Ligand‐Controlled Formation of a Low‐Valent Pincer Rhodium(I)–Dioxygen Adduct Bearing a Very Short OO Bond

Abstract

Treatment of [{Me2C6H(CH2PtBu2)2}Rh(η1‐N2)] (1a) with molecular oxygen (O2) resulted in almost quantitative formation of the dioxygen adduct [{Me2C6H(CH2PtBu2)2}Rh(η2‐O2)] (2a). An X‐ray diffraction study of 2a revealed the shortest OO bond reported for RhO2 complexes, indicating the formation of a RhIO2 adduct, rather than a cyclic RhIII η2‐peroxo complex. The coordination of the O2 ligand in 2a was shown to be reversible. Treatment of 2a with CO gas yielded almost quantitatively the corresponding carbonyl complex [{Me2C6H(CH2PtBu2)2}Rh(CO)] (3a). Surprisingly, treatment of the structurally very similar pincer complex [{C6H3(CH2PiPr2)2)}Rh(η1‐N2)] (1b) with O2 led to partial decomposition, with no dioxygen adduct being observed.

Spin State, Structure, and Reactivity of Terminal Oxo and Dioxygen Complexes of the (PNP)Rh Moiety

“…In summary, the O 2 complex is reactive, and not only with oxidants, which might have been expected if it were [13] Rh I , but these reactivity results do not strongly point to any particular oxidation state of the Rh in [Rh(O 2 )A C H T U N G T R E N N U N G (PNP)]. Rather, they reflect reaction thermodynamics, which can be favorable not only with oxidants, but also simply with "good ligands."…”

Structure and Reactivity of Dinitrogen Rhodium Complexes Containing N‐Heterocyclic Carbene Ligands

“…[32][33][34] One other example of a low-valent rhodium complex of molecular nitrogen was reported by Caulton's group featuring a unique PNP ligand, [(tBu 2 PCH 2 SiMe 2 ) 2 N]Rh(N 2 ), which has a very low ν(N-N) of 2068 cm -1 . [35] Interestingly, the three-coordinate fragment of Caulton's complex [36] and the similar (PCP)-Rh complex, [Me 2 C 6 H(CH 2 PtBu 2 ) 2 ]Rh, reported by Milstein [37] coordinate oxygen to give Rh I -O 2 complexes similar to 1a and 1b with O-O bond lengths of 1.363 and 1.365 Å, respectively.…”

“…Remarkably, 4 has such high oxidative stability that even after refluxing a tolu-ene solution of 4 in air for 24 h, no formation of 1 was observed. Since Milstein has reported Rh-O 2 complexes in which N 2 can be used to displace oxygen, [37] we have attempted to prepare 2 from 1 by refluxing a toluene solution of the oxygen adduct under a nitrogen, but with no noticeable production of the nitrogen adduct (Scheme 2). It therefore appears, that as one might expect, the carbon monoxide adduct is the most stable, while nitrogen is the most reactive.…”

A Rhodium Peroxido Complex in Mono‐, Di‐, and Peroxygenation Reactions

“…[7, 9, 16a,b] Note that the (Nheterocyclic carbene)rhodium(I) species cis-[RhCl(IMes)-(PPh 3 ) 2 ] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and the phosphine pincer rhodium(I) compound [{Me 2 C 6 H(CH 2 PtBu 2 ) 2 }Rh(h 1 -N 2 )] both react with dioxygen to give dioxygenated rhodium derivatives, but with no concomitant change in the oxidation state of the transition metal. [17] The IR spectrum of 1 a shows an absorption band at 866 cm À1 (Figure 1). This band shifts to 820 cm À1 for the 18 Olabeled isotopologue 1 b and can therefore be assigned unambiguously to the OÀO stretching vibration of the h 2 -peroxido unit.…”