Crystallographic and structural characterization of heterometalic platinum complexes Part X. Heteropolynuclear pt complexes

Abstract This review covers heteropolynuclear platinum complexes. There are over sixty examples with heterometal atoms as partners including non- transition metals, K, Cs, Mg, Ca, Sr, Tl, Sn, Pb, Zn, Cd, and transition metals: Cu, Ag, Fe, Co, Ni, Rh and Pd. In addition, there are examples for the lanthanides, Eu and Yb. The most common are Ag (x16) and K (x14). The predominant geometries for Pt(II) is square-planar and for Pt(IV) is octahedral. The overall structures are complex. In spite of the wide variety of heterometal atoms partners of platinum, there is “real” Pt-M bonds only with silver, ranging from 2.678 to 2.943(I) Å (ave 2.855 Å). The mean Pt-Pt bond distance is 2.869 Å. Graphical Abstract


Introduction
There has been increasing interest in recent years in the synthesis of heterometallic compounds. Structural details have attracted much attention from the view point of nano-scale science, supramolecular chemistry, crystal engineering and solid state chemistry properties. Weak interactions such as hydrogen-bonding, charge transfer interactions and weak metal-metal heteronuclear bonding, along with strong chemical bonds, have been found to be important in such systems. The heterometallic platinum complexes are no exception. Up to the end of the year 2000 there have been numerous published structural studies on heterometallic platinum complexes (clusters). We have already analyzed and discussed the factors which can lead to better understanding of the stereochemical interactions in the heterobinuclear to heterooligonuclear platinum clusters [1][2][3][4][5][6][7][8][9]. This review article presents a brief survey on the crystal and structural data of heteropolynuclear platinum complexes. This review together with its precursors [1][2][3][4][5][6][7][8][9], represents the first comprehensive overview of almost one thousand and five hundred heterometallic platinum complexes (clusters) for which the structures have been established by X-ray crystallographic techniques.

Heteropolynuclear Pt complex
Due to complex nature of the structures of these heteropolynuclear complexes, the systems have been classified according to the coordination number of the platinum. The complexes have been listed and referenced in order of increasing coordination number, increasing complexity of the inner coordination sphere, and increasing atomic number of the principle coordinating donor.

Complexes with PtO 4 and PtN 4 chromophores
There are over twenty heteropolynuclear platinum complexes with PtO 4 and PtN 4 chromophores for which the crystallographic and structural parameters are gathered in Table 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. c. There are four crystallographically independent molecules. d. Five-membered metallocyclic ring. e. Six-membered metallocyclic ring. and {Pt(acac) 2 } 2 . The repeating unit is shown in Fig. 1(a) and (b) where an inversion center at the midpoint of two Ag(1) atoms is observed. Two of the silver atoms, Ag(1) and Ag(1"), are quadruple bridged by triflates in µ 3 -0,0" fashion. There are four bis(oxalato)platinate salts of the general formula M x Pt(ox) 2 × yH 2 O, where M is a bivalent cation: Ni [14], Co [15], Mg [16] or Ca [17], x = 0.82 -1.0 and y = 3.5 -6.0. Their one-dimensional polymeric structures are similar. Within a platinum chain, oxalate ligands are staggered with respect to the ligands located directly above and below them. The mean Pt-Pt bond distance in these salts is 2.84 Å. The M(II) atoms are located between the planes containing {Pt(ox) 2 } 2anions and are coordinated by water molecules. The Pt(II) atoms are four-coordinated (PtO 4 ), and the M(II) atoms are six-coordinated (MO 6 ).
In polymeric trans-[(NH 3 ) 2 Pt(µ-pymo) 2 Ag(H 2 O)]NO 3 [25] the trans {(NH 3 ) 2 . Pt(pymo) 2 } moiety adopts an anticonformation. Nevertheless, the {(H 2 O)Ag(pymo) 2 } residues present a syn-conformation that leads to a meander-like global structure. In the polymer the Pt atoms alternate with the Ag atoms. The orientation of the pyrimidine ligands is head-to-tail in the trans-(NH 3 ) 2 Pt(pymo) 2 entities. Silver coordination occurs as expected at the basic N3 donor atom, leading to the polymeric structure. The pyrimidine residues in the {(H 2 O)Ag(pymo) 2 } entities display a head-to-head orientation, which is stabilized by H-bonding interaction with the amino groups bound to Pt of neighboring chain. This results in an alternating headto-tail orientation of the pyrimidine at the Pt centers, and head-to-head orientation of the pyrimidine residues at the Ag centers, which gives rise to a meander-like overall structure.
In a monoclinic deep yellow {Pt 4 Ag} n complex [28] the molecular [{(NH 3 ) 4 Pt 2 . (µ-meu) 2 } 2 Ag] 5+ cation shows crystallographic centrosymmetry, with Ag being at the inversion center. The Ag(I) atom has a square-planar coordination with four O(2) oxygens of 1-methyluracil ligands, two from each platinum dimer. Within each dimer, two cis-{Pt(NH 3 ) 2 } 2+ moieties are bridged by 1-methyluracilato ligand in a head-to-head arrangement, with Pt bound to N(3) and O(4), respectively. Thus each 1-methyluracilato ligand simultaneously bonds to three metal centers, two Pt(II) and one Ag(I). The Pt-Pt bond distance within the dimer of 2.949(2) Å is longer than the Pt-Ag bond distance, 2.787(1) Å. Adjacent complex cations are related by another inversion center, leading to a molecular structure with stacks of Pt 4 units interrupted by silver. The Pt...Pt separation between adjacent dimers is 3.246(2) Å.

Complexes with PtC 4 and PtS 4 chromophores
The crystal of orthorhombic orange yellow {Pt 3 Ag 2 } n complex [45] is an alternate pile of extended twodimensional lattices of {Pt 3 (S 2 CNEt 2 ) 6 Ag 2 } n 2+ cations and anionic layers containing ClO 4ˉ. The {Pt 3 (S 2 CNEt 2 ) 6 Ag 2 } n 2+ unit, with two extra Pt(S 2 CNEt 2 ) 2 groups and two extra Ag atoms (all labeled with an X) to show the connections between neighboring units, is shown in Fig. 6. It has C 2 symmetry, with the Pt(1)-Ag (1) [48] is shown in Fig. 7. It displays a one-dimensional infinite chain of two {K(18C6)} + cations and a {Pt(SCN) 4 } 2anion bridged alternately by N and O atoms as described below. The Pt(II) atom is located on the two fold axis and is coordinated by four S atoms from four SCN groups, and has a square-planar configuration. The K(I) atom lies almost symmetrically within the crown ether, and is also coordinated by one N atom from the SCN group of the Pt moiety (K-N, 2.837 Å). The two K(I) complex cations share an oxygen atom of a water molecule (K-O, 2.807 Å) which serves as a bridge, with K-O-K angle of 180°, to complete the one-dimensional infinite chain structure.
Inspection of the data in Table 2 reveals that the complexes crystallize in four crystal systems: orthorhombic (x2) < triclinic (x6) < tetragonal (x7) < monoclinic (x8). Each Pt(II) atom has a square-planar arrangement with different degrees of distortion. The sums of all four Pt-L bond distances are 7.87 Å for PtC 4  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. M = Ni, Cu, Zn or Cd d. Four-membered metallocyclic ring.

Complexes with heterogenous chromophores
Crystallographic and structural data for heteropolynuclear platinum complexes with heterogenous coordination spheres about platinum are gathered in Table 3. Their structures are all complex. The X-ray analysis of monoclinic orange KPt(mtso)Cl 3 [49] shows that there are two complex {Pt(mtso)Cl 3 }ˉ anions differing mostly by degree of distortion. Each Pt(II) atom has a squareplanar configuration (PtCl 3 S). In the crystal structure the potassium atoms differ from each other, with K(1) is surrounded by four chlorine atoms and one oxygen atom and K(2) by six chlorine atoms and one oxygen atom. The mean Pt-Cl bond distance located trans to the S atom is 2.322 Å, which is about 0.021 Å longer than the remaining Pt-Cl bond distances located cis to the S atom (2.301 Å). This difference is caused by the strong trans effect of the sulfoxide ligand. The structure of triclinic red K 2 Pt(ox)Cl 2 .H 2 O [21] is that of a zig-zag chain. The Pt(II) atom has a square-planar geometry (PtO 2 Cl 2 ). The chlorine is positioned above the oxalate carbonyl carbon of the adjacent complex, resulting in an electro statistically favored conformation. The Pt...Pt separations are 3.799(2) Å in the same unit cell and 3.815(2) Å in adjacent unit cells. The potassium atoms, K(1) and K(2) are eight-and seven-coordinated, respectively.

M-M [Ǻ] M-L-M [°] L-M-L [ o ]
Ref 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. Five-membered metallocyclic ring. d. Three-membered metallocyclic ring. has a square-planar environment (PtN 2 I 2 ), and the Cs(I) pseudo-octahedral (CsO 4 I 2 ). Monoclinic {PtAg} n complex [52] contains well separated {NBu 4 } + cations and [Pt(C 6 Cl 5 ) 2 (µ-Cl) 2 Ag]ˉ anions. The chain of complex anions consists of planar {Pt(C 6 Cl 5 ) 2 Cl 2 } units linked by Ag(I) atoms in which there are two types of Ag-Cl bond, those which result in Pt(µ-Cl) Ag links and those in which ortho-Cl atoms of C 6 Cl 5 units make close approaches (3.01 nd 3.09 Å) to Ag(I) atoms. The Pt...Ag separation is 3.203(1) Å. Each Pt(II) atom has a square-planar geometry (PtC 2 Cl 2 ), and the Ag(I) atom is two coordinate (AgCl 2 ).
In a linear polymer of [(PMe 2 Ph) 2 Pt(C≡CH) 2 Ag(OClO 3 )] n [54] each Pt(II) atom has a trans-square-planar arrangement (PtC 2 P 2 ). The Ag(I) atom is η 2 -coordinated to each of the two alkynyl ligands on the same face of each Pt coordination plane, forming a zig-zag chain with a perchlorate anion in each cavity along the chain. In addition to the coordinated Ag-O (2.651(1) Å), there are number of ClO 4ˉ-to-chain contacts which might help to stabilize this arrangement. The Pt...Ag separations are 3.689(6) and 3.778(6) Å.
The X-ray diffraction study of rhombohedral light brown [Pd(µ-Cl) 2 Pt{µ-C(PPh 2 ) 2 }] n [55] reveals an infinite chain structure of alternating Pd and Pt units (Fig. 9). The asymmetric unit of the crystal contains one {-PdCl 2 Pt{(PPh 2 ) 2 C}-} fragment with subsequent units in the infinite chain related by the diagonal glide in which the Pd, Pt, and carbene carbons atoms lie. The coordination units about the metal atoms are coplanar as are the adjacent units of the chain. The Pd(0) is three-(PdCl 2 C) and the Pt(II) is four (PtCl 2 P 2 ) coordinate.
The structure of orthorhombic colorless [Pt(µ-SCH 2 CH 2 Ph 2 ) 2 Ag(NO) 3 ] n [56] consists of infinite chains running along the c axis. Each silver(I) atom is coordinated almost linearly to two sulfur atoms belonging to the square-planar coordination spheres of two different platinum(II) atoms (PtS 2 P 2 ).
In monoclinic red {PtK} n complex [57] the {Pt(acac) 2 Cl}ˉ anions are linked via electrostatic inter-discrete pseudopolymeric chains parallel to the crystallographic b axis. Each unit cell contains two such chains separated by van der Waals interactions. The Pt(II) atom has a squareplanar geometry (PtO 2 CCl). The crystallographically nonequivalent linked K + cations are each six-coordinated; K(1) is coordinated by three pairs of oxygen atoms at distances   In orthorhombic yellow [Pt(CH 3 ) 2 (µ-Himpa)(µ-Br)Ag] n [59] a pseudo-octahedral coordination about the Pt(IV) atom is built up by a tridentate Himpa ligand through N, and phosphonates O atoms trans to methyl group and through carboxylate O atom trans to bromine. The bromine ligand, as well as being bonded to Pt(IV), is also bonded to Ag(1) (Ag-Br, 2.810(2) Å) with a Pt-Br-Ag bond angle of 111.5(1)°. A distorted tetrahedral stereochemistry about silver is completed by three oxygen atoms from different Himpa ligands, a carboxylate oxygen which is not bonded to platinum, a phosphate oxygen which is not protonated and also not bonded to Pt, and a phosphate oxygen which is bonded to {Pt(AgO 3 Br)}. The angles about the Ag(I) atom range from 85.5(1) to 139.5(3)°. The result is a network structure which extends throughout the unit cell in two dimensions.
A view of the extended network of monoclinic white {Pt 2 Ag 3 } n [59] is shown in Fig. 10 (3)). The planes of the coordinated carboxylate groups are at an angle of approximately 90°. Each bromine atom bridges between a platinum atom and two silver atoms, Br(1) is bonded to Pt(1), Ag(1) and Ag(3´) and Br (2) to Pt(2), Ag(2) and Ag (3)). Each Pt(IV) atom has a pseudo-octahedral environment (PtO 2 C 2 NBr). The Pt(1)-N(31), 2.04(1) Å and Pt(2)-N(32), 2.06(1) Å bond lengths trans to bromine are shorter than the Pt-N bond lengths trans to a methyl group in yellow {PtAg} n [59], with the values of 2.18(2) and 2.20(2) Å, respectively. The mean O-Pt-N "bite" angle in the white complex of 85.0° is somewhat more open than the respective angle found in the yellow complex, with the mean value of 82.6°. This can be related to the much stronger trans effect of methyl versus bromine.
The X-ray analysis of tetragonal red polymeric KPt(CN) 5 .3H 2 O complex [60] shows two different platinum atoms, Pt(II) and Pt(IV). The anion chain consists of planar {Pt(CN) 4 } 2and octahedral {Pt(CN) 6 } 2units linked by cyanide groups with mean Pt(II)-Pt(IV) bond distance of 2.92 Å. Unfortunately data for the K(I) atom is not available, but one can expect that the cations are also involved in this polymeric chain.

Conclusions
This review has classified and analyzed over sixty heteropolynuclear platinum complexes. There is only one example which contains a K(I) atom with mixed-valence platinum atoms, Pt(II) and Pt(IV) [60], and two examples in which both Pt(IV) and Ag(I) are found. In all the remaining examples the platinum is in +2 oxidation state. Each Pt(II) atom has a square-planar environment with different degrees of distortion. There is wide variability of the inner coordination sphere about the Pt(II) atoms: PtO 4 (x14), PtN 4 (x8), PtC 4 (x20), PtS 4 (x5), PtCl 3 S, PtO 2 Cl 2 , PtN 2 C 2 , PtN 2 I 2 , PtC 2 Cl 2 , PtC 2 P 2 (x3), PtS 2 P 2 , PtO 2 CCl and PtCl 2 CP. In the mixed-valence complex, Pt(II) is planar (PtC 4 ) and Pt(IV) is octahedral (PtO 2 C 2 NBr).
Analysis of the crystallographic and structural data of almost two thousand monomeric platinum coordination compounds showed [128] that about 10% of these complexes exist as isomers: distortion (65%), cis-trans (30%), mixed isomers (cis-trans plus distortion), and ligand isomerism. Despite the importance of cis-trans geometry in the chemistry of Pt(II) compared to other transition metal systems, within platinum chemistry, distortion isomerism is far more common.
It is hoped that this overview will help to focus attention on the area of platinum chemistry that could be enhanced by further study, and assist in allowing comparative behavior of the platinum atom in the situations which can arise from the wide spread use of platinum.