Polyanionic Imido-P(V) Ligands: From Transition Metal Complexes to Coordination Driven Self-Assemblies

The chemistry of the imido-anions of the main group elements has been studied for more than three decades. The imido (NR)^- group is isoelectronic to the oxo (=O) group and can coordinate with metal ions through its lone pairs of electrons. The polyimido-P(V) anions are well explored as they resemble the phosphorus oxo moieties such as H₃ PO₄ , H₂ PO₄ ^- , HPO₄ ^2- and PO₄ ^3- species. These imido anions are typically generated using strong main group organometallic reagents such as ⁿ BuLi, Et₂ Zn, Me₃ Al and ⁿ Bu₂ Mg, etc. As a result, their coordination chemistry has been restricted to reactions in anhydrous aprotic solvents for a few main group metal ions. This account presents our findings on using certain soft transition metal such Ag(I) and Pd (II) for isolating these imido-P(V) anions as their corresponding self-assembled clusters and cages. Using the various salts of Ag(I) ions in reaction with 2-pyridyl (² Py) functionalized phosphonium salts and phosphoric triamides, we obtained the mono- and dianionic form of these imido ligands {[P(N² Py)₂ (NH² Py)₂ ]^- , [P(N² Py)₂ (NH² Py)]^- , [PO(N² Py)(NH² Py)₂ ]^2- } and derived interesting examples of tri, penta, hepta and octanuclear Ag(I) clusters. Interestingly, by using the salts of Pd (II) ions, the elusive imido-phosphate trianions of the type [(RN)₃ PO]^3- (R=^t Bu, ^c Hex, ⁱ Pr) were generated in a facile one pot reaction as their corresponding tri- and hexanuclear clusters of the type {Pd₃ [(NR)₃ PO](OAc)₃ }_n (n=1 or 2). These trianions acts as a cis-coordinated hexadentate ligand for a trinuclear Pd (II) cluster and serve as the polyhedral building units for constructing hitherto unknown family of neutral cages in tetrahedral {Pd₃ [(Nⁱ Pr)₃ PO]₄ (L)₆ } and cubic {Pd₃ [(Nⁱ Pr)₃ PO]₈ (L)₁₂ } structures in the presence of suitable linker ligands (L^2- ). These cages show interesting host-guest chemistry and post-assembly reactions. Remarkably, by employing chiral tris(imido)phosphate trianions, enantiopure chiral cages of the type [(Pd₃ X*)₄ (L)₆ ], ([X*]^3- =RRR- or SSS-[PO(N(*CH(CH₃ )Ph)₃ ]^3- ), were synthesized and used for the chiral-recognition and enantio-separation of small racemic guest molecules. Some of these chiral cages were also shown to exhibit polyradical framework structures. In future, these and other similar types of cages are envisioned as potential molecular vessels for performing the reactions in their confined environment. The enantiomeric cages can be probed for asymmetric catalysis and the separation of a range of small chiral molecules.

Coordination versus hydrogen bonds in the structures of different tris(pyridin-2-yl)phosphoric triamide derivatives

The crystalline forms of tris(pyridin-2-yl)phosphoric triamide, OP[NH-2Py]3 were investigated using single crystal X-ray diffraction, both as a metal complex [Co{(O)P[NH-2Py]2[NH-2PyH]}2]Cl3, compound 1, and as the purely organic pseudopolymorphs, the pure/anhydrate OP[NH-2Py]3, 2, monohydrate 2OP[NH-2Py]3·H2O, 3 and solvate 2OP[NH-2Py]3·2DMF·H2O, 4, respectively. An improved model of the metal organic framework (MOF) structure of Cu(ii)O6 {[Cu(OCHO)3][(CH3)2NH2]} n , 5 is also reported. Compound 1 is the first example of a discrete chelate phosphoric triamide (PT) complex with an [N]3P(O)-based backbone which in the PT compound acts as a flexible tridentate ligand. The structures 1, 3 and 4 crystallize in the monoclinic space group P21/n, while the anhydrous form 2 crystallizes in the R3̄ trigonal space group. 5 which was obtained as a byproduct of the synthesis process of coordination compounds of OP[NH-2Py]3, crystallizes in the monoclinic space group I2/c. Hydrogen bonding pattern analysis for the different solid state forms of OP[NH-2Py]3 (2-4) shows a 2D sheet of the N-H⋯N linked molecules for 2, and a 1D chain and a ten-membered ring motif, both formed via the hydrogen bonds N-H⋯O, N-H⋯N and O-H⋯O, for 3 and 4, respectively. Of note is the unusual absence of the expected hydrogen bond interaction N-H⋯O[double bond, length as m-dash]P of PT compounds and also a lack of any significant π interactions in structure 2. The role of the solvent/hydrate and substituent (aminopyridine) in the formation of the expected hydrogen bond interactions in the studied solid state forms is also investigated. For these structures, crystal packing analysis using 3D Hirshfeld surface (HS), 2D fingerprint plot (FP) and enrichment ratio (E) calculations confirm a competition between the pyridinyl nitrogen and phosphoryl oxygen atoms as H-bond acceptors to construct the N-H⋯N or N-H⋯O contacts. Moreover, the significant differences between the anhydrous form of OP[NH-2Py]3, 2, and the monohydrate and solvate forms, 3 and 4, can be correlated to the pseudo-infinite (in 2) versus finite values (in 3 and 4) for the upper values of d e and d i on the related full FPs.

A novel tubular hydrogen-bond pattern in a new diazaphosphole oxide: a combination of X-ray crystallography and theoretical study of hydrogen bonds

In the structure of 2-(4-chloroanilino)-1,3,2λ⁴-diazaphosphol-2-one, C₁₂H₁₁ClN₃OP, each molecule is connected with four neighbouring molecules through (N-H)₂...O hydrogen bonds. These hydrogen bonds form a tubular arrangement along the [001] direction built from R³₃(12) and R₄³(14) hydrogen-bond ring motifs, combined with a C(4) chain motif. The hole constructed in the tubular architecture includes a 12-atom arrangement (three P, three N, three O and three H atoms) belonging to three adjacent molecules hydrogen bonded to each other. One of the N-H groups of the diazaphosphole ring, not co-operating in classical hydrogen bonding, takes part in an N-H...π interaction. This interaction occurs within the tubular array and does not change the dimension of the hydrogen-bond pattern. The energies of the N-H...O and N-H...π hydrogen bonds were studied by NBO (natural bond orbital) analysis, using the experimental hydrogen-bonded cluster of molecules as the input file for the chemical calculations. In the ¹H NMR experiment, the nitrogen-bound proton of the diazaphosphole ring has a high value of 17.2 Hz for the ²J_H-P coupling constant.

Simple entry into N-tert-butyl-iminophosphonamide rare-earth metal alkyl and chlorido complexes

New series of rare-earth metal dialkyls [(NPN^tBu)Ln(CH₂SiMe₃)₂(THF)_n] (Ln = Sc,n= 0 (2), Ln = Y,n= 1 (3)) and monoalkyls [(NPN^tBu)₂Ln(CH₂SiMe₃)] (Ln = Y (4), Nd (6), Sm (7)), [(NPN^tBu)₂Sc(THF)CH₃] (5) are achieved by protolysis of highly basic ligand Ph₂P(N-tBu)(NH-tBu) = (NPN^tBu)H (1). Further alkyl abstraction, alkyl/Cl exchange reactions and X-ray structure analyses of thus obtained new complexes are presented.

Facile formation of stable tris(imido)phosphate trianions as their tri- and hexanuclear Pd(II) complexes in protic solvents

Employing Pd(OAc)2, a facile deprotonation route to access the highly basic tris(alkylimido)phosphate trianions, [(RN)3PO](3-) (R = (t)Bu, (c)Hex, or (i)Pr), analogous to the orthophosphate (PO4(3-)) ion in polar and in protic solvents has been achieved. Trinuclear and prismatic Pd(II) clusters of these imido trianions having the formula {Pd3[(NR)3PO](OAc)3}n (n = 1 or 2) or as mixed-bridged clusters of the type {Pd3[(N(i)Pr)3PO](OAc)2(OR')}2 (R' = Me or H) were isolated exclusively in all these reactions in which the trianionic species acts as a tripodal chelating ligand to the trinuclear Pd3 unit. Reactivity studies aiming at the Pd(II) atoms in these clusters with nucleophilic reagents, such as primary amines (R″NH2), have led to a new trimeric cluster with the formula {Pd3[(NR)3PO](OAc)3(R″NH2)3} in which the tripodal coordination of the Pd-Nimido moieties remained unaffected, exemplifying the robustness of the Pd3 unit in all these clusters. We have also shown the catalytic activity of these Pd(II) complexes in Mizoroki-Heck type coupling reactions in the presence of Cu(OAc)2.