Generally, ligands are neutral or anionic like a Lewis base. Ligands having positive charge are less common. These new category of ligand species are called cationic ligands. These ligands can accept pi-electrons - Z-type ligand and donate lone pair – L-type ligand synergistically making these ligands another class in itself.
The main reason for ligand being non-cationic is the coulombic repulsion between the similarly charged positive metal centre and ligand. Even then, ligands like NO+(nitrosonium), H2N−NH3+ (hydrazinium) and NO2+(nitronium) are commonly found with a positive charge. C7H7+(tropylium) and C3H3+ (cyclopropenyl cation) are well-known aromatic carbocations that can coordinate with metal ions forming a half-sandwich or a sandwich complex.However, cationic ligands which can coordinate through the atom which has a positive charge localized on it are even less reported. These ligands show different modes of bonding like L→M σ donation, M→π back-donation and electrostatic interaction mimicking transition metal to some extent. Among this subcategory of cationic ligands, polydentate cationic ligands have only a few examples
Group 15 cationic ligand
The valence states of these elements are generally +3 and +5. Manuel Alcarazo and his group have generated several cationic phosphines. They have managed to synthesize dicationic and tricationic phosphine ligand species which were able to coordinate with gold(I) and platinum(II) cation respectively. The donating ability of these cationic phosphines was reported to be poorer than that of phosphites and only comparable to extremely toxic or pyrophoric compounds such as PF3 or P(CF3)3. Similar synthetic strategies were applied by Alcarazo to cationic arsines containing imidazolium, cyclopropenium, formamidinium and pyridinium substituents.10 It was seen that cyclopropenium substituent had considerably lesser steric requirement than the other aromatic substituent. Different coordination complexes were explored using different transition metals like Rhodium, Gold and Platinum.
1. Nitrenium cation
A pnictogen cation in its +1 monovalent state is analogous to carbene. Series of NHC analogues of nitrogens were exploited by Tulchinsky et al. as they are isolobal in nature as compared to carbene. N-heterocyclic nitrenium ion is one of the very few cationic ligands using mono-cationic nitrogen center to coordinate with electron-rich transition metal ions like Rh(I), Rh(III), Mo(0), Ru(0), Ru(II), Pd(II), Pt(II), Pt(IV), and Ag(I) .These complexes are significantly favoured in polar solvents. To maintain the transition metal centre in near proximity to L-type nitrenium donor, more L-type donors were attached to the scaffold with between the donor and nitrenium sites to favour Z-type interactions too. From IR spectroscopy it was established that these nitrenium ligands are weaker σ-donors than both the NHC and pyridine, even weaker than phosphenium! From DFT studies, it was shown that the primary bonding component came from the π-acceptor property as considerable binding energy was observed (47 kcal mol-1).[1-2]
Workers of Ray’s team explored another set of nitrenium cation scaffold where they have replaced phosphine side chains of Tulchinsky’s nitrenium with tetraazacyclotetradecane which they abbreviated as (DMC-nit)+. The five-coordinate complexes of Ni(I) and Ni(II) using (DMC-nit)+ ligand, they were able to oxidize the formate to its corresponding carbon dioxide and a proton due to fact that (DMC-nit)+ is a potential non-innocent ligand alluding to NO+ surrogates. It can reversibly undergo one electron reduction to form a neutral ligand π-radical species.3
2. Cationic phosphine
Unlike previously discussed example of nitrogen, phosphorus can coexist in different oxidation states, generally +3 and +5. Due to this reason, phosphorus found major role in catalysis. Phosphine, is by far one of the most well-established phosphorus ligands, used in metal-complex catalyst design.
The σ donor ability of the phosphines is generally modulated by replacing the substituent based on electronegativity. To increase the π-acceptance of the phosphine ligand, highly electron-withdrawing groups are attached. Prime examples being PF3, PCl3 and P(CF3)3. These phosphines are not only toxic but also P-X (X is the halogen) is quite labile making it not stable enough for industrial purposes. For the similar reasons, these compounds are not air-stable and thus pyrophoric.5
Key alternatives were to attach cationic organic groups as the substituents enabling to modulate the π-acceptor property along with forming stable P-C bond. The backdonation of phosphoryl lone pair onto cationic C make the P-C bond to have double bond characteristics making it further stabilized.[5-6]
These cationic charged phosphinic ligands are more soluble in polar and ionic liquids, which allows these species to conduct reaction even in aqueous conditions which was previously not possible with polyfluorinated phosphines.
Manuel Alcarazo and his group have ushered in the field of cationic phosphines and cationic phospholes, whose donor ability was even poorer than all the reported phosphites, only being comparable to that of toxic PF3 and P(CF3)3. They have managed to dicationic and tricationic phosphine ligand species which were able to coordinate with gold(I) and platinum(II) cation respectively, that efficiently catalyze the alkyne hydroarylation.5
Applying similar principles as that of α-cationic phosphine, Alcarazo explored α-cationic phospholes with different cationic substituents at phosphorus centre like cyclopropenium, imidazolium and iminium. These gold Au(I) complexes having cationic phospholes as ligands showed heightened electrophilicity which was able to catalyze otherwise difficult cyclomerization of tosylanylines.6
The catalytic activity of cationic phosphole depicts a similar trend for cationic phosphines characterized by similar σ-electron-releasing tendencies and low lying LUMO due to the fact that there is a good overlap of π-orbital and exocyclic σ(P-R). The π- acceptor characteristics get even more stronger with the incorporation of α-cationic substituents favouring further mixing of the σ*(P-R)-π*(ring) thus lowering the energy of the LUMO even more.
Apart from attaching cationic substituent, another strategy was to form Arduengo NHC analogues using phosphenium cations. The N-Heterocyclic phosphenium cations (NHPs) are isolobal an isostructural to that of NHCs. The key difference is NHPs are weak σ-donors and good π-acceptors due to the formal positive charge and isotropic s- (as opposed to directional sp2-) character of the “lone pair” orbital on phosphorus, whereas, NHCs are strong σ-donors and weak π-acceptors. This electronically inverse nature allows them to perform reciprocal reactions with the transition metals. This property was clearly demonstrated by the formation the NHC-NHP adduct. These NHP were able to form planar complex with iron and pyramidal complex with molybdenum. These NHPs also formed complexes with elements of group 9 and 10.7. Another report by the same group showcased the shortest ever Pd-P bond (2.1166(17)A°) through the complexation of palladium (0) and unsaturated NHP.8 Other complexes with first & second-row transition metals and coinage metals include of Cr, W, Pt, Pd and Ru.Alcarazo and his group demonstrated the huge potential of these cationic phosphines in catalysis of π-systems. They developed dicationic and tricationic phosphines with no spacer in between. These polycationic phosphines have cyclopropenium group as a substituent which can be thought as three cyclopropyl carbene stabilizing P(III) centre much alike to NHC stabilized P(III) mono- and dicationic ligands.9 These polycationic ligands were weakest lewis donor second to P(CF3)3. Using gold complex of dicationic phosphine, they were able to catalyze cycloisomerization of ortho-alkynated biaryls to its corresponding substituted phenanthrenes including some naturally occurring polyoxygenated like Bulbophyllantrin and Marylaurencinol A.10 Similarily reactivity was shown when Pt(II) complexes with tri-cationic phosphines. Not only, this π-acidic catalyst was able to synthesize phenanthrene but also able to form several homo- and heteroarenes.11
3. Cationic arsine
Figure :Different complexes of cationic arsine with transition-metal Rhodium
Cationic ligand using arsenic as the donor centre is significantly less reported than the phosphorus as cationic ligand though arsonium salt are well known.
One successful strategy was to add cationic organic substituents to the arsine. Low-valent arsenic cations were stabilized via different aromatic substituents like imidazolium, cyclopropenium, formamidinium, and pyridinium. These cationic arsines possess poorer electron-donating ability than any typical arsine or phosphine except for the fluorinated ones namely, PF3 and AsF3 as determined from Tolman electronic parameter13.
The only differences of this class of ligand from the previous cationic phosphine are only the longer As-C bond and increased s-character on lone pair of cationic Arsine, stemming from the inert pair effect which leads comparatively lower directionality and lower-energy HOMO of its lone pair accounting for poor σ donation.
Extending the reasoning of lower σ directionality of cationic arsine, the σ*(As−C) orbitals is more diffuse in nature as compared to σ*(P−C) ones thus the π-acceptor character in arsines is also diminished with respect to related phosphites
The following figure shows the list of cationic arsines. From the B3LYP-D3/def2-TZVP level computational study, it was found that all the cationic arsines (ref. figure 3, 6, 9, 10, and 13) have reduced σ-donation as compared to tri-phenyl arsine Ph3As which is evident from the lower energy of the HOMO of substituted cationic arsines. The frontier orbital analysis of HOMO shows that all of these arsines have substantial lone pair character at As.
Interestingly, cationic arsine 13 (having the pyridinium ring with very electron-withdrawing −CF3 group attached to) behaves as the weakest donor among the cationic arsines.
These cationic arsines formed complexes with Nickel, Rhodium, Platinum, Palladium and Gold. It is noteworthy that these cationic arsines when used as π-acid ligand for Pt(II) centre, it gave upto 75% yield in this Pt-catalyzed cycloisomerization, while the best ligand combination previously reported for this case - PtCl2 + GaCl3 gave a mere 12% yield of product.
Another scaffold for cationic arsine involves N-heterocyclic Arsenium ion. It was able to form a dative complex with cobalt cation.14
Authored by Dube from Prof. Alcarazo’s group, low-valent arsenic cations were stabilized via different aromatic substituents like imidazolium, cyclopropenium, formamidinium, and pyridinium. The only differences of this class of ligand from the previous one are only the longer As-C bond and comparatively lower directionality of its lone pair accounting for poor sigma-donation.
4. Cationic Stibine
Till date, there has been no report of cationic stibine though there has been computational as well synthesis reports of NHC stibinium cation extending from the spectroscopic and X-ray crystallographic results of metallo-arsines and metallo-phosphines where the metal used was Cobalt. 14
Interestingly, N-heterocyclic stibinium cation showed metal to ligand electron donation much reminiscent to Z-type ligand, one of the key features of cationic ligands.
In the respect of antimony being used as a Z-type ligand has been extensively explored by Prof. Gabbai. He has established the Antimony(V) for their Z-type bonding capabilities and thus their subsequent roles in anion sensing like fluorides. One of the prime examples is cationic complex of palladium with triarylstibine for halide sensing. This complex could be thought of coordination complex using Pd(0) and cationic stibine. 15
Group 14 cationic ligands
Cationic Carbene ligands
Another novel cationic ligand is (dppm)2CH which is reported by Langer et. al to coordinate with [ReBr(CO)5] where the former acting as a σ- and π-donating pincer-type ligand
- Reference: Facial vs. Meridional Coordination Modes in ReI Tricarbonyl Complexes with a Carbodiphosphorane-based Tridentate Ligand
Cationic silylene ligand
Cationic germylene ligand
Cationic stannylene ligand
Many examples in this long answer are of cationic ligands making a dative bond with transition metal cations which makes us overlook the overwhelmingly large coulombic repulsion at this very short distance between two cations. The coulombic force is inversely related with the square of the distance so one can only wonder how large the repulsive force might be if the distance is of the order of bond length i.e. Ao (~ 10-10 m). To put into the perspective: the Coulomb repulsion between two positive point charges at a separation of 3.1 Å is about 110 kcal/mol. If the charges are delocalized over medium-sized molecular fragments, the repulsion will be smaller but still on the order of about 40 kcal/mol. The attraction between two cations was tackled via London dispersion force along with strong dative bond. The inter cationic-ligand must also be considered to decipher the trans conformation in the case of of Ag(I) stabilized by two Ge(II) cationic ligand. The structure was mainly dominated by Coulombic repulsion between two germanium dication over steric clashes. Similar inter-ligand coulombic repulsion was found in the platinum complex by Gebbink’s group.
Cationic ligands allow a remarkable degree of control over the outcome of transition metal promoted transformations and catalysis. In cationic ligands, poor sigma donation and strong pi acceptance goes hand-in-hand that allows improvement of many transition metal catalyzed transformations. Sinhababu using germylene dication was able to perform hydroboration reaction of different aldehydes and ketones.
4 Y. Tulchinsky, M. A. Iron, M. Botoshansky, M. Gandelman, Nature Chemistry 2011, 3, 525.
12 Y. Tulchinsky, S. Kozuch, P. Saha, A. Mauda, G. Nisnevich, M. Botoshansky, L. J. W. Shimon, M. Gandelman, Chemistry - A European Journal 2015, 21, 7099.
3 F. Heims, F. F. Pfaff, S. L. Abram, E. R. Farquhar, M. Bruschi, C. Greco, K. Ray, J. Am. Chem. Soc. 2014, 136, 582.
5 M. Alcarazo, Chemistry - A European Journal 2014, 20, 7868.
12 M. Alcarazo, Accounts of Chemical Research 2016, 49, 1797.
12 E. Haldón, Á. Kozma, H. Tinnermann, L. Gu, R. Goddard, M. Alcarazo, Dalton Transactions 2016, 45, 1872.
6 T. Johannsen, C. Golz, M. Alcarazo, Angewandte Chemie - International Edition 2020.
7 C. A. Caputo, M. C. Jennings, H. M. Tuononen, N. D. Jones, 2009, 990.
8 C. A. Caputo, A. L. Brazeau, Z. Hynes, J. T. Price, H. M. Tuononen, N. D. Jones, Organometallics 2009, 28, 5261.
9 M. Patil, S. Holle, C. W. Lehmann, W. Thiel, M. Alcarazo, 2011, 12, 20758
10 J. Carreras, G. Gopakumar, L. Gu, A. Gimeno, P. Linowski, J. Petuškova, W. Thiel, M. Alcarazo, J. Am. Chem. Soc. 2013, 135, 18815.
11 J. Carreras, M. Patil, W. Thiel, M. Alcarazo, J. Am. Chem. Soc. 2012, 134, 16753.
13 J. Carreras, G. Gopakumar, L. Gu, A. Gimeno, P. Linowski, J. Petuškova, W. Thiel, M. Alcarazo, J. Am. Chem. Soc. 2013, 135, 18815.
14 S. Burck, J. Daniels, T. Gans-Eichler, D. Gudat, K. Nättinen, M. Nieger, Zeitschrift fur Anorg. und Allg. Chemie 2005, 631, 1403.
15 C. R. Wade, I.-S. Ke, F. P. Gabbaï, Angew. Chemie 2012, 124, 493.
 S. Grimme, J. P. Djukic, Inorganic Chemistry 2011, 50, 2619.
 D. J. M. Snelders, M. A. Siegler, L. S. von Chrzanowski, A. L. Spek, G. van Koten, R. J. M. K. Gebbink, Dalton Transactions 2011, 40, 2588.
 S. Sinhababu, D. Singh, M. K. Sharma, R. K. Siwatch, P. Mahawar, S. Nagendran, Dalton Transactions 2019, 48, 4094
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