Synthesis and Characterization of New Cyclam-Based Zr(IV) Alkoxido Derivatives

Abstract

In this study, new mono- and di-alkoxido zirconium(IV) complexes supported by tetradentate dianionic cyclam ligands were synthesized and characterized. These compounds were obtained by reaction of the parent Zr(IV) dichlorido species with one or two equivalents of the corresponding lithium alkoxido, whereas (^3,5-Me2Bn₂Cyclam)Zr(OPh)₂ was prepared by protonolysis of the orthometallated species (3,5-Me-C₆H₄CH₂)₂Cyclam)Zr with phenol. The solid-state molecular structures of (Bn₂Cyclam)ZrCl(O^tBu) and (^4-tBuBn₂Cyclam)Zr(OⁱPr)₂ show a trigonal prismatic geometry around the metal centers. (Bn₂Cyclam)Zr(SPh)(O^tBu) and (Bn₂Cyclam)ZrMe(OⁱPr) were prepared by reaction of (Bn₂Cyclam)ZrCl(OR) (R = ⁱPr, ^tBu) with one equivalent of LiSPh or MeMgCl, respectively. The reactions of (Bn₂Cyclam)Zr(OⁱPr)₂ and (^4-tBuBn₂Cyclam)Zr(OⁱPr)₂ with carbon dioxide suggested the formation of species that correspond to the addition of four CO₂ molecules.

1. Introduction

The use of alkoxide ligands as alternatives to the cyclopentadienyl ligand started in the 1980s. Considering electron-count rules, alkoxide and cyclopentadienyl ligands may be formally considered electronically equivalent, bonding to transition metals through one σ- and two π-donor orbitals. Regardless of this analogy, cyclopentadienyl is a carbon based and soft ligand, while alkoxides are hard ligands that form with Group 4 metals high polar and strong bonds. The latter feature was largely responsible for the extensive use of titanium and zirconium alkoxidos in catalysis [1,2]. The high polarity of M-OR bonds led to the application of several families of early transition metal complexes in the ring opening polymerization (ROP) of cyclic esters [3,4,5,6,7,8,9,10,11,12]. Our previous research on cyclam-based Zr(IV) alkoxido, phenoxido and thiophenoxido derivatives [13,14,15] also disclosed new catalytic systems for the ROP of rac-lactide strongly influenced by the nature of the OR ligands [13]. The characterization of the polymers obtained and DFT calculations clarified the important features that govern the catalytic activity and the role of OR ligands as initiators of the polymerization reactions or as supporting ligands that direct the insertion of the monomers into the Zr-N bonds of the cyclam ligand with concomitant formation of cyclam functionalized polylactide [13]. The selective insertion of heteroallenes into the Zr-N_amido bonds of (Bn₂Cyclam)Zr(OR)₂ (R = ⁱPr, ^tBu) revealed to be a suitable procedure for the N-functionalization of the cyclam ring [14]. In view of the importance of N-functionalized cyclams in several chemical [16,17,18,19,20,21] and biological [22,23,24,25,26] applications, we present here the reactivity of di-alkoxido zirconium(IV) complexes with carbon dioxide. In this manuscript, we also describe the synthesis and characterization of new mono-alkoxido zirconium(IV) complexes supported by tetradentate dianionic cyclam ligands and mixed complexes bearing one alkoxide and one methyl or thiophenoxide ligands. The formation of asymmetrically substituted cyclam-based Zr(IV) complexes of general formula (Bn₂Cyclam)Zr(X)(Y) is foreseen to display an important role in diverse catalytic transformations.

2. Materials and Methods

2.1. General Considerations

(Bn₂Cyclam)ZrCl₂, 1, [27] (Bn₂Cyclam)ZrCl(OⁱPr), 3, [13] (^3,5-Me2Bn₂Cyclam)ZrCl₂, 7, [16] (^4-tBuBn₂Cyclam)ZrCl₂, 8, [16] (3,5-Me-C₆H₄CH₂)₂Cyclam)Zr, 9 [19] (Bn₂Cyclam)Zr(OⁱPr)₂, 11 [14] and (^4-tBuBn₂Cyclam)Zr(OⁱPr)₂, 12 [15] were prepared according to previously described procedures. Lithium salts were prepared by lithiation of thiophenol or the corresponding alcohols with LiⁿBu. All other reagents were commercial grade and used without further purification. All manipulations were performed under an atmosphere of dry oxygen-free nitrogen by means of standard Schlenk and glovebox techniques. Carbon dioxide was supplied by Air Liquide and passed over a bed of molecular sieves before use. Solvents were pre-dried using 4 Å molecular sieves and refluxed over sodium-benzophenone under an atmosphere of N₂ and collected by distillation. Deuterated solvents were dried with 4 Å molecular sieves and freeze-pump-thaw degassed prior to use. NMR spectra were recorded in a Bruker AVANCE II 300 or 400 MHz spectrometer, at 296 K, referenced internally to residual proton-solvent (¹H) or solvent (¹³C) resonances, and reported relative to tetramethylsilane (0 ppm). 2D NMR experiments such as ¹H-¹³C{¹H} HSQC and ¹H-¹H COSY were performed to make all assignments. IR spectra were obtained on a Jasco FT/IR-4100 spectrometer. Elemental analyses were performed in a Fisons CHNS/O analyser Carlo Erba Instruments EA-1108 equipment (Thermo Scientific, Waltham, MA, USA) at the Analytical Laboratory of the Instituto Superior Técnico.

2.2. Synthesis and Characterization

(Bn₂Cyclam)ZrCl(O^tBu), 2: A THF solution of LiO^tBu (103 mg, 1.29 mmol) was added to a suspension of 1 (700 mg, 1.29 mmol) in the same solvent. The mixture was stirred overnight, and the yellowish suspension turned into a colorless solution. The solvent was evaporated, and the product was extracted with small volumes of warm toluene. Evaporation to dryness afforded a white solid in 62% yield (463 mg, 0.80 mmol). Crystalline material suitable for X-ray diffraction was obtained from a concentrated toluene solution at −20 °C. ¹H NMR (C₆D₆, 300.1 MHz, 296 K): δ (ppm) 7.16–7.10 (overlapping, 10H, total, PhCH₂N), 4.93 (m, ²J_H-H = 14 Hz, 1H, PhCH₂N), 4.63 (m, 1H, [C3]CH₂N), 4.52 (d, ²J_H-H = 14 Hz, 1H, PhCH₂N), 4.32 (d, ²J_H-H = 14 Hz, 1H, PhCH₂N), 4.26–4.14 (overlapping, 2H total, ²J_H-H = 14 Hz, 1H, PhCH₂N and 1H, [C3]CH₂N), 3.69 (m, 1H, [C3]CH₂N), 3.56–3.45 (overlapping, 2H total, [C2]CH₂N), 3.16 (m, 1H, [C3]CH₂N), 2.99 (m, 1H, [C2]CH₂N), 2.85–2.77 (overlapping, 2H total, 1H, [C3]CH₂N and 1H, [C2]CH₂N), 2.72–2.63 (overlapping, 3H total, 1H, [C3]CH₂N and 2H, [C2]CH₂N), 2.37–2.33 (overlapping, 2H total, [C3]CH₂N), 2.17 (m, 1H, [C2]CH₂N), 2.09 (m, 1H, [C2]CH₂N), 1.73–1.64 (overlapping, 10H total, 9H, C(CH₃)₃ and 1H, CH₂CH₂CH₂), 1.55 (m, 1H, CH₂CH₂CH₂), 1.16 (m, 1H, CH₂CH₂CH₂), 1.04 (m, 1H, CH₂CH₂CH₂). ¹³C{¹H} NMR (C₆D₆, 75.5 MHz, 296 K): δ (ppm) 133.0 (Ph), 132.9 (Ph), 128.6 (Ph), 127.9 (Ph), 76.8 (C(CH₃)₃), 57.0 (PhCH₂N), 56.7 ([C3]CH₂N), 56.4 ([C3]CH₂N), 56.2 (PhCH₂N), 56.0 ([C3]CH₂N), 54.5 ([C3]CH₂N), 53.1 ([C2]CH₂N), 51.8 ([C2]CH₂N), 48.4 ([C2]CH₂N), 48.2 ([C2]CH₂N), 33.3 (C(CH₃)₃), 25.0 (CH₂CH₂CH₂), 24.6 (CH₂CH₂CH₂). Anal. calcd for C₂₈H₄₇ClN₄OZr: C, 58.15; H, 7.49; N, 9.69. Found: C, 58.15; H, 7.55; N, 9.96.

(Bn₂Cyclam)ZrCl(OPh), 4: A THF solution of LiOPh (138 mg, 1.38 mmol) was added to a suspension of 1 (747 mg, 1.38 mmol) in the same solvent. The mixture was stirred overnight, and the yellowish suspension turned into a colorless solution. The solvent was evaporated, and the product was extracted with small volumes of warm toluene. Evaporation to dryness afforded a white solid in 56% yield (460 mg, 0.77 mmol). ¹H NMR (C₆D₆, 300.1 MHz, 296 K): δ (ppm) 7.42 (dd, ³J_H-H = 8 Hz, ⁴J_H-H = 1 Hz, 2H, o-PhOZr), 7.34 (t, ³J_H-H = 8 Hz, 2H, m-PhOZr), 7.13–7.00 (overlapping, 10H total, PhCH₂N), 6.90 (t, ³J_H-H = 8 Hz, 1H, p-PhOZr), 4.79 (m, ²J_H-H = 14 Hz, 1H, PhCH₂N), 4.59 (m, 1H, [C3]CH₂N), 4.32 (d, ²J_H-H = 14 Hz, 1H, PhCH₂N), 4.25–4.11 (overlapping, 3H total, ²J_H-H = 14 Hz, 2H, PhCH₂N and 1H, [C3]CH₂N), 3.58 (m, 1H, [C3]CH₂N), 3.54–3.41 (overlapping, 2H total, [C2]CH₂N), 3.11 (m, 1H, [C3]CH₂N), 2.99 (m, 1H, [C2]CH₂N), 2.80–2.66 (overlapping, 5H total, 2H, [C3]CH₂N and 3H, [C2]CH₂N), 2.39–2.28 (overlapping, 2H total, [C3]CH₂N), 2.18–2.05 (overlapping, 2H total, [C2]CH₂N), 1.63 (m, 1H, CH₂CH₂CH₂), 1.50 (m, 1H, CH₂CH₂CH₂), 1.17 (m, 1H, CH₂CH₂CH₂), 1.03 (m, 1H, CH₂CH₂CH₂). ¹³C{¹H} NMR (C₆D₆, 75.5 MHz, 296 K): δ (ppm) 164.7 (i-PhOZr), 133.0 (PhCH₂N), 132.8 (PhCH₂N), 132.5 (i-PhCH₂N), 132.2 (i-PhCH₂N), 129.8 (m-PhOZr), 128.3 (PhCH₂N), 120.0 (o-PhOZr), 119.0 (p-PhOZr), 56.8 ([C3]CH₂N), 56.5 (PhCH₂N), 56.3 ([C3]CH₂N and PhCH₂N), 55.2 ([C3]CH₂N), 54.8 ([C3]CH₂N), 53.3 ([C2]CH₂N), 52.2 ([C2]CH₂N), 48.7 ([C2]CH₂N), 48.4 ([C2]CH₂N), 25.2 (CH₂CH₂CH₂), 24.9 (CH₂CH₂CH₂). Anal. calcd for C₃₀H₃₉ClN₄OZr: C, 60.22; H, 6.57; N, 9.36. Found: C, 60.25; H, 6.63; N, 9.46.

(Bn₂Cyclam)Zr(SPh)(O^tBu), 5: A THF solution of LiSPh (65 mg, 0.56 mmol) was added to a suspension of 2 (300 mg, 0.55 mmol) in the same solvent. The mixture was stirred overnight, and the yellowish suspension turned into a colorless solution. The solvent was evaporated, and the product was extracted with small volumes of warm toluene. Evaporation to dryness afforded a white solid in 64% yield (228 mg, 0.35 mmol). ¹H NMR (C₆D₆, 400.1 MHz, 296 K): δ (ppm) 7.96 (d, ³J_H-H = 6 Hz, 2H, o-PhSZr), 7.18–6.92 (overlapping, 14H total, 10H, PhCH₂N, 2H, m-PhSZr and 2H, p-PhSZr), 4.78–4.71 (overlapping, 4H total, 3H, PhCH₂N and 1H, [C3]CH₂N), 4.33–4.23 (overlapping, 2H total, 1H, PhCH₂N and 1H, [C3]CH₂N), 4.09 (m, 1H, [C3]CH₂N), 3.72 (m, 1H, [C3]CH₂N), 3.52 (m, 1H, [C2]CH₂N), 3.20 (m, 1H, [C3]CH₂N), 3.03 (m, 1H, [C2]CH₂N), 2.88–2.78 (overlapping, 3H total, 1H, [C2]CH₂N and 2H, [C3]CH₂N), 2.72–2.61 (overlapping, 2H total, [C2]CH₂N), 2.40–2.31 (overlapping, 2H total, 1H, [C2]CH₂N and 1H, [C3]CH₂N), 2.19–2.17 (overlapping, 2H total, [C2]CH₂N), 1.76 (m, 1H, CH₂CH₂CH₂), 1.57 (m, 1H, CH₂CH₂CH₂), 1.46 (s, 9H, C(CH₃)₃), 1.20 (m, 1H, CH₂CH₂CH₂), 1.03 (m, 1H, CH₂CH₂CH₂). ¹³C{¹H} NMR (C₆D₆, 100.6 MHz, 296 K): δ (ppm) 133.3 (Ph), 133.0 (Ph), 132.9 (Ph), 132.6 (Ph), 128.6 (Ph), 128.4 (Ph), 127.9 (Ph), 122.7 (Ph), 77.5 (C(CH₃)₃), 57.0 (PhCH₂N), 56.3 ([C3]CH₂N), 56.1 ([C3]CH₂N and [C2]CH₂N), 55.6 (PhCH₂N), 54.5 ([C3]CH₂N), 53.4 ([C3]CH₂N), 52.3 ([C2]CH₂N), 49.4 ([C2]CH₂N), 48.5 ([C2]CH₂N), 32.4 (C(CH₃)₃), 25.0 (CH₂CH₂CH₂), 24.5 (CH₂CH₂CH₂). Anal. calcd for C₃₄H₄₈N₄OSZr: C, 62.63; H, 7.42; N, 8.59; S, 4.92. Found: C, 59.50; H, 7.72; N, 8.10; S, 5.33.

(Bn₂Cyclam)ZrMe(OⁱPr), 6: To a THF solution of 3 (112 mg, 0.20 mmol), 0.1 mL of a MeMgCl solution (2.18 M in THF) was slowly added. The mixture was refluxed overnight. The solvent was evaporated, and the light beige residue was extracted with toluene. To the toluene extract a few drops of dioxane was added. The suspension was filtered, and the solvents were evaporated to dryness affording a beige solid in 60% yield (65.3 mg, 0.12 mmol). ¹H NMR (toluene-d₈, 300.1 MHz, 296 K): δ (ppm) 7.14–6.96 (overlapping, 10H total, Ph), 4.99 (m, ³J_H-H = 6 Hz, 1H, CH(CH₃)₂), 4.78 (d, ²J_H-H = 14 Hz, 1H, PhCH₂N), 4.49–4.37 (overlapping, 2H total, 1H, [C3]CH₂N) and 1H, PhCH₂N), 4.24–4.03 (overlapping, 3H total, 1H, [C3]CH₂N) and 2H, PhCH₂N), 3.58–3.40 (overlapping, 3H total, 1H, [C3]CH₂N and 2H, [C2]CH₂N), 3.09–2.98 (overlapping, 2H total, 1H, [C3]CH₂N and 1H, [C2]CH₂N), 2.83–2.78 (overlapping, 2H total, 1H, [C3]CH₂N and 1H, [C2]CH₂N), 2.73–2.65 (overlapping, 3H total, 1H, [C3]CH₂N and 2H, [C2]CH₂N), 2.36–2.31 (overlapping, 2H total, [C3]CH₂N), 2.17–2.12 (overlapping, 2H total, [C2]CH₂N), 1.74–1.60 (overlapping, 2H total, CH₂CH₂CH₂), 1.54 (d, ²J_H-H = 6 Hz, 3H, CH(CH₃)₂), 1.48 (d, ²J_H-H = 6 Hz, 3H, CH(CH₃)₂), 1.16–1.03 (overlapping, 2H total, CH₂CH₂CH₂), 0.26 (s, 3H, Zr-CH₃). ¹³C{¹H} NMR (toluene-d₈, 75.5 MHz, 296 K): δ (ppm) 133.1 (Ph), 132.9 (Ph), 132.8 (Ph), 129.3 (Ph), 128.4 (Ph), 128.2 (Ph), 128.0 (Ph), 72.6 (CH(CH₃)₂), 56.9 (PhCH₂N or [C3]CH₂N), 56.8 (PhCH₂N or [C3]CH₂N), 56.3 (PhCH₂N or [C3]CH₂N), 56.2 (PhCH₂N or [C3]CH₂N), 56.0 (PhCH₂N or [C3]CH₂N), 54.0 ([C3]CH₂N), 52.9 ([C2]CH₂N), 51.7 ([C2]CH₂N), 48.3 ([C2]CH₂N), 48.0 ([C2]CH₂N), 27.8 (CH(CH₃)₂), 27.7 (CH(CH₃)₂), 24.8 CH₂CH₂CH₂), 24.5 (CH₂CH₂CH₂), 2.5 (Zr-CH₃) Anal. calcd for C₂₈H₄₄N₄OZr: C, 61.83; H, 8.15; N, 10.30. Found: C, 61.91; H, 8.18; N, 10.23.

(^3,5-Me2Bn₂Cyclam)Zr(OPh)₂, 10: In a J-Young NMR tube, phenol (14 mg, 0.15 mmol) was added to a toluene-d₈ solution of 9 (40 mg, 0.076 mmol). The formation of 10 was observed by NMR after 24 h. ¹H NMR (toluene-d₈, 300.1 MHz, 296 K): δ (ppm) 7.13–6.67 (overlapping, 16H total, 10H, PhOZr and 6H, PhCH₂N), 4.41 (d, ²J_H-H = 12Hz, 2H, PhCH₂N), 4.34 (d, ²J_H-H = 12 Hz, 2H, PhCH₂N), 4.22 (m, 2H, [C3]CH₂N), 3.62 (m, 2H, [C2]CH₂N), 3.18 (m, 2H, [C3]CH₂N), 2.82 (m, 2H, [C2]CH₂N), 2.69-2.61 (overlapping, 4H total, 2H, [C2]CH₂N and 2H, [C3]CH₂N), 2.54 (m, 2H, [C3]CH₂N), 2.43 (m, 2H, [C2]CH₂N), 2.14 (s, 12H, CH₃), 1.67 (m, 2H, CH₂CH₂CH₂), 1.08 (m, 2H, CH₂CH₂CH₂). ¹³C{¹H} NMR (toluene-d₈, 75.5 MHz, 296 K): δ (ppm) 164.4 (i-PhOZr), 137.7 (Ph), 130.7 (Ph), 129.1 (Ph), 128.8 (Ph), 128.5 (Ph), 125.6 (Ph), 119.9 (Ph), 56.7 ([C3]CH₂N), 56.5 (PhCH₂N), 54.6 ([C3]CH₂N), 52.3 ([C2]CH₂N), 48.5 ([C2]CH₂N), 25.0 (CH₂CH₂CH₂), 21.3 (CH₃).

{Bn₂(OCO)₂Cyclam}Zr(OCOOⁱPr)₂, 13: Carbon dioxide was bubbled into a toluene solution of 11 (125 mg, 0.21 mmol) at room temperature for 1h. The precipitate was filtered off and dried under vacuum to afford a white solid quantitatively. Anal. calculated for C₃₄H₄₈N₄O₁₀Zr: C, 53.45; H, 6.33; N, 7.33. Found: C, 53.78; H, 7.22; N, 7.36. FT-IR (KBr, cm⁻¹): 1377 and 1463 (ν_C-O).

{^4-tBuBn₂(OCO)₂Cyclam}Zr(OCOOⁱPr)₂, 14: Carbon dioxide was bubbled into a toluene solution of 12 (130 mg, 0.18 mmol) at room temperature for 1 h. The precipitate was filtered off and dried under vacuum to afford a white solid quantitatively. Anal. calculated for C₄₂H₆₄N₄O₁₀Zr: C, 57.57; H, 7.36; N, 6.39. Found: C, 57.90; H, 7.96; N, 6.34. FT-IR (KBr, cm⁻¹): 1377 and 1458 (ν_C-O).

2.3. General Procedure for X-ray Crystallography

Suitable crystals of compounds 2 and 12 were coated and selected in Fomblin^® oil under an inert atmosphere of nitrogen. Crystals were then mounted on a loop external to the glovebox environment and data was collected using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) on a Bruker AXS-KAPPA APEX II diffractometer (Bruker AXS Inc., Madison, WI, USA) equipped with an Oxford Cryosystem open-flow nitrogen cryostat. Cell parameters were retrieved using Bruker SMART and refined using Bruker SAINT software on all observed reflections [28]. Absorption corrections were applied using SADABS [29]. The structures were solved by direct methods using SIR97 [30]. Structure refinements were done using SHELXL [31], included in the WinGX-Version 1.80.01 system of programs [32]. Hydrogen atoms were inserted in calculated positions and allowed to refine in the parent atoms. Torsion angles, mean square planes, and other geometrical parameters were calculated using SHELXL [31]. Compound 2 crystalized with disordered molecules of the solvent in the asymmetric unit. As all attempts to model the disorder did not lead to acceptable solutions, the Squeeze/PLATON [33] sequence was applied. Crystallographic and experimental details of data collection and crystal structure determinations are available in

Table 1. Crystal data and details of structure refinement for compounds 2 and 12.

Parameters	2	12
Empirical formula	C28H43ClN4OZr	C38H64N4O2Zr
Formula weight	578.33	700.15
Temperature (K)	150(2)	150(2)
Crystal system, space group	Monoclinic, P21/n	Monoclinic, Cc
a, (Å)	12.311(1)	19.485(2)
b, (Å)	19.775(2)	18.109(2)
c, (Å)	14.228(2)	14.165(1)
α, (°)	90	90
β, (°)	97.297(4)	130.850(2)
γ, (°)	90	90
Volume (Å3)	3435.8(7)	3780.7(6)
Z	4	4
ρcalc (g/cm3)	1.118	1.230
μ (mm−1)	0.420	0.327
F(000)	1216	1504
Crystal size (mm3)	0.06 × 0.08 × 0.40	0.08 × 0.10 × 0.20
Θ range for data collection (°)	2.651 to 25.680	2.764 to 25.387
Limiting indices	−14 ≤ h ≤ 14 −23 ≤ k ≤ 24 −17 ≤ l ≤ 13	−23 ≤ h ≤ 23 −18 ≤ k ≤ 21 −17 ≤ l ≤ 17
Reflections collected/unique	19269/6506 [Rint = 0.0969]	18133/6687 [Rint = 0.0511]
Completeness to Θ = 25.242	99.7%	99.8%
Data/restraints/parameters	6506/24/310	6687/2/413
Goodness-of-fit on F2	0.878	1.028
Final R indexes [I ≥ 2σ(I)]	R1 = 0.0657, wR2 = 0.1606	R1 = 0.0463, wR2 = 0.0929
Final R indexes [all data]	R1 = 0.1258, wR2 = 0.1808	R1 = 0.0686, wR2 = 0.0996
Largest diff. peak and hole (e Å−3)	0.932 and −1.383	0.438 and −0.456

. Illustrations of the molecular structures were made with ORTEP-3 for Windows [34].

3. Results and Discussion

Monoalkoxido Zr(IV) complexes supported by diamido diamine cyclam based ligands were obtained by reaction of (Bn₂Cyclam)ZrCl₂, 1, with one equivalent of LiOR (R = ^tBu, 2, ⁱPr, 3, Ph, 4). The substitution of the remaining chloride in complexes 2 and 3 by thiophenoxide or methyl ligands was carried out using LiSPh or MeMgCl to afford (Bn₂Cyclam)Zr(SPh)(O^tBu), 5, and (Bn₂Cyclam)ZrMe(OⁱPr), 6, respectively. The synthetic route for the preparation of complexes 2–6 is shown in Scheme 1.

The presence of two different substituent groups in the adjacent positions of the zirconium coordination sphere in complexes 2–6 led to a reduction of symmetry from C₂ (in 1) to C₁. In accordance, the ¹H NMR spectra of complexes 2–6 reveal 20 resonances integrating to 1 proton each and 2 AB systems corresponding to the benzylic protons. In addition to the ancillary ligand resonances, the ⁱPr groups in 3 and 6 show two diastereotopic methyl resonances and one septet with ³J_H-H = 6 Hz. The ^tBu groups in 2 and 5 appear as a singlet at 1.73 and 1.46 ppm, respectively. The methyl groups coordinated to zirconium in 6 appear as a singlet at 0.26 ppm. In the ¹³C{¹H} NMR spectra, 12 resonances and 2 sets of aromatic signals attributed to the ancillary ligand carbons are observed as well as the resonances assigned to the other ligands. The ¹H and ¹³C NMR spectra of complexes 2–6 are presented in Figures S1–S4, respectively, in Supplementary Information.

Crystals of 2 suitable for single crystal X-ray diffraction were obtained from a concentrated toluene solution at −20 °C. An ORTEP depiction of its molecular structure along with selected bond lengths and angles are shown in Figure 1.

In complex 2, the zirconium is coordinated to the four nitrogen atoms of the cyclam ring, one chloride ligand and one oxygen atom of the O^tBu ligand in a trigonal prismatic geometry. The metal is located above the macrocycle at 1.098(2) Å from the average plane defined by the four nitrogen atoms. The Zr-N_amido and the Zr-N_amine bond lengths lie within the expected ranges for similar bonds in hexa-coordinated Zr(IV) complexes [13,14,15,16,18,19,27,35,36,37,38]. The Zr-Cl and Zr-O bond lengths of 2.528(3) and 1.958(4) Å, respectively, and the O(1)-Zr(1)-Cl(1) angle at 87.0(1)° are comparable with other Zr(IV) complexes based on Bn₂Cyclam ligands [13,14,16,18,19,27,38].

Dialkoxido derivatives can be obtained from the reaction of the dichlorido precursor with two equivalents of a suitable lithium alkoxido [13,14] or by protonolysis of an orthometallated species with an alcohol [18]. (^3,5-Me2Bn₂Cyclam)Zr(OPh)₂, 10, was prepared using the latter strategy by reaction of (3,5-Me-C₆H₄CH₂)₂Cyclam)Zr, 9, with two equivalents of phenol. The synthesis of (Bn₂Cyclam)Zr(OⁱPr)₂, 11, and (^4-tBuBn₂Cyclam)Zr(OⁱPr)₂, 12, was carried out by reaction of (Bn₂Cyclam)ZrCl₂, 1, and (^4-tBuBn₂Cyclam)ZrCl₂, 8, with two equivalents of LiOⁱPr, respectively. The metathesis reaction is the unique route for the synthesis of 12 because an orthometallated species similar to 9 cannot be formed in view of the stereochemical constraints imposed by the tert-butyl substituents on the para positions of the benzylic pendant arms of the macrocycle. The synthetic route for the preparation of complexes 10–12 is shown in Scheme 2.

The ¹H NMR spectra of complexes 10–12 show 10 resonances integrating for 2 protons each and corresponding to the H_anti and H_syn methylene protons of the macrocycle backbone. The AB system assigned to the benzylic protons of the cyclam pendant arms in 10 appears at 4.41 and 4.34 ppm. Two sets of aromatic resonances are observed in the spectrum of 10, which correspond to the OPh and NCH₂Ph groups. The ¹³C{¹H} NMR spectrum of 10 exhibit six resonances attributed to the ancillary ligand methylene carbons and the OPh ligand give raise to one set of resonances in agreement with a C₂ symmetry. The ¹H and ¹³C NMR spectrum of 10 is presented in Figure S5 in Supplementary Information.

Crystals of 12 suitable for single crystal X-ray diffraction were obtained from a concentrated toluene solution at −20 °C. An ORTEP depiction of its molecular structure along with selected bond lengths and angles are shown in Figure 2.

In complex 12, the zirconium is coordinated to the four nitrogen atoms of the macrocycle, and to the oxygen atoms of the isopropoxido ligands in a trigonal prismatic geometry. The four nitrogen atoms of the macrocycle define one rectangular face, as commonly reported for other cyclam based complexes [13,14,15,16,18,19,27,35,36,37,38]. The metal is located above the macrocycle at 1.222(2) Å from the average plane defined by the cyclam nitrogen atoms. The Zr-N_amido, Zr-N_amine and Zr-O bond lengths are within the ranges usually observed for this type of bonds in Zr(IV) complexes of formula (Bn₂Cyclam)Zr(OR)₂ (R = ⁱPr, ^tBu, Ph) [13,14,18,19]. The O(1)-Zr(1)-O(2) angle is in agreement with the values found in other Zr(IV) trigonal prismatic complexes supported by a tetradentate dianionic cyclam-based ligand [13,14,18,19]. The wide Zr(1)-O(1)-C(33) and Zr(1)-O(2)-C(36) angles of 161.3(5)° and 142.0(5)°, respectively, reflect the substantial π-donation of the oxygen atoms to the Zr metal center.

A preliminary study on the reactivity of (Bn₂Cyclam)Zr(OⁱPr)₂, 11, or the more soluble analogue (^4-tBuBn₂Cyclam)Zr(OⁱPr)₂, 12, with 2 equivalents of CO₂ revealed the formation of a mixture of products that were not possible to fully characterize [15]. Bubbling carbon dioxide in a toluene solution of 11 or 12 led to the formation of very insoluble white precipitates. The insolubility of the products in the most common deuterated solvents did not allow their characterization by NMR. The C, H, N elemental analyses suggest the formation of a species of empirical formulas C₃₄H₄₈N₄O₁₀Zr and C₄₂H₆₄N₄O₁₀Zr that corresponds to the addition of four CO₂ molecules to (Bn₂Cyclam)Zr(OⁱPr)₂, 11, and (^4-tBuBn₂Cyclam)Zr(OⁱPr)₂, 12, respectively. The IR spectra of both compounds (see Figures S6 and S7 in Supplementary Information) show strong absorption bands in the range 1377–1463 cm⁻¹ that were attributed to the presence of carbamate and carbonate fragments. The X-ray powder diffractogram of the solids also revealed the presence of zirconium carbonate confirming CO₂ insertion into the Zr-O bonds. Although the insolubility of the products did not allow further characterization and the data obtained do not provide detailed structural information, one may speculate that CO₂ added to both Zr-OⁱPr and Zr-N_amido bonds of the di-alkoxido Zr(IV) derivatives 11 and 12 give {Bn₂(OCO)₂Cyclam}Zr(OCOOⁱPr)₂, 13, and {^4-tBuBn₂(OCO)₂Cyclam}Zr(OCOOⁱPr)₂, 14, respectively. This reactivity pattern contrasts with that observed for the reaction of other heteroallenes with cyclam-based Zr(IV) alkoxidos. Isocyanates and CS₂ selectively insert into Zr-N_amido bonds of (Bn₂Cyclam)Zr(OⁱPr)₂ to form N-bonded ureate and dithiocarbamate fragments, respectively [14,15]. The insertion of carbon dioxide into Zr-OⁱPr bonds to form carbonate fragments may be attributed to the high oxophilicity of zirconium. The hydrolysis of compounds 13 and 14 was not possible due to their stability in protic solvents, which prevented the isolation of the corresponding N-functionalized cyclams.

4. Conclusions

Various cyclam-based Zr(IV) alkoxido derivatives have been synthesized and fully characterized. The structural characterization of the complexes in solution and in the solid state are consistent and reveals that the cyclam ligand remains tetracoordinated to the zirconium in the solution. Complexes (Bn₂Cyclam)ZrCl(O^tBu) and (^4-tBuBn₂Cyclam)Zr(OⁱPr)₂ display trigonal prismatic geometries with X-Zr-X’ angles of 87.0(1)° and 92.1(2)° and distances between the zirconium and the plane defined by the four nitrogen atoms of the cyclam of 1.098(2) and 1.222(2) Å, respectively. The Zr-N_amine and Zr-N_amido bond lengths are unaffected by the type of ligands that complete the zirconium coordination sphere (2.453(5) < Zr-N_amine < 2.518(5) Å and 2.068(5) < Zr-N_amido < 2.101(5) Å). Although in both complexes the Zr-O distances are comprised in the small range 1.958(4)–2.021(5) Å, the Zr-O-C angles assume values from 142.0(5)° to 161.8(4)°. There is no correlation between the decreasing of the Zr-O distances and the widening of the Zr-O-C angles. All compounds are thermally stable and, surprisingly, (Bn₂Cyclam)ZrMe(OⁱPr) does not undergo C-H activation of the benzyl pendant arms of the cyclam ligand with formation of orthometalated species as observed in other Zr(IV) alkyl derivatives supported by Bn₂Cyclam [17,18,19,35,38]. This observation points out that the bonding of the isopropoxide ligand to the (Bn₂Cyclam)Zr core stabilizes the Zr-C bond, a feature that may have important implications in catalysis and deserves further research. The reaction of (Bn₂Cyclam)Zr(OⁱPr)₂ and (^4-tBuBn₂Cyclam)Zr(OⁱPr)₂ with excess of carbon dioxide led to the formation of very insoluble products that are tentatively assigned as {Bn₂(OCO)₂Cyclam}Zr(OCOOⁱPr)₂ and {^4-tBuBn₂(OCO)₂Cyclam}Zr(OCOOⁱPr)₂, respectively, which might be formed from the addition of CO₂ molecules to both Zr-OⁱPr and Zr-N_amido bonds.