List of abbreviations
Heterometallic platinum complexes are a rapidly growing branch of chemistry. In many cases, these complexes have been prepared and studied by single-crystal X-ray diffraction analyses. There are almost 90 heterodinuclear Pt complexes with non-transition metals (Melník and Mikuš, 2012) and heterotrinuclear such complexes for which structural data were available. These parameters were classified and analyzed by us (Melník and Mikuš, 2013). The aim of this review is to classify and analyze structural parameters of these heterotetranuclear complexes. There are over 30 such complexes of the compositions Pt3M (10 examples), Pt2M2 (15 examples), PtSn3 (5 examples), Pt2NaAg, PtSn2Fe (2 examples), and PtSnHgGe. The data of these complexes are compared with those of heterodinuclear and heterotrinuclear examples.
There are over 30 heterotetranuclear complexes of the composition Pt3M, Pt2M2, PtM3, Pt2MM′, and PtMM′M″ for which structural parameters are available. These are analyzed and classified in this review.
Heterotetranuclear Pt3M complexes
There are 10 examples of Pt3M-type complexes, which, from the structural point of view, can be divided into (i) tetrahedral Pt3M and (ii) cyclo-(Pt-X)3 rings. Seven examples belong to the first group. The clusters adopt a tetrahedral geometry consisting of a triangular arrangement of platinum atoms capped by Zn (Stockhammer et al., 1991), Sn(Me)2(η1-O2PF2), SnF3 (Douglas et al., 1989), Te(η2-dk)(η1-OCOCF3), Tl(η2-acetylacetonate)(H2O) (Studnichenko et al., 2002), Tl (Ezomo et al., 1987), or Hg (Schwettel et al., 1990). Triangular Pt3 unit edges are bridged by three μ-CO groups (Ezomo et al., 1987; Stockhammer et al., 1991). In the remaining five clusters, each edge of a triangular Pt3 unit is double-bridged by μ-η2-dppm(P,P′) and μ3-CO (Douglas et al., 1989; Schwettel et al., 1990; Studnichenko et al., 2002). The structure of [(μ-dppm)3(μ3-CO)Pt3Tl(dk)(F3CCOO))]+ is shown in Figure 1. The mean Pt-M and Pt-Pt bond distances are 2.676 Å (M=Zn) and 2.683 Å (Stockhammer et al., 1991), 2.735 Å (Sn) and 2.620 Å (Douglas et al., 1989), 2.805 Å (Sn) and 2.639 Å (Douglas et al., 1989), 2.902 Å (Tl) and 2.638 Å (Studnichenko et al., 2002), 2.910 Å (Tl) and 2.645 Å (Studnichenko et al., 2002), 3.038 Å (Tl) and 2.667 Å (Ezomo et al., 1987), and 2.915 Å (Hg) and 2.635 Å (Schwettel et al., 1990). The inner coordination spheres about the Pt and M atoms are PtC2PZnPt2 and ZnI2Pt3, PtP2SnPt2 and SnC2Opt2, PtP2CSnPt2 and SnF3Pt3, PtP2CTlPt2 and TlO3Pt3, PtC2PTCPt2 and TlPt3, and PtP2CHgPt2 and HgPt3.
To the second group belong three colored Pt3M-type [pale yellow, M=Sn (Usón et al., 1990) or Pb (Ara et al., 2002); orange (M=Hg) (Casas et al., 2000)] clusters with a six-membered puckered Pt3X3 [X=Cl (Usón et al., 1990; Ara et al., 2002) or OH (Casas et al., 2000)] ring in which the three Pt atoms are interconnected by three X bridges and linked to the M atom by Pt to M donor-acceptor interaction. The structure of [[(C6H5)2(μ-Cl)Pt}3Pb]– is shown in Figure 2. Thus, the Pt3X3 moiety acts as a terdentate ligand to the M atom. Each Pt(II) atom has a square pyramidal coordination sphere with an apical M atom. The basal plane is defined by the ipso-C atoms of two pentafluorophenyl (C6F5) groups in all three examples and two chlorine atoms or two OH groups. The (NBu4)[[(η1-C6F5)2(μ-Cl)Pt}3Sn] (Usón et al., 1990) contains two crystallographically independent molecules within the same crystal that differ mostly by degree of distortion – an example of distortion isomerism (Melník, 1982; Melník and Holloway, 2006). The mean Pt-M bond distance elongated in the sequence 2.725 Å (M=Sn)<2.796 Å (Hg)<2.833 Å (Pb). There is a relationship between the Pt-M-Pt angle and the covalent radius of M. The former is open and the latter decreases: 87.9° (Sn, 1.41 Å)>84.8° (Pb, 1.47 Å)>78.2° (Hg, 1.51 Å).
Heterotetranuclear Pt2M2 complexes
There are 15 Pt2M2-type examples for which structural parameters are available. Their structures are complex. In monoclinic red Pt2Hg2-type derivative (Catalano et al., 2002), the geometry of the metallic skeleton is planar triangulated rhombohedral with the center of symmetry in the middle of the Hg-Hg bond (2.736 Å). The Pt-Hg bond distance is 2.814 Å and the Hg-Pt-Hg angle is 58.2°. In another two monoclinic yellow Pt2Li2- (Sebald et al., 1984) and Pt2Cd2-type complexes (Charmant et al., 1999), the four metal atoms form an approximately planar skeleton. The mean Pt-M bond distances are 2.827 Å (M=Li) and 2.960 Å (Cd). In other two monoclinic yellow Pt2Sn2-type (Albinati et al., 1983; Al-Allaf et al., 1985) and triclinic orange Pt2Sn2-type (Clark et al., 1984) derivatives, two Cl3Sn(Pet3)Pt (Albinati et al., 1983), two Cl(Me3SiN)2Sn(Pet3)Pt (Al-Allaf et al., 1985), or two Cl3Sn[μ-η2-C(CF3)=C(CF3)}Pt(PEt3) (Figure 3) (Clark et al., 1984) fragments are double-bridged by two μ-Cl atoms in the manner Pt(μ2-Cl)2Pt. The mean Pt-Sn bond distance is 2.506 Å and the mean Pt-Cl-Pt bridge angle is 87.7°.
The structure of monoclinic yellow [(η2-dppm)Pt(μ3-O)Li(thf)2]2 (Li et al., 1996) represents a μ3-O bicapped triangular LiPt2 aggregate. An intriguing planar [Pt2O2} ring has two Li(thf)2 ‘molecular pendants’ protruding vertically from the oxygen sites at opposite directions. The Pt-Pt separation is 3.112 Å. The Pt-O-Pt and Li-O-Li bridge angles are 99.0(3)° and 106.8(9)°. Another three Pt2Hg2-type complexes, [(PPh3)2Pt(μ-η2-tep)HgBr(μ3-Br)]2 (Henderson et al., 1994), [(η2-O2CNNH2)(η2-dtpp)(PtHgI(μ3-I)]2 (Falvello et al., 1997) and [(PPh3)2(μ-η2-ths)PtHgI(μ3-I)]2 (McCaffrey et al., 1998), represent a μ3-X bicapped triangular Hg2Pt aggregate. The mean Pt-X-Pt bridge angle of 96.7° is approximately 5.3° larger than the mean Hg-X-Hg bridge angle of 91.4°. The mean Pt-Hg bond distance is 2.768 Å.
In a monoclinic white Pt2Ge2 tetramer (Bender et al., 1997), two Pt(H)(PEt3)2 fragments in cis geometry are on each end of the Ge2(tfmp)4, with a Pt-Ge-Ge angle of 116.86°. The mean Pt-Ge and Ge-Ge bond distances are 2.436 and 2.466 Å. In monoclinic colorless Pt2Tl2 tetramer (Usón et al., 1997), two Pt(C6F5)3 units and two Tl(I) atoms, besides a direct Pt(II)-Tl(I) bond [2.884(1) Å], are connected by μ-acetate groups. In orthorhombic white [(PEt3)(Cl)Pt(μ-Cl)[μ-η1:η3-CH(PPh2O)2}Li]2 (Browning et al., 1981) the Pt(II) is closely planar, with the CH(PPh2O)2 group bound to the Pt(II) via C and with the Li(I) coordinated by the oxygen atoms so that the group forms a bridge between two dissimilar metals. The Li(I) is further bounded in an unusual manner involving one of the chlorines on Pt(II) forming a bridge to the Li. The six-membered [CPOLiOP} ring lies approximately perpendicular to the plane and adopts a boat conformation. The two tetrahedral (LiO3Cl) share a common edge O(1)…Oc(1)′, and the overall effect is for the Li(I) coordination to link two molecules into a compact dimer containing pairs of Pt and Li.
There is one yellow triclinic complex [(PPh3)4Pt2(μ3-S)2 Hg2(μ-Cl)2(PPh3)2](PF6)2 (Figure 4) and one yellow monoclinic complex [(PPh3)4Pt2(μ3-S)2Hg2(μ-Cl)2Cl2] (Li et al., 2000) that each contain a Pt2(μ3-S)2Hg2(μ-Cl)2 skeleton. They can be viewed as a fusion of two butterfly motifs via [Pt2S2} and [Hg2Cl2} across the Hg-S bonds. The mean Pt-Hg and Pt-Pt distances of 3.840 and 3.384 Å in the triclinic and 3.797 and 3.398 Å in the monoclinic tetramer ruled out a bond.
In the series of Pt2M2-type tetramers, the inner coordination spheres about the Pt and M atoms are with the chromophores: PtP3Hg2 and HgN2Pt2Hg (Catalano et al., 2002), PtC3PLi2 and LiC2Pt2 (Sebald et al., 1984), PtC4Cd2 and CdC4ClPt (Charmant et al., 1999), PtCl2PtSn and SnCl3Pt (Albinati et al., 1983), PtCl2PSn and SnN2ClPt (Al-Allaf et al., 1985), PtCl2P and SnCl3C (Clark et al., 1984), PtO2P2 and LiO3 (Li et al., 1996), PtP2Cs and HgBr3S (Henderson et al., 1994), PtS2CPHg and HgI3Pt (Falvello et al., 1997), PtP2OS and HgI3S (McCaffrey et al., 1998), PtP2HGe and GeC2PtGe (Bender et al., 1997), PtC3OTl and TlO2Pt (Usón et al., 1997), PtCl2CP and LiO3Cl (Browning et al., 1981), PtS2P2 and HgCl2SP (Li et al., 2000), and PtS2P2 anions. The four metal atoms are arranged in the form of a distorted Y shape with a Pt…Hg…Pt angle of 146.9(1)° and Pt…Hg…Hg (x2) angle of 106.5° (Li et al., 2000). The mean Pt-M bond distance in this Pt2M2 series increased in the sequence 2.435 Å (M=Ge)<2.508 Å (Sn)<2.798 Å (Hg)<2.827 Å (Li)<2.884 Å (Tl)<2.960 Å (Cd). In all of these examples the mean Pt…Pt separation is 3.56 Å.
Heterotetranuclear PtM3 complexes
Interestingly, such combination of metals is only found for PtSn3-type tetramers. In a triclinic yellow PtSn3-type tetramer (Al-Allaf et al., 1985), three Sn[N(SiMe3)2}2 fragments are coordinated to the Pt(0) atom with a mean Pt-Sn bond of 2.487 Å. The Pt(0) atom is three-coordinate (PtSn3) in an almost ideal Y shape with the Sn-Pt-Sn bond angles deviating from the ideal 120° only by ±0.2°. The structure of another yellow PtSn3-type tetramer (Albinati et al., 1984) contains a tetraphenylarsenium (AsPh4+) cation and an [(η4-cod)Pt(SnCl3)3]– anion. The Pt(II) atom is (4+3) seven-coordinate by a tetradentate cod [average Pt-C distance, 2.25(2) Å] and by three SnCl3 units with a mean Pt-Sn bond distance of 2.566 Å. In another PtSn3-type tetramer (Albinati et al., 1982), a trigonal bipyramidal geometry around the Pt(II) atom is created by three SnCl3 units that form a trigonal plane. The two axial positions are occupied by two AsMe3 fragments. The mean Pt-Sn bond distance is 2.622 Å. In a triclinic colorless PtSn3-type tetramer (Baumeister et al., 2000), each of the three SnMe2Cl units is connected to the central PtCl unit via one of the three CH2CH2PPh2 ligands in the manner SnC(H2)O(H2)P(Ph2)Pt. The Pt(II) atom has a distorted square planar (PtP3Cl), and each Sn(II) has a distorted tetrahedral (SnC3Cl) arrangement. In triclinic vine red PtSn3 cluster (Grassi et al., 1990) a rhombic octahedral coordination about the central Pt(II) atom is created by a terdentate methylallyl ligand and by three SnCl3 units. The mean Pt-Sn bond distance is 2.566 Ǻ.
Pt2AgNa, PtSn2Fe and PtHgSnGe complexes
A monoclinic Pt2AgNa tetramer (Zamora et al., 1999) is the only example in which besides two Pt atoms are another two different metal atoms. The structure of the tetramer consists of a [[(NH3)2Pt(μ-η2-meu)2}2AgNa(H2O)4]2+ cation (Figure 5) and ClO4– anions. The four metal atoms are arranged in the form of a distorted Y shape with a Pt…Ag…Pt angle of 146.9(1)°. The Pt-Ag bond distances are 2.847(1) Å. The Ag(I) and Na(I) are 3.57(1) Å apart. All four 1-methyluracilate (meu) nucleobases are arranged head-to-head and use their O(4) atoms as heterometal (Ag, Na) binding sites. The complex cation has a C2 symmetry axis which passes through Ag and Na. The two meu bases within one half of the cation are close to planar (dihedral angle of 4.9(8)°) and inclined by 63° (mean) with respect to the PtN4 plane. The Ag(I) is markedly out of the planes of the meu ligands, by 1.05(2) Å from the ring containing N(3) and by 1.38(2) Å from the ring containing N(3′). The Ag(I) is a center of three metallocyclic rings, two eight-membered [AgO(4)CNPtNCO(4)} and one four-membered [AgO(4)NaO(4′)}.
There are two triclinic, yellow and orange PtSn2Fe-type tetramers of the composition [(L)nPt[Sn(Me)2(μ-η1: η5-C5H4)}2Fe] (Ln=η2-dppm) (Herberhold et al., 1998) or (PPh3)2 (Herberhold et al., 1997), which have a similar structure. The (η2-dppm)Pt or (PPh3)2Pt units are connected with two Sn(Me)2 units. The mean Pt-Sn bond distances are 2.600(1) and 2.644(1) Å, respectively. Two C5H4 ligands serve as bridges between the SnMe2 units via one C atom and the iron atom via five C atoms. The Pt atom resides at the center of a distorted square plane of ligands, where the planes P(1)-Pt-P(2) and Sn(1)-Pt-Sn(2) are twisted against each other by 10.3° in the former, which is smaller than in the latter (20.7°), and the tetrahedral distortion is much less pronounced in the former. The inner coordination spheres about the metal atoms are PtP2Sn2, SnC3Pt, and FeC10 (sandwich).
A triclinic PtHgSnGe-type tetramer (Teplova et al., 1980) is the only example that contains four different metal atoms. Two fragments Sn(C6F5)3 and HgGe(C6F5)3 are bound to the Pt(PPh3)2 unit. There are three distinct metal-metal bonds, which increased in the order 2.518(1) Å (Pt-Sn)<2.534(1) Å (Hg-Ge)<2.617(1) Å (Pt-Hg). The three metal atoms are four-coordinate, PtP2SnHg (square planar), SnC3Pt, and GeC3Hg (both are tetrahedral), and mercury is linearly coordinated (HgPtGe) with a Pt-Hg-Ge angle of 177.26(6)°.
This review article classified and analyzed 34 tetranuclear complexes of Pt3M type (10 examples), Pt2M2 type (15 examples), PtSn3 type (5 examples), Pt2NaAg type, PtSn2Fe type (2 examples), and PtSnHgGe type (1 example). The structures of the tetramers are complex. In the series of Pt3M-type tetramers, two main types prevail: one in which four metal atoms form a tetrahedral geometry and the other with a six-membered puckered Pt3X3 ring with Pt-M bonds. In the series of Pt2M2-type tetramers, four metal atoms form an approximately planar skeleton, and the other one represents a μ3-X bicapped triangular MPt2 or PM2Pt aggregate. In the series of PtM3, a Y shape of the respective metal atoms prevails.
A square planar arrangement with a different degree of distortion about the Pt atoms by far prevails with some examples of coordination numbers three, five, six, and even seven. The three- (Y shape) and seven- (4+3) coordinated ones are rarities. The M atoms are three- (Y-shaped; Li, Sn, Tl, Hg), four- (tetrahedral; Li, Ge, Sn, Hg), five- (mostly trigonal bipyramidal; Sn, Zn, Hg), and six- (pseudo-octahedral; Na, Sn, Tl and Cd) coordinate. The mean Pt-M bond distance increased in the order 2.435 Å (M=Ge)<2.645 Å (Sn)<2.676 Å (Zn)<2.822 Å (Hg)<2.827 Å (Li)<2.833 Å (Pb)<2.898 Å (Tl)<2.960 Å (Cd).
There is a great variety of coordinated ligands from mono- to pentadentate ones. The most common ligands contain P and C donors. The mean Pt-L bond distance increased in the order 2.00 Å (CL)<2.025 Å (bi-NL)<2.04 Å (NL)<2.20 Å (bi-CL)<2.275 Å (PL)<2.282 Å (Cl)<2.285 Å (bi-PL)<2.305 Å (tetra-PL)<2.375 Å (bi-SL). The mean Pt-L (bridge) bond length increased in the order 2.03Å (μ-CL)<2.045 Å (μ3-O)<2.06 Å (μ-CO)<2.08 Å (μ-OH)<2.175 Å (μ3-CO)<2.365 Å (μ-SL)<2.370 Å (μ3-S)<2.375 Å (μ-Cl). The mean Hg-L bond distance increased in the order 2.365 Å (Cl)<2.417 Å (μ3-S)<2.440 Å (tetra-NL)<2.442 Å (PL)<2.465 Å (μ-SL)<2.517 Å (Br)<2.660 Å (I)<2.710 Å (μ-Cl)<2.762 Å (μ-Br)<2.952 Å (μ-I). The mean Li-L bond distance increased in the order 1.76 Å (μ3-O)<1.93 Å (OL)<2.155 Å (μ-CL)<2.19 Å (Cl); for Sn-L the order is 1.96 Å (F)<2.08 Å (NL)<2.12 Å (OL)<2.125 Å (bi-Cl)<2.305 Å (Cl).
There is a great variety of metallocycles, and the effect of both electronic and steric factors can be seen from the mean values of the L-Pt-L ‘bite’ angles, which open in the order 72.0° (PCP)<74.0° (SCS)<82.2° (CC2S)<84.5° (CP2P)<84.8° (NC2N)<85.6° (OC3S)<98.0° (PC3P). The mean PtMPt angle opens in the order of M: 54.0° (M=Tl)<60.0° (Zn)<75.5° (Cd)<78.2° (Hg)<84.8° (Pb)<100.6° (Li)<175.8°(Sn). For the mean MPtM angles the order is 58.2° (Hg)<79.3° (Li)<85.7° (Sn)<104.5° (Cd).
The complex (NBu4)[[(η1-C6F5)2(μ-Cl)Pt}3Sn] (Usón et al., 1990) contains two crystallographic independent molecules within the same crystal that differ mostly by the degree of distortion. It is an example of distortion isomerism (Melník, 1982; Melník and Holloway, 2006). A study of crystallographic and structural data of heteropentanuclear platinum complexes with non-transition metals as partners is in progress.
The authors are grateful to Michael Lutter and Klaus Jurkschat, Technical University Dortmund, Germany, for technical support.
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Published Online: 2013-03-18
Published in Print: 2013-07-01