Metal complexes of borohydride

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Metal complexes of borohydride refers to coordination complexes containing the borohydride (BH4-) ligand. The inventory is in the hundreds. [1] Although these compounds have few practical applications, they have attracted much attention for their unusual structures. [2] [3]

Contents

Bonding modes

Structure of M(BH4)4 (M = Zr, Hf, Np, Pu). Color code: blue = M, yellow = B. CSD CIF FUZBOR01.png
Structure of M(BH4)4 (M = Zr, Hf, Np, Pu). Color code: blue = M, yellow = B.
FeH(BH4)(dmpe)2, a complex of k -BH4 (Me = CH3). HFeBH4(dmpe)2.svg
FeH(BH4)(dmpe)2, a complex of κ -BH4 (Me = CH3).

The tetrahedral anion BH4- is isoelectronic with methane but more electron-rich owing to the electropositive character of boron and the negative charge. It binds to soft metal centers. Borohydride binds metals by forming M-H-B linkages. A variety of bonding modes are observed: κ1-, κ2-, and κ3- in which the BH4- is bonded via one, two, and three H atoms, respectively. Examples include Cu(κ1-BH4)(PMePh2)3, Cu(κ2-BH4)(PPh3)2, and the homoleptic complexes M(κ3-BH4)4 (M = Zr, Hf, Np, and Pu). The latter highlight the ability of borohydride, which is compact, to give complexes of very high coordination numbers, Borohydride often functions as a bridging ligand. [1]

Stereodynamics

Borohydride ligands characteristically exhibit fluxionality. They are subject to rapid "bridge-terminal exchange". For example, the room-temperature 1H NMR spectrum of [Ti(CO)4(κ3-BH4)]- shows only one hydride signal. At low temperatures, two signals in a ratio of 1:3 are resolved. [6]

Preparation

Commonly, borohydride complexes are prepared by salt metathesis reactions using potassium borohydride or sodium borohydride: [2] [3]

CuCl(PMePh2)3 + NaBH4 → Cu(κ1-BH4)(PMePh2)3 + NaCl (Me = CH3, Ph = C6H5)

The homoleptic actinide derivatives are produced using aluminium borohydride:

AnF4 + 2 Al(BH4)3 → An(κ3-BH4)4 + 2AIF2BH4 (An = actinide metal)

The metathesis is accompanied by redox in the case of Ti(IV): [7]

2 TiCl4 + 8 LiBH4 + 2 thf → 2 Ti(κ3-BH4)3(thf) + + 8 LiCl + H2 + B2H6 (thf = tetrahydrofuran)

Some metal hydride complexes react with sources of borane as well to give borohydrides.

Applications

Metal complexes of borohydride have received some attention because they are volatile. The borohydrides of the actinides were investigated for isotope separation during the Manhattan Project. [1] Some borohydride complexes have been used as hydride reducing agents. [8]

See also

References

  1. 1 2 3 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 168. ISBN   978-0-08-037941-8.
  2. 1 2 Marks, T. J.; Kolb, J. R. (1977). "Borohydride". Chem. Rev. 77: 263. doi:10.1021/cr60306a004.
  3. 1 2 Besora, M.; Lledós, A. (2008). "Coordination Modes and Hydride Exchange Dynamics in Transition Metal Tetrahydroborate Complexes". Structure and Bonding. 130: 149–202. doi:10.1007/430_2007_076. ISBN   978-3-540-78633-7.
  4. ↑ Burkmann, Konrad; Habermann, Franziska; Schumann, Erik; Kraus, Jakob; Störr, Bianca; Schmidt, Horst; Brendler, Erica; Seidel, Jürgen; Bohmhammel, Klaus; Kortus, Jens; Mertens, Florian (2024). "Structural and Thermodynamic Investigations of Zr(BH4)4 and Hf(BH4)4 between 280 K and their Decomposition Temperatures". New Journal of Chemistry. 48 (6): 2743–2754. doi:10.1039/D3NJ05601E.
  5. ↑ Bau, Robert; Yuan, Hanna S.H.; Baker, Murray V.; Field, Leslie D. (1986). "An X-Ray Study of FeH(dmpe)2(BH4): A Compound Containing a Singly-Bridged BH4 Ligand with a Bent Fe-H-B Linkage". Inorganica Chimica Acta. 114 (2): L27 –L28. doi:10.1016/S0020-1693(00)86434-8.
  6. ↑ Makhaev, Viktor D. (2000). "Structural and Dynamic Properties of Tetrahydroborate Complexes". Russian Chemical Reviews. 69 (9): 727–746. Bibcode:2000RuCRv..69..727M. doi:10.1070/rc2000v069n09abeh000580.
  7. ↑ Franz, H.; Fusstetter, H.; Nöth, H. (1976). "Äther-Addukte von Tris(boranato)-titan(III) und dimere Alkoxy-bis(boranato)-titan(III)-Verbindungen". Z. Anorg. Allg. Chem. 427: 97–113. doi:10.1002/zaac.654270202.
  8. ↑ Barda, David A. (2001). "Bis(triphenylphosphine)copper(I) Borohydride". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rb228. ISBN   0-471-93623-5.
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