The preparations and properties are described of a series of [Rh(2)](4+) complexes possessing carboxylates and N-based chelates as ligands. Treatment of Rh(2)(O(2)CR)(4)(MeOH)(2) (R = Me (1), Et (2), Ph (3) or CF(3) (4)) with 2 equiv of 2,2'-bipyridine (bpy) in refluxing MeCN leads to the [Rh(2)(O(2)CR)(2)(bpy)(2)](2+) cation (in complexes 5-9). Reaction of 1 with 1,10-phenanthroline (phen), 4,4'-dimethyl-2,2'-bipyridine (Me(2)bpy), 4,4'-diphenyl-2,2'-bipyridine (Ph(2)bpy), and 4,7-diphenyl-1,10-phenanthroline (Ph(2)phen) leads to the analogous cations of complexes 10-13. Complex 6a, [Rh(2)(OAc)(2)(bpy)(2)(MeCN)(2)](PF(6))(2).2MeCN, crystallizes in monoclinic space group P2(1)/a with the following cell parameters at -172 degrees C: a = 14.433(2) Å, b = 12.810(1) Å, c = 22.78(3) Å, beta = 104.42(3) degrees, Z = 4, and V = 3971.2 Å(3). Complex 9, [Rh(2)(O(2)CCF(3))(4)(bpy)(2)].Me(2)CO, crystallizes in triclinic space group P&onemacr; with the following cell parameters at 20 degrees C: a = 14.260(4) Å, b = 15.375(4) Å, c = 9.709(2) Å, alpha = 105.98(2) degrees, beta = 97.49(2) degrees, gamma = 70.32(2) degrees, Z = 2, and V = 1925.2 Å(3). Both 6a and 9 contain a singly-bonded [Rh(2)](4+) unit with two cis bridging RCO(2)(-) groups and two syn-bpy chelate groups in a near-eclipsed conformation about the Rh-Rh vector. Reaction of [Rh(2)(tpy)(2)(MeCN)(4)](BF(4))(4) (15) (tpy = 2,2':6',2"-terpyridine) with NBu(n)(4)(O(2)CPh) gives [Rh(2)(O(2)CPh)(tpy)(2)(MeCN)(2)](BF(4))(3).MeCN (16). Complex 16 crystallizes in triclinic space group P&onemacr; with the following cell parameters at -168 degrees C: a = 11.684(4) Å, b = 20.373(8) Å, c = 10.451(3) Å, alpha = 93.47(2) degrees, beta = 110.53(1) degrees, gamma = 100.03(2) degrees, Z = 2, and V = 2274.1 Å(3). The cation of 16 consists of a [Rh(2)](4+) unit with a bridging PhCO(2)(-) group and two chelating tpy groups. The solution (1)H NMR properties of complexes 6-13 and 16 show the complexes to retain their solid-state structures on dissolution. The electrochemical properties of 6-13 and 16 were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in MeCN. The bis(bpy) and -(phen) complexes 6-8, 10, and 11 show a reversible, one-electron reduction in the range -0.83 to -0.91 V vs ferrocene and an additional, irreversible reduction and an irreversible oxidation; the Ph(2)bpy (12) and Ph(2)phen (13) complexes show a two-electron reversible reduction at -0.61 to -0.76 and a reversible one-electron oxidation at 0.68-0.89 V. Consideration of the potentials as a function of carboxylate and chelate identity leads to the conclusion that the reductions are ligand (chelate)-based. This is supported by an extended Hückel calculation on the model compound [Rh(2)(O(2)CH)(2)(bpy)(2)(HCN)(2)](2+), which shows the LUMO to be a bpy-based orbital comprising in-phase, sigma overlap of two bpy pi orbitals, one on each of the syn-bpy groups. In contrast, complex 16 shows no reversible reduction processes. Complex 6b, [Rh(2)(OAc)(2)(bpy)(2)(MeCN)(2)](BF(4))(2), in MeCN ...

Synthetic procedures are described for the preparation of mono-2,2'-bipyridine (bpy) complexes possessing the Rh24+ core. Reaction of Rhz(OAc),(MeOH)2 (1) with 1 equiv of bpy in Me2CO produces Rhz(OAc)4(bpy) (3) in excellent yield. The compound crystallizes in the triclinic space group Pi with the following cell parameters at -172 'C: u = 9.883(3), b = 13.078(5), c = 8.323(2) A; a = 97.02(2), j 3 = 106.90(1), y = 94.29(2)'; Z = 2; V = 1015 As. The structure was solved by direct methods (MULTAN) and refined to values of conventional indices R (R,) of 4.72% (4.50%) using 2636 unique reflections with F,, > 3u(F0). The two Rh2+ centers are bridged by three q1:q1:p2AcO-groups over a Rh-Rh bond distanceof 2.475(2) A. The coordination spheres of Rh( 1) and Rh (2) are completed by chelating bpy and O A c groups, respectively. The chelating OAc-is asymmetrically bound as a consequence of the axialfequatorial positioning of its two oxygen atoms O (27) and O(29). Compound 3 undergoes a reaction on dissolution in MeOH to give [Rh2(OAc)3(bpy)(MeOH)](OAc).2MeOH (4) as determined by 'H NMR (1 D-and 2D-homonuclear J-resolved spectroscopy), elemental analysis, conductivity measurements, and reactivity studies. Reaction of Rh2(02CCF3)4(Me&0)2 (2) with 1 equiv of bpy in CHzClz, followed by recrystallization of the product from THFfH20, produces crystals of Rh2(02CCF3)4(bpy)(THF)(HzO).THF (5). Compound 5 crystallizes in the monoclinic space group Pc with the folowing cell parameters at -105 OC: u = 10.683(8), b = 9.170(8), c = 17.783(1) A; a! = 90.00, j3= 103.52(7), y = 90.00'; 2 = 2, V = 1693 The structure was solved by direct methods (MITHRIL) and refined to values of R (R,) of 5.1% (5.8%) using 2493 unique reflections with F > 3u(F). The two Rh2+ centers are bridged by two q1:q1:p2 02CCF3-ligands across a Rh-Rh separation of 2.520(3) A. Coordination about Rh(1) is completed by two terminal q1-0~CCF3-groups in equatorial sites with the axial site being occupied by a water molecule. Rh(2) is ligated by a chelating bpy group in the equatorial plane, with axial ligation being provided by a THF molecule. Reaction mixtures comprising [Rh2(0Ac)2(MeCN),j] (BF4)2 (6) and 1 equiv of bpy in MeCN lead to isolation of [R~~(OAC)~(~~~)(M~CN)~](BF~)~.M~CN (7). Compound 7 crystallizes in the triclinic space group Pi with the following cell parameters at -170 OC: u = 12.381(2), b = 14.301(2), c = 12.246(2) A; a! = 94.60(1), j 3 = 105.84(1), y = 117.42(1)'; 2 = 2; V = 1797 AS. The structure was solved by direct methods (MULTAN) and refined to values of R (R,) of 4.92% (5.74%) using 4723 unique reflections with F > 3 4 9 . The two metals are bridged by two cis q':q*:pz-OAcgroups that span a Rh-Rh distance of 2.5395(8) A. Coordination at Rh( 1) is completed by a chelating bpy in the equatorial plane and an axial MeCN ligand, while the coordination sphere of Rh(2) is completed by two equatorial MeCN molecules and an axial MeCN molecule.

The frequent occurrence of the cuboidal cluster [Fe3S4(S·Cys)3] in a variety of proteins has prompted extensive investigation of its chemical and biological properties. The biological function remains in question, but the cluster is known to sustain two reactions:  (i) electron transfer, and (ii) heterometal ion incorporation. The recent preparation of [Fe3S4(LS3)]3- (3) [Zhou, J.; Hu, Z.; Münck, E.; Holm, R. H. J. Am. Chem. Soc. 1996, 118, 1966] has permitted detailed structural, electronic, and reactivity characterization of the cuboidal [Fe3S4]0 oxidation state (LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene(3−). Redox properties (i) have been reported previously:  here reaction type (ii), resulting in the formation of cubane-type MFe3S4 clusters, has been investigated. Reaction of 3 with [M(PPh3)4]1+ affords [(Ph3P)MFe3S4(LS3)]2- (M = Cu (6), Ag (8)) while [(NC)M(PPh3)3] leads to [(NC)MFe3S4(LS3)]3- (M = Cu (7), Ag (9)). Treatment of 3 with Tl(O3SCF3) yields [TlFe3S4(LS3)]2- (10). The fragment formalism {M1+ + [Fe3S4]0} applies to 6−10, which retain the S = 2 ground state of 3. Reaction of 3 with [M(PPh3)3Cl] yields [(Ph3P)MFe3S4(LS3)]2- (M = Co (12), Ni (14)) in inner-sphere redox reactions. Clusters 12 (S = 1) and 14 (S = 3/2) are formulated as {M2+ + [Fe3S4]1-}; antiferromagnetic coupling of fragment spins gives rise to the indicated spin ground states. The reactions (ii) are metal-ion incorporation processes, a new reaction type in Fe−S chemistry. Previously, all cubane-type MFe3S4 clusters had been synthesized by spontaneous self-assembly or reductive rearrangement reactions. Cluster 7 exhibits reversible oxidation and reduction reactions; it is the only cluster that forms a stable oxidized product containing the [Fe3S4]1+ fragment. All other clusters show a reversible reduction and an irreversible or quasireversible oxidation. Potentials of the synthetic clusters are considered intrinsic to the various core units, being less influenced by environmental factors than are those in proteins. At parity of cluster charge and terminal ligation, the potential order is M = Fe < Co < Ni and Co < Ni < Cu < Ag < Tl for the [MFe3S4]2+,1+ and [MFe3S4]1+,0 core redox reactions. These orders are compared with those determined in proteins.

The recognition by Rosenberg et al. in 1969 that cis-(NH3)2-PtC12 (cisplatin) is an antitumor agent1 and its eventual approval as a chemotherapeutic drug triggered an enormous response in the chemical and medical research communities.'" During the last two decades, inorganic chemists have sought to gain insight into the mechanism whereby Pt(I1) and Pt(1V) complexes inhibit DNA replication by designing and structurally characterizing key model compound^.^.^ In recent years, increasing emphasis has been placed on the screening and tailoring of non-platinum complexes in the hope of discovering drugs that are effective against cancers other than those treated by ~isplatin.~ Among the inorganic compounds that have been documented to exhibit substantial carcinostatic activity are dinuclear complexes of Re,lO Ru,11 and Rh;9J2-18 a common feature of these is the presence of at least two bridging carboxylate ligands as depicted in the molecular drawings in Scheme 1. A goal of our research in this area is to develop the substitution chemistry of these and other biologically active dinuclear compounds with purine bases and their corresponding nucleosides and nucleotides. At theoutset of our investigation, we were aware of reports that the antitumor agent dirhodium tetraacetate reacts with adenine but not guanine bases. The latter conclusion was based partly on the observation that, upon addition of guanine and guanosine, the blue-green color of the tetraacetate complex persists. This is in contrast to the corresponding reactions of Rh2(02CCH3)4 with adenine bases, which undergo a dramatic color change from blue-green to pink.lkl* We have recently discovered, however, that Rhz(02-CCH3)4(MeOH)2 does in fact react with guanine bases in H2O or MeOH to yield substitution products that contain two purine f Michigan State University. K.R.D. is a Camille and Henry Dreyfus Teacher-Scholar, 1991-1995, and Fellow of the Alfred P. Sloan Foundation,