Strained Ruthenium Complexes Bearing Tridentate Guanidine‐Derived Ligands

Abstract

The dimer [{(η6‐p‐cymene)RuCl}2(μ‐Cl)2] (cymene=MeC6H4iPr) reacts with N,N′‐bis(p‐tolyl)‐N′′‐(2‐pyridinylmethyl)guanidine (H2L1) and N,N′‐bis(p‐tolyl)‐N′′‐(2‐diphenylphosphanoethyl)guanidine (H2L2), in the presence of NaSbF6, giving rise to chlorido compounds of formula [(η6‐p‐cymene)RuCl(H2L)][SbF6] (H2L=H2L1 (1), H2L2 (2)) in which the guanidine ligand adopts a κ2 chelate coordination mode. The related ligand (S)‐N,N′‐bis(p‐tolyl)‐N′′‐(1‐isopropyl, 2‐diphenylphosphano ethyl)guanidine (H2L3) affords mixtures of the corresponding chlorido compound [(η6‐p‐cymene)RuCl(H2L3)][SbF6] (3) together with the complexes [(η6‐p‐cymene)RuCl2(H3L3)][SbF6] (4) and [(η6‐p‐cymene)Ru(κ3N,N′,P‐HL3)][SbF6] (10) which contain phosphano‐guanidinium and phosphano‐guanidinate ions acting as monodentate and tridentate ligand, respectively. Compounds 1, 2 and mixture of 3/4/10 react with AgSbF6 rendering the cationic aqua‐complexes [(η6‐p‐cymene)Ru(H2L)(OH2)][SbF6]2 (H2L=H2L1 (5), H2L2 (6), H2L3 (7)). These aqua‐complexes exhibit a temperature‐dependent fluxional process in solution. Experimental NMR studies and DFT theoretical calculations on complex 6 suggest that the process involves the exchange between two rotamers around one of the C−N guanidine bonds. Treatment of 5–7 with NaHCO3 renders the complexes [(η6‐p‐cymene)Ru(κ3N,N′,N′′‐HL1)][SbF6] (8) and [(η6‐p‐cymene)Ru(κ3N,N′,P‐HL)][SbF6] (HL=HL2 (9), HL3 (10)), respectively, in which the HL ligand adopts a fac κ3 coordination mode. The new complexes have been characterized by analytical and spectroscopic means, including the determination of the crystal structures of the compounds 1, 2, 5, 9 and 10, by X‐ray diffractometric methods.

Guanidinates as Alternative Ligands for Organometallic Complexes

“…Interestingly, in the search for new coordination modes, guanidines have been modified with other donor groups, such as phosphanes. In this way, ruthenium (II) (Figure 40) and osmium (II) complexes were obtained [145,146]. This type of multidentate ligand allows the preparation of the iridium and rhodium guanidinato complexes as 68, which behave as a frustrated Lewis pair (FLP) capable of reversibly activating polar bonds, such as those of water, and non-polar bonds, such as those of dihydrogen (Figure 41) [147,148].…”

“…Interestingly, in the search for new coordination modes, guanidines have been modified with other donor groups, such as phosphanes. In this way, ruthenium (II) (Figure 40) and osmium (II) complexes were obtained [145,146]. In contrast, less attention has been paid to osmium guanidinato complexes [137,144], although some examples were able to transform a wide variety of aldoximes selectively and catalytically into the corresponding nitriles (Figure 39).…”

Well-Stabilized but Strained Frustrated Lewis Pairs Based on Rh/N and Ir/N Couples

“…Concomitantly, the complex features fused five- and four-membered metalacycles and the central nitrogen atom adopts a pyramidal sp 3 hybridization. A close inspection of the molecular structure of these complexes − revealed that the structural parameters within the four-membered M–N–C–N metalacycle deviate significantly from those expected for an ideal hybridization of its atoms, therefore suggesting that this ring undergoes a high strain. Consequently, the metalacycle is prone to relieve this strain by means of the cleavage of the metal-iminic nitrogen bond under mild conditions, providing a vacant site at the metal and a lone electron pair on the involved nitrogen, that is, these compounds are masked TMFLPs.…”

“…During the last years, our research group has focused on the preparation and study of the reactivity of TMFLPs based on half-sandwich compounds bearing (η 5 -C 5 Me 5 )­Rh­(III) or ( η 6 - p -cymene)­M­(II) (M = Ru, Os) fragments. , A common and pivotal feature of these TMFLPs is that the included metal centers complete their coordination sphere, thanks to a monoanionic phosphano- or pyridinyl-guanidine ligand exhibiting a fac κ 3 coordination (Scheme A), thus ensuring the stabilization of the resulting complex. Concomitantly, the complex features fused five- and four-membered metalacycles and the central nitrogen atom adopts a pyramidal sp 3 hybridization.…”