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

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(OtBu) and (4-tBuBn2Cyclam)Zr(OiPr)2 show a trigonal prismatic geometry around the metal centers. (Bn2Cyclam)Zr(SPh)(OtBu) 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.


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 σand 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 (Bn 2 Cyclam)Zr(OR) 2 (R = i Pr, t Bu) 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 monoalkoxido 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

Synthesis and Characterization
(Bn 2 Cyclam)ZrCl(O t Bu), 2: A THF solution of LiO t Bu (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. 1   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). 1

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

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 = t Bu, 2, i Pr, 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 t Bu), 5, and (Bn2Cyclam)ZrMe(O i Pr), 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 C 2 (in 1) to C 1 . In accordance, the 1 H 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 i Pr groups in 3 and 6 show two diastereotopic methyl resonances and one septet with 3 J H-H = 6 Hz. The t Bu 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 13 C{ 1 H} 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 1 H and 13 C 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 atom ring, one chloride ligand and one oxygen atom of the O t Bu ligand in a trig geometry. The metal is located above the macrocycle at 1.098(2) Å from the defined by the four nitrogen atoms. The Zr-Namido and the Zr-Namine bond len the expected ranges for similar bonds in hexa-coordinated Zr(IV) co 16,18,19,27,[35][36][37][38]. The Zr-Cl and Zr-O bond lengths of 2.528(3) an respectively, and the O(1)-Zr(1)-Cl(1) angle at 87.0(1)° are comparable wi complexes based on Bn2Cyclam ligands [13,14,16,18,19,27,38].
Dialkoxido derivatives can be obtained from the reaction of the dichlo with two equivalents of a suitable lithium alkoxido [13,14] or by prot orthometallated species with an alcohol [18]. ( 3,5-Me2 Bn2Cyclam)Zr(OPh)2, 10 using the latter strategy by reaction of (3,5-Me-C6H4CH2)2Cyclam)Zr equivalents of phenol. The synthesis of (Bn2Cyclam)Zr(O i Pr)2, tBu Bn2Cyclam)Zr(O i Pr)2, 12, was carried out by reaction of (Bn2Cyclam)Z tBu Bn2Cyclam)ZrCl2, 8, with two equivalents of LiO i Pr, respectively. T reaction is the unique route for the synthesis of 12 because an orthomet similar to 9 cannot be formed in view of the stereochemical constraints im tert-butyl substituents on the para positions of the benzylic pendant 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 t Bu 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 Bn 2 Cyclam 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-Me2 Bn 2 Cyclam)Zr(OPh) 2 , 10, was prepared using the latter strategy by reaction of (3,5-Me-C 6 H 4 CH 2 ) 2 Cyclam)Zr, 9, with two equivalents of phenol. The synthesis of (Bn 2 Cyclam)Zr(O i Pr) 2 , 11, and ( 4-tBu Bn 2 Cyclam)Zr(O i Pr) 2 , 12, was carried out by reaction of (Bn 2 Cyclam)ZrCl 2 , 1, and ( 4-tBu Bn 2 Cyclam)ZrCl 2 , 8, with two equivalents of LiO i Pr, 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.
Bn2Cyclam)ZrCl2, 8, with two equivalents of LiO Pr, respectively. The m reaction is the unique route for the synthesis of 12 because an orthometallate similar to 9 cannot be formed in view of the stereochemical constraints impose tert-butyl substituents on the para positions of the benzylic pendant arm macrocycle. The synthetic route for the preparation of complexes 10-12 is s Scheme 2. The 1 H 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 NCH 2 Ph groups. The 13 C{ 1 H} 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 C 2 symmetry. The 1 H and 13 C 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.  Figure SI5 in Information.
Crystals of 12 suitable for single crystal X-ray diffraction were concentrated toluene solution at −20 °C. An ORTEP depiction of its mo along with selected bond lengths and angles are shown in Figure 2. In complex 12, the zirconium is coordinated to the four nitrog macrocycle, and to the oxygen atoms of the isopropoxido ligands in a t geometry. The four nitrogen atoms of the macrocycle define one rec commonly reported for other cyclam based complexes [13][14][15][16]18,19,27,3 is located above the macrocycle at 1.222(2) Å from the average plane defin 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.
A preliminary study on the reactivity of (Bn 2 Cyclam)Zr(O i Pr) 2 , 11, or the more soluble analogue ( 4-tBu Bn 2 Cyclam)Zr(O i Pr) 2 , 12, with 2 equivalents of CO 2 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 C 34 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 CO 2 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 CO 2 added to both Zr-O i Pr and Zr-N amido bonds of the di-alkoxido Zr(IV) derivatives 11 and 12 give {Bn 2 (OCO) 2 Cyclam}Zr(OCOO i Pr) 2 , 13, and { 4-tBu Bn 2 (OCO) 2 Cyclam}Zr(OCOO i Pr) 2 , 14, respectively. This reactivity pattern contrasts with that observed for the reaction of other heteroallenes with cyclam-based Zr(IV) alkoxidos. Isocyanates and CS 2 selectively insert into Zr-N amido bonds of (Bn 2 Cyclam)Zr(O i Pr) 2 to form N-bonded ureate and dithiocarbamate fragments, respectively [14,15]. The insertion of carbon dioxide into Zr-O i Pr 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.

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 (Bn 2 Cyclam)ZrCl(O t Bu) and ( 4-tBu Bn 2 Cyclam)Zr(O i Pr) 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, (Bn 2 Cyclam)ZrMe(O i Pr) 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 Bn 2 Cyclam [17][18][19]35,38]. This observation points out that the bonding of the isopropoxide ligand to the (Bn 2 Cyclam)Zr core stabilizes the Zr-C bond, a feature that may have important implications in catalysis and deserves further research. The reaction of (Bn 2 Cyclam)Zr(O i Pr) 2 and ( 4-tBu Bn 2 Cyclam)Zr(O i Pr) 2 with excess of carbon dioxide led to the formation of very insoluble products that are tentatively assigned as {Bn 2 (OCO) 2 Cyclam}Zr(OCOO i Pr) 2 and { 4-tBu Bn 2 (OCO) 2 Cyclam}Zr(OCOO i Pr) 2 , respec-tively, which might be formed from the addition of CO 2 molecules to both Zr-O i Pr and Zr-N amido bonds.