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]

Bonding modes

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Structure of M(BH4)4 (M = Zr, Hf, Np, Pu).[4] Color code: blue = M, yellow = B.
FeH(BH4)(dmpe)2,[5] a complex of κ1-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

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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

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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

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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

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References

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  1. ^ a b c Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 168. doi:10.1016/C2009-0-30414-6. ISBN 978-0-08-037941-8.
  2. ^ a b Marks, T. J.; Kolb, J. R. (1977). "Borohydride". Chem. Rev. 77: 263. doi:10.1021/cr60306a004.
  3. ^ a b 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.