Methylcyclopentadienyl manganese tricarbonyl

Methylcyclopentadienyl manganese tricarbonyl

Names
IUPAC names tricarbonyl(methyl-η5- cyclopentadienyl)manganese
Other names MMT
Identifiers
CAS Registry Number	12108-13-3 Y
RTECS number	OP1450000
Properties
Chemical formula	C9H7MnO3
Molar mass	218.09 g/mol
Appearance	pale yellow liquid herbaceous odor
Density	1.38 g/cm3
Melting point	−1 °C (30 °F; 272 K)
Boiling point	232 to 233 °C (450 to 451 °F; 505 to 506 K)
Solubility in water	low
Solubility in other solvents	alkane (petrol)
Structure
Coordination geometry	Tetrahedral at Mn
Hazards
Main hazards	flammable
R-phrases	23/24/25-40
S-phrases	23-26-36/37/39-45
Flash point	96 °C; 205 °F; 369 K
US health exposure limits (NIOSH):
PEL (Permissible)	C 5 mg/m3[1]
Related compounds
Related compounds	ferrocene Mn2(CO)10 dicyclopentadiene
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Methylcyclopentadienyl manganese tricarbonyl (MMT or MCMT) is an organomanganese compound with the formula (CH₃C₅H₄)Mn(CO)₃. Marketed as a supplement to the gasoline additive tetraethyllead to increase a fuel's octane rating, MMT was later used in unleaded gasoline. Following the implementation of the Clean Air Act (United States) (CAA) in 1970, MMT continued to be used alongside TEL in the US during the phase-out of leaded gasoline (prior to TEL being finally banned from US gasoline in 1995) and was also used in unleaded gasoline until 1977. Ethyl Corporation obtained a waiver from EPA in 1995. The waiver allows the use of MMT in US unleaded gasoline (not including reformulated gasoline) at a treat rate equivalent to 8.3 mg Mn/l.^[2]

MMT has been used in Canadian gasoline since 1976 at a concentration up to 18 mg Mn/l (though the importation and interprovincial trade of gasoline containing MMT was restricted briefly during the period 1997-1998)^[3][4] and was introduced into Australia in 2000. It is sold under the tradename HiTEC® 3000.^[5] It was also marketed as Ecotane by T2 Labs.^[6]

History of use in the U.S.

Tetraethyllead (TEL) has been, and still is on a limited scale, used as an additive to increase the octane rating of gasoline. In the U.S., TEL was manufactured and marketed by the Ethyl Corporation, a joint venture between General Motors and DuPont. When the United States Environmental Protection Agency (EPA) ordered the phasing out of leaded gasoline in 1973, new fuel additives were sought. Developed as an octane improver, though initially marketed in 1958 as a smoke suppressant for gas turbines, MMT was used in 1974 as an additive in unleaded gasoline.

In 1977, the U.S. Congress amended the CAA to require advance approval by the EPA for the continued use of fuel additives such as MMT, ethanol, ETBE, etc.^[7] The new CAA amendment required a “waiver” to allow use of fuel additives made of any elements other than carbon, hydrogen, oxygen (within certain limits) and nitrogen.^[8] To obtain a waiver, the applicant has to demonstrate that the fuel additive will not lead to a failure of vehicle emission control systems.

Ethyl Corporation applied for a waiver in both 1978 and 1981 and in both cases the applications were denied because of stated concerns that MMT might damage catalytic converters and increase hydrocarbon emissions. In 1988, Ethyl began a new series of discussions with EPA to determine a program for developing the necessary data to support a waiver application. In 1990, Ethyl filed its third waiver application prompting an extensive four-year review process. In 1993, the U.S. EPA determined that use of MMT at 8.3 mg Mn/l would not cause, or contribute to, vehicle emission control system failures.^[9]

EPA ultimately denied the waiver request in 1994, however, due to uncertainty whether manganese emissions from use of MMT might present a potential public health concern.^[10]

As a result of this ruling, Ethyl Corporation began a legal battle with the EPA claiming that EPA had exceeded its authority by denying the waiver on these grounds. This was upheld by the US Court of Appeals^[11] and EPA subsequently granted a waiver which allows the use of MMT in US unleaded gasoline (not including reformulated gasoline) at a treat rate equivalent to 8.3 mg Mn/l.

Implementation of this less-toxic alternative to TEL has been controversial. Opposition from automobile manufacturers and some areas of the scientific community has reportedly prompted oil companies to voluntarily stop the use of MMT in some of their countries of operation.^[12]

MMT is currently manufactured in the U.S. by the Afton Chemical Corporation, a subsidiary of Newmarket Corporation.^[13] It is also produced and marketed as "Cestoburn" by Cestoil Chemical Inc. in Canada.

Structure and synthesis

The compound is prepared by reduction of bis(methylcyclopentadienyl)manganese using triethylaluminium. The reduction is conducted under an atmosphere of carbon monoxide.

MMT is a so-called half-sandwich compound, or more specifically a piano-stool complex (since the three CO ligands are like the legs of a piano stool). The manganese atom in MMT is coordinated to three carbonyl groups as well as to the methylcyclopentadienyl ring. These hydrophobic organic ligands make MMT highly lipophilic.

Related compounds

A variety of related complexes are known, including ferrocene, which has also been used as an additive to gasoline.^[12]

Although of no practical value, the related compound (C₅H₅)Mn(CO)₃ is also well studied. Up to two of the CO ligands in MMT can be replaced with thiocarbonyl groups, as illustrated by the compounds (CH₃C₅H₄)Mn(CS)₂CO and (CH₃C₅H₄)Mn(CS)(CO)₂.^[14]

Safety

The health considerations associated with MMT are divided into two areas; (1) MMT in its original chemical form and (2) the manganese combustion products emitted from vehicles operating on gasoline containing MMT as an octane improver.

MMT

MMT is not accessible by the general public. Indeed, a 2002 study argued that to the general public dermal absorption from accidental gasoline spills, misuse of gasoline as a solvent cleaner, and deliberate gasoline fume inhalation were the main sources of potential MMT exposure.

A 2003 study by the NICNAS in Australia suggested that MMT, in its raw concentrated form prior to its addition into gasoline, was highly toxic.^[15] Other studies have found the neat chemical to be cytotoxic and especially damaging to dopaminergic PC-12 cells.^[16] Furthermore, Toxicokinetic studies have shown that manganese in the form of neat MMT absorbs into rat blood plasma at a rate 37 times that of inorganic manganese.^[17]

Immediate symptoms of MMT exposure are mild skin and eye irritation. Short-term symptoms resulting from MMT poisoning by high exposures include giddiness, headache, nausea, and difficulties in breathing. Animal studies have shown that chronic exposure to MMT can result in damage to the liver and kidneys.^[18] Chronic exposure to high levels of manganese, typically in certain occupational activities, has also been known to cause manganism, a disease with symptoms similar to those of Parkinson's disease.

The Occupational Safety and Health Administration has set a permissible exposure limit at a ceiling of 5 mg/m³, while the National Institute for Occupational Safety and Health recommends workers not be exposed to more than 0.2 mg/m³, over an eight-hour time-weighted average.^[19]

Combustion products

The health hazards associated with manganese compounds emitted from vehicles operating on gasoline containing MMT have been debated for decades. In 1994 (reaffirmed in 1998, 2001 and 2010), Health Canada concluded that “airborne manganese resulting from the combustion of MMT in gasoline powered vehicles is not entering the Canadian environment in quantities or under conditions that may constitute a health risk”^[20] and confirmed they were taking no action with respect to MMT. The 2003 NICNAS study ruled that the airborne concentrations of manganese as a result of car emissions from vehicles using fuel containing MMT posed no health hazard.

Additional health studies, overseen by the US Environmental Protection Agency (EPA), have been conducted in order to explain the transport of manganese in the body. These studies, the most recent available, published by the Hamner Institutes for Health Sciences from 2007 through 2011 and submitted to the US Environmental Protection Agency (EPA) under the framework of the Clean Air Act, show that the body’s natural mechanisms can handle a wide range of manganese intake, whether from inhalation or ingestion.^[21] While these mechanisms can be overwhelmed if exposures to manganese are very high (as in the case of some occupational exposures), the testing confirms that the body can safely handle inhaled manganese at, and well above, levels observed when MMT is used in gasoline. The studies also indicate that MMT use is safe for the entire population including vulnerable groups such as infants and the elderly. Although the EPA will now formally review the research findings, the US Agency recommended this data as meeting the Clean Air Act’s health study’s requirements and objectives.

Vehicle manufacturer recommendations

Some manufacturers recommend against use of MMT in their vehicles, while others specifically prohibit its use.^[22]

References