Indian Journal of Chemistry Vol. 51A, Jan-Feb 2012, pp. 118-129

Cyclophosphazene- and cyclocarbophosphazene-based ligands

Vadapalli Chandrasekhar*, Atanu Dey & Subrata Kundu

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India

Email: vc@iitk.ac.in

Received 7 December 2011

In this review the utility of the inorganic ring systems, cyclophosphazenes and cyclocarbophosphazenes, to support multisite coordination platforms is described. Different types of ligands based on these ring systems and their metalation behavior is described.

Keywords: Coordination chemistry, Cyclophosphazenes, Cyclocarbophosphazenes, Multisite coordination ligands, Metalation

Cyclophosphazenes and cyclocarbophosphazenes are inorganic heterocyclic rings, the former containing alternate phosphorus and nitrogen atoms in their skeletal frame-work while the latter contain an additional heteroatom, viz. carbon (compounds 1-4).1-4 The cyclocarbophosphazenes (2) and (3) can in fact be considered as hybrids between the hexachlorocyclo-triphosphazene (1), and the cyanuric chloride (4).4

The reactions of chlorocyclophosphazenes, particularly with various types of nucleophiles, have been the subject of numerous investigations with a view to evaluate the reaction mechanisms involved as well as the product preference in terms of regio- and stereo specificity.5 This subject has been covered in the past by several critical reviews. In contrast, the reactivity of carbocyclophosphazenes has been much less investigated.6

The ring-opening polymerization (ROP) of N3P3Cl6 (1) and N3P2CCl5 (2) to afford linear polymers and the derivatization of these polymers subsequently by macromolecular substitution reactions has been a major research activity and has been covered in several recent monographs.1,7

Another facet of the chemistry of these compounds is the fact that the peripheral reactive chlorine atoms on N3P3Cl6 (1) and [{NPCl2}{NCCl}2] (3) can be utilized to construct multi-site coordination ligands which can be used in novel coordination chemistry. This aspect will be summarized in this review. There have been previously periodic critical reviews on this topic.2-3,8 Representative examples of cyclo- and carbophosphazene ligands developed in our laboratory and their metalation chemistry will be reviewed in the following account.

Cyclophosphazene-based Multi-site Coordination Ligands Pyrazolyl cyclophosphazenes

The nucleophilic substitution reactions of appropriate chlorocyclophosphazene precursors with pyrazole ligands affords a variety of pyrazolyl cyclo-phosphazenes where the number and orientation of the pyrazole substituents is different1b,2,8a. Compounds (5) and (6) are representative examples of pyrazolyl cyclophosphazenes.

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The coordination chemistry of (5) typifies the response of these ligands towards transition metal ions. Typically, 1:1 and 1:2 complexes have been obtained with transition metal salts such as CuCl2 (compounds 7 and 8).1,9 The coordination of the cyclophosphazene ligand towards Cu(II) is η3 involving two non-geminal pyrazolyl nitrogen atoms and one cyclophosphazene ring nitrogen atom present in between. The 1:2 compound (8) possesses a similar coordination environment; however, a P-N bond hydrolysis involving one of the pyrazolyl ligands is observed. Such a hydrolytic instability of phosphorus-pyrazolides is more pronounced with acyclic phosphorus systems (Scheme 1).10

The advantage of studying small molecule cyclophosphazenes is that their chemistry can be translated to the corresponding polymer analogues. Two types of polymers are possible. One is the ring-opening of N3P3Cl6 to the linear [NPCl2]n; since the

latter is hydrolytically unstable the chlorine atoms present in it have to be replaced by other substituents.1a,7 The second type of polymers are those where the cyclophosphazene unit is intact and is present as a pendant group on an organic polymer chain.1a Utilizing the latter strategy polymeric ligands containing pyrazolyl cyclophosphazenes (11) and (12) were prepared. First, the model compound (11) was prepared which was fully structurally characterized. This was followed by preparing a pyrazolyl cyclophosphazene (12) that contained a polymerizable group which could be converted to its homopolymer (13) or co-polymerized with divinylbenzene to afford a cross-linked polymeric ligand. The latter was metalated with Cu(II) to afford (14) which could be used as a recyclable catalyst for phosphate ester hydrolysis11 and plasmid DNA cleavage.12 This strategy could be readily extended to prepare other polymeric ligands containing phosphine-substituted

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cyclophosphazenes as pendant groups which could be used as catalysts in Heck-coupling reactions.13

Among other interesting derivatives of pyrazolyl cyclophosphazenes prepared by other research groups, are their Re(I) complexes. One of these (15) has a non-geminal N3 coordination mode involving two non-geminal pyrazoles and a cyclophosphazene ring nitrogen. In (16), the Re(I) is bound by two germinal pyrazolyl nitrogens and an adjacent nitrogen atom from the cyclophosphazene ring. Compound (17) which is neutral, contains a Re(I) that is bound only by pyrazole nitrogen atoms.14 Lanthanide ions also can be bound by certain types of pyrazolyl cyclophosphazene ligands.14b,15

Cyclophosphazene hydrazides

Chlorocyclophosphazenes react with N-methyl hydrazine in a regiospecific manner; a variety of cyclophosphazene hydrazides such as (18) and (19) can be prepared in this manner.16

The metalation chemistry of the cyclophosphazene hydrazides has been investigated. Compound (19) forms 2:1 complexes with a number of metal salts (Zn+2, Cd+2, Ni+2 and Co+3) as exemplified by compound (20). In this case, the two cyclophosphazene ligands come together in a η3-non-geminal N3 coordination mode affording an overall hexa-coordinate geometry at the metal center16a.

The terminal –NH2 groups on the cyclophosphazene hydrazides are amenable for further modification. A few examples are provided to illustrate this

principle. The condensation of cyclophosphazene hydrazides with ferrocenecarboxaldehyde affords cyclophosphazenes decorated with a ferrocene periphery. A representative example is the hexa-ferrocene derivative 21.16b

On the other hand the –NH2 groups can also be condensed with appropriate reagents to obtain multifunctional coordination ligands such as (22).17

Compound (22) which contains a multisite coordination arm on either end of the cyclophosphazene ring readily forms a 2:1 tetranuclear complex with Cu(II) salts.17 Cyclophosphazene hydrazides have also been used to prepare macrocycles18 as well as scaffolds to hold photoactive peripheries.19 Some of these compounds have been used effectively as efficient sensors19(a,b),20 for Cu2+ and Mg2+. Interestingly a number of acyclic phosphorus hydrazides such as (23), (24) and (25) have been utilized to prepare 1:1 and 2:3 (L:M) complexes.21

Among the various metal complexes prepared from such phosphorus-supported hydrazides, heterometallic 3d-4f complexes prepared from (24) show interesting magnetic properties with some compounds exhibiting single-molecule-magnetism behavior.21c-f The 2:3 complexes prepared from (25) exhibit 3rd order NLO behavior.21g In all of these complexes the coordination response from the ligands occurs from the imine nitrogen atoms and the phenolate oxygen atoms. In the case of (24) the –OMe group is also involved in coordination to the 4f metal ion. However, with these ligands, we could not find any situation where the

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uld be induced to bind to the metal ions.

z nes oups on cyclophosphazenes

(28) and trinuclear (29) complexes were isolated. The

P=S group coRecently, we have prepared a 2:2 complex involving (S)P[N(Me)N=CH-2C9H6N]3 and Ag(I) salts. An interesting aspect of this complex (26) is that the Ag(I) ions are not only bound by the imino and pyridine nitrogens of the ligand but also by the sulfur atom of the P=S unit.22

yridyloxy cyclophospha eP

Appending pyridyloxy grffords novel ligands wa hich allow interesting

coordination possibilities. We and others have pursued this idea with considerable effectiveness.23 Thus, utilizing N3P3(spiro-O2C12H8)(2-OC5H4N)4 we were able to isolate a rare hepta-coordinated Co(II)complex (27).23d

In other instances for example by utilizing hexakis-(2-pyridyloxy)cyclotriphosphazene, the dinuclear

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formation of the latter involves an unusual P-O bond hydrolysis.23a

Carbophosphazene-based Ligands

As mentioned above the chemistry of carbophosphazenes has not been explored very well. In particular, the ability of the carbophosphazene skeleton to serve as a scaffold to support coordination platforms has not received appropriate attention. Keeping this in view, we have prepared the ligand [{NP(3,5-Me2Pz)2}{NC(3,5-Me2Pz)}2] (30).6d, 24

Realizing that the P-N bonds are susceptible to hydrolysis upon metalation while the C-N bonds remain robust, we carried out metalation of (30) with Cu(II) salts. As anticipated, metalation leads to P-Pz hydrolysis and the in situ formed P(O)OH units condense to generate tetrameric compounds containing P-O-P and P-O-Cu linkages (Scheme 2).6d We anticipate more wide-spread utility of these types of strategies to build multi-metallic assemblies.

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More recently, we used the carbophosphazene platform to design ligands where the carbon atoms are blocked and the phosphorus centers contain the coordinating motifs.23a,d Accordingly, we assembled the pyridyloxycarbophosphazene, [{NC(NMe2)}2{NP(P-O-C5H4N)2] (33). Compound (33) reacts with Cd(II) salts affording one- and two-dimensional coordination polymers such as (34) and (35) (Figs 1 and 2). Compound (36), which is a Mn(II) complex, is a macrocycle-linked coordination polymer (Fig. 3).6e In another variation, the reaction of (33) with CdCl2 afforded 3D coordination polymers where two different one-dimensional CdCl2 layers are stabilized.25

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Fig. 1 – 1-D coordination polymer of (34). [Adapted from Ref. 6e with permission from American Chemical Society, Washington DC, USA].

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Fig. 2 – 2-D Coordination polymer of (35). [Adapted from Ref. 6e with permission from American Chemical Society, Washington DC, USA].

Fig. 3 – Macrocycle-linked coordination polymer of (36). [Adapted from Ref. 6e with permission from American Chemical Society, Washington DC, USA].

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Lately we have prepared carbophosphazenes containing guanidine substituents24 (37) and (38). Compound (38) reacts with PdCl2 to afford a hydrolytically stable metal complex47 (39). In contrast to the behavior of (38), the corresponding pyrazole derivatives of carbophosphazenes undergo P-N bond hydrolysis upon metalation. Representative examples (40) and (41) are illustrative of this behavior. Some Other Developments

In the preceding sections we have described the utility of cyclophosphazenes and cyclocarbo

and systems. Most of this account was based on the work carried out in our laboratory. There have been other groups also who have contributed to this area. Some of their work has been summarized earlier in Ref. 2.

As mentioned earlier, one of the important interests of the chemistry of cyclophosphazenes is the ring opening polymerization1 (ROP) of N3P3Cl6. There have been many suggestions that the initiation of the polymerization reaction occurs through a P-Cl bond cleavage affording a phosphazenium cation, [N3P3Cl5]+. Such a cation has not been isolated so far. However, recently a novel hexacation (42) stabilized by coordinating dimethylaminopyridine groups has been isolated.26

In recent times, group 13 metal halide adducts of AlCl3 have been isolated and structurally characterized. Thus, compound (43) represents a situation where the ring nitrogen atom of N3P3Cl6

AlCl3. Such compounds, however, do not seem to be involved in the ROP of N3P3Cl6. Conclusions

Multi-functional cyclophosphazenes and cyclo-carbophosphazenes are excellent precursors for the construction of a large number of multi-site coordination platforms. By a proper choice of nucleophile, and if necessary subsequent reactions, a library of multi-site coordination ligands is accessible by this approach. In addition, the susceptibility of the P-N bond to hydrolysis upon metalation offers an opportunity to utilize this feature deliberately to achieve the synthesis of novel transition metal ensembles. One of the challenges in this field is to design multi-site ligand systems that

-phosphazenes as supports for assembling multi-site coordination ligands. We also showed the metalation behavior of these lig

coordinatively interacts with the aluminum center of 27

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can lead to homo and heterometallic complexes having specific functions. In principle, utilizing the synthetic strategies outlined in this review exciting new functional materials can be assembled. Research in our lab is currently oriented towards the attainment of this goal.

Acknowledgement

We thank the Department of Science and Technology (DST), India, and, Council of Scientific and Industrial Research (CSIR), India, for financial support. VC is thankful to the DST for the JC Bose Fellowship. AD and SK thank CSIR, India, for fellowships.

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