Advances in Nonprecious Metal Homogeneously Catalyzed Formic Acid Dehydrogenation

: Formic acid (FA) possesses a high volumetric concentration of H 2 (53 g L − 1 ). Moreover, it can be easily prepared, stored, and transported. Therefore, FA stands out as a potential liquid organic hydrogen carrier (LOHC), which allows storage and transportation of hydrogen in a safe way. The dehydrogenation to produce H 2 and CO 2 competes with its dehydration to give CO and H 2 O. For this reason, research on selective catalytic FA dehydrogenation has gained attention in recent years. Several examples of highly active homogenous catalysts based on precious metals effective for the selective dehydrogenation of FA have been reported. Among them are the binuclear iridium-bipyridine catalysts described by Fujita and Himeda et al. (TOF = 228,000 h − 1 ) and the cationic species [IrClCp*(2,2 (cid:48) -bi-2-imidazoline)]Cl (TOF = 487,500 h − 1 ). However, examples of catalytic systems effective for the solventless dehydrogenation of FA, which is of great interest since it allows to reduce the reaction volume and avoids the use of organic solvents that could damage the fuel cell, are scarce. In this context, the development of transition metal catalysts based on cheap and easily available nonprecious metals is a subject of great interest. This work contains a summary on the state of the art of catalytic dehydrogenation of FA in homogeneous phase, together with an account of the catalytic systems based on non-precious metals so far reported.


Introduction
Platinum group metals (PGMs) have played a critical role in the development of modern homogeneous catalysis, shaping the landscape of catalyzed industrial processes, from bulk to commodity chemicals. Despite the unquestionable success of PGM-based catalysts, research on catalysts based on Earth-abundant metals (EAM) has received great interest in recent times. This renaissance of EAM-based catalysis is mainly due to concerns over sustainability, as they seem a better option than their PGM analogues owing to their global availability, low cost, and relatively low toxicity and environmental impact [1]. Particularly, 3d transition metals allow for the preparation of inexpensive and efficient catalysts from readily available sources [2][3][4][5][6][7] and, therefore, may pave the way to the development of more sustainable processes. Moreover, 3d metals do not behave simply as mimics of their 4d and 5d counterparts; on the contrary, they present a prolific reactivity mainly due to their ability to perform one-electron redox processes and straightforward ligand reorganization. To some extent, the development of well-defined 3d metal-based catalysts has been hampered by the rather unpredictable behavior of these complexes; however, the use of appropriate ligands that tame their reactivity may offer the possibility to unravel new reactions and mechanisms.
The large scale required for the implementation of global energy transformations has prompted the scientific community to aim at the development of catalysts based on abundant metals for these types of reactions. In this context, formic acid (FA) has been More recently, Beller and coworkers explored the activity of a series of cobalt complexes in the catalytic dehydrogenation of FA [27]. Initially, Co(II) complexes that had been previously reported to show activity in hydrogenation reactions were tested under various conditions, but no activity was detected [28]. The fact that the active species in  More recently, Beller and coworkers explored the activity of a series of cobalt complexes in the catalytic dehydrogenation of FA [27]. Initially, Co(II) complexes that had been previously reported to show activity in hydrogenation reactions were tested under various conditions, but no activity was detected [28]. The fact that the active species in these processes have been proposed to comprise Co(I) metal centers encouraged the authors to explore the performance of related Co(I) complexes 1-3 ( Figure 1). These complexes were isolated and tested as catalysts in a HCOOH/DMOA mixture 10/11 (DMOA = N,Ndimethyloctylamine) at 80 • C. 1 and 2 are barely active; however, 3 showed good activities, allowing total conversion after 90 min (TON = 2260). Complex 3 also proved active in aqueous solutions employing formate as base. It is worth highlighting the extreme sensitivity of this system toward electronic/steric effects, since the exchange of the i Pr substituents of the phosphane moieties by Ph groups dramatically improves the performance of the catalyst. More recently, Beller and coworkers explored the activity of a series of cobalt complexes in the catalytic dehydrogenation of FA [27]. Initially, Co(II) complexes that had been previously reported to show activity in hydrogenation reactions were tested under various conditions, but no activity was detected [28]. The fact that the active species in these processes have been proposed to comprise Co(I) metal centers encouraged the authors to explore the performance of related Co(I) complexes 1-3 ( Figure 1). These complexes were isolated and tested as catalysts in a HCOOH/DMOA mixture 10/11 (DMOA = N,N-dimethyloctylamine) at 80 °C. 1 and 2 are barely active; however, 3 showed good activities, allowing total conversion after 90 min (TON = 2260). Complex 3 also proved active in aqueous solutions employing formate as base. It is worth highlighting the extreme sensitivity of this system toward electronic/steric effects, since the exchange of the i Pr substituents of the phosphane moieties by Ph groups dramatically improves the performance of the catalyst. The high air-sensitivity of 3 prompted the authors to evaluate the activity of the in situ generated species. 3 was formed by the reduction of the parent Co(II) complex, [Co(Cl)2(PNP)] (4) (PNP = bis(2-(diphenylphosphanyl)ethyl)amine), with NaBEt3H in toluene at room temperature. Subsequently, the mixture thus generated was added to the dehydrogenation system previously charged with a water solution of FA and potassium formate at 60 °C. The use of 1 equivalent of NaBEt3H resulted in lower catalytic activities than those obtained for the isolated complex, probably due to the incomplete reduction of 4; conversely, the addition of 2 equivalents led to slightly better results than those achieved by 3. In this regard, the authors proved that the activity of 3 with 2 equivalents of KOOCH is the same as that obtained for 4 with 3 equivalents of KOOCH, which suggests a common active species in this case.
Optimization of the reaction conditions permitted to lower the amount of KOOCH to 1 equivalent relative to FA. However, the best results were obtained at 80 °C, with a 1:4 HCOOH:KOOCH ratio and 3 as catalyst, leading to a TON value of 7166 after 70 h.
The reaction mechanism was substantiated by DFT calculations using 3 as catalyst. All the species included in the Gibbs free energy profile have triplet ground states, being the singlet states too high in energy and, therefore, discarded. The formation of the active species (I1) takes place via substitution of the chloro ligand in 3 by formate, which The high air-sensitivity of 3 prompted the authors to evaluate the activity of the in situ generated species. 3 was formed by the reduction of the parent Co(II) complex, [Co(Cl) 2 (PNP)] (4) (PNP = bis(2-(diphenylphosphanyl)ethyl)amine), with NaBEt 3 H in toluene at room temperature. Subsequently, the mixture thus generated was added to the dehydrogenation system previously charged with a water solution of FA and potassium formate at 60 • C. The use of 1 equivalent of NaBEt 3 H resulted in lower catalytic activities than those obtained for the isolated complex, probably due to the incomplete reduction of 4; conversely, the addition of 2 equivalents led to slightly better results than those achieved by 3. In this regard, the authors proved that the activity of 3 with 2 equivalents of KOOCH is the same as that obtained for 4 with 3 equivalents of KOOCH, which suggests a common active species in this case.
Optimization of the reaction conditions permitted to lower the amount of KOOCH to 1 equivalent relative to FA. However, the best results were obtained at 80 • C, with a 1:4 HCOOH:KOOCH ratio and 3 as catalyst, leading to a TON value of 7166 after 70 h.
The reaction mechanism was substantiated by DFT calculations using 3 as catalyst. All the species included in the Gibbs free energy profile have triplet ground states, being the singlet states too high in energy and, therefore, discarded. The formation of the active species (I1) takes place via substitution of the chloro ligand in 3 by formate, which coordinates to the metal center by one of the O atoms while the other stablishes a hydrogen bond interaction with the proton of the amine. Subsequently, dissociation of the Co-O bond allows the formation of intermediate I2, thus allowing hydride abstraction and concomitant formation of I3 and a molecule of CO 2 . The last step brings about the formation of H 2 and the regeneration of I1 via TSI3-I1, which entails hydride protonation by a molecule of FA stabilized by a hydrogen bond between the carbonyl and N-H moieties (Scheme 2). The direct protonation of the hydride by the N-H moiety or by "free" FA were discarded owing to their high energy barriers. hydrogen bond interaction with the proton of the amine. Subsequently, dissociation of the Co-O bond allows the formation of intermediate I2, thus allowing hydride abstraction and concomitant formation of I3 and a molecule of CO2. The last step brings about the formation of H2 and the regeneration of I1 via TSI3-I1, which entails hydride protonation by a molecule of FA stabilized by a hydrogen bond between the carbonyl and N-H moieties (Scheme 2). The direct protonation of the hydride by the N-H moiety or by "free" FA were discarded owing to their high energy barriers. Scheme 2. Catalytic cycle proposed for the dehydrogenation of FA with 3. The symbol " ‡" denotes a transition state.
The latest example of a cobalt complex that catalyzes the dehydrogenation of FA has been reported by Cantat and coworkers [29]. This catalyst, analogously to Beller's (vide supra), also features an N-H moiety in the vicinity of the metal center; however, in this case no base or additives are required to achieve catalytic activity. Initially, the authors prepared Co(II) complexes 5 and 6 ( Figure 2), which were reacted with five equivalents HCOOH at 150 °C, thus leading to quantitative conversion of HCOOH into CO2 and H2. In addition, the formation of diamagnetic species 7 ( Figure 2) was detected by 1 H and 31 P{ 1 H} NMR, although its isolation was not possible by this method. The Co(I) complex 7 was prepared and successfully isolated by reaction of 5 and 6 with 3 equivalents of trimethylsilylformate at 80 °C for 16 h. The catalytic activity of complexes 5-7 (0.5 mol%) was evaluated at 80 °C using a solution of FA in 1,4-dioxane. Complex 5 showed no activity under these conditions, while 6 reached a TON value of 160 after 20 min, and 7 achieved full conversion in 10 min, which amounts to a TON of 200 with a TOF of 67 min -1 . A TON value of 458 was achieved after Scheme 2. Catalytic cycle proposed for the dehydrogenation of FA with 3. The symbol " ‡" denotes a transition state.
The latest example of a cobalt complex that catalyzes the dehydrogenation of FA has been reported by Cantat and coworkers [29]. This catalyst, analogously to Beller's (vide supra), also features an N-H moiety in the vicinity of the metal center; however, in this case no base or additives are required to achieve catalytic activity. Initially, the authors prepared Co(II) complexes 5 and 6 ( Figure 2), which were reacted with five equivalents HCOOH at 150 • C, thus leading to quantitative conversion of HCOOH into CO 2 and H 2 . In addition, the formation of diamagnetic species 7 ( Figure 2) was detected by 1 H and 31 P{ 1 H} NMR, although its isolation was not possible by this method. The Co(I) complex 7 was prepared and successfully isolated by reaction of 5 and 6 with 3 equivalents of trimethylsilylformate at 80 • C for 16 h.
Co-O bond allows the formation of intermediate I2, thus allowing hydride abstraction and concomitant formation of I3 and a molecule of CO2. The last step brings about the formation of H2 and the regeneration of I1 via TSI3-I1, which entails hydride protonation by a molecule of FA stabilized by a hydrogen bond between the carbonyl and N-H moieties (Scheme 2). The direct protonation of the hydride by the N-H moiety or by "free" FA were discarded owing to their high energy barriers. The latest example of a cobalt complex that catalyzes the dehydrogenation of FA has been reported by Cantat and coworkers [29]. This catalyst, analogously to Beller's (vide supra), also features an N-H moiety in the vicinity of the metal center; however, in this case no base or additives are required to achieve catalytic activity. Initially, the authors prepared Co(II) complexes 5 and 6 ( Figure 2), which were reacted with five equivalents HCOOH at 150 °C, thus leading to quantitative conversion of HCOOH into CO2 and H2. In addition, the formation of diamagnetic species 7 ( Figure 2) was detected by 1 H and 31 P{ 1 H} NMR, although its isolation was not possible by this method. The Co(I) complex 7 was prepared and successfully isolated by reaction of 5 and 6 with 3 equivalents of trimethylsilylformate at 80 °C for 16 h. The catalytic activity of complexes 5-7 (0.5 mol%) was evaluated at 80 °C using a solution of FA in 1,4-dioxane. Complex 5 showed no activity under these conditions, while 6 reached a TON value of 160 after 20 min, and 7 achieved full conversion in 10 min, which amounts to a TON of 200 with a TOF of 67 min -1 . A TON value of 458 was achieved after The catalytic activity of complexes 5-7 (0.5 mol%) was evaluated at 80 • C using a solution of FA in 1,4-dioxane. Complex 5 showed no activity under these conditions, while 6 reached a TON value of 160 after 20 min, and 7 achieved full conversion in 10 min, which amounts to a TON of 200 with a TOF of 67 min −1 . A TON value of 458 was achieved after 4 h by means of consecutive cycles under the reaction conditions described above. It is noteworthy that 7 also showed activity in other solvents, such as toluene or water, in the absence of additives or base, although lower TON and TOF values were recorded. Computational studies on this system revealed a reaction pathway, according to which the dangling amine moiety at the PPP ligand plays a major role, allowing outer-sphere interactions with the substrate (HCOOH) by means of hydrogen bonding (Scheme 3). The initial step entails the isomerization of 7 to give intermediate I4, where the hydride and carbonyl ligands exchange positions relative to 7. Subsequently, protonation of the hydride in I4 affords a molecule of H 2 and I5. Subsequently, although β-hydride elimination is a possible reaction pathway, the authors found that hydride abstraction aided by a molecule of FA, via intermediates I6 and I7, was a lower energy pathway. Thus, CO 2 release renders intermediate I4, closing the catalytic cycle.
absence of additives or base, although lower TON and TOF values were recorded. Computational studies on this system revealed a reaction pathway, according to which the dangling amine moiety at the PPP ligand plays a major role, allowing outer-sphere interactions with the substrate (HCOOH) by means of hydrogen bonding (Scheme 3). The initial step entails the isomerization of 7 to give intermediate I4, where the hydride and carbonyl ligands exchange positions relative to 7. Subsequently, protonation of the hydride in I4 affords a molecule of H2 and I5. Subsequently, although β-hydride elimination is a possible reaction pathway, the authors found that hydride abstraction aided by a molecule of FA, via intermediates I6 and I7, was a lower energy pathway. Thus, CO2 release renders intermediate I4, closing the catalytic cycle. Remarkably, in contrast with the mechanism described above for 3 and 7, the decarboxylation of formate without the assistance of a cooperative mechanism has been recently described for a cobalt complex [30].

Iron Homogeneous Catalysts
In 2010 Beller, Ludwig and coworkers explored the activity of a wide range of ironcarbonyl complexes in the presence of various ligands and visible-light irradiation [31]. [Fe3(CO)12] with 3 equivalents of PPh3 and tpy (terpyridine) was found to be the best performing catalyst, closely followed by [Fe2(CO)9] and [Fe(CO)3(COT)] with analogous ligand ratios (Fe/PPh3/tpy 1:1:1). It is worth noting that when these ligands were used independently, a drastic drop of the catalytic activity was observed. Remarkably, in contrast with the mechanism described above for 3 and 7, the decarboxylation of formate without the assistance of a cooperative mechanism has been recently described for a cobalt complex [30].

Iron Homogeneous Catalysts
In 2010 Beller, Ludwig and coworkers explored the activity of a wide range of ironcarbonyl complexes in the presence of various ligands and visible-light irradiation [31].  12 ] with 3 equivalents of PPh 3 and tpy (terpyridine) was found to be the best performing catalyst, closely followed by [Fe 2 (CO) 9 ] and [Fe(CO) 3 (COT)] with analogous ligand ratios (Fe/PPh 3 /tpy 1:1:1). It is worth noting that when these ligands were used independently, a drastic drop of the catalytic activity was observed.
Experiments performed by successively turning on and off the light source showed that catalytic activity was only observed under irradiation, while dark periods revealed no gas evolution, which led the authors to conclude that the reaction was photoassisted instead of simply photoinduced.
Based on NMR, IR and DFT studies the authors proposed [Fe(H)(CO) 3 (PPh 3 )] -(I8) as the active species, which forms by irradiation of [Fe(CO) 3 (PPh 3 ) 2 ] in the presence of FA. The benefit of using N-ligands is unclear since no complex containing these ligands was identified by means of stoichiometric experiments. Nonetheless, the authors claim a stabilizing role after phosphane dissociation, because the loss of a CO ligand was observed in the presence terpyridine, which may be a catalyst deactivation pathway. Once I8 is generated, protonation of its hydride ligand by FA leads to formation of the dihydrogen complex I9. Subsequently, phosphane coordination and H 2 release renders I10. Finally, β-hydride elimination closes the catalytic cycle delivering a molecule of CO 2 (Scheme 4).
Based on NMR, IR and DFT studies the authors proposed [Fe(H)(CO)3(PPh3)] -(I8) as the active species, which forms by irradiation of [Fe(CO)3(PPh3)2] in the presence of FA. The benefit of using N-ligands is unclear since no complex containing these ligands was identified by means of stoichiometric experiments. Nonetheless, the authors claim a stabilizing role after phosphane dissociation, because the loss of a CO ligand was observed in the presence terpyridine, which may be a catalyst deactivation pathway. Once I8 is generated, protonation of its hydride ligand by FA leads to formation of the dihydrogen complex I9. Subsequently, phosphane coordination and H2 release renders I10. Finally, βhydride elimination closes the catalytic cycle delivering a molecule of CO2 (Scheme 4). Shortly after, Beller, Ludwig and Laurenczy developed an Fe(II) catalyst based on the tetradentate ligand P(CH2CH2PPh2)3 (PP3), which was prepared in situ employing ratios Fe/ligand 1:1 and 1:2, with the latter showing better activities [32]. Remarkably, the reaction works without the need for base or additives, and using an environmentally benign solvent, namely, propylene carbonate. Moreover, high pressures of CO2 and H2 (up to 30 and 20 bar, respectively) showed no significant effects on the activity of the catalyst. The presence of air or the traces of water present in the solvent or FA did not affect the performance of the catalyst, but addition of chloride to the system resulted in a sharp reduction of the catalytic activity.
Several Fe-hydride complexes featuring the PP3 ligand were tested, with [Fe(H)(PP3)]BF4 (8)  Shortly after, Beller, Ludwig and Laurenczy developed an Fe(II) catalyst based on the tetradentate ligand P(CH 2 CH 2 PPh 2 ) 3 (PP 3 ), which was prepared in situ employing ratios Fe/ligand 1:1 and 1:2, with the latter showing better activities [32]. Remarkably, the reaction works without the need for base or additives, and using an environmentally benign solvent, namely, propylene carbonate. Moreover, high pressures of CO 2 and H 2 (up to 30 and 20 bar, respectively) showed no significant effects on the activity of the catalyst. The presence of air or the traces of water present in the solvent or FA did not affect the performance of the catalyst, but addition of chloride to the system resulted in a sharp reduction of the catalytic activity.
Several Fe-hydride complexes featuring the PP 3  DFT and experimental studies on the reaction mechanism led the authors to propose two competing catalytic cycles, A and B in Scheme 5. In the case of cycle A, the first step involves the protonation of the hydride ligand in 8 to afford I11 with concomitant release of H 2 . I11, which is in equilibrium with an analogous species featuring a monodentate formate, undergoes β-hydride elimination to yield I12. Finally, CO 2 dissociation closes the cycle. Regarding cycle B, coordination of formate to 8 leads to the formation of I13, which renders I14 upon protonation of the hydride and β-hydride elimination of the formate, thus generating a molecule of CO 2 . Finally, H 2 dissociation restarts the cycle.
to afford I11 with concomitant release of H2. I11, which is in equilibrium with an analogous species featuring a monodentate formate, undergoes β-hydride elimination to yield I12. Finally, CO2 dissociation closes the cycle. Regarding cycle B, coordination of formate to 8 leads to the formation of I13, which renders I14 upon protonation of the hydride and β-hydride elimination of the formate, thus generating a molecule of CO2. Finally, H2 dissociation restarts the cycle. Scheme 5. Catalytic cycle proposed for the dehydrogenation of FA using complex 8 as catalyst.
Complexes 10 and 11 (Scheme 6) can catalyze the dehydrogenation of FA in the presence of a 50 mol% Et3N at 80 °C in dioxane, allowing a TOF of 739 h -1 and a TON of 994 after 2.5 h for 11b [33]. The authors showed that the use of a Lewis acid (10 mol%) instead of Et3N gives rise to excellent catalytic activities; namely, LiBF4 renders outstanding TOF and TON values (196,728 h -1 and 983,642, respectively)-note that in the absence of additives the system proved barely active. Mechanistic studies suggest that Et3N is not directly involved in the catalytic cycle, since all the steps of the mechanism may take place in the absence of base. The improved activities in the presence of Et3N and Lewis acids, such as LiBF4, hint at a similar role in the reaction mechanism. The authors propose that the decarboxylation of 11, which is the rate liming step according to kinetic experiments, takes place via hydride abstraction assisted by a Lewis acid (instead of βhydride elimination). Rearrangement of the formate ligand yields an H-bound intermediate, which is stabilized by hydrogen bonding between the carboxylate moiety and the Lewis acid, or, in the case of Et3N, the Brønsted acid [Et3NH] + that forms in the presence of excess FA (Scheme 7). This lowers the activation energy of the decarboxylation step, thus improving the reaction rates. Complexes 10 and 11 (Scheme 6) can catalyze the dehydrogenation of FA in the presence of a 50 mol% Et 3 N at 80 • C in dioxane, allowing a TOF of 739 h −1 and a TON of 994 after 2.5 h for 11b [33]. The authors showed that the use of a Lewis acid (10 mol%) instead of Et 3 N gives rise to excellent catalytic activities; namely, LiBF 4 renders outstanding TOF and TON values (196,728 h −1 and 983,642, respectively)-note that in the absence of additives the system proved barely active. Mechanistic studies suggest that Et 3 N is not directly involved in the catalytic cycle, since all the steps of the mechanism may take place in the absence of base. The improved activities in the presence of Et 3 N and Lewis acids, such as LiBF 4 , hint at a similar role in the reaction mechanism. The authors propose that the decarboxylation of 11, which is the rate liming step according to kinetic experiments, takes place via hydride abstraction assisted by a Lewis acid (instead of β-hydride elimination). Rearrangement of the formate ligand yields an H-bound intermediate, which is stabilized by hydrogen bonding between the carboxylate moiety and the Lewis acid, or, in the case of Et 3 N, the Brønsted acid [Et 3 NH] + that forms in the presence of excess FA (Scheme 7). This lowers the activation energy of the decarboxylation step, thus improving the reaction rates.  The overall reaction mechanism entails the formation of the active species, 11, by protonation of the amide in 10 with FA and coordination of the thus formed formate. Subsequently, decarboxylation of 11 affords dihydride I15, which is protonated by FA to afford I16. Finally, release of H2 from I16 regenerates the active species 11.
The tridentate PNP ligand and the ancillary ligands about the iron center were modified to optimize the activity and stability of the catalyst [34]. However, under analogous conditions to those described above for 10 or 11 (0.1 mol% catalyst loading and 50 mol% Et3N in dioxane at 80 °C) no significant improvement was achieved with catalysts 12-14 ( Figure 3). In fact, only 13, which is closely related to 11, produced comparable results. The overall reaction mechanism entails the formation of the active species, 11, by protonation of the amide in 10 with FA and coordination of the thus formed formate. Subsequently, decarboxylation of 11 affords dihydride I15, which is protonated by FA to afford I16. Finally, release of H 2 from I16 regenerates the active species 11.
The tridentate PNP ligand and the ancillary ligands about the iron center were modified to optimize the activity and stability of the catalyst [34]. However, under analogous conditions to those described above for 10 or 11 (0.1 mol% catalyst loading and 50 mol% Et 3 N in dioxane at 80 • C) no significant improvement was achieved with catalysts 12-14 ( Figure 3). In fact, only 13, which is closely related to 11, produced comparable results. Scheme 6. Catalytic cycle proposed for the dehydrogenation of FA using 10 or 11 as catalysts. The overall reaction mechanism entails the formation of the active species, 11, by protonation of the amide in 10 with FA and coordination of the thus formed formate. Subsequently, decarboxylation of 11 affords dihydride I15, which is protonated by FA to afford I16. Finally, release of H2 from I16 regenerates the active species 11.
The tridentate PNP ligand and the ancillary ligands about the iron center were modified to optimize the activity and stability of the catalyst [34]. However, under analogous conditions to those described above for 10 or 11 (0.1 mol% catalyst loading and 50 mol% Et3N in dioxane at 80 °C) no significant improvement was achieved with catalysts 12-14 ( Figure 3). In fact, only 13, which is closely related to 11, produced comparable results.  The group of Gonsalvi developed a series of iron complexes containing racand meso-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane (rac-p4 and meso-P4) [35]. The preformed complex cis-α-[Fe(CH 3 CN) 2 (rac-P4)](BF 4 ) 2 (15)  Mechanistic studies based on NMR experiments led the authors to propose the catalytic cycle depicted in Scheme 8. The first step involves the formation of the κ 2 -formate intermediate I17, which is in equilibrium with its I18. β-hydride elimination from I19 affords the hydride intermediate I20. Finally, protonation of the hydride regenerates I17 and releases H 2 . It is noteworthy that under isochoric conditions the authors observe the formation of a carbonyl complex (17), [FeH(CO)(rac-P4)]BF 4 , which they suggest forms from the hydride species I20 by reaction with CO-generated by the competitive decomposition of HCOOH into CO and H 2 O. catalytic cycle depicted in Scheme 8. The first step involves the formation of the κ 2formate intermediate I17, which is in equilibrium with its I18. β-hydride elimination from I19 affords the hydride intermediate I20. Finally, protonation of the hydride regenerates I17 and releases H2. It is noteworthy that under isochoric conditions the authors observe the formation of a carbonyl complex (17), [FeH(CO)(rac-P4)]BF4, which they suggest forms from the hydride species I20 by reaction with CO-generated by the competitive decomposition of HCOOH into CO and H2O. Scheme 8. Catalytic cycle proposed for the dehydrogenation of FA using 16 under isobaric conditions.

Manganese Homogeneous Catalysts
In 2016, Tondreu and Boncella reported that the Mn(I) complex [Mn(PNP)(CO)2] (18; P = P i Pr2), which is isoelectronic with the Fe(II) species 10 (Scheme 6), reacts with FA to give the 1,2-addition product [Mn(PN H P)(CO)2] (19) (Scheme 9). Complex 19 proved to be an active catalyst precursor for the catalytic decomposition of FA in 1,4-dioxane. The authors observed that the addition of LiBF4 to the reaction mixture inhibits the catalytic activity. The low activity of this manganese-based FA-dehydrogenation catalytic system, in addition to the poor selectivity-formation of CO was observed-, makes it unsuitable for fuel cell applications [36].

Manganese Homogeneous Catalysts
In 2016, Tondreu and Boncella reported that the Mn(I) complex [Mn(PNP)(CO) 2 ] (18; P = P i Pr 2 ), which is isoelectronic with the Fe(II) species 10 (Scheme 6), reacts with FA to give the 1,2-addition product [Mn(PN H P)(CO) 2 ] (19) (Scheme 9). Complex 19 proved to be an active catalyst precursor for the catalytic decomposition of FA in 1,4-dioxane. The authors observed that the addition of LiBF 4 to the reaction mixture inhibits the catalytic activity. The low activity of this manganese-based FA-dehydrogenation catalytic system, in addition to the poor selectivity-formation of CO was observed-, makes it unsuitable for fuel cell applications [36]. Some years later, the strategy of using the non-innocent N-H moieties in PNP ligands was applied to prepare the species [Mn( tBu PN H NOP)(CO)2]Br (20). The deprotonation of the cationic species 20 with NaN(SiMe3)2 yields complex [Mn( tBu PNNOP)(CO)2] (21) (Scheme 10), which is an effective catalyst precursor for the dehydrogenation of FA to H2 and CO2 using chlorobenzene as solvent and Et3N as base. The highest catalytic activity for this system (TOF = 8500 h −1 ), which is comparable to some obtained using catalytic systems based on noble metals, was achieved using one equivalent of Et3N to FA at 80 °C [37]. Some years later, the strategy of using the non-innocent N-H moieties in PNP ligands was applied to prepare the species [Mn( tBu PN H NOP)(CO) 2 ]Br (20). The deprotonation of the cationic species 20 with NaN(SiMe 3 ) 2 yields complex [Mn( tBu PNNOP)(CO) 2 ] (21) (Scheme 10), which is an effective catalyst precursor for the dehydrogenation of FA to H 2 and CO 2 using chlorobenzene as solvent and Et 3 N as base. The highest catalytic activity for this system (TOF = 8500 h −1 ), which is comparable to some obtained using catalytic systems based on noble metals, was achieved using one equivalent of Et 3 N to FA at 80 • C [37].
was applied to prepare the species [Mn( tBu PN H NOP)(CO)2]Br (20). The deprotonation of the cationic species 20 with NaN(SiMe3)2 yields complex [Mn( tBu PNNOP)(CO)2] (21) (Scheme 10), which is an effective catalyst precursor for the dehydrogenation of FA to H2 and CO2 using chlorobenzene as solvent and Et3N as base. The highest catalytic activity for this system (TOF = 8500 h −1 ), which is comparable to some obtained using catalytic systems based on noble metals, was achieved using one equivalent of Et3N to FA at 80 °C [37].   Britto and Jaccob have recently reported a mechanistic study at the DFT level on the 24-catalyzed dehydrogenation of FA. They found that the formation of the Mn-formate intermediate 28 from 24 follows a dissociative mechanism (Scheme 11). The dissociation of the Brion creates a vacant site, which is occupied by a molecule of solvent (H2O). The dissociative exchange of water by the formate ion leads to intermediate 28. The energy barrier of the activation process corresponds to TS26-28 (23.5 kcal mol −1 ) [40].

Nickel Homogeneous Catalysts
Although the nickel complexes hitherto tested as catalysts for FADH show little promise, a few noteworthy examples have been reported [41]. Nickel hydride complex 31 proved able to react with FA to afford H2 and formato complex 31. Successively, 32

Nickel Homogeneous Catalysts
Although the nickel complexes hitherto tested as catalysts for FADH show little promise, a few noteworthy examples have been reported [41]. Nickel hydride complex 31 proved able to react with FA to afford H 2 and formato complex 31. Successively, 32 releases CO 2 at 50 • C to regenerate 31 (Scheme 13). Complex 31 showed activity as an FADH catalyst with a HCOOH:dimethyl-n-octylamine mixture 11:10. The best results were obtained at 80 • C using propylene carbonate as solvent (TON 3h = 626), although the authors detected CO 2 formation from solvent decomposition.

Scheme 12.
Mechanism proposal for the 28-catalyzed dehydrogenation of FA.

Nickel Homogeneous Catalysts
Although the nickel complexes hitherto tested as catalysts for FADH show little promise, a few noteworthy examples have been reported [41]. Nickel hydride complex 31 proved able to react with FA to afford H2 and formato complex 31. Successively, 32 releases CO2 at 50 °C to regenerate 31 (Scheme 13). Complex 31 showed activity as an FADH catalyst with a HCOOH:dimethyl-n-octylamine mixture 11:10. The best results were obtained at 80 °C using propylene carbonate as solvent (TON3h = 626), although the authors detected CO2 formation from solvent decomposition. Parkin and coworkers evaluated the activity of group 10 zerovalent complexes Ni(PMe3)4, Pd(PMe3)4 and Pt(PMe3)4 as catalysts in FADH. Remarkably, the nickel complex was found to be the only one to display activity, giving rise to a TON of 70 and a TOF of 1.7 h -1 in the absence of additives or base [42]. Based on mechanistic studies, the authors proposed the formation of a pentacoordinate Ni(II) species, [Ni(PMe3)3(OOCH)(H)], by oxidative addition of FA with concurrent release of a PMe3 Although experimental support was found for the former pathway, the latter cannot be discarded.

Copper Homogeneous Catalysts
The first examples of Cu-catalyzed homogeneous hydrogen production from FA/amine adducts were reported in 2014. This work studies the effect of different reaction parameters on the catalytic performance. The authors found that when using Cu(II) compounds such as Cu(AcO) 2 , Cu(OOCH) 2 , Cu(acac) 2 and Cu(NO 3 ) 2 ,no relevant changes in the activity were observed. This demonstrated that the activation step, replacement of the ancillary ligands by formate and amine, is fast and does not determine the rate of the process. The activity of these catalytic systems improves by increasing steric hindrance and by decreasing nucleophilicity, which suggests that the role of the amine is not limited to HCOOH deprotonation. These Cu-based catalytic systems are selective to the one to one H 2 :CO 2 production and only traces of CO were detected. However, the activity of all the reported examples was found to be considerably low (TOF < 1 h −1 ) [43]. Mechanistic studies at the DFT level show that the rate-determining step of the catalytic process is the amine dissociation to generate a coordination vacancy cis to the formate, which is necessary for the beta β-hydrogen elimination to give a cis-[CuH(NR 3 ) 2 (κ 1 -O-CO 2 )] intermediate. This study confirms the experimental data, which suggest that the energetic barrier of the process depends on the nature of the amine. Thus, the use of Et 3 N is preferred to NBu 3 by 1.6 kcal mol −1 (Scheme 14).
the reported examples was found to be considerably low (TOF < 1 h −1 ) [43]. Mechanistic studies at the DFT level show that the rate-determining step of the catalytic process is the amine dissociation to generate a coordination vacancy cis to the formate, which is necessary for the beta β-hydrogen elimination to give a cis-[CuH(NR3)2(κ 1 -O-CO2)] intermediate. This study confirms the experimental data, which suggest that the energetic barrier of the process depends on the nature of the amine. Thus, the use of Et3N is preferred to NBu3 by 1.6 kcal mol −1 (Scheme 14).  Recently, the potential of species [CuH(phen*)] 2− (phen* = 4,7-diphenyl-1,10phenanthroline-disul-fonic acid disodium salt) as a catalyst for the selective decarboxylation of FA has been experimentally and theoretically studied. The sulfonate sites, which can be used as linkers for the heterogenization of these species, are far from the {CuH} reactive site and therefore should not interfere with its reactivity [45].
On the other hand, Cu(I)-dinuclear species with tetradentate phosphine ligands have also been proposed as catalysts for the dehydrogenation of FA. The activity of these catalysts is moderate, with TOF values in the range of 120-700 h −1 . Mechanistic studies for these reactions suggested that asymmetric dinuclear species are formed during the catalytic process. Based on this outcomes, the authors proposed a reaction mechanism where the formate bridge changes from a Cu2(µ-κ-O,O′-O2CH) to a Cu2(µ-κ-O-O2CH) coordination mode (Scheme 16) [46]. Recently, the potential of species [CuH(phen*)] 2− (phen* = 4,7-diphenyl-1,10-phenanth roline-disul-fonic acid disodium salt) as a catalyst for the selective decarboxylation of FA has been experimentally and theoretically studied. The sulfonate sites, which can be used as linkers for the heterogenization of these species, are far from the {CuH} reactive site and therefore should not interfere with its reactivity [45].
On the other hand, Cu(I)-dinuclear species with tetradentate phosphine ligands have also been proposed as catalysts for the dehydrogenation of FA. The activity of these catalysts is moderate, with TOF values in the range of 120-700 h −1 . Mechanistic studies for these reactions suggested that asymmetric dinuclear species are formed during the catalytic process. Based on this outcomes, the authors proposed a reaction mechanism where the formate bridge changes from a Cu 2 (µ-κ-O,O -O 2 CH) to a Cu 2 (µ-κ-O-O 2 CH) coordination mode (Scheme 16) [46].
On the other hand, Cu(I)-dinuclear species with tetradentate phosphine ligands have also been proposed as catalysts for the dehydrogenation of FA. The activity of these catalysts is moderate, with TOF values in the range of 120-700 h −1 . Mechanistic studies for these reactions suggested that asymmetric dinuclear species are formed during the catalytic process. Based on this outcomes, the authors proposed a reaction mechanism where the formate bridge changes from a Cu2(µ-κ-O,O′-O2CH) to a Cu2(µ-κ-O-O2CH) coordination mode (Scheme 16) [46]. Scheme 16. Proposed catalytic cycle for Cu2-(P4) catalyzed dehydrogenation of FA.

Aluminum and Boron Homogeneous Catalysts
It is noteworthy that catalysts for the dehydrogenation of FA based on main-group elements have also been reported. Al-complex 33, which features an NNN participative ligand, proved active in a variety of solvents using Et3N as an additive [47]. The best activities were observed in tetrahydrofurane at 65 °C using a 0.006 mol% of 33 and a HCOOH:Et3N mixture (5:2), which bring about a TOF of 5200 h -1 and a TON1h of 2200. Detailed mechanistic investigation led the authors to propose the catalytic cycle depicted in Scheme 10. The pre-catalyst 33 reacts with FA to afford the resting state, 34, which presents a neutral NNN ligand formed by double protonation of the dianionic ligand in

Aluminum and Boron Homogeneous Catalysts
It is noteworthy that catalysts for the dehydrogenation of FA based on main-group elements have also been reported. Al-complex 33, which features an NNN participative ligand, proved active in a variety of solvents using Et 3 N as an additive [47]. The best activities were observed in tetrahydrofurane at 65 • C using a 0.006 mol% of 33 and a HCOOH:Et 3 N mixture (5:2), which bring about a TOF of 5200 h −1 and a TON 1h of 2200. Detailed mechanistic investigation led the authors to propose the catalytic cycle depicted in Scheme 10. The pre-catalyst 33 reacts with FA to afford the resting state, 34, which presents a neutral NNN ligand formed by double protonation of the dianionic ligand in the parent complex. β-hydride elimination generates CO 2 and an Al-hydride intermediate (35). For this step, the authors postulate a transition state where the formate ligand dissociates to interact by hydrogen bond with the N-H moiety of the NNN ligand, thus allowing hydride abstraction by the Al-center (TS34-35 Although boron is not a metal and the activities so far reported by catalysts based on this semimetal are modest, we believe that it is interesting to highlight the progress realized in this area because of the great promise for future development. Recent work by Cantat and coworkers describes the activity of dialkylborane derivatives in the presence of a HCOOH:Et 3 N mixture (5:2)

Heterodinuclear Homogeneous Catalysts
Heterodinuclear complexes featuring an Ir(III) center with a Co (36), Ni (37) or Cu (38)   The authors proposed a reaction mechanism for 37 that entails the formation of a cationic aquocomplex (I23) that reversively coordinates a formate to give (I24). Subsequently, β-hydride elimination affords I25 upon coordination of a molecule of water. Finally, protonation of the hydride ligand at the Ir center by H3O + , which is the rate limiting step, closes the catalytic cycle (Scheme 19). Scheme 18. Catalytic cycle proposed for the dehydrogenation of FA using 32 as catalyst.

Heterodinuclear Homogeneous Catalysts
Heterodinuclear complexes featuring an Ir(III) center with a Co (36), Ni (37) or Cu (38) atom in the vicinity show a dramatic increase of their activity compared to the related mononuclear Ir(III) complex 39 ( Figure 5)

Heterodinuclear Homogeneous Catalysts
Heterodinuclear complexes featuring an Ir(III) center with a Co (36), Ni (37) or Cu (38)   The authors proposed a reaction mechanism for 37 that entails the formation of a cationic aquocomplex (I23) that reversively coordinates a formate to give (I24). Subsequently, β-hydride elimination affords I25 upon coordination of a molecule of water. Finally, protonation of the hydride ligand at the Ir center by H3O + , which is the rate limiting step, closes the catalytic cycle (Scheme 19). The authors proposed a reaction mechanism for 37 that entails the formation of a cationic aquocomplex (I23) that reversively coordinates a formate to give (I24). Subse-quently, β-hydride elimination affords I25 upon coordination of a molecule of water. Finally, protonation of the hydride ligand at the Ir center by H 3 O + , which is the rate limiting step, closes the catalytic cycle (Scheme 19).

Conclusions
In recent years, several examples of outstanding catalytic systems for FADH based on EAM have been developed. The most active EAM-catalysts are those that use iron as the metal center, followed by manganese and cobalt.
Regarding the mechanisms that operate in these reactions, although participative ligands do play an important role in many cases, they are not imperative to design active catalysts. In this regard, the decarboxylation step in FADH-which usually takes place by hydride abstraction or β-elimination-is often described as the rate determining step, not only for EAM, but also for PGM. Therefore, the use of bifunctional catalysts or additives that facilitate this step may boost the catalytic activity of the system. Hydride abstraction has been described to be favored by outer-sphere interactions that stabilize a transition state whose formation requires a formate ligand to flip over, changing from a stable Ocoordination mode to an unfavorable M-H interaction. This review shows examples of this stabilization taking place by hydrogen bond between the formate and an N-H fragment of a participative ligand, or by interaction of the formate with Lewis acid additives. In both cases, the negative charge at the carboxylate moiety is stabilized, thus facilitating an M-H interaction that enables the hydride abstraction step. The H2 formation step may be assisted by ligands able to establish a hydrogen bond interaction with the carbonyl moiety of FA, which places the acidic OH proton in the proximity of the hydride, thus enabling hydride protonation by a low energy pathway. Protonation of the hydride ligand has also been proposed to be favored by the presence of water, since the hydronium ion protonates the hydride more straightforwardly than FA.
In conclusion, there is still much room for improvement in this field regarding the activity and stability of EAM-catalysts. Moreover, although some Fe systems show excellent activities, none of the examples so far reported can operate in neat FA, which is crucial for practical applications, since the use of a solvent reduces the hydrogen density Scheme 19. Catalytic cycle proposed for the dehydrogenation of FA using 37 as catalyst.

Conclusions
In recent years, several examples of outstanding catalytic systems for FADH based on EAM have been developed. The most active EAM-catalysts are those that use iron as the metal center, followed by manganese and cobalt.
Regarding the mechanisms that operate in these reactions, although participative ligands do play an important role in many cases, they are not imperative to design active catalysts. In this regard, the decarboxylation step in FADH-which usually takes place by hydride abstraction or β-elimination-is often described as the rate determining step, not only for EAM, but also for PGM. Therefore, the use of bifunctional catalysts or additives that facilitate this step may boost the catalytic activity of the system. Hydride abstraction has been described to be favored by outer-sphere interactions that stabilize a transition state whose formation requires a formate ligand to flip over, changing from a stable Ocoordination mode to an unfavorable M-H interaction. This review shows examples of this stabilization taking place by hydrogen bond between the formate and an N-H fragment of a participative ligand, or by interaction of the formate with Lewis acid additives. In both cases, the negative charge at the carboxylate moiety is stabilized, thus facilitating an M-H interaction that enables the hydride abstraction step. The H 2 formation step may be assisted by ligands able to establish a hydrogen bond interaction with the carbonyl moiety of FA, which places the acidic OH proton in the proximity of the hydride, thus enabling hydride protonation by a low energy pathway. Protonation of the hydride ligand has also been proposed to be favored by the presence of water, since the hydronium ion protonates the hydride more straightforwardly than FA.
In conclusion, there is still much room for improvement in this field regarding the activity and stability of EAM-catalysts. Moreover, although some Fe systems show excellent activities, none of the examples so far reported can operate in neat FA, which is crucial for practical applications, since the use of a solvent reduces the hydrogen density of the mixture. The examination of the catalytic systems reviewed here suggests that ligand design is fundamental to improving the catalyst performance by stabilizing the active species or by assisting key steps via outer-sphere interactions. Therefore, regarding future challenges and opportunities, EAM complexes that present ligands capable of assisting key steps of the catalytic cycle by non-covalent interactions are firm candidates for achieving enhanced activities-the final goal being the preparation of robust catalysts able to accomplish activities in neat FA comparable to those obtained by their PGM counterparts.