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The ground-state electronic structure of the trinuclear complex Cu3(dpa)4Cl2 (1), where dpa is the anion of di(2-pyridyl)amine, has been investigated within the framework of density functional theory (DFT) and compared with that obtained for other known M3(dpa)4Cl2 complexes (M = Cr, Co, Ni) and for the still hypothetical Ag3(dpa)4Cl2 compound. Both coinage metal compounds display three singly occupied x2-y2-like (delta) orbitals oriented toward the nitrogen environment of each metal atom, generating antibonding M-(N4) interactions. All other metal orbital combinations are doubly occupied, resulting in no delocalized metal-metal bonding. This is at variance with the other known symmetric M3(dpa)4Cl2 complexes of the first transition series, which all display some delocalized bonding through the metal backbone, with formal bond multiplicity decreasing in the order Cr > Co > Ni. An antiferromagnetic coupling develops between the singly occupied MOs via a superexchange mechanism involving the bridging dpa ligands. This magnetic interaction can be considered as an extension to the three aligned Cu(II) atoms of the well-documented exchange coupling observed in carboxylato-bridged dinuclear copper compounds. Broken-symmetry calculations with approximate spin projection adequately reproduce the coupling constant observed for 1. Oxidation of 1 removes an electron from the magnetic orbital located on the central Cu atom and its ligand environment; 1+ displays a much weaker antiferromagnetic interaction coupling the terminal Cu-N4 moieties via four ligand pathways converging through the x2-y2 orbital of the central metal. The silver homologues of 1 and 1+ display similar electronic ground states, but the calculated magnetic couplings are stronger by factors of about 3 and 4, respectively, resulting from a better overlap between the metal centers and their equatorial ligand environment within the magnetic orbitals.

Two compounds having six metal-metal bonded Mo(2)(4+) and Rh(2)(4+) units, the midpoints of which occupy the vertexes of an octahedron, are described here. Each of the M(2)(4+) units is also bridged by two mutually cis formamidinate groups of the type DAniF = N,N'-di-p-anisylformamidinate. These units are linked to each other by four tricarboxylate anions from trimesic acid, the centers of which form a tetrahedron that is encapsulated by the octahedron of dimetal units. Each of the neutral cages has a clathrated solvent molecule, CH(2)Cl(2) when the metal M is Mo and CH(3)CN when it is Rh. For the latter, there are CH(3)CN molecules occupying (2)/(3) of the axial sites on the Rh(2) units in a way that reduces the symmetry. These symmetrical structures appear to persist in solution as shown by the (1)H NMR spectra. The compounds [M(2)(cis-DAniF)(2)](6)[1,3,5-C(6)H(3)(CO(2))(3)](4), 1 for M = Mo and 2 for M = Rh, were crystallized as 1.22.1CH(3)CN.2.3CH(2)Cl(2) and 2.13.9CH(3)CN, respectively. In 1 there are four crystallographically independent quadruply bonded Mo-Mo distances in the narrow range 2.098-2.104 A. For 2, the six independent singly bonded Rh-Rh distances are in the range 2.428-2.438 A.

Reaction of the molecular loop [Rh(2)(cis-DAniF)(2)(CH(3)CN)](2)(malonato)(2), DAniF = N,N'-di-p-anisylformamidinate, with 2 equiv of 2,2',3,3',5,5',6,6'-octafluoro-4,4'-biphenyldinitrile gives a linear tubular structure with Rh-Rh distances of 2.435(1) A. A comparison of the structures obtained with this molecular loop but employing other linkers support the idea that longer linkers favor one-dimensional structures while shorter linkers favor two-dimensional structures because of the increased clashing between the large p-anisyl groups. Crystallographic data at 213(2) K for [[Rh(2)(cis-DAniF)(2)](2)(O(2)CCH(2)CO(2))(2)(NCC(6)F(4)C(6)F(4)CN)(2)] x 6.8nCH(2)Cl(2) are: monoclinic, P2(1)/m, a = 11.802(1), b = 28.303(2), c = 18.820(2) A, beta = 95.673(2) degrees, and Z = 2.

Supramolecular chemistry is today a major thrust area, a significant part of which is based on the use of metal atoms or ions as key elements in promoting the assembly of and dictating the main structural features of the supramolecular products. Most of the work has been done with single metal atoms or ions in this role, but considerable success has already been achieved by employing M-M bonded dimetal entities instead. We review here the work done in our laboratory. Metal-metal bonded cationic complexes of the [M(2)(DAniF)(n)(MeCN)(8-2n)]((4-n)+) type, where M = Mo or Rh and DAniF is an N,N'-di-p-anisylformamidinate anion, have been used as subunit precursors and then linked by various equatorial and axial bridging groups such as polycarboxylate anions, polypyridyls, and polynitriles. Characterization of the products by single-crystal X-ray diffraction, CV, DPV, NMR, and other spectroscopic techniques has revealed the presence of discrete tetranuclear (pairs or loops), hexanuclear (triangles), octanuclear (squares), and dodecanuclear (cages) species and one-, two-, or three-dimensional molecular nanotubes. These compounds display a rich electrochemical behavior which is affected by the nature of the linkers.

The monoanion of 2,6-bis(phenylamino)pyridine (HBPAP(-)) has been found to support quadruply bonded Cr(2)(4+) and Mo(2)(4+) units in Cr(2)(HBPAP)(4) (1) and Mo(2)(HBPAP)(4) (2). The corresponding dianion BPAP(2)(-) was able to stabilize the trinuclear complexes, (TBA)(2)Cr(3)(BPAP)(4) (3) and (TBA)(2)Ni(3)(BPAP)(4) (4), where TBA is the tetrabutylammonium cation. The dinuclear complexes have the typical paddlewheel configuration with Cr-Cr distances of about 1.87 A and a Mo-Mo distance of 2.0813(5) A and exhibit a high-field displacement of the corresponding N-H signals caused by the magnetic anisotropy of the quadruple bonds. For the trinuclear complexes, 3 has a linear chain of three chromium atoms arranged in an unsymmetrical fashion with two chromium atoms paired to give a quadruply bonded unit (Cr-Cr distance: 1.904(3) A) and an isolated, square planar Cr(II) unit at 2.589(3) A from the dimetal unit. On the other hand, the three nickel atoms in 4 are evenly spaced, having Ni.Ni distances of 2.3682(8) A. The trinuclear compounds show a twisted conformation with an overall torsion angle of about 30 degrees.

Three compounds containing two quadruply bonded Mo(2)(DAniF)(3) (DAniF = N,N'-di-p-anisylformamidinate) units linked by tetrahedral EO(4)(2-) anions (E = S, Mo, W) have been prepared and characterized by crystallography and NMR. The linkers in these [Mo(2)(DAniF)(3)](2)(mu-EO(4)) compounds hold the Mo(2) units in an approximately perpendicular orientation and mediate strong electrochemical communication between them. Each of the three compounds shows two quasireversible (mu-SO(4)) or fully reversible (mu-MoO(4), mu-WO(4)) features in its cyclic voltammogram corresponding to successive oxidation of each of its Mo(2) units. The DeltaE(1/2) values are the largest thus far measured for Mo(2)-X-Mo(2) bridged complexes and may be sufficiently large to permit isolation of the singly oxidized species.

Three new linear compounds of the type Co(3)(dpa)(4)X(2), where dpa is the anion of di(2-pyridyl)amine and X is NCS(-) (5), CN(-) (6), and N(CN)(2)(-) (7), have been prepared, and their structures and magnetic behavior have been studied. In all of them, including three different solvates of 5, the Co(3) chains are symmetrical with Co-Co distances of ca. 2.31-2.32 A. The appearance of four lines in the (1)H NMR spectra of the three compounds is also consistent with a symmetrical structure in solution. For all compounds, the magnetic behavior is quite similar with mu(eff) of ca. 1.9-2.0 micro(B) at temperatures between 1.8 and 200 K. As the temperature increases, the effective moments increase gradually, but since saturation is not reached, even at 400 K, the high-spin state cannot be assigned.

Twelve compounds containing two quadruply bonded Mo(2)(DAniF)(3) (DAniF = N,N'-di-p-anisylformamidinate) units linked by dicarboxylate anions have been prepared in high purity and good yields. All of these compounds have been characterized by crystallography and NMR. The dinuclear pairs display electrochemical behavior which is controlled by the nature of the bridging dicarboxylate group. As described by the linkers, the compounds are oxalate, 1; acetylene dicarboxylate, 2; fumarate, 3; tetrafluorophthalate, 4; carborane dicarboxylate, 5; ferrocene dicarboxylate, 6; malonate, 7; succinate, 8; propane-1,3-dicarboxylate, 9; tetrafluorosuccinate, 10; bicyclo[1.1.1]pentane-1,3-dicarboxylate, 11; and trans-1,4-cyclohexanedicarboxylate, 12.

Sixteen crystal structures have been determined for the Co3(dpa)(4)Cl2 (1) molecule in the following five crystalline solvates: 1.0.85(C2H5)(2)O.0.15CH2Cl2 (at 120, 213, 296 K); 1.C(4)H(8)O (at 120, 295 K); 1.C(6)H(6) (at 170, 213, 260, 316 K); 1.C(6)H(12) (at 120, 213, 295 K); and 1.1.75C(7)H(8).0.5C(6)H(14) (at 90, 110, 170, 298 K). For 1.0.85(C(2)H(5))(2)O.0.15CH2Cl2 the molecule of 1 is almost symmetrical at 120 K (Co-Co distances of 2.3191(3) and 2.3304(3) A) and remains so at 296 K (2.2320(3) and 2.3667(4) A). For 1.C(4)H(8)O the Co(3) chain is precisely symmetric at both 120 and 295 K though the Co-Co distances increase from 2.3111(4) to 2.3484(4) A as the temperature rises. Compound 1.C(6)H(6) is isomorphous with 1.C(4)H(8)O at 213 and 295 K and has rigorously symmetrical molecules at these two temperatures. Between 213 and 120 K the space group changes from Pccn to P2(1)/c, so that a symmetrical arrangement is no longer required and the two Co-Co distances then differ slightly (by 0.013 A). For 1.C(6)H(6) there is a phase change between 316 K (Pca2(1)) and 260 K (Pna2(1)). At all four temperatures, however, the molecule is almost symmetrical, with the two independent Co-Co distances never differing by more than 0.026 A. 1.1.75C(7)H(8).0.5C(6)H(14) contains, at all temperatures between 90 and 298 K, two crystallographically independent molecules, each of which is distinctly unsymmetrical at 298 K (Co-Co distances of 2.312(2) and 2.442(2) A for one and 2.310(2) and 2.471(2) for the other). In the first of these the distances converge to a much smaller separation (0.056 A) at 90 K while in the second the difference decreases to only 0.006 A at 90 K. Magnetic susceptibility measurements from 1.8 to 350 K indicate in each case that a gradual spin crossover, from a doublet to a quartet state, occurs over this temperature range.

The quadruply bonded compound Mo2(DpyF)4 (1), where DpyF- is the anion of N,N'-di(2-pyridyl)formamidine, has been prepared by ligand substitution reactions of Mo2(OOCCF3)4 and either the neutral ligand, HDpyF, at ambient temperature or its lithium salt, LiDpyF, under refluxing conditions. An X-ray structural analysis shows that 1 has a paddlewheel structure with a [symbol: see text] distance of 2.1108(6) A. Reaction of 1 with CoCl2 in methanol produces the paramagnetic compound [Mo2Co(DpyF)4][CoCl4].2MeOH (2). The Co(II) atom in the cation [Mo2Co(DpyF)4]2+ resides on a low-spin hexacoordinate environment (S = 1/2) with a Co...Mo separation of 2.979(6) A, suggesting there is no direct bonding interaction between the Co and Mo atoms. The Mo-Mo distance of 2.1096(5) A is similar to that in 1. Reaction of 1 and CuCl in methanol yields [Mo2Cu4(DpyF)4Cl2][CuCl2]2.2MeOHxEt2O (3). In the cation there are two copper atoms on each side of the Mo2 core. Each is coordinated to two pyridyl nitrogen atoms of the cis DpyF- ligands and loosely bridged to the other by a chloride ion. As a result, the Cu(I) atoms are not aligned with the Mo2 unit. The Cu to Mo separations are in the range 3.003(1)-3.015(1) A, and the Mo-Mo distance of 2.127(1) A is comparable to those in 1 and 2.

Four compounds consisting of molecular loops formed from two quadruply bonded Mo2(DAniF)2 (DAniF = N,N'-di-p-anisylformamidinate) units linked by two dicarboxylate anions have been prepared in high purity and essentially quantitative yields. These compounds have been characterized by crystallography and NMR spectroscopy and display electrochemical behavior dependent on the nature of the dicarboxylate anion. However, the electronic communication between the two Mo2(4+) units is not strong. As denoted by the dicarboxylate linkers, the compounds are malonate, 1, 1,4-phenylendiacetate, 2, homophthalate, 3, and trans-cyclopentane-1,2-dicarboxylate, 4.

Seven complexes obtained by reacting the quadruply bonded complex [Mo2(cis-DAniF)2(CH3CN)4](BF4)2 (DAniF = N,N'-di-p-anisylformamidinate) and (Bun4N+)2(Carb2-), where Carb2- is a dicarboxylate anion, have been found to have a ratio of dimetal unit to dicarboxylate of 1:1. As noted by the carboxylate linker, the compounds are oxalate, 1, fumarate, 2, ferrocene dicarboxylate, 3, 4,4'-biphenyldicarboxylate, 4, acetylenedicarboxylate, 5, tetrafluorophthalate, 6, and carborane dicarboxylate, 7. Structural characterization of 1-4 revealed a square of dimolybdenum units linked by the dicarboxylate anions, each having an interstice capable of accommodating specific solvent molecules. Results of NMR studies of all seven compounds are consistent with the presence of a highly symmetrical structure. These compounds display a rich electrochemical behavior that is affected by the nature of the carboxylate group.

Two compounds containing the cations Mo2(DPhIP)4n+, n = 1 or 2 and DPhIP = the anion of 2,6-diphenyliminopiperidine, have been obtained by oxidation of the quadruply-bonded Mo2(DPhIP)4 species. The first oxidation process conserves the structure but results in a slight increase of the Mo-Mo distance from 2.114(1) to 2.136(2) A in [Mo2(DPhIP)4](PF6).2CH2Cl2 (1.2CH2Cl2). However, the second oxidation process breaks the dimolybdenum bond, giving a bioctahedral complex, [Mo2(DPhIP)4](BF4)2.5CH3CN.Et2O (2.5CH3CN.Et2O), with Mo...Mo separation of 2.9954(7) A. Crystallographic data for 1.2CH2Cl2 are space group C2/c, a = 17.1891(9) A, b = 17.807(1) A, c = 24.210(2) A, beta = 106.403(4) degrees, Z = 4; for 2.5CH3CN.Et2O, space group P2(1)/n, a = 16.523(5) A, b = 27.418(5) A, c = 18.163(3) A, beta = 93.48(2) degrees, Z = 4.

The anion of 2,6-di(phenylimino)piperidine (DPhIP) has been found to support linear chains of three metal atoms. Three new compounds, [Cr3(DPhIP)4Cl]Cl.(1).5CH2Cl2.0.5H2O (1.1.5CH2Cl2.0.5H2O), [Cr3(DPhIP)4(CH3CN)]- (PF6)2.H2O.4CH3CN (2.H2O.4CH3CN), and [Cr3(DPhIP)4(F)(CH3CN)](BF4)2.5CH3CN (3.5CH3CN), have been synthesized and characterized by X-ray crystallography. Compound 1 has a linear chain of three chromium atoms arranged in an unsymmetrical fashion, with two of them forming a quadruply bonded unit (Cr-Cr distance 1.932(2) A) and the third being a non-metal-metal-bound 5-coordinate unit (Cr...Cr distance 2.659(2) A). The fifth coordination site is occupied by a chloride ion, and another chloride ion is located in the interstices of the crystal. The trimetal unit in compound 2 is structurally similar to that in compound 1 except that the axial ligand in 2 is a CH3CN molecule. Compound 3 is an oxidation product prepared by reaction of 1 with AgBF4. Here, a square pyramidal CrIII unit, FCrN4, and a Cr-Cr quadruply bonded (Cr-Cr distance 1.968(2) A) unit, with an axially coordinated acetonitrile molecule, form the trichromium chain. The CrIII...CrII separation of 2.594(2) A in 3 is too long to be considered a bonding interaction.

Reaction of the linear tricobalt compound Co3(dpa)4Cl2 (1) (dpa = di(2-pyridyl)amide) with silver hexafluorophosphate in acetonitrile yields [Co3(dpa)4(CH3CN)2][PF6]2 (2). Two crystalline forms are obtained from the same solution, namely, a monoclinic (P2(1)) form 2xCH3CNx2Et2O and a triclinic (P1) form, 2x3CH3CN. The tricobalt units in both crystals are essentially symmetrical, though this is not required by crystal symmetry, with Co-Co distances in the range 2298-2304 A. Each of the two terminal Co atoms is coordinated to an acetonitrile molecule with Co-N distances in the range 2068-2111 A at 213 K. The spiral arrangement of ligands gives an overall idealized D4 point group symmetry for the cation [Co3(dpa)4(CH3CN)2]2+ . Chiral crystals of both delta and lambda configurations in the P2(1) form have been isolated. The absolute configurations were determined by X-ray crystallography and their mirror-image circular dichroism spectra measured. The D4 symmetry of the cation appears to be preserved in solution as judged by the presence of only five proton resonance signals in the 1H NMR spectrum. Magnetic susceptibility measurements in the solid state indicates that 2 has a doublet ground state and exhibits an increase of the effective moment at high temperature (approximately 160 K) due to a spin crossover process.

The series of M2(hpp)4Cl2 complexes (hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) from M = W to M = Pt has been completed by the preparation and characterization of those with M = W, Os, and Pt. W(hpp)4Cl2 (1) has a W-W distance of 2.250(2) A, is diamagnetic, and can be assigned a W-W triple bond based on a sigma 2 pi 4 electron configuration. Os2(hpp)4Cl2 (2) has an Os-Os distance of 2.379(2) A and displays a temperature-independent paramagnetism. It can be assigned a sigma 2 pi 4 delta 2 delta*2 configuration. Pt2(hpp)4Cl2 has a Pt-Pt distance of 2.440(1) A and is diamagnetic. A bond order of 1, based on a configuration in which only the sigma* orbital is empty, is consistent with these data.

It is shown that the M2(DPhIP)4 molecules (M = Cr, Mo; HDPhIP = 2,6-di(phenylimino)piperidine) can each capture two CuI atoms to form molecules with linear Cu...M[symbol: see text] M...Cu chains. In these chains the CuI atoms have only weak interactions with the M atoms (Cr...Cu = 2.628 A; Mo...Cu = 2.615 A) even though their introduction causes marked decreases in the M[symbol: see text]M distances (from 2.265 to 1.906 A for Cr and from 2.114 to 2.078 A for Mo). These seemingly contradictory facts are explained by noting that in the M2(DPhIP)4 molecules there are strong destabilizing (to the M[symbol: see text]M bonds) donor interactions of the dangling nitrogen atoms with the pi* orbitals. When these are eliminated by the introduction of the CuI atoms, the M[symbol: see text]M bonds become stronger and shorter.

The preparation and structures of seven compounds that contain the Cr3(dpa)4(2+) core (dpa = the anion di(2-pyridyl)amide) are reported. The magnetic properties of several have been measured. In each case there are anionic ligands at each end of the Cr3(6+) chain, sometimes identical (2Cl-, 2CCPh-), sometimes different (Cl-, BF4-; Cl-, PF6-). Several of these compounds have a symmetrical arrangement of the three Cr atoms, with the two Cr-Cr distances equal at ca. 2.36 A, while others have an unsymmetrical arrangement. In the most extreme case the two Cr-Cr distances are 2.00 and 2.64 A. The electronic structures and the remarkable flexibility of the Cr3 arrangement are discussed.

Seven compounds having in common a Cr3(dpa)4(3+) core (dpa = di(2-pyridyl)amide ion) have been prepared and all shown to have an unsymmetrical chain of three Cr atoms. This chain can be described as a pair of quadruply bonded Cr(II) atoms to which a Cr(III) atom is attached. No symmetrical chain has been found, contrary to a previous preliminary report. The seven compounds have been well characterized crystallographically, and their short and long Cr to Cr distances (A, in parentheses) are: 1 [Cr3(dpa)4Cl2]Cl.2CH2Cl2.THF (2.12, 2.47), 2 [Cr3(dpa)4Cl2]AlCl4.CH2Cl2 (2.011, 2.555), 3 [Cr3(dpa)4Cl2]FeCl4.CH2Cl2 (2.009, 2.562), 4 [Cr3(dpa)4Cl2]I3.THF.2H2O (2.08, 2.49), 5 [Cr3(dpa)4Cl2]PF6.2CH2Cl2 (2.08, 2.48), 6 [Cr3(dpa)4(BF4)F]BF4.2CH2Cl2 (1.900, 2.595), 7 [Cr3(dpa)4ClF]BF4.CH2Cl2.C6H14 (2.039, 2.507). Magnetic susceptibility measurements on 1 and 2 reveal mueff = 3.85 +/- 0.05 muB from 10 to 300 K.