Manganese catalysed dehydrogenative synthesis of polyureas from diformamide and diamines

We report here the synthesis of polyureas from the dehydrogenative coupling of diamines and diformamides. The reaction is catalysed by a manganese pincer complex and releases H2 gas as the only by-product making the process atom-economic and sustainable. The reported method is greener in comparison to the current state-of-the-art production routes that involve diisocyanate and phosgene feedstock. We also report here the physical, morphological, and mechanical properties of synthesized polyureas. Based on our mechanistic studies, we suggest that the reaction proceeds via isocyanate intermediates formed by the manganese catalysed dehydrogenation of formamides.


General information
All experiments were carried out under inert atmosphere using standard Schlenk techniques unless specified. Complex 1 1 and N,N'-(octane-1,8-diyl)diformamide 2 were prepared as described by methods reported in literature. Ethyl formate, N,N'-(1,4-phenylene)diformamide, and diamines were purchased from Sigma-Aldrich, Alfa Aesar, Strem, or TCI and used as received. THF and toluene were dried using a Grubbs type solvent purification system and degassed by Freeze-Pump-Thaw under nitrogen before use.
Anhydrous anisole, diglyme and DMSO were purchased from Sigma-Aldrich and used as received.
Deuterated solvents (CDCl 3 , DMSO-d 6 , d-TFA and D 2 O) were purchased from Sigma-Aldrich and used as received. Toluene-d 8 was purchased from Sigma-Aldrich, dried over CaH 2 , distilled and degassed by three successive freeze-pump-thaw cycles before use. Schlenk flasks of 250 mL were used for the synthesis of polyureas from diformamides and diamines.
For the preparation of the MALDI samples, polyureas were dissolved in neat TFA. 0.5 µL of the resulting solution was applied to a stainless steel MALDI target plate, 0.5 µL of the matrix was co-spotted and allowed to dry. Matrix was either 2,5-dihydroxybenzoic acid or alpha-cyano-4-hydroxycinnamic acid prepared at 10 mg/mL in 50:50 acetonitrile: 0.1% TFA. MALDI data was acquired using a 12T SolariX FT-ICR (Bruker Daltonics) equipped with a Nd:YAG 355 nm laser. The sample was acquired in positive MS mode between m/z 200 and m/z 5000.
Infrared spectra (ATR-FTIR) were recorded using a MIRacle TM from Pike or using a Shimadzu IRAffinity-1.
Thermal Gravimetric Analysis (TGA) was carried out using a Netzsch STA449C with a heating rate of 10 °C/min from 30 to 600 °C in an inert gas flow. Decomposition temperatures (T d ) were recorded by TGA at 5% weight loss. 5% weight loss was taken after solvent/water/residual reactants loss. DSC were collected using a Netzsch STA449C (10 °C/min from 30 to 600 °C under a flow of nitrogen gas at 25 mL/min) or using a Netzsch DSC204 -(10 °C/min between 80-600°C under a flow of nitrogen gas at 20 mL/min, after an initial heat/cool cycle [25-120 °C at 10 °C/min with a 20 minute isothermal at 120 °C] to remove the thermal history of the sample).
NMR spectra were recorded on a Bruker AVIII-HD 500 MHz NMR spectrometer at 298 K unless otherwise specified. Residual protio solvent was used as reference for 1 H spectra in deuterated solvent samples.
Where d-TFA is used as an NMR solvent, samples were run with the presence of a D 2 O sealed capillary to aid locking when required. 31 P{ 1 H} NMR spectra were externally referenced to 85% H 3 PO 4 . All chemical S4 shifts (δ) are quoted in ppm and coupling constants (J) in Hz. NMR assignments were aided by 2D spectra ( 1 H, 1 H-COSY, 1 H, 13 C-HSQC, 1 H, 13 C-HMBC) where required.
Scanning electron microscopy (SEM) was performed on an FEI Scios dualbeam FIB/SEM operated at an acceleration voltage of 3 kV and energy dispersive X-ray spectroscopy (EDX) was performed on a JEOL JSM-IT200 equipped with an embedded JEOL EDX detector and operated at 10 kV.
GC-MS spectra were collected as solutions in HPLC grade DCM using an Agilent 8860 GC system coupled to an Agilent 5977B EI instrument. EI spectra were collected as solutions in acetonitrile using a Micromass LCT spectrometer.
Gel permeation chromatography (GPC) was performed on an Agilent 1260 InfinityLab II GPC fitted with a refractive index (RI) detector (35 °C). The single (plus guard column) Agilent PolarGel column setup was contained within an oven (35 °C). H 2 O was used as the eluent at a flow rate of 1.0 mL min −1 . Samples were dissolved in the eluent (2.0 mg mL −1 ), filtered (0.2 μm pore size) and run immediately. The calibration was conducted using a series of monodisperse poly(ethylene glycol) (M n = 194-20,000 g mol -1 ) and poly(ethylene oxide) (M n = 30,000-55,000 g mol -1 ) standards obtained from Agilent Technologies. 1 (0.01 mmol, 2 mol%), diformamide (0.5 mmol), diamine (0.5 mmol) and KO t Bu (0.04 mmol, 8 mol%) were added to a 250 mL J-Young's flask before solvent (2 mL) was added and the reaction heated to 150 °C. After 24 hours, the contents of the flask were cooled to room temperature, and the lid was slowly opened to release the gas pressure. For reactions open under N 2 atmosphere, reactions were carried out in a Schlenk flask connected to Schlenk line open to N 2 supply and attached with a bubbler. The reaction mixture (that could contain a solid) at the end of 24 h was transferred to a vial in which 5-10 mL of hexane was added. The vial was cooled to -30 o C to aid precipitation of the product. The solid product was then collected by filtration, washed with hexane (3 × 10 mL) and then dried under vacuum.

General procedure for the synthesis of diformamide
The isolated product was characterised using 1 H NMR, 13 C{ 1 H} NMR and IR spectroscopies and MALDI-FT-ICR mass spectrometry.
Note: Caution must be taken as the reaction can produce up to ~96 mL of H 2 gas (298 K).

Solubility of polyureas
The solubility of a polyureas was tested by adding 10 mg of a polyurea in a vial containing 2 mL of a solvent (CHCl 3 , THF, MeOH, acetone, H 2 O, DMF, DMSO, TFA). All polyureas reported here were found to dissolve only partially in TFA, and not in other tested solvents, with the exception of that produced from DF-1 and DA-2. In the case of the polymer produced from DF-1 and DA-2, this polymer is soluble in water by design to allow GPC analysis.  Figure S1). T d ( o C) was recorded at 5% mass loss.    Off-white solid (122mg, 79% yield).  Entry 9

Table S2
The oligomers observed via MALDI-FT-ICR MS experiments for the polymers synthesised from the reaction of diformamide with diamine in the presence of 1 (entry numbers are the same as described in Table S1). N/O = not observed The MALDI-TOF of Entry 14 was carried out on a sonicated solution (5 min) of polymer (1 mg/mL) in 1:1 MeOH:H 2 O using αcyano-4-hydroxycinnamic acid (CHCA) matrix in the presence of NaI (1 mg/mL). All other samples were prepared as 10 mg/mL solutions in 50:50 acetonitrile: 0.1% TFA using 2,5-dihydroxybenzoic acid or CHCA matrix.

Results of the MALDI-FT-ICR mass spectrometry measurements
We observe various polyureas (P-k, P-x, P-i, P-m, P-y, P-n, P-q, P-o, P-p, P-r, P-s, P-u, P-t and P-v) as described in Table S2. In the following (Figure S53 -Figure S60), the letters (k, x, i, m, y, n, q, r, s, u, t, and v) correspond to a specific polyurea and numbers correspond to the number of repeating units as described in the structure. For example, k=1 corresponds to polyurea P-k with one repeating unit in the following structure:

Mechanical characterisation of polyureas (Entries 4 and 7, Table S1)
To get reliable results from indentation tests, the contact surface must be flat. Therefore, the specimen surface was thoroughly polished with sandpapers and diamond suspensions.
All the tests were carried out using a KLA iMicro nanoindenter, equipped with a 50mN force actuator. A Berkovich tip was used. Continuous Stiffness Measurements (CSM) were performed, allowing to measure the indentation modulus E* ((E* = E/1-ν 2 ), it is used in place of the Young's modulus E when the Poisson's ratio ν is unknown) and H as a function of the indentation load.
The maximum indentation depth was set to 2000 nm.
The tests were carried out using a constant indentation strain rate of 0.1 s -1 .
The max load was held for 1 second before unloading to quantify creep.
Upon unloading, the load was held again at 10% of max load for 3 minutes to quantify thermal drift and correct the recorded value of load and depth accordingly.  Figure S87 Young's modulus vs indentation depth plot for the polyurea described in Table S1, entry 4.

Figure S88
Hardness vs indentation depth plot for the polyurea described in in Table S1, entry 4.

Figure S89
Load vs indentation depth plot for the polyurea described in in Table S1, entry 4.

Figure S90
Young's modulus vs indentation depth plot for the polyurea described in Table S1, entry 7.
S68 Figure S91 Hardness vs indentation depth plot for the polyurea described in Table S1, entry 7.

Figure S92
Load vs indentation depth plot for the polyurea described in Table S1, entry 7. Figure S83(a-c) shows SEM data from the aliphatic polyurea (made from DF2 and DA1, Entry 7, Table S1) and Figure S83(d-f) shows the SEM data from the aromatic polyurea (made from DF1 and DA1, Entry 4, Table S1). It can be seen from the lower magnification images in Figure S83(a,d) that the samples form larger agglomerations, typically in the region of tens of microns, that are composed of finer features, as shown in the higher magnification images in Figure S83  General procedure: A J-young's NMR tube is charged with 1 (10 mg, 20 μmol) and KO t Bu (1.2 eq., 2.7 mg, 24 μmol) and d 8 -toluene (0.5 mL). The sealed NMR tube is heated for 5 minutes at 110 °C to quantitatively generate Mn(PNP-iPr)(CO) 2 (2) in situ. Formamide (e.g formamide, 2 eq. 16 μL 40 μmol) is added and the NMR tube shaken vigorously.

SEM and EDX studies.
After 10 minutes at room temperature, the resulting solution is interrogated by NMR spectroscopy and intermediates 3 and 5 identified. The NMR tube is then heated to 110 °C, and the reaction progression monitored by NMR spectroscopy until a single major product (4 or 6) is obtained.    GC-MS analysis of the reaction mixture diluted in DCM post heating allows for interrogation of volatile organic components, these data are summarized in Table S4   Table S4. Summary of GC-MS data obtained from reaction mixture.

NMR spectra from in situ speciation
Using DF-1 and DA-1 with precatalyst 1 (4 mol%) and KO t Bu (8 mol%), under polymerisation conditions (anisole solvent, 170 °C). The reaction was heated for 1 hour and the resulting 31 P{ 1 H} NMR obtained. After 6 hours, the major 31 P containing species observed was free ligand, suggesting catalytic decomposition.