Synthesis of new cationic complexes of rhodium(I)

Rotaxane synthesis exploiting the M(i)/M(iii) redox couple

In the context of advancing the use of metal-based building blocks for the construction of mechanically interlocked molecules, we herein describe the preparation of late transition metal containing [2]rotaxanes (1). Capture and subsequent retention of the interlocked assemblies are achieved by the formation of robust and bulky complexes of rhodium(iii) and iridium(iii) through hydrogenation of readily accessible rhodium(i) and iridium(i) complexes [M(COD)(PPh₃)₂][BAr^F₄] (M = Rh, 2a; Ir, 2b) and reaction with a bipyridyl terminated [2]pseudorotaxane (3·db24c8). This work was underpinned by detailed mechanistic studies examining the hydrogenation of 1 : 1 mixtures of 2 and bipy in CH₂Cl₂, which proceeds with disparate rates to afford [M(bipy)H₂(PPh₃)₂][BAr^F₄] (M = Rh, 4a[BAr^F₄], t = 18 h @ 50 °C; Ir, 4b[BAr^F₄], t < 5 min @ RT) in CH₂Cl₂ (1 atm H₂). These rates are reconciled by (a) the inherently slower reaction of 2a with H₂ compared to that of the third row congener 2b, and (b) the competing and irreversible reaction of 2a with bipy, leading to a very slow hydrogenation pathway, involving rate-limiting substitution of COD by PPh₃. On the basis of this information, operationally convenient and mild conditions (CH₂Cl₂, RT, 1 atm H₂, t ≤ 2 h) were developed for the preparation of 1, involving in the case of rhodium-based 1a pre-hydrogenation of 2a to form [Rh(PPh₃)₂]₂[BAr^F₄]₂ (8) before reaction with 3·db24c8. In addition to comprehensive spectroscopic characterisation of 1, the structure of iridium-based 1b was elucidated in the solid-state using X-ray diffraction.

Square-Planar Rhodium(I) Complexes Partnered with [arachno-6-SB9H12]-: A Route toward the Synthesis of New Rhodathiaboranes and Organometallic/Thiaborane Salts

Treatment of [RhCl(eta4-diene)]2 (diene = nbd, cod) with the N-heterocyclic ligands 2,2'-bipyridine (bpy), 4,4'-dimethyl-2,2'-bipyridine (Me2bpy), 1,10-phenanthroline (phen), and pyridine (py) followed by addition of Cs[arachno-6-SB9H12] affords the corresponding salts, [Rh(eta4-diene)(L2)][SB9H12] [diene = cod, L2 = bpy (1), Me2bpy (3), phen (5), (py)2 (7); diene = nbd, L2 = bpy (2), Me2bpy (4), phen (6), (py)2 (8)]. These compounds are characterized by NMR spectroscopy and mass spectrometry, and in addition, the cod-Rh species 1 and 3 are studied by X-ray diffraction analysis. These saltlike reagents are stable in the solid state, but in solution the rhodium(I) cations, [Rh(eta4-diene)(L2)]+, react with the polyhedral anion [SB9H12]- leading to a chemistry that is controlled by the d8 transition element chelates. The nbd-Rh(I) complexes react faster than the cod-Rh(I) counterparts, leading, depending on the conditions, to the synthesis of new rhodathiaboranes of general formulas [8,8-(L2)-nido-8,7-RhSB9H10] [L2 = bpy (9), Me2bpy (10), phen (11), (py)2 (12)] and [8,8-(L2)-8-(L')-nido-8,7-RhSB9H10] [L' = PPh3, L2 = bpy (13), Me2bpy (14), phen (15); L' = NCCH3, L2 = bpy (16), Me2bpy (17), phen (18)]. Compound 13 is characterized by X-ray diffraction analysis confirming the 11-vertex nido-structure of the rhodathiaborane analogues 14-18. In dichloromethane, 1 and 3 yield mixtures that contain the 11-vertex rhodathiaboranes 9 and 10 together with new species. In contrast, the cod-Rh(I) reagent 5 affords a single compound, which is proposed to be an organometallic rhodium complex bound exo-polyhedrally to the thiaborane cage. In the presence of H2(g) and stoichiometric amounts of PPh3, the cod-Rh(I) reagents, 1, 3, and 5, afford the salts [Rh(H)2(L2)(PPh3)2][SB9H12] [L2 = bpy (19), Me2bpy (20), phen (21)]. Similarly, in an atmosphere of CO(g) and in the presence of PPh3, compounds 1-6 afford [Rh(L2)(PPh3)2(CO)][SB9H12] (L2 = bpy (22), Me2bpy (23), phen (24)]. The structures of 19 and 24 are studied by X-ray diffraction analysis. The five-coordinate complexes [Rh(L2)(PPh3)2(CO)]+ undergo PPh3 exchange in a process that is characterized as dissociative. The observed differences in the reactivity of the nbd-Rh(I) salts versus the cod-Rh(I) analogues are rationalized on the basis of the higher kinetic lability of the nbd ligand and its faster hydrogenation relative to the cod diene.

NiN2S2 complexes as metallodithiolate ligands to RhI, RhII and RhIII

Three molecular structures are reported which utilize the NiN(2)S(2) ligands -, (bis(mercaptoethyl)diazacyclooctane)nickel and -', bis(mercaptoethyl)diazacycloheptane)nickel, as metallodithiolate ligands to rhodium in oxidation states i, ii and iii. For the Rh(I) complex, the NiN(2)S(2) unit behaves as a bidentate ligand to a square planar Rh(I)(CO)(PPh(3))(+) moiety with a hinge or dihedral angle (defined as the intersection of NiN(2)S(2) and S(2)Rh(C)(P) planes) of 115 degrees . Supported by -' ligands, the Rh(II) oxidation state occurs in a dirhodium C(4) paddlewheel complex wherein four NiN(2)S(2) units serve as bidentate bridging ligands to two singly-bonded Rh(II) ions at 2.893(8) A apart. A compilation of the remarkable range of M-M distances in paddlewheel complexes which use NiN(2)S(2) complexes as paddles is presented. The Rh(III) state is found as a tetrametallic [Rh(-')(3)](3+) cluster, roughly shaped like a boat propeller and structurally similar to tris(bipyridine)metal complexes.

Synthesis, Spectroscopy and Spectroelectrochemistry of Chlorocarbonyl {1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphthene-κ2-N,N'}rhodium(I)

Reaction of the dinuclear complex [{Rh(CO)2}2(μ-Cl)2] with an α-diimine ligand, 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (iPr2Ph-bian), produces square-planar [RhCl(CO)(iPr2Ph-bian)]. For the first time, 2:1 and 1:1 α-diimine/dimer reactions yielded the same product. The rigidity of iPr2Ph-bian together with its flexible electronic properties and steric requirements of the 2,6-diisopropyl substituents on the benzene rings allow rapid closure of a chelate bond and replacement of a CO ligand instead of chloride. A resonance Raman study of [RhCl(CO)(iPr2Ph-bian)] has revealed a predominant Rh-to-bian charge transfer (MLCT) character of electronic transitions in the visible spectral region. The stabilisation of [RhCl(CO)(iPr2Ph-bian)] in lower oxidation states by the π-acceptor iPr2Ph-bian ligand was investigated in situ by UV-VIS, IR and EPR spectroelectrochemistry at variable temperatures. The construction of the novel UV-VIS-NIR-IR low-temperature OTTLE cell used in these studies is described in the last part of the paper.