Crystallographic and structural characterization of heterometallic platinum clusters. Part IX. Heterooligonuclear Pt clusters

Abstract This review classifies and analyzes over thirty heterooligonuclear platinum clusters with a wide variety of metal frameworks, from twelve to forty-four. There are thirteen heterometals (Ge, Sn, Hg, W, Mo, Ru, Rh, Pd, Os, Ni, Cu, Ag, Au) which are the partners of platinum. The clusters mostly crystallize in monoclinic (36,4%) and triclinic (30,3%) crystal classes. Their structures are complex, with platinum most commonly preferring interstitial sites, such as the centroids of icosahedrons. There are examples of distortion isomerism. The most common ligands are CO and PPh3, and it is interesting that the mean Pt-CO and M-CO bond distances are identical at 1.84 Å. In contrast, the mean Pt-μCO and M-μCO are of values of 2.02 and 1.97 Å, respectively, while the Pt-PPh3 and M-PPh3 bond distances are 2.30 and 2.28 Å, respectively. The shortest Pt-Pt, Pt-M (non-transition) and Pt-M (transition) bond distances are 2.559(2) Å, 2.412(2) Å (M = Ge) and 2.510(2) Å (M = Ni). Graphical Abstract


Introduction
The chemistry of heterometallic clusters has attracted much attention in the last few decades because of their significance in catalytic and biological functions. These compounds may serve as model systems in providing a better understanding of the underlying mechanisms of action and the development of new techniques in these fields. Among such derivatives, attention has been paid to the platinum clusters that have a variable number of metal atoms in their inner core, and which possess certain interesting properties. A considerable number of structural determinations have been carried out, which have helped to shed light on catalytic and other activity of heteronuclear platinum clusters. Because of the large number of structures involved, this review is the ninth part of a comprehensive structural overview. The earlier articles cover heterobi- [1,2], heterotri- [3], heterotetra- [4], heteropenta- [5], heterohexa- [6], heterohepta-and heteroocta- [7], and heteronona-and heterodecanuclear [8] platinum clusters. In this review undecanuclear and oligonuclear heterometallic clusters involving platinum have been classified according to the total number of metal atoms present in the unit. The heterometal atoms include both non-transition and transition metals, with the latter being more frequent. The primary source of information has been the Cambridge Crystallographic Data base up to the end of 2000.
Dark red [K(crypt)] 2 [(PPh 3 )Pt 2 Sn 9 ] derivative crystallizes as two different triclinic solvates, one with an ethylenediamine solvate molecule and one with a toluene solvate molecule in the crystal lattice [10]. Otherwise, their structures are identical. The [(PPh 3 )Pt 2 Sn 9 ] 2anion has virtual C 3v point symmetry defined by an elongated tricapped trigonal prismatic Sn 9 cluster with a linear Pt-Pt-PPh 3 rod inserted into the top triangular face. The Sn-Sn distances range from 3.002 (3)  The structure of monoclinic Pt 2 Rh 9 cluster [11] consists of well separated NEt 4 + cations and a [Pt 2 Rh 9 (µ-CO) 11 (CO) 11 ] 3anion. The metal skeleton of the anion, which has ideal D 3h symmetry, consists of three face-to-face condensed octahedra with a common edge, Pt(1)-Pt (2), coincident with the ideal three-fold axis. The Pt-Pt bond is 2.812(2) Å, and the mean Pt-Rh and Rh-Rh bond distances are 2.707 and 2.852 Å, respectively.
The structure of [PtRh 10 (µ 5 -N)(µ-CO) 10 (CO) 11 ] 2- [12] is shown in Fig. 1. The anion lies on a two-fold crystallographic axis passing through the Pt atom and the interstitial nitrogen atom. The overall idealized symmetry of the anion is C 2v . The metal atom cluster inhibits a geometry, which can be described as a two-layer arrangement, i.e. a folded hexagon of rhodium atoms {Rh(1,2,3,1´,2´,3´} centered by the platinum atom, superimposed along the C 2 axis direction on a folded square of rhodium atoms {Rh(4,5,4´,5´}. Of the 21 carbonyls, 11 are bound terminally, one per metal atom, and 10 are double-bridging on the Rh-Rh edges of the folded hexagon and square layers. The platinum forms six Pt-Rh bonds at a mean distance of 2.711 Å with the rhodium atoms of the folded hexagon. On the other hand, of four more Pt-Rh contacts, two {with Rh(4) and Rh(4´)} are significantly longer (3.180(2) Å and the remaining two, {with Rh(5) and Rh(5´)} at (3.492(2) Å), are too long be considered bonded. The Rh-Rh bond lengths range from 2.728(2) to 3.088(2) Å, the longer ones being the interlayer connections between the folded hexagon and the folded square. The interstitial nitrogen atom occupies an unusual type of cavity in which it is connected to five metal atoms, Pt-N 1.92(2) Å, Rh(4,4´)-N 2.05(1) Å and Rh(5,5´)-N 2.12(1) Å.
The cluster of composition [(H)Pt(AgNO 3 ) 2 (AuPh 3 ) 8 ] NO 3 exists in two isomeric forms [19]. Their structures differ mostly by degree of distortion and are examples of distortion isomerism [21]. There are several examples of    (2) 29.037 (7) 78.95 (1)   Footnotes:a. Where more than one chemically equivalent distance or angle is present, the mean value is tabulated. The first number in parenthesis is the e.s.d., and the second is the maximum deviation from the mean. b. The chemical identity of the coordinated atom or ligand is specified in these columns.  The structure of monoclinic brown [(η 4 -cod) 2 Pt 7 Os 6 (CO) 21 ]CH 2 Cl 2 [25] consists of layers of metal atoms (Fig. 4). A Pt(cod)-capped Os 3 triangle, on the far right sits next to a planar Pt 4 layer, which in turn lies next to a planar five-atom Os 3 Pt 2 layer on the far left of Fig, 4. The latter contains another Pt(cod) moiety bonded to four other metal atoms. The mean Os-Os bond distance of 2.86 Å is somewhat longer than the mean Pt-Pt bond distance, 2.79 Å. The Pt(2)-Pt(5) distance of 3.008(2) Å is the longest bonding distance in this cluster, involving the unusual µ 4 -Pt(cod) bridging group Pt (2).

Continued
The D 3h idealized symmetry metal atoms cluster in the [PtRh 12 (µ-CO) 12 (CO) 12 ] 4- [26] consists of a "twinned" cubooctahedral array of rhodium atoms with a central platinum atom. The [Pt 2 Rh 11 (µ-CO) 12 (CO) 12 ] 3anion [26] consists of the same cluster with one platinum atom at the center and the second platinum atom disordered on the surface. In both Pt n Rh 13-n (n = 1,2) clusters, the CO geometries imply that each surface metal atom bears one terminal CO ligand and is connected to two edgebridging CO ligands. The following bond parameters are for the tetra-anion, those of the tri-anion being of poorer quality owing to disorder. The Pt-Rh bond distances are in the range 2.743(4)-2.812(6) Å, with a mean value of 2.776 Å. The twenty-four Rh-Rh bond distances range from 2.713(8)-2.849(7) Å, have a mean value of 2.778 Å.
The metal core in triclinic dark brown [(PPh 3 ) 6 Au 6 Ag 6 Pt{Ag(µ-I) 3 ) 2 ](thf) 2 [27] can be described as a complete, platinum centered, Au 6 Ag 6 icosahedral cluster capped with two other Ag atoms (Ag(7) and Ag (8)). The six surface gold atoms form a cyclohexane style "chair" configuration, capped on top and below by two silver triangles (Fig. 5a). Each of the six PPh 3 ligands is coordinated to the six individual gold atoms in a radial  Footnotes: a. Where more than one chemically equivalent distance or angle is present, the mean value is tabulated. The first number in parenthesis is the e.s.d., and the second is the maximum deviation from the mean. b. The chemical identity of the coordinated atom or ligand is specified in these columns. c.There are two crystallographically independent clusters. Table 3: Crystallographic and structural data for heterododeca-and heterooligonuclear platinum clusters a .

Continued
fashion. In terms of the cyclohexane style "chair" analogy, they are occupying the "equatorial" position (Fig. 5b).    A monoclinic dark red [Pt 2 (AuPPh 3 ) 10 Ag 13 (µ-Cl) 5 Fig. 6. The structure consists of two identical icosahedral Pt-centered sub-units sharing a single common vertex occupied by a silver atom. Each of the gold atoms is bonded to a phosphine ligand. Atoms Ag (7) and Ag(7´) are bonded to chlorine terminally, whereas the two silver pentagons are interconnected through five bridging chlorine atoms situated in the mirror plane perpendicular to the C 5 axis. The radial Pt-Au bond distances, ranging from 2.679 to 2.689 Å, are shorter than the radial Pt-Ag bond distances of 2.789 to 2.839 Å. The peripheral Au-Au bond distances range from 2.876 to 2.886 Å. The peripheral intraicosahedral Au-Ag distances range from 2.807 to 3.045 Å, whereas the intericosahedral Ag-Ag distances ranges from 2.84 to 3.07 Å. The Pt-Ag(7)-Cl(6) bond angle (178.6°) deviates slightly from linearity.
The structure of monoclinic dark brown [PtAu 12 (PPh 3 ) 10 Ag 12 (µ-Cl) 5 (Cl) 2 ]Cl [33] is similar to Pt 2 Au 10 Ag 13 [31] and Pt 2 Au 11 Ag 12 [32]. The structures of these clusters [31][32][33] satisfy the site preference rules established for mixedmetal vertex-sharing polyicosahedral nanoclusters. The determined structures of these mixed-groups ten metal clusters shows that the site preference is a manifestation of the various and often competing bonding effects. The platinum prefers interstitial sites such as the centroids of icosahedra due to its higher cohesive energy (Pt > Au > Ag). Gold prefers the interstitial sites such as centres of icosahedra, or the shared vertices, due to its higher cohesive energy and high electronegativity (Au > Ag). The more electronegative gold prefers phosphine coordination while the more electropositive silver prefers halide coordination or halide bridging. The mean metal-metal bond distances in these clusters follow the trend of Pt-Au (2.692 Å) < Pt-Ag (2.804 Å) < Au-Au (2.881 Å) < Au-Ag (2.897 Å) < Ag-Ag (2.921 Å), which correlates (in reverse order) with the corresponding trend of the bond energies (in kJ/mol): Pt-Au (77.83) > Pt-Ag (70.88) > Au-Au (61.37) > Au-Ag (54.43) > Ag-Ag (47.48) [34].
An asymmetric unit of the co-crystallized mixture of {[NH 3 (CH 2 Ph)] 6 6 [Pt 4 Ni 37 (CO) 46 ]} 0.5 [35] consists of one anion located on a crystallographic C 3 axis with an average occupancy factor of 0.5, two [NMe 3 (CH 2 Ph)] + cations in general positions, and a disordered isopropranol molecule located on a crystallographic C 3 axis. The metal core of the [Pt 4 Ni 36 (CO) 45 ] 6anion ( Fig. 7) consists of a Pt 4 tetrahedron fully encapsulated in a shell of 36 Ni atoms belonging to a very distorted and incomplete ʋ 5    conforms to pseudo-C 3 -3 symmetry, is composed of a fourlayer hcp Pd 28 Pt core (viz. Pd 3 , Pd 7 , Pd 7 , Pd 12 and PtPd 6 layers) that contains four tetrahedrally-linked interior Pd(i) atoms, each with a localized hcp environment (Fig. 8). The pseudo-3-fold axis passes through one Pd(1), the bottom-layer centered Pt and its attached PMe 3 ligand. The Pd 28 Pt core has 12 highly distorted square pyramidal Pd 5 cavities each capped by an external Pt(PPh 3 ) fragment. The 27 carbonyl ligands consist of 18 double-bridging and 9 triple-bridging ligands. The proposed location of the 12 hydride atoms is in the 12 square-pyramidal Pd 5 cavities capped by the 12 Pt(PPh 3 ) fragments. The Pt-Pd bond distances range from 2.68-2.83 Å and Pd-Pd from 2.65-3.19 Å.

Conclusions
This review classifies and analyzes over thirty heterooligonuclear platinum clusters contain from twelve to forty-four metal atoms per unit. The clusters crystallize in five crystal classes: hexagonal (3%) < orthorhombic (12.1%) < tetragonal (18.2%) < triclinic (30.0%) < monoclinic (36.4%). The clusters are for the most part red, brown and black, some due to the metal partners of platinum, to ligand absorption or to charge-transfer bands. For example, when Ni is partner of platinum, the clusters are only of black color.
A cluster of triclinic red [(H)Pt(AgNO 3 ) 2 (AuPPh 3 ) 8 ]NO 3 [19] exists in two isomeric forms, and [(H)Pt 6 Ni 38 (CO) 48 ] [36] contains two crystallographically independent molecules within the same crystal. These isomers as well as independent molecules differ mostly by degree of distortion and all are classical examples of distortion isomerism [1]. This type of isomerism is very prevalent even in the chemistry of platinum [21].
This review together with its precursors [1][2][3][4][5][6][7][8] represents the first comprehensive overview of about 1400 heterometallic platinum clusters, illustrating a rich diversity in the chemistry of platinum. It is hoped that this overview on structural parameters will help to focus attention on areas of platinum chemistry that could be enhanced by further study and assist in predicting the properties of the platinum atom in a variety of important biological and catalytic activities. In gathering the data, it became evident that some of it is buried in the literature and effectively invisible to automated data retrieval searches for a variety of reasons. In other cases, incorrect information had been promulgated in the literature. This review hopes to organise and correct, where necessary, the information and provide a base upon which new information can grow. A similar review on heteronuclear polymeric platinum complexes is in progress.