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DOI: 10.1039/C0CC02742A (Communication) Chem. Commun., 2011, 47, 406-408

Electrophilic functionalization of well-behaved manganese monoanions supported by m-terphenyl isocyanides†‡

Mason A. Stewart , Curtis E. Moore , Treffly B. Ditri , Liezel A. Labios , Arnold L. Rheingold and Joshua S. Figueroa *
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, Mail Code 0358, La Jolla, California, 92093-0358, USA. E-mail: jsfig@ucsd.edu; Fax: +1 858 822-4446

Received 23rd July 2010 , Accepted 24th August 2010

First published on 17th September 2010

Abstract

The m-terphenyl isocyanides CNArMes2 and CNArDipp2 support five-coordinate, isocyanide/carbonyl monoanions of manganese. For CNArDipp2, a bis-isocyanide anion is available that is remarkably well behaved upon reaction with electrophiles. Most notable is the formation of an unprecedented chloride-substituted metallostannylene.

Outside of the well-studied carbonylmetalate [Mn(CO)5]−, organometallic monoanions of manganese have received limited attention. Thus except for CO, little information is available concerning the effect of ligand modification on the stability or reactivity properties of monoanionic Mn centers. A notable exception is Cooper's homoleptic penta-isocyanide manganese monoanion, [Mn(CNXyl)5]− (Xyl = 2,6-Me2C6H3).1 The latter has been shown to be thermally unstable, but when generated in situ at low temperature, it reacts with main-group electrophiles such as Ph3SnCl in a manner mirroring [Mn(CO)5]−.1 Contrastingly, reactions between [Mn(CNXyl)5]− and alkyl electrophiles are much less straightforward than for [Mn(CO)5]−, and result in complexes derived from multiple migratory-insertion processes.2 In an effort to combine the well behaved reactivity profiles of [Mn(CO)5]− with the steric and electronic variability offered by isocyanide ligands, we sought to generate mixed carbonyl/isocyanide manganese monoanions and to study their properties. Herein we report that the encumbering m-terphenyl isocyanide ligands, CNArMes2 and CNArDipp2 (Mes = 2,4,6-Me3C6H2; Dipp = 2,6-(i-Pr)2C6H3),3–7 readily furnish carbonyl/isocyanide manganese monoanions of unique construction. In addition, we show that the bis-CNArDipp2 anion, [Mn(CO)3(CNArDipp2)2]−, is particularly well-behaved upon treatment with electrophiles and can be elaborated into a unique example of a chloride-substituted metallostannylene.8

Treatment of BrMn(CO)5 with three equivalents of CNArMes2 affords after work-up the tris-isocyanide dicarbonyl complex BrMn(CO)2(CNArMes2)3 as a 9[thin space (1/6-em)]∶[thin space (1/6-em)]1 mixture of trans,mer- and cis,mer-isomers (see the ESI‡). Interestingly, attempts to reduce this mixture of BrMn(CO)2(CNArMes2)3 with Na/Hg, Mg0, Na[C10H8] or KC8 lead to intractable mixtures under a variety of conditions. However, treatment of the BrMn(CO)2(CNArMes2)3 isomeric mixture sequentially with potassium anthracenide (K[C14H10]) and 18-crown-6 (18-c-6) in THF solution resulted in the formation of [K(18-c-6)][Mn(CO)2(CNArMes2)3], albeit in low yield (20%, Scheme 1). X-ray structural determination on crystals grown from DME solution revealed the salt, [K(DME)(18-c-6)][Mn(CO)2(CNArMes2)3], in which the five-coordinate, manganese monoanion features a trigonal bipyramidal (tbp) coordination geometry with apical CO ligands (Fig. 1A). A similar tbp coordination geometry was found for the anion in Cooper's salt [K(DME)(18-c-6)][Mn(CNXyl)5], despite the fact that [Mn(CO)5]− has been crystallographically characterized in both tbp9–11 and square-pyramidal12 geometries. Notably however, CO ligands are well known to prefer the equatorial positions in tbp complexes.13 Thus the apical CO orientation in [K(DME)(18-c-6)][Mn(CO)2(CNArMes2)3] likely results from minimization of unfavorable steric interactions between the encumbering ArMes2 units, despite the ability of CNArMes2 ligands to adopt 90° separations in certain Mo6 and Co7 complexes.


Synthesis of mixed isocyanide/carbonyl manganese monoanions.
Scheme 1 Synthesis of mixed isocyanide/carbonyl manganese monoanions.

Molecular structures of (A) the anionic component of [K(DME)(18-c-6)][Mn(CO)2(CNArMes2)3], (B) [Na(NCMe)2][Mn(CO)3(CNArDipp2)2], (C) mer,trans-Cl2(Me)SiMn(CO)3(CNArDipp2)2 and (D) mer,trans-ClSnMn(CO)3(CNArDipp2)2.
Fig. 1 Molecular structures of (A) the anionic component of [K(DME)(18-c-6)][Mn(CO)2(CNArMes2)3], (B) [Na(NCMe)2][Mn(CO)3(CNArDipp2)2], (C) mer,trans-Cl2(Me)SiMn(CO)3(CNArDipp2)2 and (D) mer,trans-ClSnMn(CO)3(CNArDipp2)2.

Once crystallized at −35 °C, [K(DME)(18-c-6)][Mn(CO)2(CNArMes2)3] can be readily manipulated in the solid state. However, it decomposes over the course of 4 h at room temperature in C6D6 solution. Such thermal stability of [K(DME)(18-c-6)][Mn(CO)2(CNArMes2)3] is moderately improved relative to K[Mn(CNXyl)5], but still prevented a convenient assessment of its reactivity properties. Therefore, we sought a thermally robust isocyanide-containing manganese monoanion that could be prepared in reasonable yields.

Accordingly, we next employed the encumbering m-terphenyl isocyanide ligand CNArDipp2. Treatment of BrMn(CO)5 with two equiv. of CNArDipp2 exclusively generated the bis-isocyanide tricarbonyl complex BrMn(CO)3(CNArDipp2)2 as a yellow crystalline solid. Notably, the latter does not react with additional equivalents of CNArDipp2 at temperatures up to 80 °C in C6D6 solution. X-Ray crystallography, solution 13C{1H} NMR and FTIR spectroscopy established that BrMn(CO)3(CNArDipp2)2 possesses a trans,mer-orientation of isocyanide and carbonyl ligands (see the ESI‡). Such stereoisomerism is identical to that observed for the valence-isoelectronic molybdenum complexes Mo(NCMe)(CO)3(CNArDipp2)2 and Mo(THF)(CO)3(CNArDipp2)2.6

In contrast to BrMn(CO)2(CNArMes2)3, Na/Hg reduction of mer,trans-BrMn(CO)3(CNArDipp2)2 generated the bright red salt, Na[Mn(CO)3(CNArDipp2)2], in 47% isolated yield (Scheme 1). Importantly, Na[Mn(CO)3(CNArDipp2)2] retains its integrity in C6D6 solution at room temperature for several days. It is interesting to speculate that the combined presence of three rather than two CO units, and the steric protection offered by the CNArDipp2 ligand adequately bridges the stability extremes represented by [Mn(CNXyl)5]− and [Mn(CO)5]−. Crystallographic structure determination on crystals grown from a MeCN/toluene mixture revealed the contact ion pair [Na(NCMe)2][Mn(CO)3(CNArDipp2)2], in which the Na counterion interacts with two Dipp units and a carbonyl ligand through an asymmetric η2-O,C-interaction (Fig. 1B; d(O3–Na1) = 2.340(5) Å, d(C4–Na1) = 2.795(5) Å). The Mn center adopts a tbp geometry with apical isocyanide groups, thereby providing a contrast to the apical CO ligands found in [K(DME)(18-c-6)][Mn(CO)2(CNArMes2)3]. Despite the presence of the encumbering CNArDipp2 ligands, the apical isocyanide orientation in [Na(NCMe)2][Mn(CO)3(CNArDipp2)2] is also electronically preferred.13 This fact is demonstrated by the apical isocyanide units in the neutral d8 complex Fe(CO)3(CNMe)2, in which steric interferences are minimal.14FTIR analysis of Na[Mn(CO)3(CNArDipp2)2] in C6D6 revealed a ν(CN) stretch of 1910 cm−1 and ν(CO) stretches of 1896 and 1773 cm−1, which are consistent with the presence of substantial π-back donation from the Mn center (ν(CN) of CNArDipp2 = 2118 cm−1 (C6D6)).6 In addition, Na[Mn(CO)3(CNArDipp2)2] gives rise to C[triple bond, length as m-dash]O and C[triple bond, length as m-dash]N13C{1H} NMR resonances of 275.7 and 237.4 ppm, respectively, which are also reflective of a reduced Mn center.

Gratifyingly, Na[Mn(CO)3(CNArDipp2)2] shows well-defined reactivity with a range of electrophiles. For example, treatment of Na[Mn(CO)3(CNArDipp2)2] with HCl or methyl iodide (MeI) readily generates the corresponding hydride and methyl complexes, mer,trans-HMn(CO)3(CNArDipp2)2 and mer,trans-MeMn(CO)3(CNArDipp2)2, respectively, as thermally stable crystalline solids (Scheme 2). The robust nature of mer,trans-MeMn(CO)3(CNArDipp2)2 is particularly noteworthy since similar treatment of K[Mn(CNXyl)5] with alkyl electrophiles, including MeI, results exclusively in migratory insertion products. Thus, the RMn(CO)3(CNArDipp2)2 platform successfully mirrors the well-defined behavior of the pentacarbonyl system, RMn(CO)5,15 while featuring the steric protection attendant with m-terphenyl isocyanide ligation.


Electrophilic substitution reactions for Na[Mn(CO)3(CNArDipp2)2].
Scheme 2 Electrophilic substitution reactions for Na[Mn(CO)3(CNArDipp2)2].

Heavier main-group electrophiles also react cleanly with Na[Mn(CO)3(CNArDipp2)2]. Accordingly, treatment of Na[Mn(CO)3(CNArDipp2)2] with trichloromethylsilane (MeSiCl3) smoothly provides bright yellow complex, mer,trans-Cl2(Me)SiMn(CO)3(CNArDipp2)2 (Scheme 2, Fig. 1C). Notably, the latter is a relatively rare example of a dichloromethylsilane complex and compliments the recently reported manganese trichlorosilane complex Cl3SiMn(CO)5.16

Most interestingly however, the [Mn(CO)3(CNArDipp2)2] fragment can stabilize a chloride-substituted metallostannylene. Treatment of a dilute Et2O solution of Na[Mn(CO)3(CNArDipp2)2] with SnCl2 generates the metallostannylene complex, mer,trans-ClSnMn(CO)3(CNArDipp2)2 as a thermally stable olive-green solid (Scheme 2). The electronic absorption spectrum§ (Et2O) of mer,trans-ClSnMn(CO)3(CNArDipp2)2 features a weak band at λmax = 490 nm consistent with a 5s → 5p transition characteristic of two-coordinate stannylenes.8b,17,18 Crystallographic characterization of mer,trans-ClSnMn(CO)3(CNArDipp2)2 revealed an Mn–Sn–Cl angle of 101.53(4)° which reflects both the limited s-orbital character used by the divalent Sn center in bonding19 and the absence of any significant π-interaction between Mn and Sn.20,21 Indeed, DFT calculations on the model complex mer,trans-ClSnMn(CO)3(CNArPh2)2 are consistent with this view, revealing a non-interacting LUMO of exclusively Sn 5p character and a HOMO predominantly of Mn–Sn σ-bonding character (Fig. 2). The three next highest-lying filled molecular orbitals calculated for mer,trans-ClSnMn(CO)3(CNArPh2)2 are Mn → CO and Mn → CNR π-backbonding in character (see the ESI‡), which complete the picture of a metallostannylene derived from a classic Hoffmann-type d6ML5 fragment.13,22 This feature provides a contrast to the limited set of reported metallostannylenes, as they are exclusively based on metal-cyclopentadienyl fragments (i.e. [CpRM]).8,23,24


Selected frontier molecular orbitals calculated for the model ClSnMn(CO)3(CNArPh2)2: LUMO (top) and HOMO (bottom). ADF 2007.01; BP86/TZ2P.
Fig. 2 Selected frontier molecular orbitals calculated for the model ClSnMn(CO)3(CNArPh2)2: LUMO (top) and HOMO (bottom). ADF 2007.01; BP86/TZ2P.

Finally, mer,trans-ClSnMn(CO)3(CNArDipp2)2 represents a unique example of a metallostannylene lacking an encumbering supporting group on the Sn center. Whereas the CNArDipp2 ligands undoubtedly provide the protection necessary to stabilize the Mn–Sn–Cl unit, it is interesting to speculate that further synthetic elaboration of the ligated Sn center will be facile. Studies aimed at exploring such possibilities for mer,trans-ClSnMn(CO)3(CNArDipp2)2, in addition to further identifying the chemistry available to [Mn(CO)2(CNArMes2)3]− and [Mn(CO)3(CNArDipp2)2]−, are under way.

We are grateful to the US National Science Foundation for support (CAREER award CHE-0954710). Dr Nils Weidemann and Stephen P. George are thanked for experimental assistance. We also thank the reviewers for very helpful comments.

Notes and references

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Footnotes

† This article is part of the ‘Emerging Investigators’ themed issue for ChemComm
‡ Electronic supplementary information (ESI) available: Experimental procedures, crystallographic structure determinations and results of computational studies. CCDC reference numbers 785481–785488. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cc02742a
§ Unfortunately, repeated attempts to locate a 119Sn NMR signal for mer,trans-ClSnMn(CO)3(CNArDipp2)2 were unsuccessful. We believe this results from significant line broadening caused by the quadrupolar Mn nucleus (55Mn, I = 5/2, 100%). However, the 1H NMR spectrum (C6D6) of ClSnMn(CO)3(CNArDipp2)2 does not indicate the presence of resonances indicative of tin-hydride units (i.e.Sn(H)n), further supporting the presence of a divalent, rather than tetravalent, Sn center in mer,trans-ClSnMn(CO)3(CNArDipp2)2.
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