Transition metal clusters, which contain multinuclear metal sites connected by metal-metal bonds in a variety of geometrical structures, have attracted increasing attention due to their versatile chemical and physical properties as well as their potential to integrate multiple functions in a single molecule.[1] In particular, clusters that show linear metalmetal bonding have been regarded as promising candidates in developing nanostructured materials including molecular electronic, optical, and chemical devices. However, synthetic methods using self-assembly of metal atoms often lead to polyhedral cluster cores, and thus routes to molecules with linear metal aggregations are limited. [2][3][4][5][6][7] We have studied homo-and heterometallic dinuclear and trinuclear complexes supported by the tridentate phosphane ligand bis(diphenylphosphanylmethyl)phenylphosphane (dpmp).[8] The linearlywere prepared by site-selective incorporation of a zero-valent Pt or Pd atom into the diplatinum complex [Pt 2 (m-dpmp) 2 (XylNC) 2 ](PF 6 ) 2 .[8c] In the present study, we have examined a cluster core expansion of 1, and have successfully synthesized linear hexametallic clusters containing a redox-active Pt 2 M 2 Pt 2 metal string (M = Pt, Pd).When the linear triplatinum complex 1 a was treated with excess NaBH 4 in ethanol, and the resultant brown precipitate was extracted and stirred in CH 2 Cl 2 , the dark blue, diamagnetic hexaplatinum cluster [Pt 6 (m-H)(m-dpmp) 4 -(XylNC) 2 ](PF 6 ) 3 (2) was isolated in good yield (Scheme 1).Compound 2 was also obtained in low yield from the reaction of 1 with NaOMe. Although the Pt 2 Pd trinuclear complex 1 b failed to be expanded with NaBH 4 , it readily reacted with NaOMe in CH 2 Cl 2 /MeOH to afford dark green crystals of [Pt 4 Pd 2 (m-H)(m-dpmp) 4 (XylNC) 2 ](PF 6 ) 3 (3, Scheme 1). The changes in the ESI mass spectrum and the electronic absorption spectrum during the reaction to form 2 indicated that the initial brown compound, assigned as [Pt 3 (H) 2 (mdpmp) 2 (XylNC)] 2+ (A), was rapidly converted into [Pt 3 (H)(mdpmp) 2 (XylNC)] + (B) in CH 2 Cl 2 . It is assumed that the monohydride intermediate B undergoes coupling and concomitant partial oxidation to generate complex 2. However, the intervening species were not identified. [9] The crystal structure of 2 was determined by X-ray analysis.[10] The cluster cation of 2 has a charge of + 3 with a cluster valence electron count (CVE) of 86. It consists of six linearly ordered platinum atoms (Pt-Pt-Pt 174.87(2)-179.67(2)8) bridged by four dpmp ligands and terminated by two isocyanide molecules (Figure 1). The Pt 6 cluster core has a pseudo C 2 symmetry, and the average PtÀPt distances are 2.7041 for the outer Pt1ÀPt2 and Pt5ÀPt6 bonds (d out ), 2.7329 for the inner Pt2ÀPt3 and Pt4ÀPt5 bonds (d inn ), and 3.3092(5) for the central Pt3ÀPt4 bond (d cen ). The values for d out and d inn are comparable to those of the triplatinum complex 1 [8] and indicate the presence of PtÀPt s bonds. Although the two central platinum atoms are not supp...

Linear Pt 2 M 2 Pt 2 hexanuclear clusters [Pt 4 M 2 (μ-H)(μdpmp) 4 (XylNC) 2 ](PF 6 ) 3 (M = Pt (2a), Pd (3a); dpmp = bis-(diphenylphosphinomethyl)phenylphosphine) were synthesized by site-selective reductive coupling of trinuclear building blocks, [Pt 2 M(μ-dpmp) 2 (XylNC) 2 ](PF 6 ) 2 (M = Pt (1a), Pd (1b)), and were revealed as the first example of low-oxidationstate metal strings bridged by a hydride with M−H−M linear structure. The characteristic intense absorption bands around 583 nm (2a) and 674 nm (3a) were assigned to the HOMO−LUMO transition on the basis of a net three-center/twoelectron (3c/2e) bonding interaction within the central M 2 (μ-H) part. The terminal ligands of 2a were replaced by H − , I − , and CO to afford [Pt 6 (μ-H)(H) 2 (μ-dpmp) 4 ] + (4), [Pt 6 (μ-H)I 2 (μ-dpmp) 4 ](PF 6 ) (5), and [Pt 6 (μ-H)(μdpmp) 4 (CO) 2 ](PF 6 ) 3 ( 6). The electronic structures of these hexaplatinum cores, {Pt 6 (μ-H)(μ-dpmp) 4 } 3+ , are varied depending on the σ-donating ability of axial ligands; the characteristic HOMO−LUMO transition bands interestingly red-shifted in the order of CO < XylNC < I − < H − , which was in agreement with calculated HOMO−LUMO gaps derived from DFT optimizations of 2a, 4, 5, and 6. The nature of the axial ligands influences the redox activities of the hexanuclear complexes; 2a, 3a, and 5 were proven to be redox-active by the cyclic voltammograms and underwent two-electron oxidation by potentiostatic electrolysis to afford [Pt 4 M 2 (μdpmp) 4 (XylNC) 2 ](PF 6 ) 4 (M = Pt (7a), Pd (8a)). The present results are important in developing bottom-up synthetic methodology to create nanostructured metal strings by utilizing fine-tunable metallic building blocks.

Electron-deficient Pt2M2Pt2 hexanuclear clusters, [Pt4M2(μ-dpmp)4(XylNC)2](PF6)4 (M = Pt (7), Pd (8); dpmp = bis((diphenylphosphino)methyl)phenylphosphine), were synthesized by oxidation of hydride-bridged hexanuclear clusters [Pt4M2(μ-H)(μ-dpmp)4(XylNC)2](PF6)3 (M = Pt (2), Pd (3)) and were revealed to involve a linearly ordered Pt2M2Pt2 array joined by delocalized bonding interactions with 84 cluster valence electrons, which are discussed on the basis of DFT calculations. The central M–M distances of 7 and 8 are significantly reduced upon the apparent loss of a hydride unit from the M–H–M central part of 2 and 3, indicating that the bonding electrons in the adjacent M–Pt bonds migrate into the central M–M bond to result in a dynamic structural change during two-electron oxidation of the hexanuclear metal strings. A similar Pt6 complex terminated by two iodide anions, [Pt6I2(μ-dpmp)4](PF6)2 (9), was synthesized from [Pt6(μ-H)I2(μ-dpmp)4](PF6) (5) by treatment with [Cp2Fe][PF6]. Complexes 7 and 8 were readily reacted with the neutral two-electron donors XylNC, CO, and phosphines to afford the trinuclear complexes [Pt2M(μ-dpmp)2(XylNC)L](PF6)2 (M = Pt, L = XylNC (1a), CO (10), PPh3 (11); M = Pd, L = XylNC (1b)) through cleavage of the electron-deficient central M–M bond. When complex 7 was reacted with the diphosphines (PP) trans-Ph2PCHCHPPh2 (dppen) and Ph2P(CH2)2PPh2 (dppe), the diphosphine was inserted into the central M–M bond to afford [(XylNC)Pt3(μ-dpmp)2(PP)Pt3(μ-dpmp)2(XylNC)](PF6)4 (12), which was transformed by treatment with another 1 equiv of diphosphine into the asymmetric trinuclear complexes [Pt3(μ-dpmp)2(XylNC)(PP)](PF6)2 (13). A further ligand exchange reaction of 13a (PP = trans-dppen) provided the diphosphine-terminated symmetrical Pt3 complex [Pt3(μ-dpmp)2(L)2](PF6)2 (L = trans-dppen (14a)). Complexes 7 and 8 were also reacted with [AuCl(PPh3)] to yield the Pt2MAu heterotetranuclear complexes [Pt2MAuCl(μ-dpmp)2(PPh3)(XylNC)](PF6)2 (M = Pt (15), Pd (16)), in which the Pt2M trinuclear fragment is inserted into the Au–Cl bond in a 1,1-fashion on the central M atoms of the Pt2M2Pt2 string.

Elastic pentacopper molecular chains, [Cu5(panapy)4X2] (X=Cl (), Br ()) and [Cu5(panapy)4]X'2 (X'=BF4 (), PF6 ()), were prepared using a naphthyridine-modulated N6-donor ligand, panapy2-, and showed magnetically coupled, dynamic rearrangement of five Cu(II) ions switched by the presence/absence of halide termination.

Reduction of a mixture containing [PtCl 2 (cod)], aromatic isocyanide (RNC; R ) 2,6-xylyl (Xyl), 2,4,6-mesityl (Mes)), diphosphine (Ph 2 P(CH 2 ) n PPh 2 ; n ) 5 (dpppn), n ) 6 (dpphx)), and NH 4 PF 6 by sodium amalgam (<1%) afforded trigonal-antiprismatic hexanuclear platinum(0) clusters encapsulating two mercury(0) atoms, [Hg 2 Pt 6 (µ-RNC) 6 (µ-diphos) 3 ] (diphos ) dpphx, R ) Xyl (6a); diphos ) dpppn, R ) Xyl (8a), Mes (8b)). Complexes 6a and 8a were characterized by X-ray crystallographic analyses to demonstrate that two triangular platinum(0) units, {Pt 3 (µ-RNC) 3 }, are connected by three diphosphine ligands with long methylene chains to form a closed cage of the cluster-based metallocryptand, in which two mercury(0) atoms are incarcerated with a notably short Hg-Hg distance. In complex 8a (n ) 5), the centroids of the two Pt(0) triangles are linearly arranged with respect to the Hg-Hg axis to form a pseudo-D 3 -symmetrical Hg 2 Pt 6 antiprismatic cluster core (average Pt-Pt ) 2.6719 Å, average Pt-Hg ) 2.9417 Å, and Hg-Hg ) 2.8424(2) Å). In contrast, in complex 6a (n ) 6) the two triplatinum planes glide away from the dimercury axis to lose the D 3 symmetry (average Pt-Pt ) 2.639 Å, average Pt-Hg ) 2.923 Å, and Hg-Hg ) 2.826(2) Å). The electronic structures of the Pt 3 Hg 2 Pt 3 cluster frameworks are discussed on the basis of molecular orbital calculations with EHMO and DFT methods. Hg 2 Pt 6 clusters with an incomplete open-cage structure, [Hg 2 Pt 6 (µ-RNC) 6 (RNC) 2 (µ-diphos) 2 ] (diphos ) dpphx, R ) Xyl (7a); diphos ) dpppn, R ) Mes (9b)), were also isolated as minor products and were transformed into the closed-cage clusters 6a and 8b by treatment with an equivalent diphosphine ligand. By using dppb (n ) 4), an open-cage mixed-metal cluster with a single mercury atom, [HgPt 6 (µ-RNC) 6 (RNC) 2 (µ-dppb) 2 ] (R ) Xyl (10a), Mes (10b)), was obtained as the sole product. When the aliphatic isocyanide t-BuNC was used in the reduction with dpphx, the Hg 2 Pt 6 mixed-metal cluster was not produced, and instead, the PtHgPt trinuclear complex [HgPt 2 (µ-dpphx) 3 ] (11) was obtained in a low yield and was characterized by X-ray crystallography to demonstrate the linear trinuclear structure with a considerable attractive Pt-Hg interaction (average Pt-Hg ) 2.7930 Å and Pt-Hg-Pt ) 179.591(3)°). The electronic structures of 11 are also discussed.