New synthetic routes to face-to-face and open-book triazenide-bridged dirhodium bipyridyl complexes with the [Rh2]4+ core

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Abstract
The iodide-abstraction reaction of [{Rh2(µ-I)(CO)(bipy)(µ-RNNNR)2}2][PF6]21(bipy = 2,2′-bipyridyl, R =p-tolyl) with AgPF6 in MeCN gave [Rh2(CO)(NCMe)2(bipy)(µ-RNNNR)2][PF6]22 which slowly decarbonylated at room temperature to [Rh2(NCMe)3(bipy)(µ-RNNNR)2][PF6]23. The crystal structure of 3 shows a Rh–Rh single bond [2.534(2)Å] and one of the three terminal nitrile ligands axially attached to the bipy-bound rhodium atom. Complex 3 reacts with Na[S2CNMe2]·2H2O to give [Rh2(NCMe)(S2CNMe2)(bipy)(µ-RNNNR)2][PF6]4. In CH2Cl2, the iodide-abstraction reaction of 1 affords a green solution containing a carbonyl complex [Rh2(CO)(solv)2(bipy)(µ-RNNNR)2]2+A(solv = CH2Cl2), an analogue of 2. Complex A(solv = CH2Cl2) reacted with neutral chelating ligands to give the carbonyl-bridged complexes [Rh2(µ-CO)(L–L)(bipy)(µ-RNNNR)2][PF6]2[L–L = bipy 5, 4,4′-dimethyi-2,2′-bipyridyl (dmbipy)6, 1,10-phenanthroline (phen)7, di-2-pyridylamine (dpa)8, or Ph2PCH2CH2PPh2(dppe)9] the last of which undergoes reduction with NaBH4 to give paramagnetic [Rh2(CO)(dppe-P)(bipy)(µ-RNNNR)2][PF6]11 having a face-to-face structure with a monodentate dppe ligand. With Ph2PCH2PPh2(dppm), A(Solv = CH2Cl2) afforded [Rh2(µ-CO)(dppm)(bipy)(µ-RNNNR)2][PF6]210a X-ray studies on which, as a CH2Cl2 solvate, reveal a carbonyl-bridged open-book structure with a Rh–Rh distance of 3.179(2)Å and a large Rh–C(O)–Rh angle of 108.3°. Complex 10a equilibrates with the face-to-face, terminal carbonyl isomer [Rh2(CO)(dppm)(bipy)(µ-RNNNR)2][PF6]210b in solution. The electronic structures of the two isomers have been probed by extended-Hückel molecular-orbital calculations on the model compound [Rh2H4(CO)5]. These show (i) the absence of metal–metal bonding in 10a while 10b has a Rh–Rh σ bond, and (ii) that the CO in 10a is best viewed as more ketonic than a typical bridging carbonyl. The reaction of complex A(solv = CH2Cl2) with neutral chelating ligands also gives low yields of [{Rh2(CO)(O2PF2)(µ-O2PF2)(bipy)(µ-RNNNR)2}2]12 the crystal structure of which shows two dirhodium fragments [Rh–Rh 2.505(4)Å] linked by two O2PF2 groups bridging across axial and equatorial sites in different [Rh2]4+ moieties. The reaction of A(solv = CH2Cl2) with N–SH ligands yields [Rh2(CO)(N–S)(bipy)(µ-RNNNR)2][PF6](N–S = 1-methyl-2-sulfanylimidazolate 13, 2-sulfanylpyrimidinate 14, 2-sulfanylthiazolinate 15, or 2-sulfanylbenzimidazolate 16), and NaX gives [Rh2(CO)X2(bipy)(µ-RNNNR)2](X = Cl or NO2).

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