Cyanosilylation of Aldehydes Catalyzed by Ag(I)-and Cu(II)-Arylhydrazone Coordination Polymers in Conventional and in Ionic Liquid Media

: The novel Ag(I) and Cu(II) coordination polymers [Ag( µ 3 -1 κ O ;2:3 κ O (cid:48) ;4 κ N -HL)] n · n/2H 2 O ( 1 ) and [Cu(en) 2 ( µ -1 κ O ;2 κ N -L)] n · nH 2 O ( 2 ) [HL − = 2-(2-(1-cyano-2-oxopropylidene)hydrazinyl)benzene sulfonate] were synthesized and characterized by IR and ESI-MS spectroscopies, elemental and single crystal X-ray diffraction analyses. Compounds 1 and 2 as well as the already known complex salt [Cu(H 2 O) 2 (en) 2 ](HL) 2 ( 3 ) have been tested as homogenous catalysts for the cyanosilylation reaction of different aldehydes with trimethylsilyl cyanide, to provide cyanohydrin trimethylsilyl ethers. Coordination polymer 2 was found to be the most efficient one, with yields ranging from 76 to 88% in methanol, which increases up to 99% by addition of the ionic liquid [DHTMG][ L -Lactate].


Scheme 1. Cyanosilylation of aldehydes.
MOFs or coordination polymers are attractive not only as storage, transporter, magnetic and luminescence materials [45][46][47][48], but also as catalysts for various catalytic transformations, such as alkane/alcohol oxidation [49,50], C-C bond formation [40,44], etc., from the viewpoint of green chemistry. The design and synthesis of coordination polymer catalysts, in particular being inexpensive, highly efficient and selective with a wide range of substrates, remains a challenging goal in cyanosilylation reaction. Having this objective in mind, we intend to report the synthesis of new copper(II) and Ag(I) coordination polymers bearing an arylhydrazone ligand and their application in cyanosilylation reaction of different aldehydes with trimethylsilyl cyanide.
Moreover, ionic liquids (ILs) have wide applications in catalysis, namely as green solvents or reaction promoters [51], being able, in some cases, to enhance the reaction rate, improve the yield and selectivity in organic transformations under mild conditions. To date, a few ILs have been reported in the cyanosilylation of aldehydes [52,53]. Thus, another aim of this work is to use a cooperative action of ILs and coordination polymers in cyanosilylation reaction, a type of approach that we have successfully applied in the oxidation of alkanes [54,55] and oxidation of alcohols [56].
Moreover, ionic liquids (ILs) have wide applications in catalysis, namely as green solvents or reaction promoters [51], being able, in some cases, to enhance the reaction rate, improve the yield and selectivity in organic transformations under mild conditions. To date, a few ILs have been reported in the cyanosilylation of aldehydes [52,53]. Thus, another aim of this work is to use a cooperative action of ILs and coordination polymers in cyanosilylation reaction, a type of approach that we have successfully applied in the oxidation of alkanes [54,55] and oxidation of alcohols [56].
The asymmetric unit of 1 contains one Ag I cation, a HLanion and a crystallization water molecule. Upon symmetry expansion, a 2D polymer is revealed with every silver cation adopting a distorted trigonal pyramidal geometry (τ4 = 0.48) [58] filled by one Ncyano-and three Osulfonate-atoms ( Figure 1). Each organic ligand behaves as a bridging three donor chelators. Polymer 1 contains four membered {AgO}2 fragments which side-share with two eight-membered {AgO2S}2 metallacycles thus giving rise to infinite chains connected by the organic ligands ( Figures S1 and S2). The water molecules are trapped in these metal-organic sheets, each one donating to the Oketo atoms of two hydrazones ( Figure S3 and Table S1). The IR spectra of 1 and 2 (see experimental section) show the expected N≡C, C=O and C=N vibrations which generally occur at wavenumbers different from those of the proligand (i.e., 2206, 1645, and average 1570 cm −1 , respectively [57]), therefore attesting the involvement of the compound in coordination to the metals. The nitrile group stretching frequency ν(C≡N) in 1 is 8 cm −1 above that measured for NaHL which is consistent with its coordination. In 2, that frequency is 2 cm −1 below. Concerning the ketone vibrations, ν(C=O), they occur 16 (for 1) and 30 (for 2) cm −1 above those of the NaHL reference value, whereas ν(C=N) assume values 13 (for 1) and 26 (for 2) cm −1 above the average one for NaHL. Fragmentation peaks in MS-ESI of the compounds are related as follows: 749.20 [Mr-H 2 O+H] + (for 1) and 450.05 [Mr-H 2 O+H] + (for 2), accounting for the existence of the dinuclear and mononuclear species in solution, respectively. Elemental analyses and X-ray crystallography experiments are also in accordance with the proposed formulations.
The asymmetric unit of 1 contains one Ag I cation, a HLanion and a crystallization water molecule. Upon symmetry expansion, a 2D polymer is revealed with every silver cation adopting a distorted trigonal pyramidal geometry (τ 4 = 0.48) [58] filled by one N cyano -and three O sulfonate -atoms ( Figure 1). Each organic ligand behaves as a bridging three donor chelators. Polymer 1 contains four membered {AgO} 2 fragments which side-share with two eight-membered {AgO 2 S} 2 metallacycles thus giving rise to infinite chains connected by the organic ligands ( Figures S1 and S2). The water molecules are trapped in these metal-organic sheets, each one donating to the O keto atoms of two hydrazones ( Figure S3 and Table S1). The asymmetric unit of 2 comprises a hydrazine ligand bridging two copper cations that stand in special positions, half a chelating ethylenediamine molecule and a crystallization water molecule. Symmetry expansion reveals a 1D polymer ( Figure 2) featuring wave-like chains (base vectors [101]) with the copper cations alternating N6 and N4O2 coordination environments every 7.6836(4) Å (see Figure S4 and Table 1). The octahedral geometries of the two metals are roughly similar in terms of volume and of quadratic elongation (13.319 and 13.935 Å 3 ; 1.021 and 1.046, in this order), but differ in the angle variances (10.77 for 1 against 36.53 °2 for 2) [59]. The water molecules in 2 are trapped between the metal-organic chains ( Figure S4 and Table S1), each one donating to an Oketo and accepting from a Namine in a chain, and interacting with an Osulfonate atom in a vicinal one.
The NN and Cketo−Chydraz distances (Table 1) in the compounds considered in this work vary in the order 3 > 1 > 2 and approach single bond characters, while the C−Nhydraz and C−Oketo lengths follow the reverse sequence 2 > 1 > 3 therefore pending to double bond types. Such observations suggest electronic delocalization along the OCketoChidrazNN skeleton of the hydrazo ligand of polymer 2. This is further supported by the presence of the NH groups in polymer 1 and in complex salt 3, which are involved in RAHB interactions with Osulfonate atoms (see also Table S1). The dianionic nature of this ligand in 2 is thus allocated to their O atoms. Since the C≡N groups in 1 and 2 are engaged in coordination, their lengths slightly increased as compared to that found in 3. The M-Namine distances in 2 and 3 are comparable (Table 1) indicating that in the former this parameter was not affected by the coordination of the L 2− ligand. In 2 the axial M−Ncyano (eventually, also the M−Osulfonate) length considerably exceeds the equatorial M-Namine, conceivably due to Jahn-Teller effect. Despite the planarity of the hydrazone ligand in 1 and 2, the C≡N−M angle in the latter is considerably lower than that in the former (126.5(6) against 165.2(7)°, see Table 1). As it was found in the complex salt 3, The asymmetric unit of 2 comprises a hydrazine ligand bridging two copper cations that stand in special positions, half a chelating ethylenediamine molecule and a crystallization water molecule. Symmetry expansion reveals a 1D polymer ( Figure 2) featuring wave-like chains (base vectors [101]) with the copper cations alternating N 6 and N 4 O 2 coordination environments every 7.6836(4) Å (see Figure S4 and Table 1). The octahedral geometries of the two metals are roughly similar in terms of volume and of quadratic elongation (13.319 and 13.935 Å 3 ; 1.021 and 1.046, in this order), but differ in the angle variances (10.77 for 1 against 36.53 •2 for 2) [59]. The water molecules in 2 are trapped between the metal-organic chains ( Figure S4 and Table S1), each one donating to an O keto and accepting from a N amine in a chain, and interacting with an O sulfonate atom in a vicinal one.
The NN and C keto −C hydraz distances (Table 1) in the compounds considered in this work vary in the order 3 > 1 > 2 and approach single bond characters, while the C−N hydraz and C−O keto lengths follow the reverse sequence 2 > 1 > 3 therefore pending to double bond types. Such observations suggest electronic delocalization along the OC keto C hidraz NN skeleton of the hydrazo ligand of polymer 2. This is further supported by the presence of the NH groups in polymer 1 and in complex salt 3, which are involved in RAHB interactions with O sulfonate atoms (see also Table S1). The dianionic nature of this ligand in 2 is thus allocated to their O atoms. Since the C≡N groups in 1 and 2 are engaged in coordination, their lengths slightly increased as compared to that found in 3. The M-N amine distances in 2 and 3 are comparable (Table 1) indicating that in the former this parameter was not affected by the coordination of the L 2− ligand. In 2 the axial M−N cyano (eventually, also the M−O sulfonate ) length considerably exceeds the equatorial M-N amine , conceivably due to Jahn-Teller effect. Despite the planarity of the hydrazone ligand in 1 and 2, the C≡N−M angle in the latter is considerably lower than that in the former (126.5(6) against 165.2(7) • , see Table 1). As it was found in the complex salt 3, polymer 1 is stabilized by the intramolecular N-H···O sulfonate resonance assisted hydrogen bond (RAHB) system. The N-H···O angle of 150(11) • is significantly higher than the average O-H···O value of 149 • found in β-diketones involved in similar intra-molecular interactions [60].

Catalytic Activity of 1-3 in Cyanosilylation Reaction
Polymers 1 and 2, mononuclear complex 3 and NaHL have been tested as homogeneous catalysts in cyanosilylation reaction of benzaldehyde with trimethylsilyl cyanide (model reaction) in Ellipsoid plot of a fragment of polymer 2, drawn at 30% probability level and with atom numbering scheme. Crystallization water molecule is omitted for clarity. Symmetry operation to generate equivalent atoms: (i) −1 + x,y,−1 + z. Table 1. Selected bond distances (Å) and angles ( • ) for polymers 1 and 2, and for complex salt 3 [57].

Catalytic Activity of 1-3 in Cyanosilylation Reaction
Polymers 1 and 2, mononuclear complex 3 and NaHL have been tested as homogeneous catalysts in cyanosilylation reaction of benzaldehyde with trimethylsilyl cyanide (model reaction) in different organic solvents (tetrahydrofurane, dichloromethane, or methanol), and at room temperature (Scheme 1, Table 2). In all the experiments, higher yields are observed in MeOH (entries 1-12, Table 2). Polymer 1 can be considered as inactive towards the reaction under study in view of the obtained yields (entries 1-3, Table 2) being identical to those attained in the absence of any metal catalyst (entries 16-18, Table 2). The catalytic activities of polymer 2 and the complex salt 3 are also comparable (compare entries 4-6 with 7-9 in Table 2) suggesting disaggregation of 2 in solution giving rise to 3. Polymer 2 was chosen as the catalyst (it provides 79.9% product yield, slightly above that of 3, 75.3%) and methanol as the solvent for the following studies ( Table 3). Reaction of benzaldehyde with TMSCN provides low product yield in the presence of a metal salt, AgNO 3 or Cu(NO 3 ) 2 ·2.5H 2 O (maximum yield of 33%), and without metal catalyst (maximum yield of 25%) (entries 13-18, Table 2). With catalyst 2, a high yield of 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile is already obtained (79.0%) after 8 h, which did not increase considerably for longer times (entries 1-6, Table 3). The amount of this catalyst was varied from 1 to 9 mol %, and no considerable yield increase was observed for a catalyst load above 5% (entries 7-11, Table 3). The temperature (in the 15-55 • C range) had not a marked effect on the product yield (entries 12-15, Table 3).  Among the common organic solvents that we tested, methanol is the best one for this system. However, this solvent is considerably toxic. In order to use a greener solvent, we applied several room temperature ionic liquids (Table 4). In all experiments, the reaction proceeded smoothly to produce 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile in a moderate yield (68.7-75.2%) under catalyst-free conditions (entries 1, 6, 11, and 16), which increases (75.1-82.2%) in the presence of 2 (entries 2, 7, 12, and 17). The use, as reaction medium, of a mixture of IL with MeOH afforded the product in a higher yield in comparison to the IL alone (Table 4). In general, ILs with guanidinium cations shows a higher catalytic performance than imidazolinium or phosphonium cations ( Table 4). The ionic liquid [DHTMG][L-Lactate] afforded the product in the highest yield (92.6% yield; Table 4, entry 9).   Subsequently, the cyanosilylation reaction of para-electron-withdrawing or -donating aromatic aldehyde substrates in the presence of catalyst 2 was tested under optimized reaction conditions (entries 1-3 and 5-6 in Table 5, respectively), in MeOH or in IL+MeOH mixture. With 4-nitro-, 4-chloroor 4-bromobenzaldehyde, higher yields were obtained ( Table 5, entries 1-3), as compared to the benzaldehyde derivatives having an electron-donating substituent (methoxy or methyl) ( Table 5, entries 5 and 6), due to an increase of the electrophilicity of the carbon atom at C=O in the former case. The use of linear aliphatic aldehydes (acetaldehyde, propionaldehyde and hexanal) as substrates (entries 7-9, Table 5 (Table 5). Up to now, a solvent capable of extracting effectively the products from the IL+catalyst system was not found. Therefore, no recycling experiments were made and the reaction analysis with IL was made by using a sample of the reactional mixture containing the product and the IL (see Experimental). This method was accurate because the 1 H NMR peaks of ILs do not overlap with those of the product.
Comparing with reported homogeneous catalytic systems for the cyanosilylation reaction, there are several advantages in using 2 as a catalyst in this transformation: (i) It is available from relatively cheap starting materials (arylhydrazone and copper nitrate); (ii) it shows a higher activity (79.0%) in methanol medium in comparison to Zn(II) (30%) [61], Cu(II) (27%) [62], potassium salt of L-proline (83%) [63], etc.; and (iii) a shorter reaction time (8 h) and the convenient room temperature can be used favorably, in comparison to other cases operating for a longer reaction time (96 h) [64], at higher (40 • C) [62] or lower (even negative) (−50 • C) [65] temperatures.
The reaction mechanism can be similar to that proposed for some reported examples with related catalytic systems [1,66,67]. Moreover, the reaction possibly can be promoted by cooperative action of coordination and noncovalent interactions, which increase the electrophilic character of carbon atom at the carbonyl group of the aldehyde towards the nucleophilic addition of the cyano moiety with the assistance of a tetrel bonding (Scheme 3), and migration of the silyl group to the oxygen followed by product liberation.

Materials and Instrumentation
All the chemicals were obtained from commercial sources (Aldrich, St. Louis, MO, USA) and

Materials and Instrumentation
All the chemicals were obtained from commercial sources (Aldrich, St. Louis, MO, USA) and used as received. Sodium 2-(2-(1-cyano-2-oxopropylidene)hydrazinyl)benzenesulfonate (NaHL) and [Cu(H 2 O) 2 (en) 2 ](HL) 2 (3) were synthesized according to the reported procedures [57]. Infrared spectra (4000-400 cm −1 ) were recorded on a Vertex 70 (Bruker, Billerica, MA, USA) instrument in KBr pellets. Carbon, hydrogen, and nitrogen elemental analyses were carried out by the Microanalytical service of Instituto Superior Técnico. The 1 H and 13 C NMR analyses were performed on a Bruker Avance II + 300 (Bruker, Billerica, MA, USA) spectrometer, which operates at 300.130 and 75.468 MHz for 1 H and 13 C, respectively. The chemical shifts are recorded in ppm in reference to tetramethylsilane. Electrospray mass spectra (ESI-MS) experiments were run by using an ion-trap instrument (Varian 500-MS LC Ion Trap Mass Spectrometer, Palo Alto, CA, USA) containing an electrospray ion source. In order to perform the electrospray ionization, the optimization of the drying gas and flow rate was undertaken in accord to the particular sample with 35 p.s.i. of nebulizer pressure. Scanning was recorded from m/z 0 to 1100 in a methanol solution. The compounds were seen in the positive mode (capillary voltage = 80-105 V).

Synthesis of 1
1 mmol (289 mg) of NaHL was dissolved in 25 mL of methanol, and 2 drops of HNO 3 (70%) and 1 mmol (170 mg) of AgNO 3 were added, and the system was then stirred for 10 min. After ca. 3 d at room temperature, orange crystals precipitated and were filtered off and dried in air.

Synthesis of 2
1 mmol (289 mg) of NaHL was dissolved in 25 mL of methanol, 0.12 mL (2 mmol) en and 1 mmol (232 mg) of Cu(NO 3 ) 2 ·2.5H 2 O were added, and the system was then stirred for 10 min. After ca. 2 d at room temperature, greenish yellow crystals precipitated and were filtered off and dried in air.

Crystal Structure Determination
Intensity data for compounds 1 and 2 were collected at 150 K using a Bruker SMART APEX-II diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Data were collected using omega scans of 0.5 • per frame and full sphere of data were obtained. Cell parameters were retrieved using Bruker SMART software and refined using Bruker SAINT [68] on all the observed reflections. Absorption corrections were applied using the SADABS program [69]. The structures were solved by direct methods using SIR97 package [70] and refined with SHELXL-2018/3 [71]. Calculations were performed using the WinGX System-Version 2014-1 [72]. The hydrogen atoms of water molecules and hydrazine (in 1) or ethylenediamine (in 2) were found in the difference Fourier map and the isotropic thermal parameters were set at 1.5 times the average thermal parameters of the belonging oxygen or nitrogen atoms, with their distances restrained by using the DFIX and DANG commands. Hydrogen atoms bonded to carbon atoms were included in the refinement using the riding-model approximation with the Uiso(H) defined as 1.2 Ueq of the parent aromatic atoms, and 1.5U eq of the parent carbon atoms for methyl. Compound 2 was refined as a 2-component twin; the unaccounted twinning was resolved by using the TwinRotMat program in Platon [73]. Least square refinements with anisotropic thermal motion parameters for all the non-hydrogen atoms were employed. The details of the crystallographic data for 1 and 2 are summarized in Table 6. Selected bond distances and angles for polymers 1 and 2 as well as, for comparative purposes, complex salt 3 are shown in Table 1. Ellipsoid plots are presented in Figures 1 and 2   This IL was synthesized in two steps: (i) 3.6 mmol of L-proline were dissolved in 50 mL of methanol and 3.6 mmol of crystalline NaOH were added. The reaction was left stirred for 24 h; (ii) After sodium prolinate was formed, 3 mmol of [P 6,6,6,14 ]Cl were added, and the mixture was stirred for 24 h at room temperature. After that time, methanol was removed under low pressure and the mixture was re-dissolved in dichloromethane. The precipitated NaCl was filtered off and the dichloromethane was evaporated. Then, the product was dried under vacuum during 24 h. A yellow oil was obtained (67% yield). 1

General Procedure for Catalytic Studies
In a typical cyanosilylation experiment, to a solution of benzaldehyde (0.4 mmol), catalyst (1-3) (1-8 mol %) in any of the solvents [CH 2 Cl 2 , THF, MeOH or IL; 2 mL], trimethylsilyl cyanide (TMSCN) (0.6 mmol) was added dropwise. The mixture was stirred continuously for a certain amount of time. The solvent was then evaporated (in the case of CH 2 Cl 2 , THF and MeOH) and the residue was analyzed by 1 H-NMR spectroscopy in CDCl 3 , in order to evaluate the yield of the products [39,40]. In the case of the reactions with IL, the analysis was made by taking directly a sample of the reactional mixture, which was analyzed by 1 H-NMR in CDCl 3 . For the reactions in a IL+MeOH mixture, MeOH was evaporated and a sample of products+IL was taken for analysis. The adequacy of this procedure was confirmed by using blank 1 H NMR analyses with 1,2-dimethoxyethane (0.10 mmol) as an internal reference [benzaldehyde (0.10 mmol) and TMSCN) (0.15 mmol)] ( Figure S6). The internal standard method showed the absence of side products

Conclusions
We have prepared two new silver(I) and copper(II) coordination polymers 1 and 2, and applied them, with the known mononuclear copper(II) complex 3, as catalysts in cyanosilylation reaction of several aldehydes with TMSCN. The Cu II coordination polymer 2 showed the highest activity for the cyanosilylation reaction in methanol and in a mixture of [DHTMG][L-Lactate]:MeOH (1:10, v/v) achieving yields up to 99% at room temperature. Electron-withdrawing substituents, such as -NO 2 , -Cl, -Br, in the para position of the aromatic aldehyde provide higher product yields, whereas electron-donating groups (-CH 3 , -OCH 3 ) inhibit the reaction. Cooperative actions of coordination and noncovalent interactions are proposed for the C=O activation in the aldehyde substrate. The use of a IL+MeOH mixture also increases the catalytic activity in comparison with the case of MeOH alone or another organic solvent.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/9/3/284/s1, Figure S1. Fragments of the 2D network in compound 1; Figure S2. Fragment of a 1D infinite chain in compound 1; Figure S3. Fragment of two 2D sheets of polymer 1 with intercalated water molecules; Figure S4. Fragments of the 1D chain in compound 2 viewed perpendicular to the ab plane; Figure S5. Fragment of chains of polymer 2 with intercalated water molecules (represented in space filling model); Table S1. Hydrogen bonding distances and angles for 1 and 2.  Authors are thankful to the Portuguese NMR Network (IST-UL Centre) for access to the NMR facility and the IST Node of the Portuguese Network of mass-spectrometry for the ESI-MS measurements. This work also was supported by the "RUDN University Program 5-100".

Conflicts of Interest:
The authors declare no conflicts of interest.