Iconography of Icosahedra. Calculations of Metallic Energies and Relative Stabilities of Stereoisomers of Binary Icosahedral Clusters

Nanosized Pd37(CO)28{P(p-Tolyl)3}12 containing geometrically unprecedented central 23-atom interpenetrating tri-icosahedral palladium kernel of double icosahedral units: its postulated metal-core evolution and resulting stereochemical implications

Pd37(CO)28{P(p-Tolyl)3}12 (1) was obtained in approximately 50% yield by the short-time thermolysis of Pd10(CO)12{P(p-Tolyl)3}6 in THF solution followed by crystallization via layering with hexane under N2. The low-temperature (100 K) CCD X-ray diffraction study of 1 revealed an unusual non-spheroidal Pd37-atom polyhedron, which may be readily envisioned to originate via the initial formation of a heretofore non-isolated central Pd23 kernel composed of three interpenetrating trigonal-planar double icosahedra (DI) that are oriented along the three bonding edges of its interior Pd3 triangle. This central Pd23 kernel is augmented by face condensations with two additional phosphorus-free and 12 tri(p-C6H4Me)phosphine-ligated Pd atoms, which lower the pseudo-symmetry of the resulting 37-atom metal core from D(3h) to C2. The 12 P atoms and 28 bridging CO connectivities preserve the pseudo-C2 symmetry. The central Pd23 kernel in 1 provides the only crystallographic example of the 23-atom member of the double icosahedral family of "twinned" interpenetrating icosahedra (II), which includes the 19-atom two II (1 DI), the 23-atom three II (3 DI), the 26-atom four II (6 DI), and the 29-atom five II (9 DI). The n-atoms of these DI models coincide exactly with prominent atom-peak maxima of 19, 23, 26, and 29, respectively, in the mass spectrum of charged argon clusters formed in a low-temperature free-jet expansion. The only previous crystallographically proven 26- and 29-atom DI members are the central pseudo-T(d) tetrahedral Pd26 kernel (4 II, 6 DI) in the PMe3-ligated Pd29Ni3(CO)22(PMe3)13 (2) and the central pseudo-D(3h) trigonal-bipyramidal Pd29 kernel (5 II, 9 DI) in the PMe3-ligated Pd35(CO)23(PMe3)15 (3). Two highly important major stereochemical implications are noted: (1) The formation of geometrically identical idealized architectures for these three II palladium kernels with corresponding DI models constructed for the charged argon clusters provides compelling evidence that the nature of delocalized Pd-Pd bonding in these II (and presumably other nanosized) Pd clusters, in which each zerovalent Pd atom individually has a closed-subshell 4d (10) ground state, may likewise (as in argon clusters) be viewed primarily in terms of (considerably stronger) attractive dispersion interactions. (2) The existence of the 23-atom II Pd23 kernel in 1 provides an essential heretofore "missing" geometrical link as an intermediate in the same sequential growth pathway to give the 26- and 29-atom II Pd(n) kernels found in 2 and 3, respectively. Accommodation of the 12 bulky P(p-Tolyl)3 ligands around the entire 37-atom palladium core necessitates an extended metal surface that originates from the pseudo-2D trigonal-planar Pd23 kernel found in 1. The much smaller PMe3 ligands in 2 and 3 would sterically allow further sequential transformations of an initially formed 23-atom II intermediate palladium kernel into the 26-atom spheroidal II palladium kernel in 2 or further into the 29-atom semi-spheroidal II palladium kernel in 3, but with smaller total metal-atom nuclearities of 32 and 35, respectively.

Atom distributions in binary atom clusters: a perturbational approach and its validation in a case study

An approach to describe heteroatomic clusters A(n)B(N-n) as perturbed homoatomic ones is presented. By first treating the homoatomic systems A(N) or/and B(N) and subsequent application of first-order perturbation theory it is possible to estimate relative stabilities of the 2(N) possible distributions of the atom types A and B at the N atomic sites in a very efficient manner. The approach was tested considering Ir(n)Pt(13-n) as an example (treated with density functional methods). One observes good correlation between relative stabilities estimated from the homoatomic cases and those obtained from explicit treatments of the binary systems. Moreover we rationalize the observed correlation of atom type and atom position in Ir(n)Pt(13-n).

Syntheses and structural analyses of variable-stoichiometric Au-Pt-Ni carbonyl/phosphine clusters, Pt3(Pt(1-x)Ni(x))(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 and Pt2(Pt(2-y)Ni(y))(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2, with ligation-induced site-specific Pt/Ni substitutional disorder within butterfly-based Pt3(Pt(1-x)Ni(x))Au2 and Pt2(Pt(2-y)Ni(y))Au2 core-geometries

In ongoing attempts of directed synthesis of high-nuclearity Au-Pt carbonyl/phosphine clusters with [Ni6(CO)12]2- used as reducing agent and CO source, we have isolated and characterized two new closely related variable-stoichiometric trimetallic clusters, Pt3(Pt(1-x)Ni(x))(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 (1) and Pt2(Pt(2-y)Ni(y))(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2). Their M4Au2 cores may be envisioned as substitutional disordered butterfly-based M4Au2 frameworks (M = Pt/Ni) formed by connections of the two basal M(B) atoms with both (Au-Au)-linked Au(PPh3) moieties. Based upon low-temperature CCD X-ray diffraction studies of eight crystals obtained from different samples, ligation-induced site-specific Pt/Ni substitutional disorder (involving formal insertion of Ni in place of Pt) in a given crystal was found to occur only at the one OC-attached basal M(B) site in 1 or at both OC-attached basal M(B) sites in 2 corresponding to a crystal composite of the Pt3(Pt(1-x)Ni(x))Au2 core in 1 or of the Pt2(Pt(2-y)Ni(y))Au2 core in 2; the Ph3P-attached M(B) site (M(B) = Pt) in 1 and two wingtip M(w) sites (M(w) = Pt) in 1 and 2 were not substitutionally disordered. The resulting variable stoichiometry of the M4Au2 core in 1 may be viewed as a crystal composite of two superimposed individual stereoisomers, Pt4(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 (1a) and Pt3Ni(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 (1b), in the averaged unit cell of a given crystal. Likewise, 2 represents the crystal-averaged composite of three individual stereoisomers, Pt4(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2a), Pt3Ni(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2b), and Pt2Ni2(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2c). Formal Ni substitution for Pt at only the basal M(B) site(s) in the four crystal composites each of 1 and 2 was found to vary widely from 17% to 79% Ni in 1 and from 21% to 95% Ni in 2. Nevertheless, reasonably close Pt/Ni occupancy factors were found within each of the four pairs of composite crystals selected from samples obtained from duplicate syntheses. Both 1 and 2 may be formally derived from the electronically equivalent classic butterfly Pt4(mu2-CO)5(PPh3)4 cluster by replacement of its bridging mu2-CO ligand spanning the basal M(B)-M(B) edge with two one-electron donating (Au-Au)-linked AuPPh3 moieties along with the substitution of a terminal CO in place of one or both M(B)-attached PPh3 ligands in 1 and 2, respectively; site-specific Pt/Ni substitutional disorder occurs only at the CO-attached M(B) sites. The variable-stoichiometric 1 and 2 re also electronically equivalent and geometrically related to the crystal-ordered butterfly-based Pt4(mu2-CO)4(PR3)4(mu3-HgX)2 clusters (R3 = Ph3, MePh2; X = CF3, Br, I).

Nanosized Au2Pd41(CO)27(PEt3)15containing two geometrically unprecedented 13-coordinated Au-centered (μ13-Au)Pd13polyhedra connected by triangular face-sharing and three interpenetrating 12-coordinated Pd-centered (μ12-Pd)Au2Pd10icosahedra: geometrical change in centered polyhedra induced by Au/Pd electronegativity-mismatch

The synthesis, isolation, and stereochemical characterization of Au(2)Pd(41)(CO)(27)(PEt(3))(15)(1) are described. This nanosized Au(2)Pd(41) cluster (maximum metal-core diameter, 1.04 nm) was originally obtained with Au(2)Pd(21)(CO)(20)(PEt(3))(10) as low-yield by-products together with Pd(145)(CO)(x)(PEt(3))(30)(x approximately 60) from the reaction of Pd(PEt(3))(2)Cl(2) and Au(PPh(3))Cl in DMF with NaOH under CO atmosphere. The subsequent preparation of Au(2)Pd(21)(CO)(20)(PEt(3))(10) in greatly improved yields (preceding article) thereby provided the starting material that led to the isolation of 1 in reasonable yields (54%) from an overnight refluxing of the preformed Au(2)Pd(21) cluster in THF under N(2). Both the composition (subsequently ascertained from elemental analysis) and molecular geometry of 1 were unequivocally established from a low-temperature CCD X-ray diffraction study, which revealed a cubic unit cell of P2(1)3 symmetry with four molecules of 1 and four co-crystallized triphenylphosphine oxide molecules each lying on a crystallographic three-fold axis. The entire Au(2)Pd(41) core of pseudo-C(3h) symmetry may be viewed as a central Au(2)Pd(29) fragment of pseudo-D(3h) symmetry composed of two heretofore geometrically unknown 13-coordinated Au-centered (mu(13)-Au)Pd(13) polyhedra that share a common internal Pd(i)(3) triangular face perpendicular to the C(3) principal axis and of three three-fold-related interpenetrating 12-coordinated Pd-centered (mu(12)-Pd)Au(2)Pd(10) icosahedra. A comparative analysis of this central Au(2)Pd(29) fragment in with an internal Au(i)(2)Pd(i)(3) trigonal bipyramid vs. the corresponding central Pd(29) fragment in the known homopalladium Pd(35)(CO)(23)(PMe(3))(15) (2) with an internal Pd(i)(5) trigonal bipyramid resulting from five interpenetrating 12-coordinated Pd-centered [(mu(12)-Pd)Pd(12)] icosahedra is particularly illuminating; it provides a striking illustration of the remarkable observed difference between Pd- vs. Au-centered polyhedra which is attributed to a large electronegativity-mismatch in radial bonding interactions that occurs upon replacement of the Pd-centered atom with a highly electronegative Au-centered atom. The entire Au(2)Pd(41) core-geometry is obtained by additional face-condensations of 12 tetracapping Pd(cap) atoms. This cluster is stabilized by 15 PEt(3) ligands and 27 doubly- and triply-bridging CO ligands. A close geometrical resemblance between the three three-fold-related Au(2)Pd(14) moities within the Au(2)Pd(41) core in 1 and the entire Au(2)Pd(14) core in the known [Au(2)Pd(14)(CO)(9)(PMe(3))(11)](2+) dication (3) is observed; resulting stereochemical implications are given.

Generation of AuPd22/Au2Pd21analogues of the high-nuclearity Pd23(CO)20(PEt3)10cluster containing 19-atom centered hexacapped-cuboctahedral (ν2-octahedral) metal fragment: structural-to-synthesis approach concerning formation of Au2Pd21(CO)20(PEt3)10

Reactions of Pd(PEt(3))(2)Cl(2) and Au(PPh(3))Cl in DMF with NaOH under CO atmosphere gave rise to the unique capped three-shell homopalladium Pd(145)(CO)(x)(PEt(3))(30)(x approximately 60) and two neutral Au-Pd clusters: Au(2)Pd(21)(CO)(20)(PEt(3))(10) (1) and Au(2)Pd(41)(CO)(27)(PEt(3))(15)(following article). Similar reactions with Pd(PMe(3))(2)Cl(2) being used in place of Pd(PEt(3))(2)Cl(2) afforded Au(2)Pd(21)(CO)(20)(PMe(3))(10) (2), the trimethylphosphine analogue of, and the electronically equivalent [AuPd(22)(CO)(20)(PPh(3))(4)(PMe(3))(6)](-) monoanion (3) as the [PPh(4)](+) salt. Each of these three air-sensitive 23-atom heterometallic Au-Pd clusters was obtained in low yields (7-25%); however, their geometrical similarities with the known cuboctahedral-based homopalladium Pd(23)(CO)(20)(PEt(3))(10) (4), recently obtained in good yields from Pd(10)(CO)(12)(PEt(3))(6), suggested an alternative preparative route for obtaining. This "structure-to-synthesis" approach afforded 1 in 60-70% yields from reactions of Pd(10)(CO)(12)(PEt(3))(6) and Au(PPh(3))Cl in DMF with NaOH under N(2) atmosphere. Both the compositions and atomic arrangements for 1, 2 and 3 were unambiguously established from low-temperature single-crystal CCD X-ray crystallographic determinations in accordance with their nearly identical IR carbonyl frequencies. Cluster 1 was also characterized by (31)P[(1)H] NMR, cyclic voltammetry (CV) and elemental analysis. The virtually identical Au(2)Pd(21) core-architectures of 1 and 2 closely resemble that of 4, which consists of a centered hexa(square capped)-cuboctahedral Pd(19) fragment of pseudo-O(h) symmetry that alternatively may be viewed as a centered Pd(19)nu(2)-octahedron (where nu(n) designates (n + 1) equally spaced atoms along each edge). [AuPd(22)(CO)(20)(PPh(3))(4)(PMe(3))(6)](-) (3) in the crystalline state ([PPh(4)](+) salt) consists of two crystallographically independent monoanions 3A and 3B; a superposition analysis ascertained that their geometries are essentially equivalent. A CV indicates that reversibly undergoes two one-electron reductions and two one-electron oxidations; these reversible redox processes form the basis for an integrated structural/electronic picture that is compatible with the existence of the electronically-equivalent 1-3 along with the electronically-nonequivalent 4 (with two fewer CVEs) and other closely related species.

Assessment of relative stabilities of positional isomers of polyhedral heteronuclear clusters via a simplified method of bond energy calculations based on tight-binding approach and adjacent matrix method: applications to binary icosahedral clusters

A new and simple method for assessing the relative stabilities of various positional isomers of a given heteronuclear cluster is described. The method is based on a tight-binding approach in conjunction with an adjacent matrix methodology (TBAM). The usefulness of the method is illustrated by bond energy calculations of a number of binary icosahedral clusters, including noncentered icosahedral A(n)B(12)(-)n clusters comprising main-group elements B, C, N, and S as well as B- and A-centered icosahedral A(n)B(13)(-)n clusters that consist of transition metals, Au, Ag, Ni, and Pt atoms. The latter results are compared with the previously reported molecular mechanics calculations based on Lennard-Jones potential and with experimental results, whenever possible. The trends of the total bond energies obtained by the two methods are nearly parallel in all cases, indicating that the relative stabilities predicted by the two methods follow the same order. The TBAM approach provides a simple and efficient way of predicting the relative stabilities of various positional isomers of a given cluster, particularly for clusters where the number of positional isomers is so large that it cannot be handled manually. The total bond energies exhibit a stepwise progression. Each step is characterized by a set of A-A, B-B, and A-B bonds which uniquely determines the total bond energy and, hence, the stability. The step formation implies that positional isomers of a given cluster geometry can be categorized by sets of numbers of A-A, B-B, and A-B bonds, or simply the numbers of the minority (either A-A or B-B) bonds. Three site preference rules, the strong-bond rule, the heterobond rule, and the big-hole rule, were formulated based on these model calculations. These rules are useful in rationalizing and/or predicting the relative stabilities of various positional isomers of a given cluster geometry.