Molybdenum (VI) Imido Complexes Derived from Chelating Phenols: Synthesis, Characterization and ɛ-Caprolactone ROP Capability

Reaction of the bulky bi-phenols 2,2′-RCH[4,6-(t-Bu)2C6H2OH]2 (R = Me L1MeH2, Ph L1PhH2) with the bis(imido) molybdenum(VI) tert-butoxides [Mo(NR1)(NR2)(Ot-Bu)2] (R1 = R2 = 2,6-C6H3-i-Pr2; R1 = t-Bu, R2 = C6F5) afforded, following the successive removal of tert-butanol, the complexes [Mo(NC6H3i-Pr2-2,6)2L1Me] (1), [Mo(NC6H3i-Pr2-2,6)2L1Ph] (2) and [Mo(Nt-Bu)(μ-NC6F5)(L1Me)]2 (3). Similar use of the tri-phenol 2,6-bis(3,5-di-tert -butyl-2-hydroxybenzyl)-4-methylphenol (L2H3) with [Mo(NC6H3i-Pr2-2,6)2(Ot-Bu)2] afforded the oxo-bridged product [Mo(NC6H3i-Pr2-2,6)(NCMe)(μ-O)L2H]2 (4), whilst use of the tetra-phenols α,α,α′,α′-tetrakis(3,5-di-tert-butyl-2-hydroxyphenyl)-por -m-xylene L3pH4/L3mH4 led to {[Mo(NC6H3i-Pr2-2,6)2]2(μ-L3p)} (5) or {[Mo(NC6H3i-Pr2-2,6)2]2(μ-L3m)} (6), respectively. Similar use of [Mo(NC6F5)2(Ot-Bu)2] with L3pH4 afforded, after work-up, the complex {[Mo(NC6F5)(Ot-Bu)2]2(μ-L3p)}·6MeCN (7·6MeCN). Molecular structures of 1, 2·CH2Cl2, 3, 4·6MeCN, 6·2C6H14, and 7·6MeCN are reported and these complexes have been screened for their ability to ring open polymerize (ROP) ε-caprolactone; for comparative studies the precursor complex [Mo(NC6H3i-Pr2-2,6)2Cl2(DME)] (DME = 1,2-dimethoxyethane) has also been screened. Results revealed that good activity is only achievable at temperatures of ≥100 °C over periods of 1 h or more. Polymer polydispersities were narrow, but observed molecular weights (Mn) were much lower than calculated values. OPEN ACCESS Catalysts 2015, 5 1929


Introduction
There remains significant academic interest in the design of new catalyst systems capable of producing biodegradable polymers [1][2][3].This interest stems in part from issues relating to the inertness of polyethylene and subsequent landfill issues, but also from the potential for such biodegradable polymers to be employed in other areas such as the biomedical arena.This is typified by polycaprolactone (PCL), which has found application in tissue engineering and possesses drug permeability [4].The formation of PCL via the ring opening polymerization (ROP) of ε-caprolactone using metal complexes as initiators, usually in the form of alkoxides, is a favoured synthetic route for PCL formation.However, despite the continued interest in such systems, the catalysts deployed in ROP tend to be either based on main group species, primarily of aluminum, or are based on a select number of transition metals, lanthanides or, more recently, systems utilizing alkali/alkaline earth metals.By contrast, the more earth-abundant metals, have received far less attention.We were attracted to the potential use of molybdenum for the ROP of cyclic esters given its excellent track record over the last couple of decades in ring opening metathesis polymerization (ROMP), as well as its low cost and toxicity.Such ROMP studies have revealed the ability of the molybdenum complexes to promote living polymerizations, and to tolerate a wide range of functionalities, which bodes well for the proposed studies herein [5].Central to such chemistry has been the use of high valent bis(imido) species, due to their ease of preparation and facile modification.The variety of precursor anilines available means that there is much scope for controlling both the steric and electronic properties of the resultant imido group at the metal, which in turn can influence both the catalytic activity and properties of the polymer products.With this in mind, we recently reported the use of molybdenum chelate complexes derived from the oxydianiline [(2-NH2C6H4)2O], and found that that for the ROP of ε-caprolactone, conversion rates were good (>90%) at high temperatures (100 °C) [6].As part of that study, a siloxide complex was also isolated and was found to be active without the need for the addition of external alcohol; for the chloride species the addition of benzyl alcohol was necessary to generate an alkoxide.Previous use of molybdenum species in the ROP of cyclic esters is somewhat limited [7][8][9][10][11][12].Given this, we have now extended our studies to high-valent molybdenum imido phenolate chemistry, where again the expectation is that the addition of an external alcohol would not be necessary for ROP activity.We report the use of bulky di-phenols in combination with bulky organoimido groups which allows for the isolation of mono-nuclear four coordinate complexes, whilst variation of the imido group can lead to bridged di-nuclear species.The use of tri-and tetra-phenolates has also been explored, and in the case of the latter, in the form of the para and meta pro-ligands, the possibility of possible cooperative effects has been investigated.The complexes prepared/screened herein are shown in Scheme 1.

Di-Phenolate Compounds
The interaction of [Mo(NC6H3i-Pr2-2,6)2(Ot-Bu)2] (formed in situ from [Mo(NC6H3i-Pr2-2,6)2Cl2(dme)] and a slight excess of LiOt-Bu) and the di-phenol 2,2′-CH3CH[4,6-(t-Bu)2C6H2OH]2 (L 1Me H2) in diethyl ether readily gives multigram quantities of [Mo(NC6H3i-Pr2-2,6)2L 1Me ] (1) in good yield (ca. 70%).Stoichiometrically 1 is formed via the loss of two molecules of tert-butanol, which can be removed during the reaction by removing volatiles in-vacuo and then adding more solvent (diethyl ether) and repeating the process several times.Small golden-yellow prisms of 1 suitable for an X-ray structure determination using synchrotron radiation were grown from a saturated heptane solution at ambient temperature.The molecular structure is shown in Figure 1 with bond lengths and angles given in the caption; crystallographic data is presented in Table 1.There is one molecule in the asymmetric unit.The space group is chiral and essentially a single enantiomer has crystallized out.The geometry about the Mo atom is essentially tetrahedral with distortions from ideal varying from 104.93 (12) to 119.03(10)°.
Use of the mixed-imido precursor [Mo(Nt-Bu)(NC6F5)(Ot-Bu)2] with L 1Me H2 led, following work-up, to the orange complex [Mo(Nt-Bu)(NC6F5)L 1Me ]2 (3), which was readily crystallized from a saturated acetonitrile solution on prolonged standing (2 days) at ambient temperature.The molecular structure (Figure 3) revealed that half of the complex comprises the asymmetric unit.The molecule lies on a centre of symmetry i, and possesses asymmetric imido (C6F5) bridges, the latter arising given the differing trans environments.The terminal tert-butylimido groups are near linear [Mo-N(1)-C(1) 178.0(2)°].There is literature precedent for bending of C6F5N groups in preference to tert-butylimido groups when present in the same complex, which is attributed to the more electron-releasing nature of the latter [14].Furthermore, bridging arylimido groups have been structurally characterized in a complex also containing linear, terminal tert-butylimido ligation [24].The eight membered ring chelates each adopt a flattened chair-like conformation; the "bite angle" of the chelates are 117.20(9)°.

Tri-Phenolate Compound
When the tri-phenol 2,6-bis (3,5- ) was isolated from a saturated acetonitrile solution on prolonged standing at ambient temperature.The presence of the oxo bridges was thought to be the result of fortuitous hydrolysis (also resulting in the elimination of aniline).The molecular structure of 4 is shown in Figure 4, with selected bond lengths and angles given in the caption.Half of the complex and three acetonitrile molecules comprise the asymmetric unit.The molecule resides on a centre of symmetry i, and possesses asymmetric oxo bridges.Each molybdenum centre exhibits a distorted octahedral environment, for example Mo(1) is 0.3349(6) Å out of the O4 plane.Of the three acetonitrile molecules of crystallization, two lie in clefts of the phenol/di-phenolate ligand, namely those solvent molecules containing N(4) and N(5); the other containing N(3) lies between molecules of 4. The bonding mode of the tri-phenol derived ligand in 4 is reminiscent of that observed for the tungsten(VI) complex [W(eg)2L a H] (eg = 1,2-ethanediolato, L a H = doubly deprotonated form of 2,6-bis(3,5-dimethyl -2-hydroxybenzyl)-4-t-butylphenol) and the niobium complexes [NbCl3(NCMe)L b H] and [NbCl(NCMe)L a/b H]2 (L b H = doubly deprotonated form of 2,6-bis(4-methyl-6-t-butylsalicyl) -4-t-butylphenol [26,27].The eight membered ring chelates adopts a boat-like conformation; the "bite angle" of the chelate is 93.43(7)°, which is much smaller than observed in 1-3 (ca.119°) due to the higher coordination number of 6 as opposed to 4 or 5.In solution however, the 1 H NMR spectrum (in CDCl3, C6D6 or CD3CN) of 4 contained only two resonances for the tert-butyl groups, which is not consistent with the unsymmetrical nature of the tri-phenol derived ligand observed in the solid state.

Tetra-Phenolate Compounds
The synthetic methodology was then extended to the relatively unexplored tetra-phenols α,α,α′,α′-tetrakis(3,5-di-tert-butyl-2-hydroxyphenyl)-p-xylene L 3p H4 and α,α,α′,α′-tetrakis(3,5-di-tert -butyl-2-hydroxyphenyl)-m--xylene L 3m H4 [28].Treatment of either L 3p H4 or L 3m H4 with 6), respectively in moderate to good yield.Red plate-like crystals of 6•2C6H14 suitable for an X-ray structure determination were obtained on recrystallization from a saturated hexane solution at 0 °C.The molecular structure of 6 is shown in Figure 5 with selected bond lengths and angles given in the caption; crystallographic data are given in Table 1.The molecule lies on a 2-fold axis that passes through the vector C(32)-C (33).Each molybdenum centre is four coordinate and exhibits a distorted tetrahedral geometry with angles in the range 105.30(7) to 115.06(5)°, the largest angle being associated with the chelate.Each eight membered ring chelate adopts a flattened chair-like conformation.The imido groups are categorized as linear, with that at N(1) lying to the lower end of the range associated with linearity [Mo-N(1)-C(34) 156.29 (16)°].The difference between the imido angles here in 6 is ca.13.6°, which compares favorably with that in 1 (ca.13.0°) and is slightly smaller than that observed in 2 (ca.15

.0°).
There is tendency in these complexes for the shorter Mo-N bond length to be associated with the larger Mo-N-C angle; a similar situation has been observed in molybdenum and tungsten imido alkylidene chemistry [29][30][31][32].The distance between Mo centers in this system is 10.588 Å. Surprisingly, when the imido precursor employed was [Mo(NC6F5)2(Ot-Bu)2] with L 3p H4, the reaction proceeded via loss of aniline rather than alcohol, which must be due to the differing electronics associated with the C6F5 group.Crystals of {[Mo(NC6F5)(Ot-Bu)2]2(μ-L 3p )}•6MeCN (7•6MeCN) were grown from a saturated acetonitrile solution.The molecular structure is shown in Figure 6, with selected bond lengths and angles given in the caption.Each molybdenum centre in 7 is five coordinate, bound by the chelate, one imido group and two tert-butoxide ligands.There are two similar metal complexes and twelve acetonitrile molecules in the asymmetric unit.The geometries at the metal can best be described as distorted trigonal pyramidal with the imido group and one of the chelate phenoxide oxygen atoms occupying axial positions.The imido group is slightly bent  The molecular structure of the complex [Mo(NC6H3i-Pr2-2,6)2Cl2(dme)] (8) has also been determined and is given in the (see ESI, Figure S1, Tables S1 and S2).A number of such mononuclear bis(imido) dichloro molybdenum(VI) complexes have been structurally characterized; a search of the CSD revealed 14 hits [22].

Polymerization Screening
Complexes 1-8 have been screened for their ability to act as catalysts for the ring opening polymerization (ROP) of ε-caprolactone and the results are presented in Table 2.At temperatures below 80 °C, the systems were inactive.At 80 °C, the systems utilizing 1, 2, 6 and 7 exhibited moderate activity with conversions of about 45%-50%, whilst the combination of 8/BnOH exhibited good conversion (ca.85%).At 100 °C, there was little or no activity for reaction times of less than one hour.In most cases, excellent conversions were achieved over 6 h, and little was gained by prolonging the reaction time beyond this point.Although the complexes 1-7 are phenolates (and 7 also a tert-butoxide), we have screened them both in the presence and absence of benzyl alcohol (BnOH) to monitor if this is beneficial or not.The presence or absence of BnOH had little effect on %conversion or control (eg runs 30 v 31 and 39 v 40), though depending on the temperature there was either an increase or decrease in the observed molecular weight (Mn).All systems produced polyesters with narrow dispersities with unimodal characteristics (Mw/Mn 1.08 to 1.72); those at the higher end of the range were associated with increases in the CL:cat:BnOH ratio (runs 17, 19 and 20) and are perhaps indicative of some transesterification reactions occurring under such conditions.Such ratio changes (for 3) however led to little change in the %conversion.
In terms of structure-activity relationships, in the case of 1 versus 2, the presence of the bulkier phenyl group in the bridge of the di-phenol appears to have only a slight effect with 1 exhibiting a better conversion at 100 °C over 6 h (99% cf.95%).The bridging imido complex 3 exhibits activity on a par with 1 containing the same di-phenolate ligand.Analysis of the results for the tetra-phenolate systems 5 to 7 indicates that at 100 °C over various reaction times, the meta system out-performs the para system (i.e., 5 v 6; runs 24 v 29, 26 v 32, 27 v 33), which is tentatively assigned to the closer proximity of the metal centers in 6 and thus an enhanced cooperative effect.A comparison of the use of different imido group in the meta system (6 v 7) is not possible given the different structures adopted, however it is evident that meta system 7 is comparable with 6 over 6 or 12 h and is superior over shorter reaction periods (runs 35 and 36).There appears to be no advantage in having two metals present over one (5, 6 cf. 1, 2).Interestingly, the bis(imido) dichloride complex 8, in the presence of BnOH, also exhibits excellent conversions at 100 °C when employed for 3 h or more.
In general, the observed polymer molecular weights were lower than expected, which indicates that in most cases, there were significant trans-esterification reactions occurring.Such a trend has been noted previously when using molybdenum-based species [6][7][8][9][10][11][12].The MALDI-ToF spectra of the resultant PCL revealed (see ESI Figure S2) a major series of peaks with separation 114 g•mol −1 (i.e., the monomer) with evidence of a secondary minor set of peaks resulting from hydrolysis under ionization conditions.Examination of the 1 H NMR spectrum (see ESI Figure S3) of the same samples revealed peaks at δ 5.10 and 3.65 assigned to benzyl ester and hydroxymethylene end groups.Interestingly for 3, a plot of number average molecular weight (Mn) and monomer conversion was approximately linear, which was suggestive of a well-controlled polymerization, this despite the apparent trans-esterification processes present (see ESI Figure S4).
Comparison of these systems with other molybdenum-based catalysts reveals that it is typical for high temperatures (≥80 °C) to be employed to achieve activity.Neutral chelate complexes derived from the oxydianiline [(2-NH2C6H4)2O] can achieve good conversion rates (>90%) at high temperatures (100 °C) over 12h; the tetra-nuclear siloxide complex performed best achieving a conversions >90% over 1 h [6].Of the other Mo-based systems known, bis(salicylaldehydato)dioxomolydenum operates effectively at 110 °C in mesitylene, whilst ammonium decamolybdate functions as a melt at 150 °C [9,12].As observed herein, such molybdenum systems are susceptible to trans-esterification processes.

Procedure for ROP
Typical polymerization procedures in the presence of one equivalent of benzyl alcohol (Table 1, run 1) are as follows.A toluene solution of 1 (0.010 mmol, in 1.0 mL toluene) and BnOH (0.010 mmol) were added into a Schlenk tube in the glove-box at room temperature.The solution was stirred for 2 min, and then ε-caprolactone (2.5 mmol) along with 1.5 mL toluene was added to the solution.The reaction mixture was then placed into an oil bath pre-heated to the required temperature, and the solution was stirred for the prescribed time.The polymerization mixture was then quenched by addition of an excess of glacial acetic acid (0.2 mL), and the resultant solution was then poured into methanol (200 mL).The resultant polymer was then collected on filter paper and was dried in vacuo.

Crystallography
X-ray diffraction data for 1 were collected using synchrotron radiation at Daresbury Laboratory, Station 9.8, using silcon 111 monochromated radiation and a Bruker 1K CCD detector.X-ray diffraction data for 2 and 8 were collected using a Stoe & Cie GmbH, Darmstadt, Germany.Diffraction data for 3 and 4•6MeCN were collected on a Bruker AXS GmbH, Karlsruhe, Germany.Diffraction data for 6•2C6H14 and 7•6MeCN were collected on a Rigaku Corp., Tokyo, Japan.All data collections except that for 1 utilised monochromated Mo-Kα radiation and ω-scans.Standard procedures were employed

Table 1 .
Crystallographic data for the complexes 1