Claims:WE CLAIM:
1. A POCNP-Iridium Pincer complex catalyst for alkane dehydrogenation, said catalyst being represented by Formula-I

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
2. The POCNP-Iridium Pincer complex catalyst as claimed in claim 1, wherein the catalyst is bicyclic, and is represented by Formula-XI

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
3. The POCNP-Iridium Pincer complex catalyst as claimed in any one of claims 1 and 2, wherein the alkyl groups are at least one independently selected from the group consisting of methyl, ethyl, isopropyl, tert-butyl, cyclopentyl, cyclohexyl, and adamantyl.
4. The POCNP-Iridium Pincer complex catalyst as claimed in claim 1 being a product of reaction of a POCNP Pincer ligand of Formula-II and an iridium salt,

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
5. The POCNP-Iridium Pincer complex catalyst as claimed in claim 2 being a product of reaction of a POCNP Pincer ligand of Formula-XII and an iridium salt,

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
6. The POCNP-Iridium Pincer complex catalyst as claimed in any one of claim 4 and 5, wherein the iridium salt is at least one selected from the group consisting of chlorobis(cyclooctene)iridium(I)dimer [Ir(COE)2Cl]2, and bis(1,5-cyclooctadiene)diiridium(I) dichloride [Ir(COD)Cl]2.
7. A process for preparing POCNP-Iridium Pincer complex catalyst as claimed in claim 1, the process comprising the following steps:
(a) mixing a POCNP Pincer ligand of Formula-II, and an iridium salt in a first fluid medium to obtain a first reaction mixture,

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups;
(b) heating the first reaction mixture to obtain a first product mixture, followed by the separation of the first fluid medium from the first product mixture to obtain a crude POCNP-Iridium Pincer complex catalyst; and
(c) purifying the crude POCNP-Iridium Pincer complex catalyst to obtain the POCNP-Iridium Pincer complex catalyst.
8. A process for preparing POCNP-Iridium Pincer complex catalyst as claimed in claim 2, the process comprising the following steps:
(a) mixing a POCNP Pincer ligand of Formula-XII, and an iridium salt in a first fluid medium to obtain a first reaction mixture,

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups;
(b) heating the first reaction mixture to obtain a first product mixture, followed by the separation of the first fluid medium from the first product mixture to obtain a crude POCNP-Iridium Pincer complex catalyst; and
(c) purifying the crude POCNP-Iridium Pincer complex catalyst to obtain the POCNP-Iridium Pincer complex catalyst.
9. The process as claimed in any one of claims 7 and 8, wherein the first reaction mixture is heated at a temperature in the range of 100 °C to 150 ?C.
10. The process as claimed in any one of claims 7 and 8, wherein the molar ratio of the POCNP pincer ligand to the iridium salt is in the range of 2:1 to 2.1:1.
11. The process as claimed in any one of claims 7 and 8, wherein the first fluid medium is at least one selected from the group consisting of toluene, xylene and mesitylene.
12. The process as claimed in any one of claims 7 and 8, wherein the step (b) of purifying the crude involves extraction with n-pentane and n-hexane.
13. A process for preparing the POCNP Pincer ligand of Formula-II as claimed in claim 4, the process comprising reacting a compound of Formula-III;

wherein, R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups;
with a halodialkylphosphine of Formula-IV,
R'R"PX - Formula-IV
wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and X is a halogen selected from the group consisting of Cl, Br, and I.
14. The process as claimed in claim 13, wherein the compound of Formula-III is 2-hydroxycarbazole.
15. A process for preparing the POCNP Pincer ligand of Formula-XII as claimed in claim 5, the process comprising reacting a compound of Formula-XIII;

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups;
with a halodialkylphosphine of Formula-IV,
R'R"PX - Formula-IV
wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and X is a halogen selected from the group consisting of Cl, Br, and I.
16. The process as claimed in any one of claims 13 and 15, wherein the halodialkylphosphine is at least one selected from the group consisting of chlorodiisopropylphosphine, and di-tert-butylchlorophosphine.
17. The process as claimed in any one of claims 13 comprising the following steps:
i. stirring a mixture comprising the compound of Formula-III and a first base in a second fluid medium to obtain a resultant mixture;
ii. adding a mixture of the halodialkylphosphine of Formula-IV, and the second fluid medium to the resultant mixture to obtain a second reaction mixture and stirring the second reaction mixture to obtain a second product mixture;
iii. separating the second fluid medium from the second product mixture to obtain a residue containing the POCNP Pincer ligand; and
iv. extracting the residue with a third fluid medium, followed by removal of the third fluid medium from the extract to obtain the POCNP Pincer ligand.
18. The process as claimed in claim 15 comprising the following steps:
i. stirring a mixture comprising the compound of Formula-XIII and a first base in a second fluid medium to obtain a resultant mixture;
ii. adding a mixture of the halodialkylphosphine of Formula-IV, and the second fluid medium to the resultant mixture to obtain a second reaction mixture and stirring the second reaction mixture to obtain a second product mixture;
iii. separating the second fluid medium from the second product mixture to obtain a residue containing the POCNP Pincer ligand; and
iv. extracting the residue with a third fluid medium, followed by removal of the third fluid medium from the extract to obtain the POCNP Pincer ligand.
19. The process as claimed in any one of claims 17 and 18, wherein the molar ratio of the compound of Formula-III or Formula XIII to the halodialkylphosphine is in the range of 1:2 to 1:2.2.
20. The process as claimed in any one of claims 17 and 18, wherein the second fluid medium is tetrahydrofuran.
21. The process as claimed in any one of claims 17 and 18, wherein the molar ratio of the compound of Formula-III or Formula XIII to the first base is in the range of 1:2 to 1:2.2.
22. The process as claimed in any one of claims 17 and 18, wherein the first base is at least one selected from the group consisting of potassium tert-butoxide, and sodium tert-butoxide.
23. The process as claimed in any one of claims 17 and 18, wherein in the step (ii), the second reaction mixture is stirred at a temperature in the range of 10 ?C to 50 ?C for a period in the range of 2 hours to 20 hours.
24. The process as claimed in any one of claims 17 and 18, wherein the third fluid medium is selected from the group consisting of n-pentane, n-hexane, and n-heptane.
25. A process for dehydrogenation of alkane using the POCNP-Iridium Pincer complex catalyst as claimed in any one of claims 1 and 2, the process comprising heating a mixture of an alkane, a hydrogen acceptor and a second base in a fourth fluid medium at a temperature in the range of 160 ?C to 210 ?C in the presence of the POCNP-Iridium Pincer complex catalyst, to obtain a dehydrogenated alkane.
26. The process as claimed in claim 25, wherein the hydrogen acceptor is tert-butylethylene.
27. The process as claimed in claim 25, wherein the second base is at least one selected from the group consisting of potassium tert-butoxide, and lithium tert-butoxide.
28. The process as claimed in claim 25, wherein the fourth fluid medium is at least one selected from the group consisting of toluene, xylene and mesitylene.
, Description:FIELD
The present disclosure relates to a catalyst, particularly iridium based Pincer complex catalyst.
DEFINITIONS
As used in the present disclosure, the following terms are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used indicate otherwise.
A POCNP Pincer ligand is a tridentate ligand that binds with a metal at three sites; one phosphorous atom of phosphinite group, one phosphorous atom of phosphinous amide group and a carbon of an aromatic framework. The phosphinite group is in the form of a phosphorous-oxygen (P-O) bond and phosphinous amide group is in the form of a phosphorous-nitrogen (P-N) bond.
Turnover number (TON) of a catalyst is calculated as amount of the desired product formed per unit amount of the catalyst.
BACKGROUND
A Pincer ligand is a type of chelating agent that binds to three adjacent coplanar sites, usually on a transition metal in a meridional configuration. The Pincer complex obtained from the complexation of the Pincer ligand and a transition metal attain a planer framework. The Pincer complexes catalyze chemical transformations with high selectivity.
A POCNP Iridium Pincer complex catalyst can be used for dehydrogenation of alkanes to corresponding olefins. It is desired that the POCNP Iridium Pincer complex catalyst has high catalytic activity for dehydrogenation of alkanes. The catalytic activity of a catalyst can be calculated as catalyst turnover number (TON), which is calculated as the molar ratio of the amount of olefin produced to the amount of the catalyst used. Thus, it is desired that the catalyst has a high TON.
Further, it is desired that the POCNP Iridium Pincer complexes are stable at high temperature, since the dehydrogenation reactions are generally carried out at high temperature.
Conventional processes for the preparation of Iridium Pincer complexes are associated with various drawbacks such as being multistep and tedious processes and having low yield.
There is, therefore, felt a need to provide a POCNP Iridium Pincer complex catalyst that has high catalytic activity and that is stable at high temperature, and a simple process for preparing the POCNP Iridium Pincer complex catalyst with high yield.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
It is another object of the present disclosure to provide POCNP-Iridium Pincer complex catalyst that has high catalytic activity.
It is yet another object of the present disclosure to provide POCNP-Iridium Pincer complex catalyst that is stable at high temperature.
It is still another object of the present disclosure to provide a simple process for preparing POCNP-Iridium Pincer complexes in high yield.
Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY
In first aspect, the present disclosure provides a POCNP-Iridium Pincer complex catalyst for alkane dehydrogenation.
In accordance with an embodiment of the present disclosure, the catalyst is represented by Formula-I.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
In accordance with another embodiment of the present disclosure, the catalyst is bicyclic, and is represented by Formula-XI.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
The alkyl groups are at least one independently selected from the group consisting of methyl, ethyl, isopropyl, tert-butyl, cyclopentyl, cyclohexyl, and adamantyl.
In accordance with an embodiment of the present disclosure, the POCNP-Iridium Pincer complex catalyst of the present disclosure is a product of reaction of a POCNP Pincer ligand of Formula-II and an iridium salt.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
In accordance with another embodiment of the present disclosure, the POCNP-Iridium Pincer complex catalyst of Formula-XI is a product of reaction of a POCNP Pincer ligand of Formula-XII and an iridium salt.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
The iridium salt is at least one selected from the group consisting of chlorobis(cyclooctene)iridium(I)dimer [Ir(COE)2Cl]2, and bis(1,5-cyclooctadiene)diiridium(I) dichloride [Ir(COD)Cl]2.
In second aspect, the present disclosure provides a process for preparing POCNP-Iridium Pincer complex catalyst.
In accordance with an embodiment of the present disclosure, the process comprises the following steps:
A POCNP Pincer ligand of Formula-II, and an iridium salt are mixed in a first fluid medium to obtain a first reaction mixture.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
The first reaction mixture is heated to obtain a first product mixture, followed by the separation of the first fluid medium from the first product mixture to obtain a crude POCNP-Iridium Pincer complex catalyst.
The crude POCNP-Iridium Pincer complex catalyst is purified to obtain the POCNP-Iridium Pincer complex catalyst.
In accordance with another embodiment of the present disclosure, the process comprises the following steps.
A POCNP Pincer ligand of Formula-XII, and an iridium salt are mixed in a first fluid medium to obtain a first reaction mixture.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
The first reaction mixture is heated to obtain a first product mixture, followed by the separation of the first fluid medium from the first product mixture to obtain a crude POCNP-Iridium Pincer complex catalyst.
The crude POCNP-Iridium Pincer complex catalyst is purified to obtain the POCNP-Iridium Pincer complex catalyst.
The first reaction mixture is heated at a temperature in the range of 100 °C to 150 ?C.
The molar ratio of the POCNP pincer ligand to the iridium salt is in the range of 2:1 to 2.1:1.
The first fluid medium is at least one selected from the group consisting of toluene, xylene and mesitylene.
The step of purifying the crude POCNP-Iridium Pincer complex catalyst involves extraction with n-pentane and n-hexane.
In third aspect, the present disclosure provides a process for preparing the POCNP Pincer ligand.
In accordance with an embodiment of the present disclosure, the process comprises reacting a compound of Formula-III;

wherein, R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups;
with a halodialkylphosphine of Formula-IV,
R'R"PX - Formula-IV
wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and X is a halogen selected from the group consisting of Cl, Br, and I.
In an exemplary embodiment of the present disclosure, the compound of Formula-III is 2-hydroxycarbazole.
In accordance with another embodiment of the present disclosure, the process comprises reacting a compound of Formula-XIII;

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups;
with a halodialkylphosphine of Formula-IV,
R'R"PX - Formula-IV
wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and X is a halogen selected from the group consisting of Cl, Br, and I.
In an exemplary embodiment of the present disclosure, the compound of Formula-XIII is 6-hydroxyindole.
The halodialkylphosphine is at least one selected from the group consisting of chlorodiisopropylphosphine, and di-tert-butylchlorophosphine.
More specifically, the process for preparing the POCNP Pincer ligand of Formula-II comprises the following steps.
A mixture comprising the compound of Formula-III and a first base in a second fluid medium is stirred to obtain a resultant mixture.
A mixture of the halodialkylphosphine of Formula-IV, and the second fluid medium is added to the resultant mixture to obtain a second reaction mixture and the second reaction mixture is stirred to obtain a second product mixture.
The second fluid medium is separated from the second product mixture to obtain a residue containing the POCNP Pincer ligand.
The residue is extracted with a third fluid medium, followed by removal of the third fluid medium from the extract to obtain the POCNP Pincer ligand of Formula-II.
More specifically, the process for preparing the POCNP Pincer ligand of Formula-XII comprises the following steps.
A mixture comprising the compound of Formula-XIII and a first base in a second fluid medium is stirred to obtain a resultant mixture.
A mixture of the halodialkylphosphine of Formula-IV, and the second fluid medium is added to the resultant mixture to obtain a second reaction mixture and the second reaction mixture is stirred to obtain a second product mixture.
The second fluid medium is separated from the second product mixture to obtain a residue containing the POCNP Pincer ligand.
The residue is extracted with a third fluid medium, followed by removal of the third fluid medium from the extract to obtain the POCNP Pincer ligand of Formula-XII.
The molar ratio of compound of Formula-III or Formula XIII to the halodialkylphosphine is in the range of 1:2 to 1:2.2.
The second fluid medium is tetrahydrofuran.
The molar ratio of the compound of Formula-III or Formula XIII to the first base is in the range of 1:2 to 1:2.2.
The first base is at least one selected from the group consisting of potassium tert-butoxide, and sodium tert-butoxide.
The second reaction mixture is stirred at a temperature in the range of 10 ?C to 50 ?C for a period in the range of 2 hours to 20 hours.
The third fluid medium is selected from the group consisting of n-pentane, n-hexane, and n-heptane.
In fourth aspect, the present disclosure provides a process for dehydrogenation of alkane using the POCNP-Iridium Pincer complex catalyst of the present disclosure. The process comprises heating a mixture of an alkane, a hydrogen acceptor and a second base in a fourth fluid medium at a temperature in the range of 160 ?C to 210 ?C in the presence of the POCNP-Iridium Pincer complex catalyst, to obtain a dehydrogenated alkane.
The hydrogen acceptor is tert-butylethylene.
The second base is at least one selected from the group consisting of potassium tert-butoxide, and lithium tert-butoxide.
The fourth fluid medium is at least one selected from the group consisting of toluene, xylene and mesitylene.
DETAILED DESCRIPTION
A Pincer ligand is a tridentate ligand that binds with a metal at three sites in one plane. A POCNP Pincer ligand provides a binding site via phosphorous atom of the phosphinite group, a binding site via phosphorous atom of the phosphinous amide group, and a binding site through a carbon on aromatic framework, for complexation with the metal. Thus, a Pincer ligand forms complex with the metal through two metal-phosphorous (M-P) bonds and one metal–carbon bond [M-C(aryl) bond].
The conventional processes for preparation of Iridium Pincer complexes are associated with various drawbacks such as being multistep and tedious processes, and having low yield.
The present disclosure envisages a simple process for preparing the POCNP-Iridium Pincer complexes with high yield. Further, POCNP-Iridium Pincer complexes that have high catalytic activity and that are stable at high temperature are envisaged.
In first aspect, the present disclosure provides a POCNP-Iridium Pincer complex catalyst for alkane dehydrogenation.
In accordance with an embodiment of the present disclosure, the catalyst is represented by Formula-I.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
In accordance with another embodiment of the present disclosure, the catalyst is bicyclic, and is represented by Formula-XI.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
The alkyl groups are at least one independently selected from the group consisting of methyl, ethyl, isopropyl, tert-butyl, cyclopentyl, cyclohexyl, and adamantyl.
In accordance with one embodiment of the present disclosure, both R1 and R2 are isopropyl.
In accordance with second embodiment of the present disclosure, both R1 and R2 are tert-butyl.
The POCNP-Iridium Pincer complex catalysts of the present disclosure can be used for dehydrogenation of alkanes. These catalysts have high catalytic activity and they dehydrogenate alkane with high turnover number (TON).
It is observed that the POCNP-Iridium Pincer complex catalysts of the present disclosure are stable up to 210 ?C. Thus, the POCNP-Iridium Pincer complexes of the present disclosure have high thermal stability.
In accordance with an embodiment of the present disclosure, the POCNP-Iridium Pincer complex catalyst of the present disclosure is a product of reaction of a POCNP Pincer ligand of Formula-II and an iridium salt.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
In accordance with another embodiment of the present disclosure, the POCNP-Iridium Pincer complex catalyst of Formula-XI is a product of reaction of a POCNP Pincer ligand of Formula-XII and an iridium salt.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
The oxygen atom and the nitrogen atom of the POCNP Pincer ligands of the present disclosure are separated by 3 carbon atoms. Both the nitrogen atom and the oxygen atom of the heterocyclic aromatic compound bear a dialkylphosphine group each. Due to this spatial relationship between the oxygen atom and the nitrogen atom, phosphorous atoms of the phosphinite group and the phosphinous amide group are suitably positioned for the formation of a POCNP-metal Pincer complex.
The iridium salt is at least one selected from the group consisting of chlorobis(cyclooctene)iridium(I)dimer [Ir(COE)2Cl]2, and bis(1,5-cyclooctadiene)diiridium(I) dichloride [Ir(COD)Cl]2.
In accordance with one embodiment of the present disclosure, the iridium salt is [Ir(COE)2Cl]2.
In second aspect, the present disclosure provides a process for preparing POCNP-Iridium Pincer complex catalyst.
In accordance with an embodiment of the present disclosure, the process comprises the following steps.
A POCNP Pincer ligand of Formula-II, and an iridium salt are mixed in a first fluid medium to obtain a first reaction mixture.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
The first reaction mixture is heated to obtain a first product mixture, followed by the separation of the first fluid medium from the first product mixture to obtain a crude POCNP-Iridium Pincer complex catalyst.
The crude POCNP-Iridium Pincer complex catalyst is purified to obtain the POCNP-Iridium Pincer complex catalyst.
In accordance with another embodiment of the present disclosure, the process comprises the following steps.
A POCNP Pincer ligand of Formula-XII, and an iridium salt are mixed in a first fluid medium to obtain a first reaction mixture.

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups.
The first reaction mixture is heated to obtain a first product mixture, followed by the separation of the first fluid medium from the first product mixture to obtain a crude POCNP-Iridium Pincer complex catalyst.
The crude POCNP-Iridium Pincer complex catalyst is purified to obtain the POCNP-Iridium Pincer complex catalyst.
The first reaction mixture is heated at a temperature in the range of 100 °C to 150 ?C.
In accordance with one embodiment of the present disclosure, the first reaction mixture is heated at 135 °C.
The molar ratio of the POCNP pincer ligand to the iridium salt is in the range of 2:1 to 2.1:1.
In accordance with one embodiment of the present disclosure, the molar ratio of the POCNP pincer ligand to the iridium salt is 2:1.
The first fluid medium is at least one selected from the group consisting of toluene, xylene and mesitylene.
In accordance with one embodiment of the present disclosure, the first fluid medium is toluene.
The step of purifying the crude POCNP-Iridium Pincer complex catalyst involves extraction with n-pentane and n-hexane.
In accordance with one embodiment of the present disclosure, the step of purifying the crude POCNP-Iridium Pincer complex catalyst involves extraction with n-pentane.
The yield of the purified POCNP-Iridium Pincer complex catalyst is in the range of 60% to 71%.
The process for preparing the POCNP-Iridium Pincer complex catalysts of the present disclosure is simple. Further, the process of the present disclosure provides POCNP-Iridium Pincer complex catalysts in high yield.
In third aspect, the present disclosure provides a process for preparing the POCNP Pincer ligand.
In accordance with an embodiment of the present disclosure, the process comprises reacting a compound of Formula-III;

wherein, R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups;
with a halodialkylphosphine of Formula-IV,
R'R"PX - Formula-IV
wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and X is a halogen selected from the group consisting of Cl, Br, and I.
In an exemplary embodiment of the present disclosure, the compound of Formula-III is 2-hydroxycarbazole.
In accordance with another embodiment of the present disclosure, the process comprises reacting a compound of Formula-XIII;

wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and R1 and R2 are at least one independently selected from the group consisting of hydrogen, electron withdrawing groups, and electron donating groups;
with a halodialkylphosphine of Formula-IV,
R'R"PX - Formula-IV
wherein, R? and R? are at least one independently selected from the group consisting of C1 to C10 alkyl groups; and X is a halogen selected from the group consisting of Cl, Br, and I.
In an exemplary embodiment of the present disclosure, the compound of Formula-XIII is 6-hydroxyindole.
The halodialkylphosphine is at least one selected from the group consisting of chlorodiisopropylphosphine, and di-tert-butylchlorophosphine.
In accordance with one embodiment of the present disclosure, the halodialkylphosphine is chlorodiisopropylphosphine.
The halodialkylphosphines used in the process of the present disclosure are non-pyrophoric and cheaper as compared to the corresponding dialkylphosphines, which can be used for preparing certain Pincer ligands.
More specifically, the process for preparing the POCNP Pincer ligands of Formula-II comprises the following steps.
A mixture comprising the compound of Formula-III and a first base in a second fluid medium is stirred to obtain a resultant mixture.
A mixture of the halodialkylphosphine of Formula-IV, and the second fluid medium is added to the resultant mixture to obtain a second reaction mixture and the second reaction mixture is stirred to obtain a second product mixture.
The second fluid medium is separated from the second product mixture to obtain a residue containing the POCNP Pincer ligand.
The residue is extracted with a third fluid medium, followed by removal of the third fluid medium from the extract to obtain the POCNP Pincer ligand of Formula-II.
More specifically, the process for preparing the POCNP Pincer ligand of Formula-XII comprises the following steps.
A mixture comprising the compound of Formula-XIII and a first base in a second fluid medium is stirred to obtain a resultant mixture.
A mixture of the halodialkylphosphine of Formula-IV, and the second fluid medium is added to the resultant mixture to obtain a second reaction mixture and the second reaction mixture is stirred to obtain a second product mixture.
The second fluid medium is separated from the second product mixture to obtain a residue containing the POCNP Pincer ligand.
The residue is extracted with a third fluid medium, followed by removal of the third fluid medium from the extract to obtain the POCNP Pincer ligand of Formula-XII.
The molar ratio of compound of Formula-III or Formula XIII to the halodialkylphosphine is in the range of 1:2 to 1:2.2.
In accordance with one embodiment of the present disclosure, the molar ratio of the compound of Formula-III to the halodialkylphosphine is 1:2.
The second fluid medium is tetrahydrofuran.
The molar ratio of the compound of Formula-III or Formula XIII to the first base is in the range of 1:2 to 1:2.2.
In accordance with one embodiment of the present disclosure, the molar ratio of the compound of Formula-III to the first base is 1:2.05.
The first base is at least one selected from the group consisting of potassium tert-butoxide, and sodium tert-butoxide.
In accordance with one embodiment of the present disclosure, the first base is potassium tert-butoxide.
The second reaction mixture is stirred at a temperature in the range of 10 ?C to 50 ?C for a period in the range of 2 hours to 20 hours.
In accordance with one embodiment of the present disclosure, the second reaction mixture is stirred at 25 ?C for 12 hours.
The POCNP Pincer ligands obtained in the process of the present disclosure may contain two rotamers. These two rotamers can be at least partially separated by recrystallization from the second fluid medium.
The third fluid medium is selected from the group consisting of n-pentane, n-hexane, and n-heptane.
In accordance with one embodiment of the present disclosure, the third fluid medium is n-pentane.
The process of the present disclosure provides the POCNP Pincer ligands with a yield in the range from 80% to 94%.
The present disclosure provides a simple process for preparing the POCNP Pincer ligands. Further, yield of POCNP ligand prepared by the process of the present disclosure is high.
In fourth aspect, the present disclosure provides a process for dehydrogenation of alkane using the POCNP-Iridium Pincer complex catalyst of the present disclosure. The process comprises heating a mixture of an alkane, a hydrogen acceptor and a second base in a fourth fluid medium at a temperature in the range of 160 ?C to 210 ?C in the presence of the POCNP-Iridium Pincer complex catalyst, to obtain a dehydrogenated alkane. The dehydrogenated alkane can be alkenes and/or diene.
The hydrogen acceptor is tert-butylethylene.
The second base is at least one selected from the group consisting of potassium tert-butoxide, and lithium tert-butoxide.
In accordance with one embodiment of the present disclosure, the second base is potassium tert-butoxide.
The fourth fluid medium is at least one selected from the group consisting of toluene, xylene and mesitylene.
In accordance with the embodiments of the present disclosure, the alkane is at least one selected from the group consisting of C1 to C10 alkanes.
In accordance with first exemplary embodiment of the present disclosure, the POCNP-Iridium Pincer complex is used for dehydrogenation of n-butane. The products of dehydrogenation are butenes and butadiene. The butenes are a mixture of 1-butene, cis-2-butene and trans-2-butene.
In accordance with second exemplary embodiment of the present disclosure, the alkane is a C4-stream containing a mixture of n-butane and isobutane. The POCNP-Iridium Pincer complex of the present disclosure dehydrogenates isobutane, which is a branched alkane, in preference to n-butane, which is a straight chain alkane.
The POCNP-Iridium Pincer complex of the present disclosure can be immobilized on supports selected from the group consisting of alumina, silica, zeolites, resins, and metal surfaces. The immobilization may involve physical adsorption process or covalent bond linkage.
The POCNP-Iridium Pincer complex of the present disclosure have high catalytic activity. Alkane dehydrogenation is carried out in the presence of the POCNP-Iridium Pincer complex of the present disclosure with a catalyst turnover number in the range of 100 to 1500.
The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.

Experiments:
Experiment-1: Synthesis of POCNP-Iridium Pincer complex 31

A glass pressure tube was charged with [IrCl(COE)2]2 (0.47 g, 0.53 mmol), 30 (0.5 g, 1.06 mmol) (see experiment no. 2), and 15 mL of toluene to obtain a first reaction mixture. The tube was sealed, placed in an oil bath, and heated at 135 °C for 24 hours to obtain a first product mixture. Toluene was removed under reduced pressure to obtain a residue. The residue was extracted with three portions of n-hexane (50 mL). Three n-hexane extracts were combined. The combined extract was subjected to evaporation under reduced pressure. The residue obtained after distillation was dried under reduced pressure to obtain 31.
Experiment 2 – synthesis of POCNP Pincer ligand 30

To a solution of 2-hydroxycarbazole (1g, 5.46 mmol) in THF (10 mL), a suspension of KOtBu (1.24g, 11 mmol) in THF (10 mL) was added drop-wise to obtain a resultant mixture, and the resultant mixture was stirred at 25 ?C for 2 hours. A solution of di-tert-butylchlorophosphine (1.98 g, 11 mmol) in THF was added dropwise to the resultant mixture to obtain a second reaction mixture, and the second reaction mixture was stirred at 25 ?C for 12 hours obtain a second product mixture. THF was evaporated under reduced pressure to obtain a residue and the residue was extracted trice with n-pentane (10 mL). The n-pentane extracts were mixed and the combined n-pentane extract was subjected to evaporation under reduced pressure. The residue was dried under reduced pressure to obtain 30.

Experiment-3: Synthesis of POCNP-Iridium Pincer complex 41

A glass pressure tube was charged with [IrCl(COE)2]2 (0.5 g, 0.56 mmol), 40 (0.475 g, 1.14 mmol) (see experiment no. 4), and 15 mL of toluene to obtain a first reaction mixture. The tube was sealed, placed in an oil bath, and the mixture heated at 135 °C for 24 hours to obtain a first product mixture. Toluene was removed under reduced pressure to obtain a residue and the residue was extracted with three portions of n-pentane (50 mL). The n-pentane extracts were combined and the combined extracts were subjected to evaporation under reduced pressure to obtain 41.

Experiment no. 4 – synthesis of POCNP Pincer ligand 40

To a solution of 2-hydroxycarbazole (1g, 5.46 mmol) in THF (10 mL), a suspension of KOtBu (1.24g, 11 mmol) in THF (10 mL) was added dropwise and the resultant mixture was stirred at 25 ?C for 2 hours. A solution of chlorodiisopropylphosphine (1.68 g, 11 mmol) in THF was added dropwise to the resultant mixture to obtain a second reaction mixture and the second reaction mixture was stirred at 25 ?C for 12 hours obtain a second product mixture. THF was evaporated under reduced pressure from the second product mixture and the residue was extracted thrice with n-pentane (10 mL). Three n-pentane extracts were combined and the combined extract was subjected to evaporatation under reduced pressure to obtain 40 in the form of two conformers in the ratio of approximately 1:1 (2.07 g).

Experiment-5: Synthesis of POCNP-Iridium Pincer complex 21

A glass pressure tube was charged with [IrCl(COE)2]2 (613 mg, 0.684 mmol), 20 (500 mg, 1.36 mmol) (see experiment 6), and 15 mL of toluene to obtain a first reaction mixture. The tube was sealed, placed in an oil bath, and heated at 135 °C for 24 hours to obtain a first product mixture. Toluene was removed under reduced pressure to obtain a residue. The residue was extracted three times with n-pentane (50 mL). Three n-pentane extracts were combined. The combined extract was subjected to evaporation under reduced pressure to obtain 21.

Experiment-6 : Synthesis of POCNP Pincer ligand 20

To a solution of 6-hydroxyindole (0.5 g, 3.75 mmol) in THF (10 mL), a suspension of KOtBu (0.86 g, 7.66 mmol) in THF (10 mL) was added and the resultant mixture was stirred at 25 ?C for 10 hours. A solution of chlorodiisopropylphosphine (1.17 g, 7.66 mmol) in THF was added dropwise to the resultant mixture to obtain a second reaction mixture and the second reaction mixture was stirred at 25 ?C for 12 hours to obtain a second product mixture. THF was evaporated under reduced pressure to obtain a residue and the residue was extracted twice with n-pentane (20 mL). The n-pentane extracts were combined. The combined extract was subjected to evaporation under reduced pressure to obtain 20 (1.29 g).

Experiment-7: Dehydrogenation of alkane using 41
To a Parr reactor, were added tbutyl-ethylene (TBE) (16 mL), potassium tert-butoxide (6 mg), 41, and mesitylene (8 mL) under inert atmosphere. The Parr reactor was cooled to -70 °C and charged with n-butane through a molecular sieve 4? trap. The reaction mixture was heated at 190 °C for 2 hours under stirring. The Parr reactor was cooled to 0 °C and the content was collected, weighed and analyzed by gas chromatography (GC).
The conversion was calculated as the total amount of the olefins obtained, which was the sum of 1-butene, cis-2-butene, trans-2-butene, and butadiene. The catalyst turnover number (TON) was calculated as a molar ratio of the total amount of olefin produced during an experiment to the amount of the catalyst used during that experiment. The results are shown in Table-1.

Table 1: Dehydrogenation of n-butane
Sr. No Catalyst Wt % n-butane: catalyst Butane Conversion (%) Products butadiene selectivity* TON
trans-2-butene 1-butene cis-2-butene Butadiene
1 0.33 3321 17.0 9.0 2.6 4.9 0.5 2.8 564
2 0.15 7306 11.0 5.9 1.8 3.2 0.1 1.3 803
*Butadiene selectivity = amount of butadiene/ total amount of olefins
*Catalyst Wt% is with respect to the amount of n-butane

The dehydrogenation of n-butane with a n-butane to catalyst ratio of 3321 gave the dehydrogenated products with the TON of 564 (Table 1, Sr. No. 1). On increasing the n-butane:catalyst ratio from 3321 to 7306 (Table 1, Sr. No. 2), the TON increased whereas, the butane conversion and butadiene selectivity decreased.

Experiment-8: Dehydrogenation of a C4 feed
To a Parr reactor, were added tbutyl-ethylene (TBE) (16 mL), potassium tert-butoxide (6 mg) , 41 as a catalyst and mesitylene (8 mL) under inert atmosphere. The Parr reactor was cooled to -10 °C and charged with a C4 feed (containing 70% n-butane and 29% isobutane) through a molecular sieve 4? trap. The reaction mixture was heated at 190 °C for 2 hours under stirring.
The Parr reactor was cooled to 0 °C and the content were collected, weighed and analyzed by GC. The results are shown in Table-2.

Table 2: Dehydrogenation of C4 feed

Sr No Catalyst Wt % feed: cat Butane Conversion (%) Iso-Butane Conversion (%) Products % BD
TON
trans-2-butene 1-butene cis-2-butene Butadiene
1 0.29 3124 16.4 26.3 6.1 1.9 3.4 0.3 2.5 389
*%BD = amount of butadiene/ total amount of olefins
*Catalyst Wt% is with respect to the amount of feed
The dehydrogenation of C4-feed with a n-butane to catalyst ratio of 3124 gave the dehydrogenated products with the TON of 389. Isobutane was preferentially dehydrogenated.
Experiment-8: Dehydrogenation of alkane using 21
To a Parr reactor, were added tbutyl-ethylene (TBE) (16 mL), 21, potassium t-butoxide (6 mg) and mesitylene (8 mL) under inert atmosphere. The Parr reactor was cooled to -70 °C and charged with n-butane through a molecular sieve 4? trap. The reaction mixture was heated at 190 °C for 2 hours under stirring. The Parr reactor was cooled to 0 °C and the content was collected, weighed and analyzed by gas chromatography (GC).
The conversion was calculated as the total amount of the olefins obtained, which was the sum of 1-butene, cis-2-butene, trans-2-butene, and butadiene. The catalyst turnover number (TON) was calculated as a molar ratio of the total amount of olefin produced during an experiment to the amount of the catalyst used during that experiment. The results are shown in Table-3.

Table 3: Dehydrogenation of n-butane
Sr. No Catalyst Wt % n-butane: catalyst Butane Conversion (%) Products butadiene selectivity* TON
trans-2-butene 1-butene cis-2-butene Butadiene
1 0.45 2435 25.4 12.1 4.0 7.0 2.3 9.1 617
2 0.208 5313 11.4 6.1 1.7 3.4 0.2 1.4 604
*Butadiene selectivity = amount of butadiene/ total amount of olefins
*Catalyst Wt% is with respect to the amount of n-butane

The dehydrogenation of n-butane with a n-butane to catalyst ratio of 2435 provided the dehydrogenated products with the TON of 617 (Table 3, Sr. No. 1). There was no significant change in turnover numbers on increasing the catalyst ratio to 5313 (Table 1, Sr. No. 2). However, the butane conversion and the butadiene selectivity decreased with an increase in the catalyst ratio.

TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of POCNP-Iridium Pincer complex catalyst that:
- are prepared by a simple process;
- can be used for alkane dehydrogenation with high turnover number; and
- shows high thermal stability.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results. While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Variations or modifications to the formulation of this invention, within the scope of the invention, may occur to those skilled in the art upon reviewing the disclosure herein. Such variations or modifications are well within the spirit of this invention.
The numerical values given for various physical parameters, dimensions and quantities are only approximate values and it is envisaged that the values higher than the numerical value assigned to the physical parameters, dimensions and quantities fall within the scope of the invention unless there is a statement in the specification to the contrary.
While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.