NEW SYNTHETIC ROUTES TO FACE-TO-FACE AND OPEN-BOOK TRIAZENIDE-BRIDGEDDIRHODIUM BIPYRIDYL COMPLEXES WITH THE [RH2]4+ CORE

The iodide-abstraction reaction of [{Rh2(mu-I)(CO)(bipy)(mu-RNNNR)2}2] [PF6]2 1 (bipy = 2,2'-bipyridyl, R = p-tolyl) with AgPF6 in MeCN gave [Rh2(CO)(NCMe)2(bipy)(mu-RNNNR)2][PF6]2 2 which slowly decarbonylated at room temperature to [Rh2(NCMe)3(bipy)(mu-RNNNR)2][PF6]2 3. The crys tal structure to 3 shows as Rh-Rh single bond [2.534(2) angstrom] 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 [R h2-(NCMe)(S2CNMe2)(bipy)(mu-RNNNR)2][PF6] 4. In CH2Cl2, the iodide-abs traction reaction of 1 affords a green solution containing a carbonyl complex [Rh2(CO)(solv)2(bipy)(mu-RNNNR)2]2+ A (solv = CH2Cl2), an anal ogue of 2. Complex A (solv = CH2Cl2) reacted with neutral chelating li gands to give the carbonyl-bridged complexes [Rh2(mu-CO)(L-L)(bipy)(mu -RNNNR)2][PF6]2 [L-L = bipy 5, 4,4'-dimethyl-2,2'-bipyridyl (dmbipy) 6 , 1,10-phenanthroline (phen) 7, di-2-pyridylamine (dpa) 8, or Ph2PCH2C H2PPh2 (dppe) 9] the last of which undergoes reduction with NaBH4 to g ive paramagnetic [Rh2(CO)(dppe-P)(bipy)(mu-RNNNR)2][PF6] 11 having a f ace-to-face structure with a monodentate dppe ligand. With Ph2PCH2PPh2 (dppm), A (solv = CH2Cl2) afforded [Rh2(mu-CO)(dppm)(bipy)(mu-RNNNR)2 ][PF6]2 10a X-ray studies on which, as a CH2Cl2 solvate, reveal a carb onyl-bridged open-book structure with a Rh-Rh distance of 3.179(2) ang strom and a large Rh-C(O)-Rh angle of 108.3-degrees. Complex 10a equil ibrates with the face-to-face, terminal carbonyl isomer [Rh2(CO)(dppm) (bipy)(mu-RNNNR)2][PF6]2 10b in solution. The electronic structures of the two isomers have been probed by extended-Huckel molecular-orbital calculations on the model compound [Rh2H4(CO5]. These show (i) the ab sence of metal-metal bonding in 10a while 10b has a Rh-Rh sigma bond, and (ii) that the CO in 10a is best viewed as more ketonic than a typi cal bridging carbonyl. The reaction of complex A (solv = CH2Cl2) with neutral chelating ligands also gives low yields of [{Rh2(CO)(O2PF2)(mu -O2PF2)(bipy)(mu-RNNNR)2}2] the crystal structure of which shows two d irhodium fragments [Rh-Rh 2.505(4) angstrom] linked by two O2PF2 group s bridging across axial and equatorial sites in different [Rh2]4+ moet ies. The reaction of A (solv = CH2Cl2) with N-SH ligands yields [Rh2(C O)(N-S)(bipy)(mu-RNNNR)2][PF6] (N-S = 1-methyl-2-sulfanylimidazolate 1 3, 2-sulfanylpyrimidinate 14, 2-sulfanylthiazolinate 15, or 2-sulfanyl benzimidazolate 16), and NaX gives [Rh2(CO)X2(bipy)(mu-RNNNR)2] (X = C l or NO2).