 0.99 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 158.8 (3-N₂C̲₃H^tBu₂), 152.3 (5-N₂C̲₃H^tBu₂), 102.7 (4-N₂C̲₃H^tBu₂), 74.0 (HC̲(^tBu₂pz)₂), 32.9 (3-N₂C₃H(C̲Me₃)₂), 32.1 (5-N₂C₃H(C̲Me₃)₂), 31.4 (3-N₂C₃H(CM̲e̲₃)₂), 30.7 (5-N₂C₃H(CM̲e̲₃)₂), 6.7 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 1541 (m), 1531 (m), 1361 (s), 1334 (m), 1319 (w), 1249 (s), 1230 (m), 1203 (w), 1125 (w), 1084 (w), 1059 (w), 1012 (w), 999 (m), 867 (w), 844 (w), 652 (w). EI-MS: m/z = 465 (6%) [M]⁺, 429 (100%) [M − Cl]⁺, 372 (82%) [M − SiMe₂Cl]⁺. Anal. found (calcd for C₂₅H₄₅ClN₄Si): C, 64.5 (64.6); H, 9.7 (9.8); N, 11.9 (12.0)%.

HC(^tBu₂pz)₂SiMe₂N(H)R (R = ⁱPr or ^tBu) illustrated for R = ⁱPr. To a stirred solution of HC(^tBu₂pz)₂SiMe₂Cl (4.00 g, 8.60 mmol) in Et₂O (60 mL) at 0 °C, was added NH₂ⁱPr (2.20 mL, 25.8 mmol). The resultant colourless suspension was allowed to warm to 23 °C and stirred for 20 h. The suspension was filtered, dried under reduced pressure and extracted into pentane (3 × 20 mL). The volatiles were removed under reduced pressure to afford HC(^tBu₂pz)₂SiMe₂N(H)ⁱPr as a thick, colourless oil. Yield: 4.03 g (96%). ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 6.99 (1 H, s, H̲C(^tBu₂pz)₂), 6.01 (2 H, s, N₂C₃H̲^tBu₂), 3.09 (1 H, sept, ³J_HH = 6.2 Hz, H̲CMe₂), 1.40 (18 H, s, 3-N₂C₃H^t̲B̲u̲₂), 1.15 (18 H, s, 5-N₂C₃H^t̲B̲u̲₂), 0.96 (6 H, d, ³J_HH = 6.2 Hz, HCM̲e̲₂), 0.37 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 158.1 (3-N₂C̲₃H^tBu₂), 153.9 (5-N₂C̲₃H^tBu₂), 102.5 (4-N₂C̲₃H^tBu₂), 76.5 (HC̲(^tBu₂pz)₂), 43.3 (HC̲Me₂), 32.4 (3-N₂C₃H(C̲Me₃)₂), 32.2 (5-N₂C₃H(C̲Me₃)₂), 30.8 (3-N₂C₃H(CM̲e̲₃)₂), 30.5 (5-N₂C₃H(CM̲e̲₃)₂), 28.3 (HCM̲e̲₂), 0.7 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 3363 (w), 2712 (w), 1536 (s), 1394 (s), 1362 (s), 1343 (s), 1316 (s), 1276 (m), 1213 (s), 1168 (s), 1131 (s), 1067 (m), 1021 (s), 1003 (s), 931 (w), 880 (m), 828 (m), 685 (w). EI-MS: m/z = 487 (75%) [M]⁺, 430 (100%) [M − N(H)ⁱPr]⁺. Anal. found (calcd for C₂₈H₅₃N₅Si): C, 69.0 (68.9); H, 11.1 (11.0); N, 14.2 (14.4)%.

HC(^tBu₂pz)₂SiMe₂N(H)Xyl. To a stirred solution of NH₂Xyl (1.06 mL, 8.60 mmol) in Et₂O (20 mL) at −78 °C, was added ⁿBuLi (5.37 mL, 8.60 mmol; 1.6 M in hexanes) over a 10 minutes period. The resultant colourless suspension was stirred for 1.5 h at −78 °C to afford LiN(H)Xyl which was reacted on in situ. To a stirred solution of HC(^tBu₂pz)₂SiMe₂Cl (4.00 g, 8.60 mmol) in Et₂O (50 mL) at −78 °C, was added LiN(H)Xyl over a 10 minutes period. The resultant yellow suspension was allowed to warm to 23 °C and stirred for 22 h. The suspension was filtered, the solid washed with Et₂O (3 × 10 mL) and the volatiles removed under reduced pressure to give a yellow oily solid which was extracted into pentane (3 × 10 mL) and dried under reduced pressure. The solid was recrystallised from pentane (20 mL) at −30 °C, filtered and dried under reduced pressure to afford HC(^tBu₂pz)₂SiMe₂N(H)Xyl as an orange solid. Yield: 4.44 g (94%).¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.18 (1 H, s, H̲C(^tBu₂pz)₂), 7.00 (2 H, d, ³J_HH = 7.0 Hz, 3-C₆H₃Me₂), 6.87 (1 H, t, ³J_HH = 7.0 Hz, 4-C₆H₃Me₂), 6.02 (2 H, s, N₂C₃H̲^tBu₂), 3.55 (1 H, s, NH), 2.12 (6 H, s, C₆H₃M̲e̲₂), 1.35 (18 H, s, 3-N₂C₃H^t̲B̲u̲₂), 1.12 (18 H, s, 5-N₂C₃H^t̲B̲u̲₂), 0.42 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 158.3 (3-N₂C̲₃H^tBu₂), 154.2 (5-N₂C̲₃H^tBu₂), 143.2 (1-C₆H₃Me₂), 133.0 (2-C₆H₃Me₂), 128.7 (3-C₆H₃Me₂), 122.7 (4-C₆H₃Me₂), 102.9 (4-N₂C̲₃H^tBu₂), 75.4 (HC̲(^tBu₂pz)₂), 32.4 (3-N₂C₃H(C̲Me₃)₂), 32.2 (5-N₂C₃H(C̲Me₃)₂), 30.8 (3-N₂C₃H(CM̲e̲₃)₂), 30.4 (5-N₂C₃H(CM̲e̲₃)₂), 19.7 (C₆H₃M̲e̲₂), 1.3 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 3319 (w), 1593 (w), 1537 (m), 1392 (w), 1363 (m), 1341 (m), 1316 (m), 1276 (w), 1237 (m), 1215 (m), 1176 (w), 1159 (w), 1129 (w), 1094 (w), 1070 (w), 1001 (m), 902 (m), 877 (m), 828 (m), 792 (s), 759 (s). EI-MS: m/z = 429 (100%) [M − N(H)C₆H₃Me₂]⁺. Anal. found (calcd for C₃₃H₅₅N₅Si): C, 72.2 (72.1); H, 10.2 (10.1); N, 12.7 (12.6)%.

[Li{HC(^tBu₂pz)₂SiMe₂NR}]_n (R = ⁱPr (1), ^tBu (2), Ph (3), Xyl (4)) illustrated for R = Xyl (4). HC(^tBu₂pz)₂SiMe₂N(H)Xyl (0.620 g, 1.13 mmol) was dissolved in pentane (40 mL) and cooled to −78 °C. ⁿBuLi (0.840 mL, 1.36 mmol; 1.6 M in hexanes) was diluted with pentane (10 mL), similarly cooled and then added dropwise to the pro-ligand solution to afford a colourless precipitate. The reaction mixture was stirred for 6 h and then allowed to stand overnight, whereby crystallisation took place. The colourless solid was filtered off, washed with cold (−78 °C) pentane (3 × 10 mL) and dried under reduced pressure to afford complex 4 as a colourless powder. Yield: 0.44 g (70%). ¹H NMR (C₆D₆, 299.9 MHz, 293 K): 7.39 (2 H, d, ³J_HH = 7.4 Hz, 3-C₆H₃Me₂), 6.89 (1 H, t, ³J_HH = 7.4 Hz, 4-C₆H₃Me₂), 6.56 (1 H, s, H̲C(^tBu₂pz)₂), 5.91 (2 H, s, N₂C₃H̲^tBu₂), 2.39 (6 H, s, C₆H₃M̲e̲₂), 1.23 (18 H, s, 5-N₂C₃H^t̲B̲u̲₂), 1.16 (18 H, s, 3-N₂C₃H^t̲B̲u̲₂), 0.13 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): 159.7 (3-N₂C̲₃H^tBu₂), 157.2 (1-C̲₆H₃Me₂), 153.2 (5-N₂C̲₃H^tBu₂), 131.8 (2,6-C̲₆H₃Me₂), 127.9 (3-C̲₆H₃Me₂), 115.4 (4-C₆H₃Me₂), 101.3 (4-N₂C̲₃H^tBu₂), 71.5 (HC̲(^tBu₂pz)₂), 32.8 (5-N₂C₃H(C̲Me₃)₂), 32.0 (3-N₂C₃H(C̲Me₃)₂), 31.6 (5-N₂C₃H(CM̲e̲₃)₂), 30.3 (3-N₂C₃H(CM̲e̲₃)₂), 21.01 (C₆H₃M̲e̲₂), 3.24 (SiMe) ppm. ⁷Li NMR (toluene-d₈, 194.3 MHz, 218 and 293 K): 2.1 (NL̲i̲) ppm. IR (NaCl plates, Nujol): 1587 (w), 1544 (m), 1418 (s), 1359 (m), 1297 (s), 1235 (m), 1207 (m), 1097 (w), 1064 (w), 1023 (w), 994 (m), 982 (m), 829 (m), 821 (m), 776 (m), 755 (s), 709 (w), 693 (w), 615 (w), 530 (w) cm⁻¹. EI-MS: m/z: 555 [M]⁺ (8%), 429 [M − N(Li)Xyl]⁺ (100%), 371 [M − SiMe₂N(Li)Xyl]⁺ (8%), Anal. Found (calcd for C₃₃H₅₄LiN₅Si): C, 71.4 (71.3); H, 9.7 (9.8); N, 12.7 (12.6)%.

Mg{HC(^tBu₂pz)₂SiMe₂NR}Me (R = ⁱPr (5), Ph (6)) illustrated for R = ⁱPr (5). To a stirred solution of HC(^tBu₂pz)₂SiMe₂N(H)ⁱPr (0.500 g, 1.02 mmol) in benzene (30 mL), was added MeMgCl (1.02 mL, 3.07 mmol; 3.0 M in thf). The resultant yellow solution was stirred for 38 h. Volatiles were removed under reduced pressure to give a white solid, which was extracted into pentane (3 × 10 mL) and dried under reduced pressure. The solid was recrystallised from pentane (15 mL) at −80 °C, filtered and dried under reduced pressure to afford complex 5 as a colourless solid. Yield = 0.35 g (65%). Diffraction-quality crystals were grown from a saturated pentane solution. ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 6.52 (1 H, s, H̲C(^tBu₂pz)₂), 5.96 (2 H, s, N₂C₃H̲^tBu₂), 3.61 (1 H, sept, ³J_HH = 6.3 Hz, H̲CMe₂), 1.54 (6 H, d, ³J_HH = 6.4 Hz, HCM̲e̲₂), 1.52 (18 H, s, 3-N₂C₃H^t̲B̲u̲₂), 1.16 (18 H, s, 5-N₂C₃H^t̲B̲u̲₂), 0.05 (6 H, s, SiMe), −0.20 (3 H, s, MgMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 163.2 (3-N₂C̲₃H^tBu₂), 153.1 (5-N₂C̲₃H^tBu₂), 102.9 (4-N₂C̲₃H^tBu₂), 69.8 (HC̲(^tBu₂pz)₂), 46.8 (HC̲Me₂), 33.0 (HCM̲e̲₂), 32.3 (3-N₂C₃H(C̲Me₃)₂), 32.1 (5-N₂C₃H(C̲Me₃)₂), 31.6 (3-N₂C₃H(CM̲e̲₃)₂), 30.7 (5-N₂C₃H(CM̲e̲₃)₂), 1.2 (SiMe), −3.6 (MgMe). IR (NaCl plates, Nujol mull, cm⁻¹): 2795 (w), 2599 (w), 1615 (w), 1526 (m), 1423 (m), 1348 (m), 1305 (m), 1244 (m), 1220 (m), 1209 (m), 1139 (m), 1111 (m), 1061 (s), 1029 (m), 899 (m), 829 (m), 755 (s), 733 (w), 710 (w). EI-MS: m/z = 429 (100%) [HC(^tBu₂pz)₂SiMe₂]⁺. Anal. found (calcd for C₂₉H₅₅MgN₅Si): C, 65.9 (66.2); H, 10.4 (10.5); N, 13.1 (13.3)%.

General procedure for the synthesis of Mg{HC(Me₂pz)₂SiMe₂NR}ⁿBu (R = ⁱPr (7), ^tBu (8), Ad (9), Ph (10)) illustrated for R = Ph (10). To a stirred solution of HC(Me₂pz)₂SiMe₂N(H)Ph (0.500 g, 1.41 mmol) in benzene (30 mL), was added MgⁿBu₂ (1.41 mL, 1.41 mmol; 1.0 M in heptanes). The resultant orange solution was stirred for 12 h. Volatiles were removed under reduced pressure, the solid washed with cold pentane (3 × 5 mL, −78 °C) and dried under reduced pressure to afford complex 10 as a bright orange solid. Yield = 0.48 g (78%). Diffraction-quality crystals were grown from slow diffusion of hexane into a concentrated toluene solution. ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.35 (2 H, app. t, app. ³J_HH = 7.8 Hz, 3-C₆H₅), 7.28 (2 H, app. d, app. ³J_HH = 7.2 Hz, 2-C₆H₅), 6.79 (1 H, t, ³J_HH = 6.9 Hz, 4-C₆H₅), 5.28 (2 H, s, N₂C₃H̲Me₂), 5.04 (1 H, s, H̲C(Me₂pz)₂), 2.27 (2 H, m, 2-MgⁿBu), 2.09 (6 H, s, 3-N₂C₃HM̲e̲₂), 1.90 (2 H, m, ³J_HH = 7.2 Hz, 3-MgⁿBu), 1.57 (6 H, s, 5-N₂C₃HM̲e̲₂), 1.30 (3 H, t, ³J_HH = 7.3 Hz, 4-MgⁿBu), 0.49 (2 H, m, 1-MgⁿBu), 0.05 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 157.7 (1-C₆H₅), 150.4 (3-N₂C̲₃HMe₂), 139.5 (5-N₂C̲₃HMe₂), 129.5 (3-C₆H₅), 121.7 (2-C₆H₅), 115.0 (4-C₆H₅), 105.9 (4-N₂C̲₃HMe₂), 63.3 (HC̲(Me₂pz)₂), 33.4 (2-MgⁿBu), 32.7 (3-MgⁿBu), 14.7 (1-MgⁿBu), 12.9 (3-N₂C₃HM̲e̲₂), 10.3 (5-N₂C₃HM̲e̲₂), 8.8 (4-MgⁿBu), −1.4 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 2925 (s), 2361 (m), 1587 (s), 1553 (s), 1489 (w), 1457 (w), 1377 (m), 1291 (m), 1183 (m), 1078 (s), 1047 (s), 990 (s), 942 (s), 827 (m), 804 (w), 768 (s), 738 (m), 694 (m). EI-MS: m/z = 376 (100%) [M − ⁿBu]⁺. Anal. found (calcd for C₂₃H₃₅MgN₅Si): C, 63.7 (63.7); H, 8.1 (8.1); N, 16.2 (16.1)%.

General procedure for the synthesis of Mg{HC(^tBu₂pz)₂SiMe₂NR}ⁿBu (R = ⁱPr (11), Ph (12)) illustrated for R = Ph (12). To a stirred solution of HC(^tBu₂pz)₂SiMe₂N(H)Ph (0.50 g, 0.96 mmol) in benzene (30 mL), was added MgⁿBu₂ (1.92 mL, 1.92 mmol; 1.0 M in heptanes). The resultant yellow solution was stirred for 18 h. Volatiles were removed under reduced pressure to afford a colourless solid, which was washed with cold pentane (3 × 5 mL, −78 °C) and dried under reduced pressure. The solid was recrystallised from pentane (15 mL) at −80 °C, filtered and dried under reduced pressure to afford complex 12 as a colourless solid. Yield = 0.38 g (66%). ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.29 (2 H, app. t, app. ³J_HH = 7.6 Hz, 3-C₆H₅), 7.2 (2 H, d, ³J_HH = 8.2 Hz, 2-C₆H₅), 6.82 (1 H, t, ³J_HH = 7.3 Hz, 4-C₆H₅), 6.52 (1 H, s, H̲C(^tBu₂pz)₂), 5.96 (2 H, s, N₂C₃H̲^tBu₂), 2.08 (2 H, m, 2-MgⁿBu), 1.83 (2 H, app. sext, app. ³J_HH = 7.2 Hz, 3-MgⁿBu), 1.47 (18 H, s, 3-N₂C₃H^t̲B̲u̲₂), 1.27 (3 H, m, 4-MgⁿBu), 1.10 (18 H, s, 5-N₂C₃H^t̲B̲u̲₂), 0.49 (2 H, m, 1-MgⁿBu), 0.15 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 164.0 (3-N₂C̲₃H^tBu₂), 157.1 (1-C₆H₅), 153.8 (5-N₂C̲₃H^tBu₂), 129.0 (3-C₆H₅), 124.4 (2-C₆H₅), 116.2 (4-C₆H₅), 103.2 (4-N₂C̲₃H^tBu₂), 69.2 (HC̲(^tBu₂pz)₂), 33.0 (3-N₂C₃H(C̲Me₃)₂), 32.8 (2-MgⁿBu), 32.6 (3-MgⁿBu), 32.2 (5-N₂C₃H(C̲Me₃)₂), 31.5 (3-N₂C₃H(CM̲e̲₃)₂), 30.7 (5-N₂C₃H(CM̲e̲₃)₂), 15.8 (1-MgⁿBu), 14.6 (4-MgⁿBu), 0.33 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 1587 (s), 1534 (w), 1486 (s), 1425 (w), 1367 (m), 1284 (s), 1222 (w), 1209 (m), 1173 (w), 1148 (w), 1059 (m), 1027 (m), 995 (m), 957 (s), 871 (w), 825 (m), 800 (s), 791 (m), 764 (m). EI-MS: m/z = 429 (60%) [HC(^tBu₂pz)₂SiMe₂]⁺. Anal. found (calcd for C₃₅H₅₉MgN₅Si): C, 69.7 (69.8); H, 9.9 (9.9); N, 11.7 (11.6)%.

Mg{HC(Me₂pz)₂SiMe₂NR}{N(SiHMe₂)₂} (R = ⁱPr (13), ^tBu (14), Ph (15)) illustrated for R = ⁱPr (13). To a stirred solution of Mg{N(SiHMe₂)₂}₂ (0.450 g, 1.56 mmol) in benzene (20 mL), was added a solution of HC(Me₂pz)₂SiMe₂N(H)ⁱPr (0.500 g, 1.56 mmol) in benzene (20 mL). The resultant orange solution was stirred for 16 h. Volatiles were removed under reduced pressure, the solid washed with cold pentane (3 × 5 mL, −78 °C) and dried under reduced pressure to afford complex 13 as an orange solid. Yield = 0.50 g (68%). ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 5.41 (2 H, sept, ³J_HH = 3.0 Hz, N(SiH̲Me₂)₂), 5.34 (2 H, s, N₂C₃H̲Me₂), 5.00 (1 H, s, H̲C(Me₂pz)₂), 3.85 (1 H, sept, ³J_HH = 6.3 Hz, H̲CMe₂), 2.36 (6 H, s, 3-N₂C₃HM̲e̲₂), 1.55 (6 H, s, 5-N₂C₃HM̲e̲₂), 1.49 (6 H, d, ³J_HH = 6.3 Hz, HCM̲e̲₂), 0.55 (12 H, d, ³J_HH = 3.4 Hz, N(SiHM̲e̲₂)₂), −0.90 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 149.9 (3-N₂C̲₃HMe₂), 139.4 (5-N₂C̲₃HMe₂), 105.8 (4-N₂C̲₃HMe₂), 63.8 (HC̲(Me₂pz)₂), 47.5 (HC̲Me₂), 31.5 (HCM̲e̲₂), 13.8 (3-N₂C₃HM̲e̲₂), 10.4 (5-N₂C₃HM̲e̲₂), 4.6 (N(SiHMe₂)₂), 1.3 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 2925 (s), 2361 (m), 2067 (w), 1553 (m), 1506 (w), 1459 (s), 1377 (s), 1315 (s), 1280 (m), 1242 (s), 1164 (s), 1132 (m), 1045 (m), 945 (m), 892 (s), 826 (m), 800 (m), 756 (m), 729 (m). EI-MS: m/z = 474 (100%) [M]⁺, 342 (20%) [M − N(SiHMe₂)₂]⁺. Anal. found (calcd for C₂₀H₄₂MgN₆Si₃): C, 50.2 (50.5); H, 8.7 (8.9); N, 17.6 (17.7)%.

Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiHMe₂)₂} (16). To a solution of Mg{N(SiHMe₂)₂}₂ (0.17 g, 0.59 mmol) in benzene (15 mL) was added slowly HC(^tBu₂pz)₂SiMe₂N(H)Ph (0.31 g, 0.59 mmol) in benzene (15 mL). The mixture was heated to 60 °C and stirred for 24 h. Volatiles were removed under reduced pressure to give a pale-yellow solid. Recrystallisation from a saturated pentane solution at −30 °C yielded complex 16 as an analytically pure, off-white solid. Yield: 0.21 g (53%). Diffraction-quality crystals were grown from a saturated pentane solution at −30 °C. ¹H NMR (C₆D₆, 299.9 MHz, 293 K): 7.26 (2 H, app. t, app. ³J_HH = 7.9 Hz, 3-C₆H₅), 7.01 (2 H, d, ³J_HH = 7.1 Hz, 2-C₆H₅), 6.84 (1 H, app. t, app. ³J_HH = 7.9 Hz, 4-C₆H₅), 6.30 (1 H, s, H̲C(^tBu₂pz)₂), 6.03 (2 H, s, N₂C₃H̲^tBu₂), 5.24 (2 H, sept, ³J_HH = 2.7 Hz, SiMe₂H̲), 1.59 (18 H, s, 3-N₂C₃H^t̲B̲u̲₂), 1.11 (18 H, s, 5-N₂C₃H^t̲B̲u̲₂), 0.38 (12 H, ³J_HH = 2.7 Hz, SiHM̲e̲₂), −0.05 (6 H, s, SiM̲e̲₂). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): 165.5 (3-N₂C̲₃H^tBu₂), 156.8 (1-C₆H₅), 155.9 (5-N₂C̲₃H^tBu₂), 128.8 (3-C₆H₅), 126.4 (2-C₆H₅), 117.5 (4-C₆H₅), 104.7 (4-N₂C̲₃H^tBu₂), 69.0 (HC̲(^tBu₂pz)₂), 33.1 (5-N₂C₃H(C̲Me₃)₂), 33.0 (3-N₂C₃H(C̲Me₃)₂), 31.6 (5-N₂C₃H(C̲Me₃)₂), 31.4 (3-N₂C₃H(C̲Me₃)₂), 3.7 (SiHM̲e̲₂), 0.4 (SiM̲e̲₂). IR (NaCl plates, Nujol, cm⁻¹): 2854 (s), 2058 (m), 1586 (w), 1261 (w), 1093 (s), 976 (w), 886 (m), 759 (w). EI-MS: m/z 676 [M − Mg{N(SiHMe₂)₂} − NPh]⁺ (20%). Anal. found (calcd for C₃₅H₆₄MgN₆Si₃): C, 62.0 (62.1); H, 9.6 (9.5); N, 12.4 (12.4)%.

Mg{HC(Me₂pz)₂SiMe₂NR}{N(SiMe₃)₂} (R = ⁱPr (17), ^tBu (18), Ph (19)) illustrated for R = ⁱPr (17). To a stirred solution of Mg{N(SiMe₃)₂}₂ (0.480 g, 1.41 mmol) in benzene (20 mL), was added a solution of HC(Me₂pz)₂SiMe₂N(H)Ph (0.500 g, 1.41 mmol) in benzene (20 mL). The resultant yellow solution was stirred for 14 h. Volatiles were removed under reduced pressure, the solid washed with cold pentane (3 × 5 mL, −78 °C) and dried under reduced pressure to afford complex 17 as an pale-orange solid. Yield = 0.58 g (77%). Diffraction-quality crystals were grown from slow diffusion of diethyl ether into a concentrated toluene solution. ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.30 (2 H, t, ³J_HH = 7.6 Hz, 3-C₆H₅), 6.98 (2 H, d, ³J_HH = 7.5 Hz, 2-C₆H₅), 6.83 (1 H, t, ³J_HH = 7.2 Hz, 4-C₆H₅), 5.37 (2 H, s, N₂C₃H̲Me₂), 4.99 (1 H, s, H̲C(Me₂pz)₂), 2.42 (6 H, s, 3-N₂C₃HM̲e̲₂), 1.49 (6 H, s, 5-N₂C₃HM̲e̲₂), 0.41 (18 H, s, N(SiMe₃)₂), −0.13 (SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 157.0 (1-C₆H₅), 150.6 (3-N₂C̲₃HMe₂), 140.0 (5-N₂C̲₃HMe₂), 129.2 (3-C₆H₅), 125.3 (2-C₆H₅), 117.0 (4-C₆H₅), 106.2 (4-N₂C̲₃HMe₂), 62.9 (HC̲(Me₂pz)₂), 14.1 (3-N₂C₃HM̲e̲₂), 10.4 (5-N₂C₃HM̲e̲₂), 6.3 (N(SiMe₃)₂), −1.1 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 2972 (w), 2728 (m), 1581 (s), 1521 (s), 1419 (m), 1377 (s), 1312 (s), 1243 (s), 1172 (m), 1096 (s), 1005 (s), 959 (m), 882 (m), 826 (s), 773 (m). EI-MS: m/z = 536 (40%) [M]⁺, 376 (85%) [M − N(SiMe₃)₂]⁺. Anal. found (calcd for C₂₅H₄₄MgN₆Si₃): C, 55.8 (55.9); H, 8.3 (8.3); N, 15.6 (15.7)%.

Mg{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiMe₃)₂} (20). To a solution of Mg{N(SiMe₃)₂}₂ (0.300 g, 0.87 mmol) in benzene (15 mL) was added slowly HC(^tBu₂pz)₂SiMe₂N(H)Ph (0.45 g, 0.87 mmol) in benzene (15 mL). The mixture was heated to 60 °C and stirred for 6 h. Volatiles were removed under reduced pressure to give a pale-yellow solid. Recrystallisation from a saturated pentane solution at −30 °C yielded complex 20 as an analytically pure, off-white solid. Yield: 0.30 g (49%). ¹H NMR (C₆D₆, 299.9 MHz, 293 K): 7.24 (2 H, app. t, app. ³J_HH = 7.9 Hz, 3-C₆H₅), 6.98 (2 H, d, ³J_HH = 7.1 Hz, 2-C₆H₅), 6.83 (1 H, app. t, app. ³J_HH = 7.9 Hz, 4-C₆H₅), 6.21 (1 H, s, H̲C(^tBu₂pz)₂), 6.05 (2 H, s, N₂C₃H̲^tBu₂), 1.61 (18 H, s, 3-N₂C₃H^t̲B̲u̲₂), 1.07 (18 H, s, 5-N₂C₃H^t̲B̲u̲₂), 0.36 (18 H, SiM̲e̲₃), −0.08 (6 H, s, SiM̲e̲₂). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): 165.4 (3-N₂C̲₃H^tBu₂), 156.8 (1-C₆H₅), 156.3 (5-N₂C̲₃H^tBu₂), 128.4 (3-C₆H₅), 126.9 (2-C₆H₅), 118.0 (4-C₆H₅), 104.8 (4-N₂C̲₃H^tBu₂), 68.5 (HC̲(^tBu₂pz)₂), 32.8 (5-N₂C₃H(C̲Me₃)₂), 32.6 (3-N₂C₃H(C̲Me₃)₂), 31.4 (5-N₂C₃H(C̲Me₃)₂), 31.2 (3-N₂C₃H(C̲Me₃)₂), 6.1 (SiM̲e̲₃), 0.2 (SiM̲e̲₂). IR (NaCl plates, Nujol, cm⁻¹): 2726 (s), 2670 (m), 1589 (w), 1549 (w), 1196 (w), 1167 (w), 1067 (s), 951 (w), 837 (m). Anal. found (calcd for C₃₇H₆₈MgN₆Si₃): C, 59.1 (63.0); H, 9.2 (9.7); N, 11.2 (11.9)%. This was the best of several attempts using recrystallised material.

Ca{HC(Me₂pz)₂SiMe₂NR}{N(SiMe₃)₂}(thf) (R = ⁱPr (21), ^tBu (22), Ph (23)) illustrated for R = Ph (23). To a stirred solution of Ca{N(SiMe₃)₂}₂(thf)₂ (0.710 g, 1.41 mmol) in benzene (20 mL), was added a solution of HC(Me₂pz)₂SiMe₂N(H)Ph (0.500 g, 1.41 mmol) in benzene (20 mL). The resultant dark orange solution was stirred for 12 h. Volatiles were removed under reduced pressure, the solid washed with cold pentane (3 × 5 mL, −78 °C) and dried under reduced pressure to afford complex 23 as a brown solid. Yield = 0.65 g (74%). ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.27 (2 H, t, ³J_HH = 7.6 Hz, 3-C₆H₅), 6.87 (2 H, app. d, app. ³J_HH = 7.8 Hz, 2-C₆H₅), 6.71 (1 H, app. t, app. ³J_HH = 7.6 Hz, 4-C₆H₅), 5.45 (2 H, s, N₂C₃H̲Me₂), 5.20 (1 H, s, H̲C(Me₂pz)₂), 3.74 (4 H, br s, OCH̲₂CH₂), 2.38 (6 H, s, 3-N₂C₃HM̲e̲₂), 1.72 (6 H, s, 5-N₂C₃HM̲e̲₂), 1.34 (4 H, br s, OCH₂CH̲₂), 0.45 (18 H, s, N(SiMe₃)₂), 0.07 (SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 158.8 (1-C₆H₅), 150.2 (3-N₂C̲₃HMe₂), 139.9 (5-N₂C̲₃HMe₂), 129.2 (3-C₆H₅), 121.7 (2-C₆H₅), 113.9 (4-C₆H₅), 106.4 (4-N₂C̲₃HMe₂), 68.7 (OC̲H₂CH₂), 64.7 (HC̲(Me₂pz)₂), 25.4 (OCH₂C̲H₂), 14.4 (3-N₂C₃HM̲e̲₂), 11.0 (5-N₂C₃HM̲e̲₂), 6.0 (N(SiMe₃)₂), 2.9 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 2854 (w), 2361 (m), 1576 (s), 1521 (s), 1496 (w), 1458 (s), 1377 (s), 1259 (m), 1019 (m), 975 (m), 881 (m), 837 (w), 722 (s), 667 (m). EI-MS: m/z = 460 (25%) [M − NPh(thf)]⁺. Anal. found (calcd for C₂₉H₅₁CaN₆OSi₃): C, 55.7 (55.8); H, 8.2 (8.2); N, 13.4 (13.5)%.

Ca{HC(^tBu₂pz)₂SiMe₂NPh}{N(SiMe₃)₂} (24). To a stirred solution of Ca{N(SiMe₃)₂}₂(thf)₂ (0.48 g, 0.96 mmol) in benzene (20 mL), was added a solution of HC(^tBu₂pz)₂SiMe₂N(H)Ph (0.50 g, 0.96 mmol) in benzene (20 mL). The resultant orange solution was stirred at 60 °C for 8 h. Volatiles were removed under reduced pressure, the solid washed with cold pentane (3 × 5 mL, −78 °C) and dried under reduced pressure to afford complex 24 as a pale-yellow solid. Yield = 0.41 g (60%). ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.25 (2 H, t, ³J_HH = 7.6 Hz, 3-C₆H₅), 6.79–6.76 (3 H, overlapping t and d, ³J_HH = 7.9 Hz and ³J_HH = 7.1 Hz, 4-C₆H₅ and 2-C₆H₅), 6.27 (1 H, s, H̲C(^tBu₂pz)₂), 5.99 (2 H, s, N₂C₃H̲^tBu₂), 1.44 (18 H, s, 3-N₂C₃H^t̲B̲u̲₂), 1.17 (18 H, s, 5-N₂C₃H^t̲B̲u̲₂), 0.28 (18 H, s, N(SiMe₃)₂), 0.03 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 164.1 (3-N₂C̲₃H^tBu₂), 157.1 (1-C₆H₅), 156.6 (5-N₂C̲₃H^tBu₂), 129.6 (3-C₆H₅), 125.3 (2-C₆H₅), 116.7 (4-C₆H₅), 103.9 (4-N₂C̲₃H^tBu₂), 71.1 (HC̲(^tBu₂pz)₂), 33.3 (3-N₂C₃H(C̲Me₃)₂), 32.8 (5-N₂C₃H(C̲Me₃)₂), 31.8 (3-N₂C₃H(CM̲e̲₃)₂), 30.9 (5-N₂C₃H(CM̲e̲₃)₂), 5.8 (N(SiMe₃)₂), 1.4 (SiMe). IR (NaCl plates, Nujol mull, cm⁻¹): 1584 (w), 1549 (w), 1533 (w), 1364 (m), 1197 (w), 1171 (w), 1046 (s), 995 (w), 955 (m), 879 (m), 703 (w), 589 (w). EI-MS: m/z = 429 (40%) [HC(^tBu₂pz)₂SiMe₂]⁺. Anal. found (calcd for C₃₇H₆₈CaN₆Si₃): C, 61.6 (61.6); H, 9.4 (9.5); N, 11.7 (11.7)%.

Zn{HC(Me₂pz)₂SiMe₂NR}Me (R = ⁱPr (25), ^tBu (26), Ph (27)) illustrated for R = Ph (27). To a stirred solution of HC(Me₂pz)₂SiMe₂N(H)Ph (0.500 g, 1.41 mmol) in benzene (30 mL), was added ZnMe₂ (2.11 mL, 4.23 mmol; 2.0 M in toluene). The resultant light yellow solution was stirred for 15 h. Volatiles were removed under reduced pressure, the solid washed with cold hexanes (3 × 5 mL, −78 °C) and dried under reduced pressure to afford complex 27 as a white solid. Yield = 0.52 g (85%). Diffraction-quality crystals were grown from slow diffusion of diethyl ether into a concentrated toluene solution. ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.40–7.31 (4 H, overlapping 2 × m, 3-C₆H₅ and 2-C₆H₅), 6.81 (1 H, m, 4-C₆H₅), 5.23 (2 H, s, N₂C₃H̲Me₂), 5.00 (1 H, s, H̲C(Me₂pz)₂), 1.97 (6 H, s, 3-N₂C₃HM̲e̲₂), 1.51 (6 H, s, 5-N₂C₃HM̲e̲₂), 0.13 (3 H, s, ZnMe), 0.01 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 163.6 (1-C₆H₅), 149.4 (3-N₂C̲₃HMe₂), 138.5 (5-N₂C̲₃HMe₂), 129.4 (3-C₆H₅), 121.1 (2-C₆H₅), 115.1 (4-C₆H₅), 105.7 (4-N₂C̲₃HMe₂), 62.8 (HC̲(Me₂pz)₂), 12.8 (3-N₂C₃HM̲e̲₂), 10.3 (5-N₂C₃HM̲e̲₂), −1.5 (SiMe), −9.8 (ZnMe). IR (NaCl plates, Nujol mull, cm⁻¹): 2923 (w), 2360 (m), 1576 (s), 1521 (w), 1489 (w), 1457 (s), 1376 (s), 1313 (w), 1290 (s), 1248 (m), 1141 (w), 991 (w), 949 (s), 853 (w), 835 (m), 771 (m), 722 (w). EI-MS: m/z = 431 (100%) [M]⁺, 416 (20%) [M − Me]⁺. Anal. found (calcd for C₂₀H₂₉N₅SiZn): C, 55.4 (55.5); H, 6.8 (6.8); N, 16.1 (16.2)%.

Zn{HC(^tBu₂pz)₂SiMe₂NPh}Me (28). To a stirred solution of HC(^tBu₂pz)₂SiMe₂N(H)Ph (0.50 g, 0.96 mmol) in benzene (30 mL), was added ZnMe₂ (1.44 mL, 2.88 mmol; 2.0 M in toluene). The resultant clear, colourless solution was stirred at 60 °C for 3 days. Volatiles were removed under reduced pressure, the solid washed with cold hexanes (3 × 5 mL, −78 °C) and dried under reduced pressure to afford complex 28 as a colourless solid. Yield = 0.41 g (72%). Diffraction-quality crystals were grown from a pentane solution. ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.32–7.23 (4 H, overlapping 2 × m, 3-C₆H₅ and 2-C₆H₅), 6.79 (1 H, tt, ³J_HH = 6.5 Hz and ⁴J_HH = 1.9 Hz, 4-C₆H₅), 6.5 (1 H, s, H̲C(^tBu₂pz)₂), 6.01 (2 H, s, N₂C₃H̲^tBu₂), 1.45 (18 H, s, 3-N₂C₃H^t̲B̲u̲₂), 1.12 (18 H, s, 5-N₂C₃H^t̲B̲u̲₂), 0.41 (3 H, s, ZnMe), 0.18 (6 H, s, SiMe). ¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 162.9 (3-N₂C̲₃H^tBu₂), 157.0 (1-C₆H₅), 153.1 (5-N₂C̲₃H^tBu₂), 129.1 (3-C₆H₅), 123.0 (2-C₆H₅), 115.8 (4-C₆H₅), 103.2 (4-N₂C̲₃H^tBu₂), 68.5 (HC̲(^tBu₂pz)₂), 32.9 (3-N₂C₃H(C̲Me₃)₂), 32.3 (5-N₂C₃H(C̲Me₃)₂), 31.6 (3-N₂C₃H(CM̲e̲₃)₂), 30.6 (5-N₂C₃H(CM̲e̲₃)₂), 0.0 (SiMe), −3,4 (ZnMe). IR (NaCl plates, Nujol mull, cm⁻¹): 3143 (w), 3060 (w), 1588 (s), 1557 (m), 1542 (m), 1528 (s), 1489 (s), 1396 (w), 1364 (m), 1357 (m), 1291 (s), 1211 (m), 1178 (w), 1169 (m), 1152 (w), 1130 (w), 1075 (w), 1060 (m), 1023 (m), 993 (m), 961 (s), 867 (w), 843 (s), 793 (m), 765 (m), 751 (m), 709 (w), 692 (m). EI-MS: m/z = 429 (100%) [HC(^tBu₂pz)₂SiMe₂]⁺. Anal. found (calcd for C₃₂H₅₃N₅SiZn): C, 63.8 (63.9); H, 8.8 (8.9); N, 11.6 (11.7)%.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

A. D. S and M. G. C. thank the EPSRC (Grant EP/P502667/1 and EP/P503876/1 respectively) and M. L. B. thanks the University of Oxford and the European Union Alβan Programme for financial support (Grant E05D059950MX). The authors also thank Chemical Crystallography for use of the diffractometers and help with structure solutions.

Notes and references

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Footnotes

† This paper is dedicated in admiration and friendship to Professor F. Geoff N. Cloke FRS.

‡ Electronic supplementary information (ESI) available: Further experimental details, X-ray crystallographic details, CIF files and DFT calculation details. CCDC 1885899–1885910. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt04985h

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