Postsynthetic modifications of [2,2,2-(H)(PPh3)2-closo-2,1-RhSB8H8] with Lewis bases: cluster modular tuning.

It has been demonstrated that the reaction of [2,2,2-(H)(PPh3)2-closo-2,1-RhSB8H8] () with PPh3 affords the boron substituted rhodathiaborane-PPh3 adduct, [6,6-(PPh3)2-9-(PPh3)-arachno-6,5-RhSB8H9] (). Building upon this reaction, we report herein that the 10-vertex hydridorhodathiaborane reacts with the Lewis bases, PCy3, py, 2-Mepy, 2-Etpy, 3-Mepy and 4-Mepy to form the rhodathiaborane-ligand adducts, [6,6-(PPh3)2-9-(L)-arachno-6,5-RhSB8H9], where L = PCy3 (), 2-Mepy (), 2-Etpy (), py (), 3-Mepy () or 4-Mepy (), and [8,9-µ-(H)-9-(PPh3)2-8-(L)-arachno-9,6-RhSB8H8], where L = py (), 3-Mepy () or 4-Mepy (). The selectivity of the reactions depended on the nature of the entering Lewis bases, affording the 6,5-isomers, , , and as single products for PPh3, PCy3, 2-Mepy and 2-Etpy; and mixtures of the 6,5-/9,6-regioisomers, /, / and / for py, 3-Mepy and 4-Mepy, respectively. The molecular structures of both regioisomers were characterized by X-ray diffraction analysis for the 6,5-isomers, and , and for the 9,6-isomers, and . Variable temperature NMR studies of the reaction between and PPh3 or 2-Mepy demonstrated that at low temperatures there is formation of the 9,6-species that subsequently isomerizes to the 6,5-regioisomer, indicating that for the more sterically hindered Lewis bases, PPh3, 2-Mepy and PCy3, the latter isomer is more stable and accessible through an intramolecular {Rh(PPh3)2} vertex flip. The formation of both isomers with py, 3-Mepy and 4-Mepy indicates that the kinetic and thermodynamic energies of the 6,5 and 9,6 rhodathiaborane-ligand adducts are similar for these Lewis bases. Lewis base bonding to exo-polyhedral boron vertices results in a change of the metal coordination from pseudo-octahedral Rh(iii) in to pseudo-square planar Rh(i) in the adducts. The chemistry described here highlights the remarkable structural flexibility of these polyhedral boron-containing compounds, their modular architecture and their easy postsynthetic modification.

[1,1-(η(2)-dppe)-3-(NC5H5)-closo-1,2-RhSB9H8]: conformational lability and reactivity with H2 upon protonation.

Metallaheteroboranes are versatile compounds that can be conveniently modified and eventually tailored by ligand modification at either the metal centre or the boron vertices. Recently, we have discovered that protonation of some rhodathiaboranes affords cationic clusters with interesting reaction chemistry. In order to tune the reactivity of some of these polyhedral boron-based compounds, we have prepared air-stable orange [1,1-(η(2)-dppe)-3-(NC5H5)-closo-1,2-RhSB9H8] (2) by the treatment of the known hydridorhodathiaborane [8,8,8-(H)(PPh3)2-9-(NC5H5)-nido-8,7-RhSB9H9] (1) with dppe. The new 11-vertex rhodathiaborane, 2, reacts readily with triflic acid (TfOH) in CH2Cl2 to give orange cationic [8,8-(η(2)-dppe)-9-(NC5H5)-nido-8,7-RhSB9H9](+) (3). VT NMR experiments have allowed the characterization of a structural closo ↔ nido tautomerism, which involves hapticity changes in the ligation of the {SB9H9-(NC5H5)} moiety to the {Rh(dppe)} fragment, with the proton moving between the Rh(1)-B(3) and the B(9)-B(10) edges of the closo- and nido-isomers, respectively. The proton enhances the stereochemical non-rigidity and Lewis acidity of 3 versus the neutral 2. This modification of the chemical and structural basis permits the efficient heterolytic splitting of the H-H bond, leading to the formation of new hydridorhodathiaborane isomers [8,8,8-(H)(η(2)-dppe)-µ-8,9-(H)-9-(NC5H5)-nido-8,7-RhSB9H10](+) ()4 that are in equilibrium with the reactants, H2 and 3.

Rhodathiaborane reaction cycles driven by C2H4 and H2: synthesis and characterization of [(H)2(PPh3)RhSB8H7(PPh3)] and [(η(2)-C2H4)(PPh3)RhSB8H7(PPh3)].

New 10-vertex rhodathiaboranes are reported to exhibit reversible reaction chemistry leading to the formation of stoichiometric cycles driven by oxidation/reduction chemistry of the polyhedral boron-based clusters with ethelyne and dihydrogen.

NH3-promoted ligand lability in eleven-vertex rhodathiaboranes.

The reaction of the 11-vertex rhodathiaborane, [8,8-(PPh3)2-nido-8,7-RhSB9H10] (1), with NH3 affords inmediately the adduct, [8,8,8-(NH3)(PPh3)2-nido-8,7-RhSB9H10] (4). The NH3-Rh interaction induces the labilization of the PPh3 ligands leading to the dissociation product, [8,8-(NH3)(PPh3)-nido-8,7-RhSB9H10] (5), which can then react with another molecule of NH3 to give [8,8,8-(NH3)2(PPh3)-nido-8,7-RhSB9H10] (6). These clusters have been characterized in situ by multielement NMR spectroscopy at different temeperatures. The variable temperature behavior of the system demonstrates that the intermediates 4-6 are in equilibrium, involving ligand exchange processes. On the basis of low intensity signals present in the (1)H NMR spectra of the reaction mixture, some species are tentatively proposed to be the bis- and tris-NH3 ligated clusters, [8,8-(NH3)2-nido-8,7-RhSB9H10] (7) and [8,8,8-(NH3)3-nido-8,7-RhSB9H10] (8). After evaporation of the solvent and the excess of NH3, the system containing species 4-8 regenerates the starting reactant, 1, thus closing a stoichiometric cycle of ammonia addition and loss. After 40 h at room temperature, the reaction of 1 with NH3 gives the hydridorhodathiaborane, [8,8,8-(H)(PPh3)2-nido-8,7-RhSB9H9] (2), as a single product. The reported rhodathiaboranes show reversible H3N-promoted ligand lability, which implies weak Rh-N interactions, leading to a rare case of metal complexes that circumvent "classical" Werner chemistry.

Unusual cationic rhodathiaboranes: synthesis and characterization of [8,8,8-(H)(PR3)2-9-(Py)-nido-8,7-RhSB9H10]⁺ and [1,3-µ-(H)-1,1-(PR3)2-3-(Py)-isonido-1,2-RhSB9H8]⁺.

The treatment of the hydridorhodathiaboranes, [8,8,8-(H)(PR3)2-9-(Py)-nido-8,7-RhSB9H9], where PR3 = PPh3 (2), PMePh2 (3), PPh3 and PMe2Ph (4), or PMe3 and PPh3 (5), and [8,8,8-(H)(PMePh2)2-9-(PMePh2)-nido-8,7-RhSB9H9] (6), with TfOH affords [8,8,8-(H)(PR3)2-9-(Py)-nido-8,7-RhSB9H10](+) cations, where PR3 = PPh3 (12), PMePh2 (13), PPh3 and PMe2Ph (14), or PMe3 and PPh3 (15), and [8,8,8-(H)(PMePh2)2-9-(PMePh2)-nido-8,7-RhSB9H10](+) (16). Compounds 13 and 14 lose H2 to give [1,3-µ-(H)-1,1-(PR3)2-3-(Py)-isonido-1,2-RhSB9H8](+), where PR3 = PMe2Ph (18), PPh3 and PMe2Ph (21), or PMePh2 (22). Similarly, the 11-vertex rhodathiaboranes, [1,1-(PR3)2-3-(Py)-1,2-RhSB9H8], where PR3 = PPh3 (7), PMe2Ph (8), PMe3 (9), or PPh3 and PMe3 (10), react with TfOH to give the corresponding cations, [1,3-µ-(H)-1,1-(PR3)2-3-(Py)-isonido-1,2-RhSB9H8](+), where PR3 = PPh3 (17), PMe2Ph (18), PMe3 (19), or PPh3 and PMe3 (20). Four conformers of 20 are studied by X-ray diffraction methods and DFT-calculations, identifying packing motifs that stabilize different metal-thiaborane linkages, and energy variations that are involved in these conformational changes. It is demonstrated that the proton induces nonrigidity on these clusters as well as an enhancement of their Lewis acidity.

Heterolytic H2 activation on a carbene-ligated rhodathiaborane promoted by isonido-nido cage opening.

A new mechanism of H2 activation is reported to occur on a carbene-ligated rhodathiaborane that features metal-thiaborane bifunctional synergistic effects. The key is the creation of vacant coordination sites by an isonido-nido structural transformation leading to the heterolytic H-H bond splitting.

Proton-assisted hydrogen activation on polyhedral cations.

The treatment of [1,1-(PR3 )2 -3-(Py)-closo-1,2-RhSB9 H8 ] (PR3 =PMe3 (2) or PPh3 and PMe3 (3); Py=pyridine) with triflic acid (TfOH) affords [1,3-µ-(H)-1,1-(PR3 )2 -3-(Py)-1,2-RhSB9 H8 ](+) (PR3 =PMe3 (4) or PMe3 and PPh3 (5)). These products result from the protonation of the 11-vertex closo-cages along the Rh(1)B(3) edge. These unusual cationic rhodathiaboranes are stable in solution and in the solid state and they have been fully characterized by multinuclear NMR spectroscopy. In addition, compound 5 was characterized by single-crystal X-ray diffraction. One remarkable feature in these structures is the presence of three {Rh(PPh3 )(PMe3 )}-to-{η(n) -SB9 H8 (Py)} (n=4 or 5) conformers in the unit cell, thus giving an uncommon case of conformational isomerism. [1,1-(PPh3 )2 -3-(Py)-closo-1,2-RhSB9 H8 ] (1), that is, the bis-PPh3 -ligated analogue of compounds 2 and 3, is also protonated by TfOH, but, in marked contrast, the resulting cation, [1,3-µ-(H)-1,1-(PPh3 )2 -3-(Py)-1,2-RhSB9 H8 ](+) (6), is attacked by a triflate anion with the release of a PPh3 ligand and the formation of [8,8-(OTf)(PPh3 )-9-(Py)-nido-8,7-RhSB9 H9 ] (9). The result is an equilibrium that involves cationic species 6, neutral OTf-ligated compound 9, and [HPPh3 ](+) , which is formed upon protonation of the released PPh3 ligand. The resulting ionic system reacts readily with H2 to give cationic species [8,8,8-(H)(PPh3 )2 -9-(Py)-nido-8,7-RhSB9 H9 ](+) (7). This reactivity is markedly higher than that previously found for compound 1 and it introduces a new example of proton-assisted H2 activation that occurs on a polyhedral boron-containing compound.

Reactions of 11-vertex rhodathiaboranes with HCl: synthesis and reactivity of new Cl-ligated clusters.

Reactions of [8,8,8-(H)(PPh(3))(2)-9-(Py)-nido-8,7-RhSB(9)H(9)] (1), [1,1-(PPh(3))(2)-3-(Py)-closo-1,2-RhSB(9)H(8)] (2), and [1,1-(CO)(PPh(3))-3-(Py)-closo-1,2-RhSB(9)H(8)] (4), where Py = Pyridine, with HCl to give the Cl-ligated clusters, [8,8-(Cl)(PPh(3))-9-(Py)-nido-8,7-RhSB(9)H(9)] (3) and [8,8,8-(Cl)(CO)(PPh(3))-9-(Py)-nido-8,7-RhSB(9)H(8)] (5), have recently demonstrated the remarkable nido-to-closo redox flexibility and bifunctional character of this class of 11-vertex rhodathiaboranes. To get a sense of the scope of this chemistry, we report here the reactions of PR(3)-ligated analogues, [8,8,8-(H)(PR(3))(2)-9-(Py)-nido-8,7-RhSB(9)H(9)], where PR(3) = PMePh(2) (6), or PPh(3) and PMe(3) (7); and [1,1-(PR(3))(2)-3-(Py)-closo-1,2-RhSB(9)H(8)], where PR(3) = PPh(3) and PMe(3) (8), PMe(3) (9) or PMe(2)Ph (10), with HCl to give Cl-ligated clusters. The results demonstrate that in contrast to the PPh(3)-ligated compounds, 1, 2, and 3, the reactions with 6-10 are less selective, giving rise to the formation of mixtures that contain monophosphine species, [8,8-(Cl)(PR(3))-9-(Py)-nido-8,7-RhSB(9)H(9)], where PR(3) = PMe(3) (11), PMe(2)Ph (12), or PMePh(2) (15), and bis-ligated derivatives, [8,8,8-(Cl)(PR(3))(2)-9-(Py)-nido-8,7-RhSB(9)H(9)], where PR(3) = PMe(3) (13) or PMe(2)Ph (14). The {RhCl(PR(3))}-containing compounds, 3, 11, 12, and 15, are formally unsaturated 12 skeletal electron pair (sep) clusters with nido-structures. Density functional theory (DFT) calculations demonstrate that the nido-structure is more stable than the predicted closo-isomers. In addition, studies have been carried out that involve the reactivity of 3 with Lewis bases. Thus, it is reported that 3 interacts with MeCN in solution, and it reacts with CO and pyridine to give the corresponding Rh-L adducts, [8,8,8-(Cl)(L)(PPh(3))-9-(Py)-nido-8,7-RhSB(9)H(9)], where L = CO (5) or Py (20). On the other hand, the treatment of 3 and 5 with Proton Sponge (PS) promotes the abstraction of HCl, as [PSH]Cl, from the nido-clusters, and the regeneration of the parent closo-species, completing two new stoichiometric cycles that are driven by Brønsted acid/base chemistry.