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Transition metal pyridine complexes
Transition metal pyridine complexes
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Transition metal pyridine complexes
Structure of [Ru(NH3)5py]2+, illustrating the steric avoidance of the 2,6-protons and the cis ligands.[1]

Transition metal pyridine complexes encompass many coordination complexes that contain pyridine as a ligand. Most examples are mixed-ligand complexes. Many variants of pyridine are also known to coordinate to metal ions, such as the methylpyridines, quinolines, and more complex rings.

Bonding

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With a pKa of 5.25 for its conjugate acid, pyridine is about 15x less basic than imidazole. Pyridine is a weak pi-acceptor ligand. Trends in the M-N distances for complexes of the type [MCl2(py)4]2+ reveal an anticorrelation with d-electron count.[2] Low-valent metal complexes of pyridines are known, e.g. IrI(diene)(pyridine)Cl. The role of pyridine as a Lewis base extends also to main group chemistry. Examples include sulfur trioxide pyridine complex SO3(py) and pyridine adduct of borane, BH3py.

Pyridine is classified as L ligand in the covalent bond classification method. In the usual electron counting method, it is a two-electron ligand. With respect to HSAB theory, it is intermediate softness, reflecting its small but significant properties as a pi-acceptor ligand.

Classification of metal-pyridine complexes

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Many metal pyridine complexes are known. These complexes can be classified according to their geometry, i.e. octahedral, tetrahedral, linear, etc.

Octahedral complexes

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trans-[MCl2(pyridine)4]n+ is a common type of transition metal pyridine complex.
Chloro(pyridine)cobaloxime.
Crabtree's catalyst.

Owing to the relatively wide C-N-C angle, the 2,6-hydrogen atoms interfere with the formation of [M(py)6]z complexes. A few octahedral homoleptic pyridine complexes are known. These complex cations are found in the salts [Ru(py)6]Fe4(CO)13 and [Ru(py)6](BF4)2.[3][4] Some compounds with the stoichiometry M(py)6(ClO4)2 have been reformulated as [M(py)4(ClO4)2].(py)2 [5]

A common family of pyridine complexes are of the type [MCl2(py)4]n+. The chloride ligands are mutually trans in these complexes.

MCl2(pyridine)4 complexes
formula CAS RN key properties Preparation
TiCl2(pyridine)4 131618-68-3 blue, triplet
dTi-N=2.27 Å, dTi-Cl = 2.50 Å (thf solvate)[6]		 TiCl3(thf)3 + KC8 + py[7]
VCl2(pyridine)4 15225-42-0 purple[8] VCl3 + Zn + py[9]
CrCl2(pyridine)4 51266-53-6 green
dCr-Cl = 2.80 Å dCo-Cl = 2.16 Å 		CrCl2 + py[10]
MnCl2(pyridine)4 14638-48-3 1.383
FeCl2(pyridine)4 15138-92-8 yellow
dFe-Cl = 2.43 Å		 FeCl2 + py[2]
CoCl2(pyridine)4 13985-87-0 blue
dCo-Cl = 2.44 Å		 CoCl2 + py[2]
[CoCl2(pyridine)4]Cl 27883-34-7 green (hexahydrate)
dCo-Cl = 2.25 Å, dCo-N = 1.98 Å[11]
as [CoCl3(py)]− salt		 CoCl2(pyridine)4 + Cl2[12]
NiCl2(pyridine)4 14076-99-4 blue
dNi-Cl = 2.44 Å		 NiCl2 + py[2]
NbCl2(pyridine)4 168701-43-7 dNb-N = 2.22 Å, dNb-Cl = 2.51 Å NbCl4(thf)2 + KC8 + py[6]
[MoCl2py)4]Br3 Br3− salt[13] yellow
dMo-Cl= 2.41 Å, dMo-N=2.20 Å
TcCl2py)4 172140-87-3 purple
dTc-Cl = 2.41 Å, dTc-N = 2.10 Å[14]		 TcCl4py2 + Zn + py
RuCl2(pyridine)4 16997-43-6 red-orange
dRu-N=2.08 Å, dRu-Cl=2.40 Å		 RuCl3(H2O)x + py [15]
[RhCl2(pyridine)4]+ 14077-30-6 (Cl− salt) yellow RhCl3(H2O)3 + py + cat. reductant[16]
OsCl2(pyridine)4 137822-02-7 brown
dOs-Cl = 2.40 Å, dOs-N= 2.068 Å		 K3OsCl6 + py + (CH2OH)2/140 °C[17]
[IrCl2(pyridine)4]+ yellow
1.35 Å (chloride.hexahydrate)[18]

The tris(pyridine) trihalides, i.e., [MCl3(py)3] (M = Ti, Cr, Rh[19] Ir), are another large class of M-Cl-py complexes.

Four-coordinate complexes

[edit]
Collins reagent, the complex CrO3(pyridine)2, is a reagent in organic chemistry.[20]

Four-coordinate complexes include tetrahedral and square planar derivatives. Examples of homoleptic tetrahedral complexes include [M(py)4]n+ for Mn+ = Cu+,[21] M = Ni2+,[22] Ag+,[23] and Ag2+.[24] Examples of homoleptic square planar complexes include the d8 cations [M(py)4]n+ for Mn+ = Pd2+,[25] Pt2+,[26] Au3+.[27]

Ni(ClO4)2(3-picoline)2 can be isolated in two isomers, yellow, diamagnetic square planar or blue, paramagnetic tetrahedral.[28]

Mn(II) and Co(II) form both tetrahedral MCl2py2 and octahedral MCl2py4 complexes, depending on conditions:[29]

MCl2py2 + 2 py → MCl2py4

Two- and three-coordinate complexes

[edit]

Many examples exist for [Au(py)2]+.[27] [Ag(py)3]+ and [Cu(py)2]+ are also precedented.[30][27]

Pi-complexes

[edit]

The η6 coordination mode, as occurs in η6 benzene complexes, is observed only in sterically encumbered derivatives that block the nitrogen center.[31]

[edit]

Picolines

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Many substituted pyridines function as ligands for transition metals. The monomethyl derivatives, the picolines (2-, 3-, and 4-picoline), are best studied. 2-Picolines are sterically impeded from coordination.[28]

2,2'-bipy

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Main article: Transition metal complexes of 2,2'-bipyridine

Coupling of two pyridine rings at their 2-positions gives 2,2'-bipyridine, a widely studied bidentate ligand. A number of differences are apparent between pyridine and bipyridine complexes. Many [M(bipy)3]z complexes are known, whereas analogous [M(py)6]z complexes are rare and apparently labile. Bipyridine is a redox-noninnocent ligand, as illustrated by the existence of complexes such as [Cr(bipy)3]0. The pyridine analogues of such complexes are unknown. The dichloro complexes [MCl2(bipy)2]n+ tend to be cis, as exemplified by RuCl2(bipy)2. In contrast, the complexes [MCl2(py)4]n+ are always trans.

Imidazoles

[edit]
Main article: Transition metal imidazole complex

Imidazoles comprise another major series of N-heterocyclic ligands. Unlike pyridines, imidazole derivatives are common ligands in nature.

Applications and occurrence

[edit]

Crabtree's catalyst, a popular catalyst for hydrogenations, is a pyridine complex.

Although transition metal pyridine complexes have few practical applications, they are widely used synthetic precursors. Many are anhydrous, soluble in nonpolar solvents, and susceptible to alkylation by organolithium and Grignard reagents. Thus CoCl2(py)4 has proven very useful in organocobalt chemistry[32][33] and NiCl2(py)4 useful in organonickel chemistry.[34]

References

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  1. ^ Shin, Yeung-gyo K.; Szalda, David J.; Brunschwig, Bruce S.; Creutz, Carol; Sutin, Norman (1997). "Electronic and Molecular Structures of Pentaammineruthenium Pyridine and Benzonitrile Complexes as a Function of Oxidation State". Inorganic Chemistry. 36 (14): 3190–3197. doi:10.1021/ic9700967. PMID 11669976.
  2. ^ a b c d Long, Gary J.; Clarke, Peter J. (1978). "Crystal and Molecular Structures of trans-Tetrakis(pyridine)dichloroiron(II), -Nickel(II), and -Cobalt(II) and trans-Tetrakis(pyridine)dichloroiron(II) Monohydrate". Inorganic Chemistry. 17 (6): 1394–1401. doi:10.1021/ic50184a002.
  3. ^ Templeton, Joseph L. (1979). "Hexakis(pyridine)ruthenium(II) tetrafluoroborate. Molecular structure and spectroscopic properties". Journal of the American Chemical Society. 101 (17): 4906–4917. doi:10.1021/ja00511a020.
  4. ^ Lichtenberg, Crispin; Adelhardt, Mario; Wörle, Michael; Büttner, Torsten; Meyer, Karsten; Grützmacher, Hansjörg (2015). "Mono- and Dinuclear Neutral and Cationic Iron(II) Compounds Supported by an Amidinato-diolefin Ligand: Characterization and Catalytic Application". Organometallics. 34 (12): 3079–3089. doi:10.1021/acs.organomet.5b00395.
  5. ^ Soldatov, Dmitriy V.; Ripmeester, John A. (1998). "Hexapyridine Addition Compounds of Metal(II) Salts (Nickel and Magnesium Perchlorates, Magnesium Nitrate): Their Supramolecular Nature, and Anion Coordination of the Metal Cation [1]". Supramolecular Chemistry. 9 (3): 175–181. doi:10.1080/10610279808034984.
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